Design of Customised Medical Implants by Layered Manufacturing.

S F Khan, K W Dalgarno

School of Mechanical and Systems Engineering, Newcastle University, NE1 7RU, UK

AbstractThe utilisation of Layered Manufacturing (LM) and CAD for medical applications had been reviewed in this paper. It highlights a number of researches describing the implementation of LM and CAD processes in medical applications and also described the design process for customised medical implants by layered manufacturing. The paper then examines the feasibility of designing of new implants in mandibular bone based on bio ceramics and new fixation methods. It describes the requirements for these bone implants and the processing steps required to develop the implants. This paper then concludes by outlining future work.

1. Introduction

Layered Manufacturing is a technique of fabricating parts by additive method. A 3D model generated in a CAD system is sliced into 2D profile by the software in the LM machine. The sliced-layers of the model are then added one layer at a time onto the build platform by the LM machine until a 3D part is produced. Main LM technologies used are stereolithography (SLA), laser sintering such as Selective Laser Sintering (SLS), Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM); Fused Deposition Model (FDM); 3D ink jet printing (3DP) techniques such as Sanders ModelMaker™, Z-Corp Ink Jet System™, 3DS Multi-Jet Modeling™, and 3DObject PolyJet™; and electron beam melting (EBM). LM is largely being used to produce prototypes and functional parts in the engineering and manufacturing industries. Since it inception in 1986, numerous engineering and manufacturing applications utilizing LM have been documented and researched. In LM technologies, various applications in fields which are not traditionally associated with engineering and manufacturing have opened up such as in architectural and medical modeling, artistic creation [10, 36, 48] and historical restoration work [4]. The main applications are in the evaluation, visualization, validation, form fitting and functional testing in the early stage of product development process as well as tooling aids (refer to Fig 1) [39]. Only about 12% are in the Rapid Manufacturing application with growth expected in its application for customised parts. This will be particularly relevant to fields such as the medical and healthcare industry.

The principle pulling point of utilizing LM for fabrication and manufacturing parts in this field is that LM systems can produce parts of almost any geometrical complexity with relatively minimal tooling cost and time as well a significant reduction in requirements of technical expertise. The removal of tooling will reduce the cost at early stages of the product development process and avoid the lead times imposed by

tooling. Minor or substantial changes to part geometry during the course of design will not incur the times and costs of producing new tooling [11, 26].

2. LM technologies in Medical Applications

Numerous researches have documented the use of LM in medical applications. In general LM technologies for medical applications can be categorized as follows:-

visualisation and surgical planning customized orthoses and protheses implant/replacement scaffoldings and tissue engineering drug delivery and micron-scale medical devices

2.1 Visualisation and surgical planning

LM systems can be used to produce physical models of parts of the human anatomy and biological structures to assist in surgery planning or testing as well as for communication. 3DP system can be used to produce coloured medical models (Fig.2) to enhance learning for students in the classroom as well as for researchers. It can be used to better illustrate the anatomy, allow viewing of internal structures and much better understanding of some problems or procedures [11, 26]. The possibility to mark different structures in different colors in a 3D physical model not only well suited for teaching purpose but can be very useful for surgery planning. The medical models generated using LM systems plays a vital role in providing tactile interaction for the surgeon with patient anatomy prior to the operation. This facilitates the surgeon in planning and performing complex pre-operative surgical procedures and simulations( Fig. 3). When using medical models, the surgeon will have a chance to study the bony structures of the patient before the surgery, to increase surgical precision, to decrease time of procedures and risk during surgery as well as costs and also to predict possible problem that may arise during operation [11]. As compared to conventional MRI assessment, medical models allow a more acceptable judgement over the feasibility of diagnosis and procedural planning [1].

Fig. 1 LM Applications [39].

2.2 Customized orthoses and protheses implant/replacementSince every patient is unique, LM systems are used in fabrication of personalised implants for reconstructive and plastic surgery. Due to the inherent strength of LM technologies to fabricate complex geometry, it is very easy to manufacture custom implants. The model can be used as the custom implant itself or of the implant. Winder et al. [37] and D’Urso et al. [7] successfully used implants model fabricated using LM systems as a master model for reconstructive surgery of a skull defect. They claim there is a reduction on operating time and excellent outcome at ‘reasonable’ cost. The capability of LM to customise implants to quickly fit into a patient's unique size is a great advantage. Hip sockets, knee joints and spinal implants could greatly benefit from this. He et al. [15] claims that the composite hemi-knee joint prosthesis (Fig. 4) reconstructed using LM are accurate to within a maximum tolerance of 0.206 mm. It fitted well and matched with the surrounding tissues, in particular to the lower tibial knee joint. Chang et al [4], Eggbeer et al [9] , Kruth et al [21] and Bibb et al [2] had demonstrated the use of LM technologies in dental applications to be viable as it can improved the speed, quality and efficiency.

2.3 Scaffoldings and tissue engineeringWith it ease of fabricating internal structure, LM technologies are ideal for generating implants with special geometrical characteristics, such as scaffolds for the restoration of tissues [18] as shown in Fig. 5. Scaffolds are porous supporting structures used as transplantation of tissue cells for the rapid and guided growth of new tissue in damaged or defective bones of the patient. 3D scaffold fabrication techniques for tissue engineering has been used for the last 30 years with less favourable success due

Fig. 3 Biomodel use in pre-surgery planning [46].

Fig. 2 Left: CAD image - Right: Medical model [47].

a b c

Fig. 4 SL pattern of a hemi-knee joint (a) as a master for the titanium joint (b) which implanted in the femur bone (c) [15].

to the lack of mechanical strength, no assurance of interconnection channels and uncontrolled pore size [18, 40]. Hence in order to for tissue reconstruction, scaffolds must have interconnected macro and micro-channels with high porosity and adequate pore size to facilitate cell seeding and diffusion of both cells and nutrients throughout the whole structure. These characteristics are required in producing scaffold for tissue engineering. LM has been studied by numerous researchers as a fabrication choice in constructing 3D scaffolds to guide the development tissue culture both in vivo and in vitro [13, 17, 33]. Armillota et al. [1], Hollister [17], Hutmacher et al [18], and Yang et al [40] have demonstrated and rationalised the use of LM technologies in scaffold design for tissue engineering to be viable, cost effective and practical. Yang et al [24] state the advantages of using this technique in designing and

fabricating scaffolds. Fig. 6 illustrates a scaffold tissue engineering proposed for reconstruction of a mandibular implant. LM systems like SLS, FDM, and 3DP have proved to be suitable for fabricating controlled porous structures for use in tissue engineering. LM technologies have contributed significantly to the field of scaffoldings and tissue engineering through the use of biomaterials [20].

2.4 Medical devices and Drug delivery systems

Another application of LM techniques is in fabricating medical devices and drug delivery systems. Skull defect and dental implant are restoration process that requires

a bFig. 6 Layered manufactured scaffold tissue engineering (a) used in reconstruction of mandibular implant (b). [42]

a b c

Fig. 5 LM femur with scaffold (a) [27], (b) [17] and mandibular scaffold implant (c) [45]

detailed planning and high accuracy in implant placement. Hence surgical guidance aids are required. Sarment et al. [27] and Di Giacomo et al. [6] investigated the use of SL surgical guides (Fig. 7) to accurately place dental implants and concluded that there is a significant improvement in implant placement. Ruppin et al [29] claim that LM fabricated surgical guides are comparable to optical tracking system and in agreement other researches on accuracy in computer aided surgery for implant. Bibb et al [2] studied the use of LM to fabricated removable partial dental (RPD) framework for retaining artificial replacement teeth in the oral cavity. The patterns produced were deemed by a qualified and experienced dental technician to be a satisfactory fit and comparable with those produced by expert pattern technicians. In the study, the stiffer patterns produced by SL were easy to handle, were accurate, and produced satisfactory results. And Tay et al. [35] claim that an actual prosthetic socket fabricated using an FDM system (Fig. 8) provides an acceptable degree of comfort, and clinical trial confirmed the viability of fabricate prosthetic socket using FDM technology. Besides medical devices, LM methods are also use to produce drug-delivery system like oral tablet. Rowe et al [28], Leong et al [22] and Low et al [24]demonstrated the possibility of building oral tablet that controlled specific and precise drug delivery by using SL.

a b cFig. 7 SLA surgical guide (a) and the constructed surgical guide in CAD (b) [29] fabricated guide (c) [43].

a b

Fig. 8 Below-the-knee Selective laser sintering prosthetic (a) [44] and FDM total knee replacement (b) [41]

3. Implementation of LM and CAD in medical applications

The development of medical images into 3D models as a tool to help practitioners visualize 2D images has contributed to the development of a new methodology in fabricating medical parts. Medical imaging provides important data of various body structures for diagnostic reasons. These data can be used to obtain geometrical information of the body structures for three-dimensional modeling. CT and MRI images of various structures from conventional hospital scanners provided the input data for commercially available software packages. The image data are visualised, segmented and three dimensionally reconstructed. Solid models can then be generated for use in CAD systems. The generated models incorporating tissues of interest can be imported into a CAE environment for further CAD modelling and finite element analysis. That environment also serves as a platform for conversion to a readable format by rapid prototyping systems. LM systems are then used to produce the physical medical models. The integration of Medical Imaging, CAD, FEA and LM has been presented as a realistic method for modeling and designing various body structures in medical applications. Hieu et al [38], Gopakumar [14] and Lohfeld et al [23] in the study of designing cranial and maxillofacial implant claim that there is a reduction time in implementing the integrated approach of Medical Imaging, CAD, FEA and LM for fabricating personalised medical implants. The common theme in Hieu et al [16], Gopakumar [14] and Lohfeld et al [23] methodology of this approach is illustrated in Fig. 9.

However Starly et al [33] highlight a more comprehensive approach that generate CAD models from scan images. Integrated slice software in LM is used as an interface between the STL file generated from CAD modeled implants and the LM machine. It allows the user to specify the attributes for the LM to build the physical model.

4. Implants design for mandibular bone based on bioceramics and new fixation methods

Bone is a living tissue; considered as a composite material, it comprises of trabecular (cancellous) bone and cortical (compact) bone. It has the capability of healing and remodelling. It will respond with adaptation in its structure to loading stress or injury such as fracture. However, bone is unlikely to remodel itself in major losses cause by trauma, cancer, congenital abnormalities or bone deficiency. Most of these types of major bone repairs are treated by grafting which uses the patient’s own bone (autografts) or donor bone (allografts). The need of further surgery, risk of transmitted disease and limited material from donor site pose some limitations to the current practices [25, 34, 32]. Synthetic substitutes using metal, ceramic, polymer and composites are currently been use to overcome these limitations. Bioceramics are most frequently used in scaffold manufacturing and hard tissue implants within bones, joints and teeth. Bioceramics have the basic chemical composition akin to natural bone. Goodridge et al [13] and Dyson et al [8] have demonstrated the feasibility of using bioceramics for use as bone substitute. The design of the implant must take in to consideration biocompatibility, mechanical properties, cost effective

Fig. 9 LM and CAD methodology adapted to design and manufacture of implants

Modeling in CAD and FEA

Commercial CAD/CAE Solid or NURBS based parametric Modeler for design and analysis.

Capture Patient Data

Scan Images from CT, MRI

Process Scan data

Medical Modeler Software

Export data in suitable formatIGES, SSL, STL, DXF, 3DS

LM of Medical Implant

SLA, SLS, SLM, FDM,3DP.EBM

Export data in RP format

STL (Binary or ASCII)

Direct Method Indirect Method

manufacturability and process [19] as well as an accurate fit that requires minimal or no healthy bone removal. Furthermore, the bone variations in material and mechanical properties are dependence on location and function [12, 31]. Hence a promising method of manufacturing implants is through LM. This paper proposes a LM and CAD methodology to design customised implants of mandibular bone. The process flow is as shown in Fig 10.

Fig. 10 LM and CAD methodology for implants design in mandibular bone based on bioceramics and new fixation

Process Scan data

Capture patient data

Export data in suitable format

Modeling in CAD and FEA

Create Library of 3D cellular microstructure

Selection of microstructure

Boolean Operation

Dense Implant Implant with Microstructure

Export data in RP format

LM of Medical Implant

Post process

Customised Bioceramics Implants

Verify data RP file before uploading into LM system

Normally most of the fixation methods for mandibular reconstruction and fracture system consist of drill bits, plate bending forceps, plate holding forceps, plate cutters, cannulae, taps, countersinks, plate bending pliers, plate cutters, drill guides and screwdrivers to facilitate the placement of screws and modification of plates. The implant for reconstruction is secured in place by plate and screw. However, this paper proposed a design that integrates the fixation method into the implant. This is derived from the advantages of LM. A lower jaw bone model in STL format was imported to the CAD to design an implant. The lower jaw is non-defective and was obtained from a secondary source. A simulated defective section was created on the jaw by cutting a section of it in the CAD software (Fig. 11).

Several joints are proposed as shown in Fig 12. Type (a), (b), and (c) joints rely on rapid bonding to secure the implant while type (d) uses conventional screws.

Fig. 11. A section cut out for implant.

The design of the implants will need to incorporate interconnected controlled channels in order to promote bone growth. Bignon et al [3], Woesz et al [38] and Chu et al [5] indicated that there are significant correlation between micro- and macro-porosity as well as controlled channels with bone response and mechanical properties.

5. Conclusion and Future work

The LM and CAD approach for medical applications had proved to be viable and promises potential benefits as demonstrated by the numerous researches conducted. Precise customised bioceramic implants can reduce the removal of healthy bone, eliminate the need for bone grafting, and promote effective planning of implantation. The future work to be undertaken should be related to finding suitable bioceramics’ material and mechanical properties that adhere to the requirements of successful bone growth. In addition, further work will be conducted to propose new fixation methods and assembly. This will involved FEA to determine the strength of the propose fixation joint as well as of the implant itself.

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