14

CHAPTER 2

LITERATURE REVIEW

2.1 RAPID PROTOTYPING FOR MEDICAL APPLICATION

Gibson et al (2006) illustrated a number of instances where RP and

associated technology had been successfully used for medical applications.

In these studies, they have been discussed, how the technology had been

applied in order to solve medical related problems. It is found that RP has

been helpful in a number of ways to solve medical problems. They also

pointed out some limitations in the technology and how to improve the

technology in future. The study mainly suggested

Increase the machine builds speed.

The machines ought to be capable of building usable models

in less than half a day.

It is important to have full integration of software and

hardware to suit medical use (machine, language

compatibility).

The model obtained from RP must be strong and flexible that

can take a lot of physical handling (need for better materials).

Develop a new material and machines that facilitate

flexibility, changing material feed.

15

Singare et al (2005) attempted to develop a computer assisted

pre-fabricated implant design and manufacturing system to improve the

aesthetic outcome in chin surgery. Fabrication of custom fabricated chin

augmentation implant using medical RP methods was presented. Based on CT

data, the inner and outer surface of the prosthesis was designed to fit the bone

surface exactly. SLA was used for the production of the physical model. The

surgical planning was performed using the implants and skull models. The

resulting SLA implant is used for the production of a mould, which is used to

cast the Titanium part. Some of the findings are,

This approach showed significant results in chin

augmentation.

Intra-operative fit and esthetic was achieved.

The operating time was considerably reduced.

Post operatively, the patients experienced the restoration of a

natural chin contour.

There were no complications and no implant has to be

removed.

Also, the study suggested that the implant delivery time can be

reduced if RP can directly fabricate a bio medical implant.

RP developed for building components from CAD in the motor

industry, are now being applied in medicine to build models of human

anatomy from high resolution multiplanar imaging data such as CT. The

established technique of stereo lithography and the more recent Selective

Laser Sintering (SLS), both build up an object layer by layer. Models have

applications in surgical planning, design of customised implants and training.

Preliminary experience of using the SLS technique for medical applications is

16

described, addressing questions regarding image processing, data transfer and

manufacture. Pilot models, built from nylon, included two skulls (a child with

craniosynostosis and an adult with hypertelorism) and a normal femur which

was modelled for use in a bioengineering test of an artificial hip. The

dimensions of the models were found to be in good agreement with the CT

data from which they were built-for the child’s skull the difference between

the model and the CT data was less than l.0 ±.05 mm in each direction. A

combination of existing software packages may be used for data conversion.

Ideally, image data of high spatial resolution was used. The pilot models

generated sufficient clinical interest for the technique to be pursued in the

orthopaedic (Berry et al (1997)).

Wei Sun and Pallavi Lal (2002) have reviewed the recent

development on computer aided tissue engineering. The utilisation of

computer-aided technologies in tissue engineering has evolved in the

development of a new field of Computer Aided Tissue Engineering (CATE).

This article reviews recent development and application of enabling computer

technology, imaging technology, Computer-Aided Design and Computer-

Aided Manufacturing (CAD and CAM), and RP technology in tissue

engineering, particularly, in computer-aided tissue anatomical modeling, 3D

anatomy visualisation and 3D reconstruction, CAD based anatomical

modeling, computer-aided tissue classification, computer-aided tissue

implantation and prototype modeling assisted surgical planning and

reconstruction.

Implant design and fabrication using model a new approach by

selective laser melting. Initially, RP has been used for preoperative planning

of surgical operations. But nowadays, bio compatible materials are used for

replacement of soft and hard tissues in surgery. This study shows a new

application of a laser beam for RP using Selective Laser Melting (SLM)

17

process for production of detailed anatomic macro and micro structures in

dense steel and Titanium (Wehmoller et al (2005)). They reconstructed 3

regions such as (i) the complete lower jaw including teeth, mandibular joint

and the canal of mandibular nerve. (ii) 3rd

lumbar vertebra consisting of the

vertical body and its process. (iii) a middle segment of the tubular femoral

bone.

They have concluded that complex geometries and structures

demonstrate and demand the possibilities of this new technology, which in the

near future will help to revolutionise not only the geometrical configuration

but also reduce the cost and manufacturing time of permanent metal body

implants.

Richard Bibb and Dominic Eggbeer (2006) studied to explore the

application of RM to the production of patient specific, custom-fitting

Removable Partial Denture (RPD) alloy frameworks. RPDs are metal

frameworks designed to retain artificial replacement teeth in the oral cavity. It

was undertaken by a case study. A RPD was designed using CAD software,

based on a digitally scanned cast produced from an impression of the patient’s

mouth. It was then exported as an STL file for preparation by direct

manufacture using SLM. 316L Stainless Steel and Chromium-Cobalt alloy

materials were used to fabricate accurate framework in RP. Then the accuracy

of fit and function on the patient cast and on the patient in clinic were

assessed. It confirms that an RM approach could produce fully functional,

precisely fitting RPD frameworks for specific individual patients. This study

provides some practical guidance for the custom fitting applications.

Fiankang He et al (2006) attempted for custom fabrication of a

composite, hemi-knee joint based on RP and RT techniques. The 3D freeform

model of a femur bone was reconstructed based on CT images via reverse

engineering and the accuracy was evaluated. The negative image of artificial

18

bone was designed with interconnected micro structures (250 - 300 µm). SLA

process was used to fabricate the epoxy resin mould of a hemi-knee joint and

the negative pattern of an artificial bone. Based on these moulds, a Titanium

alloy hemi-knee joint and a porous bio ceramic artificial bone were fabricated

by quick casting. They concluded that,

The 3D reconstructed freeform model of the femur bone

conformed to the original anatomy within a maximum of

deviation 0.206 mm.

The sintered artificial bone had inter connected micro pores

(250 µm) and micro channels (300 µm).

The substitute matched well with the surrounding tissues and

bones with sufficient mechanical strength.

Also, the study suggested that the further in-vivo research is needed

to provide the evidence for tissue growth into the ceramic structures and long

term viability and stability of the implant.

Deon De Beer (2005) developed a methodology to manufacture

patient-specific models (lead masks) to be used as protective shields during

cancer treatment, using 3D photography, RP and metal spraying. It is also

intended to reduce the trauma experienced by the patient, by removing any

physical contact as with conventional methods, and also to reduce the

manufacturing lead time. In this paper, 3D photography of the patient is input

to the RP process and LS 380 polyamide is used as a material to fabricate the

mould of the shield. Compared to conventional fabrication process, the

process and material used is less environmental risk and cost also not more

than the conventional method.

19

Singare et al (2006) described CAD and RP system for the

fabrication of customised maxillofacial implant. Design methods for medical

RP of custom-fabricated were presented. Helical CT data were used to create

a 3D model of the patient skull. The individual shape of the implant was

designed in CAD environment using 3D model and fabricated by stereo

lithography RP process. This was tried on a patient with a large mandible

defect, subjected to reconstruction with pre-fabricated implant resulting from

initial surgical failure with hand contoured reconstruction plate. Some of the

findings are

Custom made implant fit well the defect.

Excellent mandible symmetry and stability were achieved.

Patient was able to eat and no saliva drooping after surgery.

They have concluded that this method allows accurate fabrication

of the implant. Using this technique, the physical model of the implant is

fitted on the skull model, so that the surgeon can plan and rehearse the

surgery in advance and a less invasive surgical procedure and less time-

consuming reconstructive and an adequate aesthetic can result. Also, this

method reduces the risk of a second intervention and the psychological stress

of the patient will be eliminated.

Michele Truscott et al (2007) attempted to justify the proposal of a

new method using CAD/CAM/RP related technologies to substitute lost or

damaged bone regions through the use of CT to CAD to .STL manipulation.

RP technology is used to support rapid product development. This narrates

how the integrated product development research group of the central

University of Technology is applying various CAD/CAM/RP technologies to

support a medical team to save limbs rather than amputating as last attempt at

20

a stage when conventional medical techniques or practices may not apply any

longer.

A case study where RP related technologies were used to support

medical product development for a patient with severe injuries from a road

accident is discussed. It considers direct metal free form fabrication and its

potential to revolutionise the medical industry. Some of the findings are,

Customised implants can be manufactured to engineering

tolerances enabling surgeons to operate with much higher

levels of confidence and precision.

This eliminates the need to use bone harvested from a second

site.

Eliminates time consuming modification of off-the-shelf

devices during the surgery.

This study proves the necessity of patient-specific designs and

hence, patient-specific implants in certain instances, so as to minimise trauma,

speed up recovery and maximise potential to regain full use of affected limbs.

The need for concurrent engineering concept for medicinal application can be

seen.

Lohfield et al (2007) started the Engineering Assisted Surgery

(EAS) as a route for digital design and manufacturing of customised implants

prostheses prior to surgical procedures. RP and more recently RM is a part of

EAS. Transfer of CT data, design of prostheses, FE analysis, RM from

Titanium and quality control were discussed. A method for designing large

maxillofacial implants in a virtual environment is presented without the use of

physical bio models and direct manufacture of the final implant using RM is

21

explored. Also a computational analysis of the implant had been discussed to

verify its mechanical stability.

They concluded that, the lean design achievable using the route

presented, enables one to make large, but light weight prostheses and the

overall process can make a significant contribution to improving the life

quality of the patient. The cost and time effective design and manufacture

route benefit all stake holders.

Nicholas Herbert et al (2005) performed a preliminary investigation

in the development of 3D printing of prosthetic sockets. They developed

patient-specific socket using RP technique and proved that the socket

fabricated using this technique is advantageous in saving time, cost and

accuracy of the socket than the other manual methods. The design and

manufacture of a prosthetic socket traditionally has been a manual process

that relies on the use of plaster of Paris casts to capture the shape of the

patient’s residual limb and then artisan fabrication techniques to manufacture

the socket. CAD and manufacturing technologies have overcome some of the

shortcomings of the traditional process, but the final manufacture of the

prosthetic socket is still performed manually. RP, a relatively new class of

manufacturing technologies, creates physical models directly from 3D

computer data.

Previous research into the application of RP systems to the

manufacture of prosthetic sockets has focused on expensive, high end

technologies that have proven too expensive. This paper investigates the use

of a cheaper, low end RP technology known as 3D printing. Under normal

circumstances, these printed components are weak and relatively fragile.

However, comfortable prosthetic sockets manufactured with 3D printing have

been used in preliminary fittings with patients.

Lohfield et al (2005) reviewed the definition of a bio model based

on which different specific types of bio models are identified, viz. virtual bio

22

models, computational bio models and physical bio models. They have

discussed both physical and virtual bio models of bone and presented a review

of model generator methodologies giving examples of typical bio model

applications.

The use of macro scale bio models for such issues as the

design and preclinical testing of surgical implants and

preoperative planning is discussed.

At the micro scale, bio models of trabecular bone are

examined and the link with scaffolds for tissue engineering is

established.

They concluded that both milling and RP allow the production of

macro scale bio models with sufficient accuracy for clinical applications, RP

has proven to be more flexible regarding feature size and bio models

complexity. Simulation with a virtual bio models is non-destructive, which

allows one to evaluate a much wider range of alternatives to analysis the same

structure. Finally, the benefits of integrating the use of different types of bio

models are reviewed for the further research in bio mechanics and bio

materials (Barker et al (1993) and Bill et al (1995)).

Trainia et al (2008) attempted for Direct Laser Metal Sintering

(DLMS) as a new approach to fabrication of an isoelastic, functionally graded

material for manufacture of porous Titanium dental implants. This work

focuses on a Titanium alloy implants incorporating a gradient of porosity,

from the inner core to the outer surface, obtained by laser sintering of metal

powder with a particle size of 1-10 µm. Surface appearance, microstructure,

composition, mechanical properties and fractography were evaluated. The

analyses were performed by SEM and energy dispersive X-ray spectroscopy.

The flexure strength and surface roughness was determined.

They have concluded that,

23

DMLS proved to be an efficient means of construction of

dental implants with a functionally graded material which is

better adapted to the elastic properties of the bone.

Such implants should minimise stress shielding effects and

improve long-term performance.

Webb (2000) has reviewed the RP techniques in the medical and

bio medical sector. The evolution of RP technology is briefly discussed, and

the application of RP technologies to the medical sector is reviewed.

Although the use of RP technology has been rather delayed in the medical

arena, the potential of the technique is seen to be widespread. Various uses of

the technology within surgical planning, prosthesis development and bio

engineering are discussed. Possible drawbacks are noted in some applications,

owing to the poor resolution of CT slice data in comparison with that

available on RP machines, but overall, the methods are seen to be beneficial

in all areas, with one early report suggesting large improvements in

measurement and diagnostic accuracy as a result of using RP models.

Jiankang He et al (2006) attempted a custom fabrication approach

combining CAD, Computer Aided Engineering (CAE) and CAM techniques

for constructing a novel composite tibial hemi-knee joint. Anatomical

modeling was used to provide the computer model with specific geometry for

individuals and the Finite Element Method (FEM) was adopted to understand

the loading distribution on each component of the composite substitute. RP

was employed to build the negative patterns, based on which the Titanium

alloy tibial tray and the porous artificial bone were custom fabricated through

quick casting and powder sintering techniques. The results confirm that the

Titanium alloy component bears most of the loading while the artificial bone

shares little, which could prevent it from fracturing in vivo. The final porous

24

artificial bone had controllable micro channels and random micro pores,

which ensures full interconnectivity and is expected to address the biological

consideration. Also, clinical application reveals that the composite tibial

hemi-knee joint has enough mechanical strength and can fit with the upper

hemi-knee joint. This novel approach provides a new way to repair large bone

defects in the loading sites.

2.1.1 Surgical planning using RP model

Chua Chee Kai et al (1998) have attempted for Rapid Prototyping

Assisted Surgery Planning to improve the quality of operations, reduce the

risk to patients and reduce the pain experienced by patients. RP technique is

used to fabricate a representative, physical 3D model which enhance

interpretation, visual and physical evaluation and the rehearsal and planning

of the surgical steps before a surgical operation is carried out in order to

eradicate the trauma. This describes the procedures involved in the conversion

of CT scan data to a useful physical model.

Models fabricated via RP techniques can be used for one specific

area of engineering applications like reverse engineering.

De momi et al (2005) developed hip joint anatomy virtual and

stereolithographic reconstruction for pre-operative planning of total hip

replacement. Starting from the MRI, the 3D surface model of both pelvis and

femur was built and the surgical operation was virtually performed. Gait

analysis data’s were added to visualise the pathologic movement of the hip

joint. The resulting triangular mesh was sufficiently accurate to allow the

building of the stereo lithographic model of the joint by means of RP

technique. They concluded that the custom implant fabricated using SLA

25

according to the specific characteristics that precisely fits a patient at

reasonable cost.

Jamieson et al (1995) have studied how RP can assist in the

development of new orthopaedic products. They suggested that orthopaedics

appears to be an attractive market for RP systems manufacturers but noted

that the geometry of bones allows the manufacture of implant prototypes

using conventional machining. The highly individual nature of the products

developed for different customers and the difficulties in instrument

optimisation do provide possible avenues for the use of RP technology in

orthopaedic standard and custom products specifically in the early conception

phases and in instrumentation. They compared the medical instruments

fabricated using conventional and RP methods. From the comparison, it

identified that the instruments fabricated via RP is cost effective, lighter and

more ergonomic.

Chua et al (1999) have compared the RP and virtual prototyping in

product design and manufacturing. The study investigated the suitability and

effectiveness of both technologies in the various aspects of prototyping.

They concluded that the RP model is more useful as a visualising

tool. An actual part always gives a better perception of size and shape than an

image on a screen. In fact, RP assembly helps in determining the placement of

parts in a VP assembly. VP provides a quick iterative design process, where

problems can be rectified immediately whenever indicated from analysis.

Solving the problems in the VP domain helps reduce physical prototyping

costs and time. VP has high initial investment costs in hardware and software

and demands skilled and experienced operators to extract the full benefit from

the software.

26

Banchong Mahaisavariya et al (2006) report two cases using RP

model for surgical planning of corrective osteotomy for cubitus varus. This

new application enables the surgeon to choose the proper configuration and

location of the osteotomy that will be most appropriate to individual patient.

Petzold et al (1999) presented RP technology in medicine as basics

and applications. Numbers of medical case studies were presented. These

studies show that how medical models built with RP technologies represent a

new approach for surgical planning and simulation. These techniques allow

one to reproduce anatomical objects as 3D physical models, which give the

surgeon a realistic impression of complex structure before a surgical

intervention.

Ming-Yih Lee et al (2002) developed custom implant design for

patients with cranial defects. The main indications for cranial reconstruction

of these patients are cosmetic reasons and for protection of intracranial

structure from mechanical impact. The recent development and advancement

of 3D CT has been very useful for evaluation of the cranio-facial

dysmorphology and surgical planning. Poly Methyl Metha Acrylate (PMMA)

is used to fabricate implant through a RP stereo lithography technique

(Chua et al (1998)).

They have concluded that because of the accuracy of the physical

model, the surgeon has a good understanding of the cranial defect and precise

fitting implants can be fabricated in order to re establish skull contours for the

patient. In addition, the excellent fitting and fixation techniques have reduced

operating time significantly.

D’Hauthuille et al (2005) evaluated the two Computer Assisted

Surgery (CAS) techniques to guide a mandibular distraction surgical

procedure. 3D reconstruction using CT images are used to develop

customized RP template and CAS unit to study the positioning and screwing

27

of the device. The accuracy of distracted facial bone displacement depends on

the preoperative clinical assessment, surgical planning and technique. It

concluded that the RP technique appears to be more satisfactory than other

technique.

2.2 RAPID MANUFACTURING FOR MEDICAL APPLICATION

Jayanthi Parthasarathy et al (2010) attempted to evaluate the

mechanical properties of porous Titanium (Ti-6AL-4V) structures with

Electron Beam melting (EBM). This paper discusses an image based micro

structural analysis and the mechanical characterisation of porous Ti-6Al-4V

structures fabricated using the EBM RM process. SEM studies have indicated

the complete melting of the powder material with no evidence of poor

inter-layer bonding. Micro CT scan analysis of the samples indicate well

formed Titanium struts and fully interconnected pores with porosities varying

from 49.75 % - 70.32 %. Compression tests of the samples showed effective

stiffness values ranging from 0.57 ± (0.05) - 2.92 ± (0.17) GPa and

compressive strength values of 7.28 ± (0.93) - 163.02 ± (11.98) MPa. For

nearly the same porosity values of 49.75 % and 50.75 %, with a variation in

only the strut thickness in the sample sets, the compressive stiffness and

strength decreased significantly from 2.92 GPa to 0.57 GPa (80.5 %

reduction) and 163.02 MPa to 7.28 MPa (93.54 % reduction) respectively.

The grain density of the fabricated Ti-6Al-4V structures was found to be

4.423 g/cm3 equivalent to that of dense Ti-6Al-4V parts fabricated using

conventional methods.

In this paper they have concluded that from a mechanical strength

view point, the porous structures produced by the EBM process present a

promising RM process for the direct fabrication of customised Titanium

implants for enabling personalised medicine.

28

Murr et al (2009), have studied the microstructure and mechanical

behaviour of Ti-6Al-4V produced by rapid-layer manufacturing, for bio

medical applications. The microstructure and mechanical behavior of simple

product geometries produced by layered manufacturing using the EBM

process and SLM process are compared with those characteristic of

conventional wrought and cast products of Ti-6Al-4V. In this article the

microstructures are characterised utilising Optical Metallography (OM),

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy

(TEM), and included (hcp), (bcc) and 1 (hcp) martensite phase regimes

which give rise to hardness variations ranging from HRC 37 to 57 and tensile

strengths ranging from 0.9 to 1.45 GPa. The advantages and disadvantages of

layered manufacturing utilising initial powders in custom building of bio

medical components by EBM and SLM in contrast to conventional

manufacturing from Ti-6Al-4V wrought bar stock are discussed.

They have concluded that EBM and SLM produced products

representing precursors for bio medical implants have demonstrated the

ability to build simple geometries having a microstructure which gives rise to

a mechanical behavior similar to and superior to wrought or cast Ti-6Al-4V

products. Tensile strengths have ranged from 1 to 1.45 GPa, a 50 % increase

over wrought products; with corresponding elongations ranging from 25 % to

4.4 %. Corresponding residual hardnesses ranged from HRC 37 to 54.

Jayanthi Parthasarathy et al (2011) reviewed a design for the

additive manufacture of functionally graded porous structures with tailored

mechanical properties for bio medical applications. CAD/CAM based layered

manufacturing and additive manufacturing techniques of metals have found

applications in near-net-shape fabrication of complex shaped parts with

tailored mechanical properties for several applications. Especially with the

onset of newer processes such as EBM and DMLS, revolutionary advances

29

may be achieved in material substitution in the medical implant industry. These

processes must be suitably developed and tested for the production of medical

grade substitutions. In this article, we discuss a design process for creating

periodic cellular structures specifically targeted for biomedical applications.

Electron beam melting is used to fabricate the parts. Evaluation of the

mechanical properties is performed and compared with design parameters.

Compression tests of the samples show effective stiffness values ranging from

0.57 (± 0.05) to 2.92 (± 0.17) GPa and compressive strength values of

7.28 (± 0.93) to 163.02 (± 11.98) MPa. Substituting these values for simulation

of biomechanical performance of patient-specific implants illustrates the

compatibility and matched functional performance characteristics of highly

porous parts at a safety factor of 5 and an effective reduction in weight. These

developments are unique for the construction of maxillofacial and craniofacial

implants. The novel design strategy also lends itself very well to metal additive

manufacturing technologies. Implants designed and fabricated with this design

strategy and manufacturing process would have mechanical properties

equivalent to the part they replace and restore better function and esthetics as

against the currently used methods of reconstruction.

Liciane Sabadin Bertol et al (2010), reported medical design by

DMLS of Ti-6Al-4V. Design and manufacturing of customised implants prior

to surgery are described in this study. Implant shape and functional

requirements are established by digital data based on CT scans and mirroring

operations. The design process of customised mandible prosthesis is

illustrated as well as its manufacturing process (DMLS) and dimensional

control. Laser sintering process and its constraints for the production of

customised implants in Titanium alloy (Ti-6Al-4V) with complex geometry

and internal structures are reported. Important parameters and restrictions in

the production of complex parts, including support structures, maximum over

hanging angle and internal structure are also described.

30

2.3 BIO MATERIALS FOR BONE AND IMPLANT

The past half century has seen explosive growth in the use of

medical implants. Orthopedic, cardiac, oral, maxillofacial and plastic

surgeons are typical medical specialists treating millions of patients each year

by implanting devices varying from pace makers, artificial hip joints, breast

and dental implants, to implantable hearing aids. All such medical implants

make use of special materials, known as bio materials, defined as ‘‘materials

intended to interface with biological systems to evaluate, treat, augment or

replace any tissue, organ, or function of the body’’ (Williams, The Williams

Dictionary of Biomaterials, Liverpool University Press, Liverpool, 1999).

While the priority for the first generation of bio materials was inertness with

living tissues (stability criterion), the field is shifting towards biologically

active systems in order to improve their performance and to expand their use.

Bio materials can be combined as scaffolds with cells (i.e. tissue engineering),

growth factors or genetic material in order to trigger tissue regeneration.

Bio materials and prototyping (both virtual and physical) are

becoming more and more prevalent in medicine. These materials and

technologies are used to design, develop and manufacture medical devices

and instruments and to fabricate drug dosage forms. These processes are also

increasingly being used by surgeons to plan complex operations, especially in

the craniofacial and maxillofacial areas. Further, prototyping shows promise

in the area of tissue engineering through the use of bio materials including the

direct manufacture of biologically active implants.

Amel Farzad et al (2007) examined in-body corrosion fatigue

failure of a Stainless Steel (SS) orthopaedic implant with a rare collection of

different damage mechanisms. Different investigations for various damages

like crevice corrosion pitting, initiation of cracks from these pits, inter-

granular surface cracking inside the crevice and SCC-like branched cracks

31

damages were observed using visual assessments, hardness testing,

stereoscopy, metallographic, quantometry, SEM fractography and EDS

micro-analysis to analyse the failure mechanism and its causes. The main

failure mechanism was determined to be corrosion fatigue assisted by crevice

corrosion pitting. The failure is sequenced by cracking, crack propagation and

surface failure.

Young Taeg Sul (2003) studied the significance of the surface

properties of oxidised Titanium to the bone response: Special emphasise on

potential bio chemical bonding of oxidised Titanium implant. The aim of the

study is to investigate bone tissue reactions to various surface oxidised

features of Titanium implants. Three test screw shaped implants such as S, P

and Ca were prepared by the micro-arc oxidation method. The surface

characterisations were performed with X-ray photo electron spectroscopy,

Auger electron spectroscopy; Scanning Electron Microscopy and top scan 3D.

Based on the comparative analysis of the surface characteristics resulting in

different bone responses between all groups, it was concluded that surface

chemistry and topography, separately or together play important roles in the

bone response to the oxidised implants.

Karen Burg et al (2000) reviewed the bio material developments for

bone tissue engineering. The development of bone tissue engineering is

directly related to changes in materials technology. While the inclusion of

materials requirements is standard in the design process of engineered bone

substitutes, it is also critical to incorporate clinical requirements in order to

engineer a clinically relevant device. This review presents the clinical need

for bone tissue-engineered alternatives to the present materials used in bone

grafting techniques, a status report on clinically available bone tissue-

engineering devices and recent advances in bio materials research.

32

Larsson et al (1996) have investigated bone response to surface

modified Titanium implants and studied on the early tissue response to

machined and electro polished implants with different oxide thicknesses. In

this work, machined and electro polished samples with and without thick

anodically formed surface oxides were prepared, surface characterised and

inserted into the cortical bone of rabbits. Scanning electron microscopy,

scanning Auger electron microscopy and Atomic force microscopy revealed

marked differences in oxide thickness, surface topography and roughness. It

showed that all the implants were in contact with bone and had a large

proportion of bone within the threads at 6 weeks.

Gradzka Dahlkea et al (2008) have studied the modification of

mechanical properties of sintered implant materials on the base of Co-Cr-Mo

alloy. The issue of joint alloplastics has been a subject of research and studies

of different fields of engineering such as mechanical, materials and chemical.

Despite many attempts to improve the design of prosthetic implant and

materials, the major problem with all designs of joint replacement prostheses

is wear. Of the existing materials in common use Co-Cr-Mo alloys

(Spriano et al (2005), Onate et al (2002) and Sheeja et al (2001)) are widely

used in implants such as prosthetic hips and knees due to their mechanical

properties, good wear and corrosion resistances as well as bio compatibility.

Results show that composites can be the alternative materials for bio medical

applications. Also, it can be stated that biofunctional characteristics of

sintered composites are better than of cast metallic bio materials (interface

considered).

Vamsi Krishna et al (2007) have developed low stiffness porous Ti

structures for load bearing implants. The need for unique mechanical and

functional properties coupled with manufacturing flexibility for a wide range of

metallic implant materials necessitates the use of novel design and fabrication

33

approaches. In this work, they have demonstrated that application of proposed

design concepts in combination with Laser Engineered Net Shaping (LENS)

can significantly increase the processing flexibility of complex-shaped metallic

implants with three-dimensionally interconnected, designed and functionally

graded porosities down to 70 % vol., to reduce effective stiffness for load

bearing implants. The microstructure of the samples was examined using both

optical microscopy and SEM. They concluded that the young’s modulus and

0.2 % proof strength of these porous Ti samples having 35-42 % vol. porosity

are found to be similar to those of human cortical bone (i.e.) stiffness of

implant is a vital factor apart from compatibility.

Garrett Ryan et al (2006) reviewed the fabrication methods of

porous metals for use in orthopaedic applications. Implant stability is not only

a function of strength but also depends on the fixation established with

surrounding tissues (Robertson et al (1976)). Previously, stability was

primarily achieved using screws and bone cements. Recently, improved

fixation with low elastic modulus can be achieved by bone tissue growing into

and through a porous matrix of metal, bonding in this way the implant to the

bone host. Depending on the porosity, moduli can even be tailored to match

the modulus of bone closer than solid metals can, thus reducing the problems

associated with stress shielding (Zhong Zhong Chen et al (2004)). Finally,

extensive body fluid transport through the porous scaffold matrix is possible,

which can trigger bone ingrowths, if substantial pore interconnectivity is

established. Over the years, a variety of fabrication processes have been

developed, resulting in porous implant substrates that can address unresolved

clinical problems. The advantages of metals exhibiting surface or bulk

porosity have led researchers to conduct systematic research aimed at

clarifying the fundamental aspects of interactions between porous metals and

hard tissue. Also, this review explains all known different methods for

fabricating such porous metallic scaffolds.

34

John Alan Hunt et al (2005) studied the design and production of

Co-Cr alloy implants with controlled surface topography by CAD/CAM

method. Improved fixation and increased longevity are still important

performance criteria in the development of orthopaedic prostheses. Two

different process routes, conventional casting and SLS were employed, each

process yielded implants that had identical surface topology but different

microstructures. Hydroxyapatite (HA) was used to coat some samples by

plasma spraying. The amount of bone growth was quantified using image

analysis. Plasma spray HA coated samples promoted better osteogenesis and

integration than uncoated samples. They concluded that SLS production of a

Co-Cr implant with a shaped surface and plasma HA coating provided a

surface that was able to enhance osteogenesis and tissue implant cohesion

early in new bone formation. Significance of HA coating is providing tissue

implant cohesion can be seen.

Horatiu Rotaru et al (2006) used Silicone rubber mould cast

polyethylmethacrylate-HA plate for repairing a large skull defect. A large

custom made cranial implant was produced using data acquired from 3D CT,

RP and casting in a silicone rubber mould. They have observed that the

cranial plate fitted precisely into the defect and needed no correction at the

time of surgery. The stability of the reconstruction plate was increased by the

presence of thin margins allowed by silicone rubber elasticity. No

complications occurred and the final functional and aesthetic results were

good. The use of 3D imaging and RP allow precise repair of large skull

defects, with good aesthetic and functional results. At the same time, silicone

rubber moulds permit the production of very minute details needed not only

for cosmetic reasons but for reconstruction plate stability as well.

35

2.4 BIO MECHANICS OF BONE AND IMPLANT

Glitsch and Baumann (1997) have developed a 3D model of the

lower limb containing 47 muscles was to study the differences between a 2D

and 3D approach for determining internal loads, the role of the dynamic joint

representation and the behaviour of different load bearing criteria in walking

and running. The problem of redundancy of the musculo-skeletal system was

resolved by applying inverse dynamics and static optimisation methods.

Different hypothetical load bearing capabilities of hinged, spherical and

intermediate joint types for the knee and the ankle joints were tested. It was

found that even almost planar movements such as walking and running are

associated with significant 3D intersegment moments, especially in the frontal

plane. Thus, a 2D approach may underestimate internal loads up to 60 %.

They have suggested that pure hinged joints are inappropriate for

modeling the dynamical joint function of the knee and ankle joints. A more

flexible joint representation in combination with a squared muscle stress

minimisation criterion predicted a lot of muscle activation which was also

found in the EMC patterns. The results indicate the importance of muscular

joint stabilisation in natural human movements.

George Duda et al (1997) have studied the internal forces and

moments in the femur bone during walking. The forces exerted by the soft

and hard tissues of the thigh together represent a system in equilibrium. This

balance of loads must be considered when the system components are

examined individually and independently. However, in many bio mechanical

analyses of the thigh and the femur is studied without considering soft tissue

loading. To improve the understanding of femoral loading, a 3D model was

developed. Taking into account all thigh muscles, body weight and contact

forces at the hip, patello-femoral and knee joints, the internal loads of the

bone were calculated. Some of the findings were

36

Internal loads of the femur decreased as a result of muscle

activity at the hip and at the knee.

The load reduction could be up to 50 % of the internal forces

at the hip, depending on gait phase and

Maximal forces were found between 40 and 60 % of the

stance phase, whereas maximal torsional moments occurred

shortly after heel strike.

They have concluded that

In general, the bone is loaded axially, rather than in bending,

with maximum shear forces at the proximal and distal ends.

From one gait phase to another, the femur experiences

alternating, rather than one sided bending load.

Simon et al (2003) have focused on the influence of the implant

material stiffness on stress distribution and micromotion at the interface of

bone defect implants. They hypothesised that a low stiffness implant with a

modulus closer to that of the surrounding trabecular bone would yield a more

homogeneous stress distribution and less micromotion at the interface with

the bony bed. To prove this hypothesis, 3D, non-linear, anisotropic FE model

has been generated. The interface was described by face-to-face contact

elements. A physiological load condition was assumed and contact pressures,

shear stresses, and shear movements at the interface for two implants of

different stiffness were evaluated.

FEA result shows the stress distribution was more homogeneous for

the low stiffness implant. The maximum pressure for the composite implant

(2.1 MPa) was lower than for the Titanium implant (Sykaras et al (2000)) (5.6

MPa). Contrary to hypothesis, more micromotion occured for the composite

(up to 6 mm) than for the Titanium implant (up to 4.5 mm). However, for

37

both implants peak stresses and micromotion were in a range that predaicts

adequate conditions for the osseointegration.

Marco Viceconti et al (2006) have reviewed the status and key

issues on biomechanics modeling of the musculoskeletal apparatus. The aim

of this review paper is to report on the current state of the art in creating in

silico humans able to simulate the biomechanics of the human body at all

scales of interest. The focus is on the musculoskeletal apparatus, although

much of what is written is valid also for the biomechanical modeling of other

organs. The state of the art of computational biomechanics at body, organ,

tissue and cell levels is briefly described and the most recent achievements in

the area of multiscale models are discussed.

In conclusion, the challenges to be faced to realise a true living

human model are summarised. It is evident that the demands associated with

some of these challenges greatly exceed the potential currently possessed by

the computational biomechanics research community. Thus, to tackle them it

will be necessary not only to co-ordinate all efforts in a coherent way, but also

to mobilise much greater financial and human resources than are currently

available.

Mason et al (2008) have reviewed patellofemoral joint forces as

they might apply to implant design, methodologies for estimating forces on

the patella and estimates of the forces, as reported in the literature, are

summarised. Two methodologies exist for studying joint loads; one that

measures kinematics in vivo and uses analysis to estimate the joint loads and

another that measures ground reaction forces and uses analysis to estimate the

joint loads.

Koehle and Hull (2008) have communicated a method of

calculating physiologically relevant joint reaction forces during forward

38

dynamic simulation of movement from an existing knee model. In the

commonly used SIMM software, which includes a complete musculoskeletal

model of the lower limbs, the reaction forces at the knee are computed. These

reaction forces represent the bone-on-bone contact forces and the soft tissue

forces (e.g. ligaments) other than muscles acting at the joint. Hence objectives

were to develop a method of calculating physiologic knee joint reaction forces

using the knee model incorporated into the SIMM software and to

demonstrate the differences in the forces returned by SIMM. This result

illustrates that the physiologic knee joint reaction forces are profoundly

different than the forces returned by SIMM. However physiologic knee joint

reaction forces can be computed with post processing of SIMM results.

Au et al (2005) have studied a parametric analysis of fixation post

shape in tibial knee prostheses. A primary concern of Total Knee

Replacement (TKR) is aseptic loosening of the tibial component, which may

be caused by shielding of mechanical stresses in the bone and may require

subsequent revision surgery. A 3D FE model has been developed to study

bone and interface stresses for four different tibial prosthesis designs. The

model described here incorporates orthotropic and heterogeneous bone

properties with physiologically representative loading conditions. Results

from this model indicate that stress distribution is affected by the

incorporation of anisotropy and spatial variation of bone properties.

Convergence testing was performed to ensure model accuracy. The

significance of shape and position on bone stress can be seen.

Heller et al (2001) have presented musculo-skeletal loading

conditions at the hip during walking and stair climbing. Musculo-skeletal

loading plays an important role in the primary stability of joint replacements

and in the biological processes involved in fracture healing. However, current

knowledge of musculo-skeletal loading is still limited. In the past, a number

39

of musculo-skeletal models have been developed to estimate loading

conditions at the hip. So far, a cycle-to-cycle validation of predicted musculo-

skeletal loading by in vivo measurements has not been possible. The aim of

this study was to determine the musculo-skeletal loading conditions during

walking and climbing stairs for a number of patients and compare these

findings to in vivo data.

On the basis of CT and X-ray data, individual musculo-skeletal

models of the lower extremity were developed for each patient. Muscle and

joint contact forces were calculated using an optimisation algorithm. In all

cases, the comparison of in vivo measured and calculated hip contact forces

showed good agreement.

Thus, the authors consider the presented approach as a useful

means to determine valid conditions for the analysis of prosthesis loading,

bone modeling or remodeling processes around implants and fracture stability

following internal fixation. It opens up the prospect of investigating the

implant bone load sharing and primary stability of implants under loading

conditions which approximate most realistically to the in vivo loading

conditions of walking and stair climbing.

Duda et al (2001) have attempted to study the mechanical boundary

conditions of fracture healing: borderline indications in the treatment of

unreamed tibial nailing. Unreamed nailing favors biology at the expense of

the achievable mechanical stability. It is therefore of interest to define the

limits of the clinical indications for this method. The extended usage of

unreamed tibial nailing resulted in reports of an increased rate of

complications, especially for the distal portion of the tibia. The goals of this

work were to gain a thorough understanding of the load sharing mechanism

between unreamed nail and bone in a fractured tibia, to identify the

mechanical reasons for the unfavourable clinical results and to identify

40

borderline indications due to bio mechanical factors. The loading of the bone,

the loading of the implant and inter fragmentary strains were computed. The

findings of this study show that with all muscle and joint contact forces

included, nailing leads to considerable unloading of the interlocked bone

segments. Unreamed nailing of the distal defect results in an extremely low

axial and high shear strain between the fragments. The results suggest that

mechanical conditions are advantageous to unreamed nailing of proximal and

mid diaphyseal defects. It is to be assured that the thickness accordingly the

stiffness of the implant can also contribute to the concept of fracture healing.

The currently used surgical treatment of a bone tumour in the

extremities shows some shortcomings in providing a restoration of the

mechanical strength of the bone and the containment of the used filling

materials. The use of a medical image based, preformed and custom made

Titanium membrane (Bonfield et al (1987)) screwed onto the periosteal side

of the bone is introduced.

In this work, the design process and the bio mechanical evaluation

of membrane was discussed. The mechanical behaviour at various locations is

tested experimentally. From the two performed tests it identifies that a

Titanium membrane without a wave pattern, of 0.3 mm thickness, fixed to the

bone with seven trabecular bone screws (4 mm diameter and 28 mm length) is

capable of carrying the anticipated mechanical loads on the reconstructed

tibia. The feasibility of preformed Titanium membranes for bone

reconstruction in tumour surgery is demonstrated (Pattijn et al (2002)).

Hurwitz et al (1998) have examined the dynamic knee loads during

gait predict proximal tibial bone distribution. This study tested the validity of

the prediction of dynamic knee loads based on gait measurements. The

relationship between the predicted loads at the knee and the distribution of

41

bone between the medial and lateral sides of the tibia was examined. The

motion and external forces and moments at the knee were measured during

gait and a statically determinate muscle model was used to predict the

corresponding forces on the medial and lateral tibial plateaus. Bone mineral

content present in the tibial bone is studied using X-Ray absorptiometry at

four regions 20 mm below proximal and 20 mm above distal end on medial

and lateral regions. Distally neither the subject characteristics nor the gait

moments and predicted forces were significant predictors of the bone

distribution. The lack of a correlation distally may be reflective of the forces

being more evenly distributed further from the tibial plateau. While it has

long been suggested that the adduction moment is the primary determinate of

the distribution of load between the medial and lateral plateaus, this is the first

evidence of its relationship to the underlying bone distribution.

Amit Gefen (2003) has studied the consequences of imbalanced joint

muscle loading of the Femur and Tibia from bone cracking to bone loss. Bone

is a living tissue requiring regular mechanical stress stimulation to maintain

its mass and organisation, but excessive stresses may damage its structural

integrity. Stresses and strains, due to imbalanced joint-muscle forces, that are

above or below critical levels may cause acute or accumulated bone damage.

In this study, 3D anatomically accurate femur and tibia modeled using Solid

Works were utilised to determine alterations in bone stresses during two

situations of imbalanced joint-muscle loading, due to fatigue related muscular

weakness during intensive physical exercise and exposure to micro gravity

during a space flight. Substantial alterations in stress levels were observed in

both conditions. Importantly, the findings demonstrate that surface and

internal stress distributions in the real 3D anatomy of the femur and tibia

during locomotion can be obtained using realistic biomechanical models, and

that these models are applicable in identifying the relevant joint-muscle

imbalance mechanisms involved in the onset of bone damage.

42

The anisotropy of Young's modulus in human cortical bone

(Hoffmeister et al (2000)) was determined for all spatial directions by

performing co-ordinate rotations of a 6 by 6 elastic stiffness matrix. The

elastic stiffness coefficients were determined experimentally from ultrasonic

velocity measurements on 96 samples of normal cortical bone removed from

the right tibia of eight human cadavers. Analysis revealed a substantial

anisotropy in Young's modulus in the plane containing the long axis of the

tibia, with maxima of 20.9 GPa parallel to the long axis, and minima of

11.8 GPa perpendiculars to this axis. A less pronounced anisotropy was

observed in the plane perpendicular to the long axis of the tibia. This can be

attributed to the different stress less experienced by the bone in axial direction

compared to lateral direction. To display results for the full 3D anisotropy of

cortical bone, a closed surface was used to represent Young's modulus in all

spatial directions.

Moreo et al (2007) have developed a computational model for

living interfaces with bone implants. The model is able to qualitatively

capture the evaluative behaviour of bony interfaces: deterioration and bone

ingrowths. They assumed that the evolution of the variables that define the

mechanical state of the interface can be formulated following the principles of

Continuum Damage Mechanics (CDM) with the additional feature. Within the

present study, the femoral component of total hip non cemented arthroplasties

has been analysed by means of 3D FEA. The dependence of the bone

ingrowths pattern on the stem stiffness has been studied, concluding that

stiffer stems improve primary fixation. Moreover, a sensitivity analysis has

been performed studying the influence of patient activity, stem surface

finishing and other model parameters. Overall, the model is able to reproduce

the progressive deterioration and osseointegration of living bony interfaces,

obtaining results that qualitatively agree with clinical observations.

43

During the operation of total hip arthroplasty, when the cement

polymerises between the stem implant and the bone, residual stresses are

generated in the cement at an early post operative situation. The purpose of

this study was to determine whether including residual stresses at the stem

cement interface of cemented hip implants affected the cement stress

distributions due to externally applied loads. FEA was in 3D and used non-

linear contact elements to represent the debonded stem cement interface. The

results showed that the inclusion of the residual stresses at the interface had

up to a 4 fold increase in the von Mises cement stresses compared to the case

without residual stresses (Nuno and Avanzolini (2002)). Since there is no

chemical bond at the interface between the stem and cement, the interface

resistance depends on friction, thus, radial compressive stresses developed by

cement curing play a direct role.

Frost (1994) reviewed Wolff’s law and bone’s structural

adaptations to mechanical usage to an overview for clinicians. He stated that,

overloading the bone can increase micro damage and this can happen in

pathologic fractures and in bone overloaded by endoprostheses and internal

fixation implants. The basic activities of growth, modeling and remodeling

determine the architecture and strength of bones. Living bone may depend

more on strain than stress to generate the signals that control its biological

reactions to mechanical loads. The design and use of all load bearing bone

implants should keep the strains of the supporting bone below its micro

damage thresholds. Both modeling and remodeling can respond in its own

way to mechanical and other influences.

Ulrich Hansen et al (2008) have studied the effect of strain rate on

the mechanical properties of human cortical bone. Bone mechanical

properties are typically evaluated at relatively low strain rates. However, the

strain rate related to traumatic failure is likely to be orders of magnitude

44

higher and this higher strain rate is likely to affect the mechanical properties.

Human femoral cortical bone was tested longitudinally at strain rates ranging

between 0.14-29.1 s1 in compression and 0.08-17 s

1 in tension. Young’s

modulus generally increased, across this strain rate range, for both tension and

compression. Strength and strain (at maximum load) increased slightly in

compression and decreased (for strain rates beyond 1 s1) in tension. The

behaviour seen in compression is broadly in agreement with past literature,

while the behaviour observed in tension may be explained by a ductile to

brittle transition of bone at moderate to high strain rates. Refinement of

structure associated with strain rate could attribute to this.

One of the major failure modes of cementless acetabular

components is the loosening of the acetabular cup, which is mostly

attributable to insufficient initial stability (Miyoshi et al (2002)). A

hemispherical cup with a porous coating which is inserted with press-fit

fixation and secured with several screws is one of the most widely used

approaches. Many studies have found that bone screws are very helpful aids

for cup fixation, but the optimal surgical technique for inserting screws has

not been clearly reported. In this study, hemispherical cups were fixed into

blocks of foam bone with zero to three screws. The experimental results

indicate that increasing the number of screws enhances the cup stability in the

case of ideal screwing (i.e., with no eccentricity). However, the presence of

angular eccentricity significantly reduced the stability of the cup, while 1 mm

of offset eccentricity produced an even greater impact (Hsu et al (2006)). This

study demonstrates that both the skill of screw insertion and the number of

screws have significant effects on cup stability.

Hsu et al (2007) have attempted to investigate the effects of the

number of screws, bone quality and friction coefficient of the acetabular cup

on the initial stability under normal walking using 3D FEM of pelvic bone

45

and acetabular cup components. A commercially available hemispherical

acetabular cup with five screw holes was used as the default model. The

stiffness of the pelvis and the friction coefficient of the cup were

systematically varied, within a realistic range, to assess the initial stability of

the acetabular cup. The simulations showed that the inserted screws provide

only a localised reduction in the relative micromotion between the cup and

pelvis therefore inserting several screw closed together might not be useful.

Changes in the pelvic stiffness have a non linear effect on the initial stability

of acetabular cup and the subchondral bone provides good support for fixation

of the cementless cup. They concluded that the friction coefficient of the

acetabular cup plays a limited role, comparing with the factor of bone quality,

in resisting relative micromotion in the cup pelvis interface. Interfacial micro

motion (slip) can be influenced by friction (a function of texture) and stiffness

of contact models. To facilitate micromotion or slip over the interface, pitch

distance and number of screws to be used plays a contributory factor.

2.5 IMPACT ANALYSIS

Dawson et al (1998) have attempted analysis of the structural

behaviour of the pelvis during lateral impact for different peak forces in the

range of 5,520 to 15,550 N to correspond to the velocities and impulses of

real world accidents using the FEM. In this work, 3D FEM were created from

CT data to study lateral impact fractures of the pelvis having the complex

geometry and material properties of the pelvis. From the FEA results, the

locations of structurally significant regions of the pelvis to failure were

identified. The impact force which induced fracture of the pelvic bone was

8,610 N. The region which failed first in left lateral impact was the right pubic

ramus. The fracture pattern was a variant of the lateral compression pelvic

injury. The results suggest that the anterior structures of the pelvis are the

most sensitive regions. The energy absorbed by the pelvis prior to failure was

46

8.98 J. FEM may be used to determine the strength and energy absorbing

capability of the pelvis for lateral impact loading. Energy absorbed and

energy of formation decide material stability.

Amein Alsuezi (2005) has developed and validated an anatomically

based plastic kinematic FEM of the knee joint for vehicle-pedestrian collision

injury investigation. This paper presents an anatomically based FEM of the

human knee joint to investigate knee injury in vehicle-pedestrian collisions.

The model was created from knee CT of a 25 year old male. Vector contours

of the femur, tibia, fibula and patella were created based on axial slices with a

5 mm span of the CT scans. Extrapolation of sagittal layers helped resolve

encountered difficulties. Solid surfaces were transferred as IGES format to the

HyperMesh pre-processor commonly used for mesh generation. Final model

assemblage was carried out and meshed by 4-node tetrahedral elements. The

mesh was checked and further refined to satisfy several FE criteria. This

model is an improvement on published bio mechanical FE knee models. It is

validated against data in published literature.

Wood et al (2004) have studied vehicle–pedestrian collisions:

validated models for pedestrian impact and projection. The most important

factor in pedestrian injuries from vehicle collisions is the strain rate. In cases

where the impact configuration can be ascertained, the most common method

now used to determine vehicle speed involves the pedestrian projection

distance. The more traditional method of using tyre brake marks is losing

applicability as ABS braking systems become more common. The two most

common impact configurations are wrap projection and forward projection,

these being determined by the vehicle and pedestrian geometry and the initial

conditions of the impact. In this paper, two models are presented for

pedestrian forward and wrap projection impacts. These models are predicated

on separating the total projection distance into the individual projection

47

distances occurring during three principal phases of the collision. The models

are novel as they use a rigid single-segment body representation of the

pedestrian, include explicit modeling of the impact phase and also allow for

uncertainty in the input parameters. Published data are used to provide

distributions for the input variables such as pedestrian and vehicle masses,

etc. The model predictions of impact speed from overall projection distance

are validated by comparison with real world accident data.

2.6 FINITE ELEMENT ANALYSIS IN MEDICAL

APPLICATIONS

Blake Latham and Goswami (2004), Bufford and Goswami (2004),

Slonaker and Goswami (2004) and Monasky and Goswami (2004) have

attempted to study the effect of geometric parameters in the design of hip

implants with respect to the development of stress. The parameters include:

head diameter, neck diameter, and neck angle are considered and two 3D

models were drawn, one modular and one integrated implant. These models

were then altered geometrically one variable at a time and FEA was performed

on the models. In total, twenty assemblies of implant, bone cement and bone

were tested. Stress spectrum was drawn for each case under a combination of

hip dimensions. The significance of neck angle in enhancing the performance

of implants can be seen. They concluded that more kinematics testing should be

performed to simulate in vivo conditions, so as to ensure durability of implants.

A quantitative assessment of bone tissue stresses and strains is

essential for the understanding of failure mechanisms associated with

osteoporosis, osteoarthritis, loosening of implants and cell-mediated adaptive

bone remodeling processes. Normally, stresses and strains should be

distributed rather evenly over the trabecular architecture.

48

For the stance phase of walking an average tissue principal strain in

the VOI of 279 strains was found, with a standard deviation of 212 µ strain.

The standard deviation depended not only on the hip force magnitude, but

also on its direction. In more than 95 % of the tissue volume, the principal

stresses and strains were in a range from zero to three times the averages, for

all hip force directions. This indicates that no single load creates even stress

or strain distributions in the trabecular architecture, excessive values occurred

at few locations only and the maximum tissue stress was approximately half

the value reported for the tissue fatigue strength. These results thus indicate

that trabecular bone tissue has a safety factor of approximately two for hip

joint loads that occur during normal activities (Van Rietbergan et al (1999)).

This study also reveals for better understanding of mechanically induced

processes in bone

Oguz Eraslan and Ozgur Inan (2009) have evaluated the effect of

thread design on stress distribution in a solid screw implant using 3D FEA.

The bio mechanical behaviour of implant thread plays an important role on

stresses at implant bone interface. Information about the effect of different

thread profiles upon the bone stresses is limited. In this study, four types of

3D mathematical models simulating four different thread form configurations

(V-thread, buttress, reverse buttress and square thread) for a solid screw

implant was prepared with supporting bone structure. A 100 N static axial

occlusal load was applied to occlusal surface of abutment to calculate the

stress distributions. Solid works and Cosmos works structural analysis

programs were used for FEA. The analysis of the von Mises stress values

revealed that maximum stress concentrations were located at loading areas of

implant abutments and cervical cortical bone regions for all models. Stress

concentration at cortical bone (18.3 MPa) was higher than spongious bone

(13.3 MPa) and concentration of first thread (18 MPa) was higher than other

threads (13.3 MPa). It was seen that, while the von Mises stress distribution

49

patterns at different implant thread models were similar, the concentration of

compressive stresses were different. The present study showed that the use of

different thread form designs did not affect the von Mises concentration at

supporting bone structure. However, the compressive stress concentrations

differ by various thread profiles. Unlike the case of engineering joints, to

ensure continuous interface and contact friction V thread may be preferred.

Harris et al (2005) have performed a FEA on intramedullary nail Vs

osteosynthesis plates for femoral fracture healing and stabilisation and

assesses how these differences are affected by the size and consolidation of

the fracture. Simulations were performed using a femur model with a 1 mm

and 3 mm diaphyseal fracture at two stages of consolidation. In this work, the

fracture was stabilised with plate or an intramedullary nail made of SS, Ti or a

composite material. The von Mises stress in Ti implants is lower than the SS

implants, and in the implants decreased with fracture consolidation. Nails can

bear higher stresses than plates, though these differences were reduced in

unstable fractures and fixation choice is critical for weak fractures. Results

suggest that

Plates induce fracture healing through intramembranous

ossification without fracture callus formation.

Nails, on the other hand, induce endochondral ossification

with fibrous tissue formation.

The composite implants have mechanical limitations, but

increasing their yield stress could overcome these drawbacks.

Significance of ossification is highlighted.

Mohammed Rafiq Abdul Kadir et al (2009) have attempted to study

the interface micromotion of cementless hip stems in simulated hip

arthroplasty. The design of hip prostheses has evolved over time due to

50

various complications found after hip replacement surgery. The currently

commercially available cementless femoral stems can be categorised into one

of three major types, straight cylindrical, tapered rectangular and anatomical.

Each type proposes a unique concept to achieve primary stability-a major

requirement for bone healing process. Virtual analyses have been made on

individual implants, but comparison between the three major types is required

to determine the strength and weaknesses of the design concepts. The size of

the three implants was carefully designed to fit and fill the canal of a femur

reconstructed from a CT image dataset. Hip arthroplasty was simulated

virtually by inserting the hip stem into the femoral canal. FEM was used in

conjunction with a specialised sub-routine to measure micromotion at the

bone-implant interface under loads simulating physiological walking and stair

climbing. Another sub-routine was used to assign bone properties based on

the gray scale values of the CT image.

The results suggested that all the three types of cementless hip

stems were found to be stable under both walking and stair climbing

activities. Large micromotion values concentrated around the proximal and

distal part of the stems. They concluded that the three major types of hip

stems were compared in this study and all of them were found to be stable

after simulated physiological activities. Interfacial bone loss was simulated to

better predict the stability of the stems.

Benedikt Helgason et al (2007) have attempted to compare the

results from subject-specific FEA of a human femur to experimental

measurements, using two different methods for assigning material properties

to the FE models. A modified material mapping strategy allowing for spatial

variation of material properties within the elements and Young’s modulus

surface corrections is presented and compared to a more conventional

strategy, whereby constant material properties are assigned to each element.

51

The accuracy of the superficial stress-strain predictions was evaluated against

experimental results from 13 strain gauges and five different load cases. Five

different FE meshes were used for convergence analysis. Both methods

predicted stresses with acceptable accuracy with the conventional method

performing slightly better. The modified method performed better in strain

prediction.

Novel techniques have been developed to convert 3D image data,

obtained from a range of imaging modalities (MRI, CT, Ultrasound, confocal

microscopy), automatically into numerical meshes suitable for FEA and

Computational Fluid Dynamic (CFD) analysis. The different case studies

illustrate how complex biological models can be modeled with not only a much

higher degree of accuracy but also in a fraction of the time than was previously

possible (Philippe Young (2005)). If the system is coupled with RP hardware, it

is also possible to produce a solid polymer or metal facsimile of the object.

Ying Tie et al (2005) present a non uniform, periodic closed

B-spline approximation algorithm for the fabrication of a medical pelvic

model, based on RP and also give the finite element evaluation of the pelvic

model. RP requires correct data structure in STL files to process the slices.

The non uniform periodic closed B-spline curve approximation method was

applied to processing CT data. The precision and size of STL files was

improved to optimise the RP model of the pelvis. Finally, the model of the

pelvis was evaluated with the FEM. Results suggest that a high similarity has

been achieved in terms of shape, size and bio mechanical properties of the

pelvic model and the normal one, which validates our argument that RP with

non uniform, periodic closed B-spline algorithm, is suitable for the fabrication

of a pelvic model, which will prove to be significant in the design of pelvic

prostheses. The significance of surfacing of scanned model feature.

52

Travis Burgers et al (2009) performed an in vitro and finite element

study on initial fixation of a femoral knee component. Loosening is the

primary cause of total knee arthroplasty implant failure; therefore, to

investigate this failure mode, femoral knee components were implanted in

vitro on three cadaveric femurs. Bone-implant FEM was created to predict the

initial fixation of the interface of each femur and was successfully measured

with the strain-gauged implants. Specimen-specific FE models were

calibrated using the in vitro strain measurements and used to assess initial

fixation. Initial fixation was shown to increase with bone density. The

geometry of the implant causes the distal femur to deform plastically. It also

causes higher stresses in the lateral side and higher pressures on the lateral

surfaces. The implementation of plasticity in the bone material model in the

FE model decreased these strains and pressures considerably from a purely

elastic model, which demonstrated the importance of including plasticity.

Also, the significance of grounding of implant is highlighted.

Brandi Carr and Tarun Goswami (2009) reviewed knee implants

models and bio mechanics. Materials used for knee implants are selected to

balance strength requirements with bio compatibility needs. While use of

materials such as Ti (Pilliar (1987) and Clemow, et al (1981)) alloys, Co-Cr

and ultra high molecular weight polyethylene have led to improved implant

designs, wear, loosening and other factors continue to limit the performance

of knee implants. 2D and 3D FEM have been developed which include the

artificial knee and portions of the surrounding biological materials to

investigate this interaction. FEA has been used to predict implant bio

mechanical behaviour under various static and dynamic loading conditions.

Prosthetic joint implants currently in use exhibit high failure rates.

Realistic computer modeling of prosthetic implants provides an opportunity

for orthopedic bio mechanics researchers and physicians to understand

53

possible in vivo failure modes, without having to resort to lengthy and costly

clinical trials. The research presented here is part of a larger effort to develop

realistic models of implanted joint prostheses. The example used here is the

thumb Carpo Meta Carpal (CMC) joint. The work, however, can be applied to

any other human joints for which prosthetic implants have been designed.

Preliminary results of prosthetic joint loading, without surrounding human

tissue (i.e., simulating conditions under which the prosthetic joint has not yet

been implanted into the human joint), are presented, based on a 3D, non linear

FEA of three different joint implant designs (Nielsen et al (1995)).

Ola Harrysson et al (2007) describe an approach where the custom

design is based on a CT scan of the patient's joint. The proposed customised

design for both the articulating surface and the bone-implant interface to

address the most common problems found with conventional knee-implant

components. FEA is used to evaluate and compare the proposed design of a

custom femoral component with a conventional design. The proposed design

shows a more even stress distribution on the bone-implant interface surface,

which has reduce the uneven bone remodeling that, can lead to premature

loosening. MIMICS software is used to reconstruct the 3D object from 2D CT

images and Geomagic studio V7.0 was used to convert .stl format into

NURBS format that can be exported as a solid CAD model.

The proposed custom femoral component design has the following

advantages compared with a conventional femoral component.

The resurfacing of the patella during surgery or gait change is

not necessary as the articulating surface closely mimics the

shape of the distal femur.

Because of uniform stress distribution, bone remodeling is

even and the risk of premature loosening might be reduced.

54

Because the bone-implant interface can accommodate

anatomical abnormalities at the distal femur, the need for

surgical interventions (Emoto, et al (2000)) and fitting of filler

components is reduced and

Given that the bone-implant interface is customised, about

60 % bones saved from resurfacing.

The primary disadvantages are the time and cost required for the

design and the possible need for a surgical robot to perform the bone

resection. Some of these disadvantages may be eliminated by the use of RP

technologies, especially the use of EBM technology for quick and economical

fabrication of custom implant components.

David Bennett and Tarun Goswami (2008) performed FEA on six

hip stem designs. The hip implant designs were then analysed at forces

ranging from 2.5 to 7 KN. These forces were selected because a typical gait

cycle generates forces up to 6 to 7 times of the body weight in the hip joint.

The design objective for a hip stem is to have a low stress, displacement and

wear at a very high fatigue life. The FEA results were compared for various

stem designs assuming a rectangular cross section, which is made up of bio

compatible materials such as Titanium, Co-Cr, SS and bone-cement.

Sowmianarayanan et al (2008) illustrated FEA of a subtrochanteric

fractural femur with Dynamic Hip Screw (DHS), Dynamic Condylar Screw

(DCS) and Proximal Femur Nail (PFN) implants - A comparative study. In

this work, the bio mechanical behaviour of the femur bone with three

different implant configurations for simple transverse subtrochanteric fracture

and the intact femur using FEA was studied. The simulation also included

modeling of the cortical defect near the fracture. Convergence study was

performed on the intact femur prior to the fractured femur analysis to validate

55

the femur model and hence 3 mm element size was chosen. Different contact

interface surfaces were considered. For all the regions of the bone, the

material properties were assigned as linearly elastic isotropic. Loads due to

body weight and various muscles at the proximal femur were considered for

the analyses.

An estimation of the critical depth of the cortical defect based on

the von Mises stress was detained using this study on the DHS implant. The

three implants were for deflection, stress and strains. The displacement and

principal stress on the proximal femur have been compared for all the implant

models. The stresses on the critical screws for DCS and DHS have also been

compared.

Johnson and Young (2005) have conducted simulation of the

response of the human head to impact on a patient-specific RP model and

comparison with analytical and FEM. In this work, an experimental study of

the response of the human head to impact is presented. A RP model of human

head was generated based on MRI scan data. The physical model was

subjected to low velocity impacts using a metallic pendulum and a sensitivity

test were performed to explore the influence of various parameters including

mass and velocity of impactor on the response. The experimental response

characteristics were compared with predictions from FE models generated

from the same MRI data set. The results from the experimental tests closely

matched those predicted by both the analytical and the FE models and thus

provide with substantive corroboration of all three approaches.

Baris Simsek et al (2006) have evaluated the effects of different

inter implant distances on the stress distribution around endosseous implants

in posterior mandible by 3D FEA. In this work, 3D FEA models representing

mandible and ITI implant (Straumann, Waldenburg, Switzerland) were

simulated. The distances in between units were set at 0.5 to 2.0 cm. Loads

56

were applied to each of these designs. The principal stresses (tensile and

compressive stress) on each model were calculated using MSC MARC FEA

solver software. They have concluded that

The magnitude of the stress was influenced by complex

factors such as the direction of loads and the distance between

adjacent fixtures.

The stress occurring around fixtures differ simultaneously

with inter implant distance.

Bleecha et al (2005) have demonstrated how plate positioning

impacts the bio mechanics of the open wedge tibial osteotomy by FEA. A

numerical model of the medial open wedge tibial osteotomy based on the

FEM was developed. Two plate positions such as the plate being fixed in a

medial position and in an antero-medial position were tested numerically. The

simulation took into account soft tissues preload, muscular tonus and maximal

gait load.

2.7 SCOPE AND OBJECTIVE

SCOPE

From the literature survey, the significance of RP for medical

applications, in terms of implant design, manufacturing of custom made

implants, incorporating bio compatible materials which will facilitate

improved surgical planning can be seen. Pre-fabricated implants are available

in a standard range of sizes and shapes which do not conform to the geometric

shape of the patient’s bone and are selected to near suitable during surgery.

This needs more surgical interventions and time. This may results in gap

between bone and implant which cause micro movements with subsequent

bone resorption and loss of fixation and discomfort to patient. Patient-specific

57

implants are necessary to overcome these situations and are manufactured on

a prescription basis and hence unique for each patient. The bio mechanical

considerations while designing the implants, improves the longevity of the

implant and provides stability to the bone-implant construct. Sharing of load

by implants creates favourable mechanical conditions for faster healing.

OBJECTIVE

The scope of the present work is to fabricate patient-specific implant

using RP techniques. Also to analyse and evaluate the mechanical behaviour

of bone with patient specific implants using FEA technique. Accordingly, the

objectives of the present work are:

Fabrications of patient-specific implant using RM.

Conversion of CT scan data into CAD model.

Modeling of patient-specific implant.

Simulation of impact condition on proximal end of tibia

bone to predict the fracture risk zone.

FEA and evaluation of fractured bone fixed with patient-

specific implant by varying thickness, position and bio

compatible material.

Fabrication of patient-specific implant by RP techniques.

Measurements of patient-specific implant for dimensional

stability

2.8 METHODOLOGY

Block diagram of the methodology adopted for the study is

schematically illustrated in Figure 2.1.

58

Model verification (MIMICS)

Patient-specific implant (CATIA)

Lateral side

Antero-lateral side

MODELING, SIMULATION AND ANALYSIS OF PATIENT-SPECIFIC IMPLANTFOR RAPID MANUFACTURING

Modeling, Simulation, Analysis and Fabrication of patient-specific

implant using FEA and RP

CT data

Image processing (MIMICS)

CAD model (CATIA)

Reverse Engineering

Pre-processing (HyperMesh)

Simulation (HyperMesh)

Lateral impact

Fracture

Fixation of implant with fractured bone

Pre-fabricated implant

Patient-specific implant-criteria

1. Thicknesses

2. Bio compatible materials3. Positions

Finite Element Analysis (FEA) (ANSYS)

Mechanical behaviour such as stress, strain & displacement at

Intact condition

Fixed with pre-fabricated implant

Fixed with patient-specific implant

Rapid Manufacturing

Fabrication

Measurements

Results & Discussion

Conclusion

CAD (CATIA)

Pre-fabricated implant

Figure 2.1 Methodology