design and development of a medical device to improve the assembly of head/neck taper junctions in...
TRANSCRIPT
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UNIVERSITY OF BATH
DEPARTMENT OF MECHANICAL ENGINEERING
DESIGN AND DEVELOPMENT OF A MEDICAL
DEVICE TO IMPROVE THE ASSEMBLY OF
HEAD/NECK TAPER JUNCTIONS IN MODULAR
TOTAL HIP REPLACEMENTS
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COPYRIGHT
Attention is drawn to the fact that copyright of this dissertation rests with the
author. This copy of the dissertation has been supplied on condition that
anyone who consults it is understood to recognise that its copyright rests with
its author and that no quotation from this dissertation and no information
derived from it may be published without the prior written consent of the
author.
This dissertation may be available for consultation within the University
Library and may be photocopied or loaned to other libraries for the purpose of
consultation.
CHEATING AND PLAGIARISM
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ABSTRACT
There has been a significant failure rate in modular total hip replacements
(MTHR) over the past few years, particularly with the use of large diameter
Metal on Metal (MoM) bearings. Various studies have shown that sub-optimal
strength of the head-neck taper junction plays an important role in these high
failure rates.
The purpose of this project is to design and develop a medical device to
improve the assembly of this taper junction with an overall aim to reduce the
occurrence of early revision surgeries on MTHRs. The device aims to ensure
axial alignment of the head and neck tapers before providing an adjustable
impact force between 4KN and 6KN to achieve the strongest possible junction
assembly, with the target of reducing the incidence of fretting and corrosion at
thi j ti d it i t d bl
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CONTENTS
1. INTRODUCTION 1
1.1 What is a Total Hip Replacement? 1
1.2 What are THRs used to treat? 2
1.3 What is a Modular Total Hip Replacement? 3
1.4 Why are they Modular? 4
1.5 How are MTHRs implanted? 4
1.6 How are MTHR assembled? 5
1.8 What is the problem effecting MTHR? 6
2 AIMS AND OBJECTIVES 8
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4.3.2Generation of PDS 39
5. DEVELOPMENT & EVALUATION 40
5.1 Concept Generation and Evaluation 40
5.1.1 Initial Product Design Specification (PDS) 41
5.1.2 Discretisation of Design Challenge 42
5.1.3 Radial Thinking 43
5.1.4 Visual Concept Analysis 44
5.1.5 Critical Assessment and Selection 52
5.1.6 Further Investigation of Powering Concept 55
5.1.7 Development of Final Powering Concept 60
5.1.8 Mechanical Feasibility of Chosen Concept 67
5.1.9 Development of Proof-Of-Concept Testing Rig 73
5.2 Detailed Design 79
5 2 1 Solid Modelling 79
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7.4 Discussion 95
8. CONCLUSIONS 96
10. REFERENCES 98
11. APPENDICES 101
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NOMENCLATURE
N = Newtons
KN = Kilo Newtons
F = Force
m = Mass
g = acceleration due to gravity (9.81m/s)
m/s = meters per second
Kg = Kilograms
t = Time (in seconds)
t = Impact duration
v = Velocity
v = change in velocity
k = Spring Stiffness (in N/m)
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LIST OF FIGURES AND TABLES
FIGURES
Figure 1: Illustrated Hip Replacement; Before and After [2] ....................................... 1
Figure 2: Charnleys Low Friction Arthroplasty [4] ..................................................... 2
Figure 3: Illustration of Normal Vs. Arthritic Hip [5] .................................................... 3
Figure 4: Exploded View of MTHR Assembly [7] ....................................................... 3
Figure 5: Illustration of MTHR Surgical Procedure [11] .............................................. 4
Figure 6: Orthopaedic Mallet and Impactor [12] ......................................................... 6
Figure 7: Example of matched and mismatched taper angles [20] .......................... 13
Figure 8: Stuart Pughs Design Process Model [33]................................................. 25
Figure 9: Orthopaedic Surgeon Survey Introduction ................................................ 28
Fi 10 S S Q1 28
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Figure 23: Device for Handling Hip joint Heads [39] Figure 24: Head
holder and impactor [40] .................................................................................. 36
Figure 25: Method of applying Femoral head Resurfacing [41] Figure 26: Nail
gun Patent 1 [42] ............................................................................................. 36
Figure 27: Nail gun Patent 2 [43] Figure 28: Wire Shelf
Driver [44] ........................................................................................................ 36
Figure 29: Automatic Centre Punch [44] .................................................................. 37
Figure 30: Inserter jaw for knee prosthesis impaction and extraction [45] ................ 37
Figure 31: Comparison in grading between EU and USA Device Classification [48] 38
Figure 32: Initial sketch to capture ideas ................................................................. 42
Figure 33: Development of Objective B ................................................................... 43
Figure 34: Development of Objective C ................................................................... 44
Figure 35: Three Prong Flexible Support and Centring Cone Sketch ...................... 45
Figure 36: Semi-Cup + Centring Cone Sketch ......................................................... 45
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Figure 51: The Solenoid Powered Nail Gun [49] ...................................................... 57
Figure 52: Electric Powered Nail Gun [49] ............................................................... 58
Figure 53: Can-Crushing Device [50] ...................................................................... 59
Figure 54: Adapted Juicer Sketch ............................................................................ 60
Figure 55: Adapted Can Crusher Sketch ................................................................. 61
Figure 56: Corkscrew Lever System Sketch ............................................................ 62
Figure 57: Twisting Adjustment and Release Concept Sketch ................................. 63
Figure 58: Gearbox Style Spring Compression Adjustment Concept Sketch ........... 63
Figure 59: Spring Compression adjustment System Sketch .................................... 64
Figure 60: Slotted Trigger Release Mechanism Sketch ........................................... 65
Figure 61: Firearm Trigger Concept Sketch ............................................................. 65
Figure 62: Handle Trigger System Sketch ............................................................... 65
Figure 63: Final Developed Concept Sketch ........................................................... 66
Figure 64: Tubular Casing Surrounding Spring Sketch ............................................ 67
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Figure 78: Base plate fixed to table with and without Rubber Base ......................... 86
Figure 79: Table of Data recorded from Instron ....................................................... 88
Figure 80: Test 2, Steel, 2KN (with rubber base) ..................................................... 89
Figure 81: Extrapolated Load Cell Data, >2KN ........................................................ 90
Figure 82: Extrapolated Load Cell Data, 4KN and 6KN ........................................... 91
Figure 83: Images of Failed PVC and Rubber Tips at 4KN Load ............................. 92
TABLES
Table 1: Data from Pennock et al. (2002) study [29], [10] ........................................ 17
Table 2: Data from Lavernia et al. (2009) Study [30], [10]........................................ 18
Table 3: Data from Heiney et al. (2009) Study [31], [10] .......................................... 19
Table 4: Data from Rehmer et al. (2012) Study [32], [10]......................................... 21
T bl 5 I iti l PDS 41
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1. INTRODUCTION
1.1 What is a Total Hip Replacement?
There are two types of Hip Replacement surgery, Hip Resurfacing and Total Hip
Replacement (THR). This project focusses on the THR procedure, which is also
known as Total Hip Arthroplasty. THRs are among the most common orthopaedic
procedures performed today [1]. The THR procedure involves removing the femoral
head (top of the thigh bone) and a layer of bone from in and around the acetabulum
(hip socket) and replacing them with artificial materials, thus resulting in an artificial
hip joint. The before and after pictures of a hip joint that has undergone a THR is
shown in Figure 1 [2] below, with the original diseased hip joint shown on the left
and the new replacement joint shown on the right.
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The first modern THRs were designed by John Charnley in the 1960s, which
stemmed from his paper Surgery of the Hip Joint - present and future
developments [3] published in 1960. Charnleys THR consisted of a high density
polyethylene cup that was fixed inside the hip joint socket and a stainless steel
component that made up the artificial femoral head and stem which slotted into the
patients femur. This low friction arthroplasty [4] hip design was first implanted in
November 1962 and can be seen in Figure 2 [4] below.
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Figure 3: Illustration of Normal Vs. Arthritic Hip [5]
1.3 What is a Modular Total Hip Replacement?
Modular THRs (MTHR) were introduced in the 1970s [6]. The previous leg (femoral)
component used in Charnleys original design was separated into head and stem
components and the previous plastic cup was separated into shell and liner
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1.4 Why are they Modular?
Modularisation was introduced into the design of THRs in the 1970s [8] to allow
more flexibility in material selection/combination and component sizing to ensure a
more individually suited THR for each patient. It also allows surgeons to reduce
inventory [9] and simplifies revision surgeries [8]. Several different material choices
and combinations are available to surgeons. For the stem and head; Cobalt Chrome
or Ceramic Heads can be used on Titanium stems. For the bearing combinations;
Metal on Metal (MoM), Ceramic on Ceramic (CoC) and Ceramic on Metal (CoM),
and finally Ceramic or Metal heads can also be used on Ultra High Molecular
Weight Polyethylene (UHMWPE). [10]
1.5 How are MTHRs implanted?
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Step A involves making an incision to gain access to the joint area and dislocating
or disarticulating [11] the femoral head from the acetabulum (or hip socket).
Step B involves cutting off the femoral head with a surgical saw.
Step C involves reaming out the acetabulum and the femur to prepare them to
receive the shell and stem respectively.
Step D involves the introduction of the prosthetic components and the final image in
the bottom right hand side of the figure shows the fully installed THR.
1.6 How are MTHR assembled?
The order in which the components are introduced in this procedure is important to
note. The acetabulum shell is first introduced and fixed in place before the stem is
inserted into the femur. There are two types of stem designs; cemented, where
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stem taper using a mallet and impactor (usually tipped with a softer material than
the head so as not to damage the surface of the head). An example of the sort of
mallet and impactor commonly used is shown in Figure 6 [12] below.
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product recalls. Large diameter (>36mm) MoM bearings are by far the worst
offenders when it comes to early revisions and product recalls. The use of large
diameter MoM bearings amplifies an existing issue regarding the strength of the
head/neck taper junction more so than other joint material and geometry selections.
Large diameter bearings produce an increase in the torque in the joint, as a larger
frictional torque is generated since there is a longer lever arm acting between the
fulcrum or centre of the joints rotation and the surface where the head makes
contact with the liner, especially MoM. This increase in force about the junction,
leads to increased levels of fretting wear and corrosion, which causes the liberation
of prosthesis material and hence early revision surgeries (as the human body has
an adverse reaction to the presence of these foreign particles).
Fretting corrosion and wear can still occur in all material and geometry combinations
but the accelerated and extreme instances found in some of the large diameter
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2. AIMS AND OBJECTIVES
Aim: To design and develop a medical device to improve the assembly of
head/neck taper junctions in MTHRs with an overall aim to contribute to the
reduction of early revision surgeries for MTHRs
Objectives:
1. Review relevant literature to gain greater understanding/scope of problem
2. Establish User Needs and Design Requirements
3. Produce Product Design Specification (PDS)
4. Generate and evaluate design concepts
5. Design and develop a prototype for proof-of-concept
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3. LITERATURE REVIEW
This section of the report contains the findings from the literature review carried out
on the failure of MTHRs due to head/neck taper junctions. The review spread out
beyond the borders of this specific issue to ensure an understanding of the bigger
picture could be taken into account before focussing on the specific problem itself
towards the end of the review. The findings are now presented under two headings,
Understanding the Problem and why it is occurring, followed by How to reducing
or eliminate the problem.
3.1 Understanding the problem and why it is occurring
Before trying to solve the problem it is essential to take the time to fully understand
the background and history of the problem and the reason behind its occurrence.
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diameter of the bearing. This increase in torque caused wear and fretting corrosion
which again led to the liberation of metal particles and thus patient complications, as
with the first failure method. The reason why the second failure method is the most
important to this project is because the use of large diameters in MoM bearings is
not unique to the ASR design and so plays a role in the failure rates of various other
MTHR designs. Both Henghan et al. (2012) [14] and Langton et al. (2011) [13]
agree on this point and Langton et al. go on to suggest that bearing diameters of
36mm or greater are most at risk to this failure method.
Smith et al. (2102) [15] concluded, following an in depth analysis of National Joint
Registry Data covering England and Wales, that MoM bearings are more likely than
other bearing material combinations to fail, and also found that their failure rates
were increasing proportional to increasing bearing diameter size. Langton et al.
(2011) [13] also came to the same conclusion in their study into the failure of the
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greatest frictional moments followed by Metal on UHMWPE and then CoC with the
lowest frictional moments, thus adding to the growing evidence pointing at the
failings of MoM bearings. Langton et al. (2011) [13] found that as the trend in
increasing bearing diameter grew, there was no increase in the diameter of the neck
taper to counter the associated increase in torque. In a different study by Langton et
al. (2012) [18] they noted that neck diameters actually decreased as larger and
larger bearings sizes became available, thus exacerbating the problem. The
reasoning behind reducing the diameter of the neck taper was to increase the range
of motion of the prosthesis.
One of the biggest studies of MTHR neck/taper junctions was carried out by
Goldberg et al. (2002) [19] and looked into various different aspects and failure
methods in this junction. One of the recommendations that they put forward
following their research was to increase the neck taper diameter with an aim to
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Now that the severity and reasoning behind the failure MTHRs has been established
the next step is to look at who has influence or control over the reduction or
elimination of the problem.
3.2 How to reduce or eliminate the problem
This part of the chapter looks at who has control over or influence on the key factors
that contribute towards the strength of the head/neck taper junction. This part
finishes with an in depth analysis of four particularly relevant studies with an aim to
establishing the optimum conditions and provisions for assembling a head/neck
taper junction.
3.2.1 What influence do Manufacturers have on the assembly?
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Figure 7: Example of matched and mismatched taper angles [20]
Figure 7 Part A shows the correct fit with the maximum contact area between the
head and neck tapers. Figure 7 Part B on the right shows a poor fit where there is a
significant taper angle mismatch leaving low contact area creating stress
concentrations and room for micro-motion (also described as toggling) which leads
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resistant materials plays an important part in the improvement of these designs,
along with the recognition that only a very small mismatch is required to begin a
cycle of fretting corrosion and wear.
In a study which involved measuring the forces required to disassemble three
different model of MTHRs, which had been retrieved from patients undergoing
revision surgeries, Lieberman et al. (1994) [23] made an interesting discovery. One
of the models required a much greater force than the other two to be disassembled
and was the only model type of the three examined not to show any signs of
corrosion after a 78 month period. These particular MTHRs had a different assembly
history from the other two model types in that they had been assembled by the
manufacturer and were supplied to the surgeon in a preassembled condition. These
MTHRs had been shrink fitted with a sealant, applied during this assembly.
Lieberman et al. (1994) [23] believe the greater junction strength and resistance to
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3.2.2 What influence do Surgeons have on the assembly?
The fit of the spigot head is noted as the most important source of error in
Fessler and Frickers (1989) [21] study into the Stresses in Alumina Universal
Heads of Femoral Prosthesis. Bobyn et al. (1994) [24] would agree with their
statement since they found a reduction in taper surface contact area and an
increase in wear and fretting corrosion in two Modular Femoral Prosthesis, after
assembling them both using one fifth of the manufacturers recommended assembly
force and exposing them to the sort of cyclic loading that they would experience in-
vivo. Thus the impact load applied has a significant influence on the fit or the
assembly of the head on the neck. A study carried out by Goldberg and Gilbert
(2003) [25] entitled In vitro corrosion testing of hip tapers concluded that the
proper seating of the head onto the neck increases the forces required to cause
micro-motion, and hence wear and fretting corrosion. A study by Mroczkowski et al.
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The impact force is not the only important factor that the surgeon has influence over
during assembly. The axial alignment when placing the head on the neck taper prior
to impact and the axial alignment of the impact delivered is also extremely
important. Both Callaway et al. (1995) [27] and Pansard et al. (2012) [28] traced
back the failure of a number of MTHRs to incorrect fitting of the head on the neck
taper by examining retrieved MTHRs removed during revision surgeries. They both
found that their retrieved Hip Replacements had failed due to extreme corrosion
caused by incorrect fitting of the head during original assembly. Due to varying
manufacturing tolerances between different brands, it is also strongly recommended
not to mix different manufacturers components as this can result in poorly fitted
parts that can reduce the life of the prosthesis.
Four key papers are now discussed with a focus on the effects of the impact/s
applied during the assembly of the head/neck taper junction with an aim to
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Table 1: Data from Pennock et al. (2002) study [29], [10]
This study looks at the effects of varying the magnitude of the impact force, the
order in which the different impact forces are applied and the total number of
impacts delivered during assembly and their effect on the resulting junction strength
(determined by pull-off tests). This study also looked at the effects of wet and dry
taper surfaces on junction strength, but the wetted samples were not included in this
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[29]. Pennock et al. (2002) [29] also states the importance of axial alignment when
delivering the impacts, to ensure that all of the force is transmitted during the
impaction. One of the findings taken from their study (especially when the wet
tapers were taken into consideration) was that they noted an increase in junction
strength with increasing impact magnitude [29].
The next study was carried out by Lavernia et al. (2009) [30] and looked mainly at
the effects of blood and fat contamination on the taper surfaces and the effect they
had on the junctions strength. However, as they used control or dry tapers for
comparison the data recorded for these was of benefit to this investigation and has
been recorded in Table 2 [10] below.
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this study can be considered to be a more realistic representation of the average
magnitude of a surgeons impact as they recorded the impact forces applied by 8
different surgeons as opposed to the previous study by Pennock et al. (2002) [29]
that only used 11 impacts by a single surgeon. The result is a 27% (approx.)
decrease of the force used by Pennock et al (2002). It is worth noting that this study
does not vary the impact magnitude or the number of impacts applied, but does give
a good representation of the average pull-off force for the prescribed magnitude with
a single impaction and provides another average value for surgeon impaction
magnitude, which will both be of use later when comparing this study to those that
follow on in this part of the chapter. The overall study showed that a clean and dry
taper provides the optimum assembly condition to facilitate maximum junction
strength.
The next study was carried out by Heiney et al (2009) [31] and had a much larger
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assembly forces. The average surgeons impact force applied during assembly in
this study is roughly twice that of either of the two previous studies showing a large
range of magnitudes arising across the different surgeons used to create the
averages in each study.
Heiney et al. (2009) [31] found there to be a difference in junction strength between
using one impaction and two impactions but found no difference when applying
more than two impactions. This finding compliments one of the findings from the
study by Pennock et al. (2002) [29], in that the first impaction provides the majority
of the junction strength with subsequent impacts providing a small but additional
increase. Heiney et al. (2009) [31] also found that the junction strength increased
along with the impact magnitude thus adding weight to this original finding by
Pennock et al (2002) [29]. It is unfortunate that neither Pennock et al (2002) [29]
nor Heiney et al. (2009) [31] provided the impact forces in newton values instead of
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studies. The relevant data acquired from this study is shown in Table 4 [10] as
follows.
Table 4: Data from Rehmer et al. (2012) Study [32], [10]
Using pull-off and twist-off disassembly tests, Rehmer et al. (2012) [32] found that a
single impact of a minimum of 4KN (kilo-newtons) was required to ensure optimum
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head connections on a modular shoulder prosthesis. In this study, they use a
surgeon to try to acquire an average assembly impact force for which they can
design a drop rig. The drop rig is then used to assemble their specimens with a
constant impact force but under different conditions (i.e. dry or wet) prior to
disassembly testing. The surgeon assembled 6 shoulder taper junctions using a
mallet and impactor (the same as is used in a modular femoral hip assembly), and
Loch et al. (1994) [8] then measured the force required to pull the joints apart. They
repeated this process with the surgeon and the same six specimens 16 times to
acquire their average pull-off value, which they then used to set a drop rig to
assemble the test specimens to replicate this average pull-off value (under control
conditions). It is acknowledged the average pull-off values may have been affected
by the reduction in strength that can be experienced when repeatedly assembling
and disassembling the same specimens. The pull-off forces, from the surgeon s
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head is correctly seated on the neck taper before providing a single impact of no
less than 4KN, which is axially aligned with the taper axis. The impact should also
not exceed 6KN to ensure that it does not stray into the region where it could cause
internal damage to the patient or damage to the tapers, as mentioned previously.
The findings from this review will be used to guide the design of the device
proposed in this project, and to aid the assembly of MTHRs. The next step in the
process is to establish the user needs and hence design requirements for such a
device.
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4. USER NEEDS & DESIGN REQUIREMENTS
It is now clear from the Literature Review that there is room for improvement in the
assembly of head/neck taper junctions in MTHRs. This room for improvement
grows and becomes a serious problem when considered in the use of large
diameter bearings, particularly MoM bearings. Even if manufacturers applied perfect
tolerances, surfaces finishes and optimum neck taper diameters it is still essential to
correctly assemble this taper junction to benefit from these improvements. It is clear
from the wide ranging impact forces applied by different surgeons that it is unfair to
expect them to be able to repeatedly provide the very specific forces and alignments
required for the optimum assembly of MTHRs using the current tools at their
disposal (mallet and impactor). Therefore the development of a new device is
completely justified.
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producing a Product Design Specification (PDS), which can then be used to guide
the next stage of the project.
4.1 Fundamental Design Requirements
These design requirements have been extracted directly from the findings in the
literature review and form the foundation and basis for the entire design, i.e. the
design must achieve all of these requirements to be successful. The fundamental
design requirements are listed as follows.
1. Ensure axially aligned seating of head on neck taper axis prior to impaction
2. Impact must be delivered in axial alignment with neck taper axis
3. Deliver impact force of between 4KN and 6KN, adjustable to 0.5KN
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minimum force, compared to larger metal taper junctions that may require slightly
more force for optimum assembly.
The fourth requirement involves trying to concentrate the impact to the taper
junction and not down the stem where it could cause damage to the stem/femur
interface. It is also intended to ensure the efficient transfer of impact energy into the
junction and not to have it wasted through dissipation into the surrounding region.
4.2 Establishing and Defining User Needs
Since the fundamental design requirements had now been established, the next
step was to look beyond these fundamentals to establish other design requirements.
It is extremely important to involve the end user in the design of anything to ensure
that it meets their specific needs, so a survey was used to try to gather design
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Figure 9: Orthopaedic Surgeon Survey Introduction
After the introduction to the survey, the first question posed attempted to gain an
understanding of the size of the range of different hip implants that were being used
and whether or not they were cemented or cement-less. This would influence
whether or not the device would be designed solely for use with a very popular
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The next task was to establish the range of femoral head sizes. This was important
to establish so that the device could be designed to facilitate the most common
head sizes. This also fulfils the purpose of establishing a general impression of the
current use of large diameter (>36mm) MoM bearings, given their associated
problems previously mentioned in the literature review. The wording and layout of
this question is shown in Figure 11 below.
Figure 11: Surgeon Survey, Q2
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Figure 12: Surgeon Survey, Q3
The next question aims to establish the size of the access area or incision in the
patient that the device must fit and function inside. The average size is 10cm, so this
question looks to see if many surgeons work under or above this incision size. This
question is shown in Figure 13 below.
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Figure 14: Surgeon Survey, Q5
The next question was not so much based on the establishment of design
requirements for the device, but more so at gathering data to compare with the four
studies listed at the end of the literature review. It was acknowledged that the
responses could not be looked upon too strongly, as the information provided by the
surgeons is opinion-based and thus is quite subjective. This question is shown in
Figure 15 below.
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question was provided in a format where the participant rates the level of
importance out of 10. This question is shown in Figure 16 below.
Figure 16: Surgeon Survey, Q7
The final question allowed the surgeons to propose any features that they felt
should be included in the design of the device. The intention of this question was to
give the user an opportunity to directly propose things that were of importance to
them so that the design would have some sort of user-centred-design approach.
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However, a brief analysis of the survey has shown that there is a significant amount
of very relevant data available which would be of great value to the future
development of the device. A summary of the survey response is contained in
Appendix 1.
The original plan had been to gather the findings from the literature review and the
feedback from the survey and allow this to contribute to the PDS, but as mentioned
above this was not possible so for this reason none of the feedback from the survey
influenced the PDS. The PDS will now be discussed in more detail in the next
section.
4.3 Product Design Specification
The purpose of a PDS is to provide the designer with a list of design requirements
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Figure 18: Stuart Pughs Design Core [33]
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4.3.2 Commercially available designs and patent research
As mentioned previously, the only commercially-available designs for the assembly
of the head/neck taper junctions are the orthopaedic hammer and impactor
methods. An in-depth patent search was carried out to establish what other like-
minded or similar and applicable designs already existed. Samples of some of the
more interesting designs are shown as follows. Some of these have made an
influence on the design of the device rig as can be seen later on in the Design and
Development stage.
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Figure 23: Device for Handling Hip joint Heads [39] Figure 24: Head holder and impactor [40]
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this case involved delivering an impact or maintaining an alignment. Rough notes
were taken during the patent search to keep track of any good ideas, which could
then be applied directly or manipulated to fit into the device proposed in this project.
Figure 29 and Figure 30, shown below; display two more applicable technologies
that could be of use for the concept generation stage later in the project.
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4.3.1 Medical device standards review
Medical Device Classifications exist to grade the level of risk that a medical device
poses to a patient or user; the higher the grading, the more stringent the regulations
imposed on the development and manufacture of the device. There are different
classification systems for both the EU (EU/ISO [46]) and USA (FAA/ISO [47]), and
as the risk or grading increases, so too do the design regulations imposed by the
standardisation bodies to meet their audit requirements. The two grading streams
are illustrated side by side in an extract from medical Device Design by P.J.
Ogrodnik [48], as shown in Figure 31 below.
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for clinical trials, so the standards review is of greater value further down the line in
future clinical design and development of this device.
4.3.2 Generation of PDS
The PDS pooled all of the design requirements acquired through the project so far,
and so took information from the Literature Review, the Surgeons Survey (although
left open, pending response from participants), the Standards Review, and the
patent and existing design research. The PDS is a working document, and can be
added to and edited as future work progresses on the development of the device
proposed in this project. The most up-to-date version of the PDS is included in
Appendix 2 [10].
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5. DEVELOPMENT & EVALUATION
This chapter of the report looks at the generation, evaluation and development of a
concept for the device proposed in this project. It then examines the concept
generation, development, detailed design and manufacture of a proof-of-concept
Testing Rig, to verify the functionality of the overall device concept established in
the first phase.
5.1 Concept Generation and Evaluation
Since the PDS created in the previous chapter was quite detailed and in-depth, not
all of the points covered will be relevant at this stage in the design. It is for this
reason that the PDS was condensed down into its more critical attributes. This
created a less constrained environment to work in when generating creative
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5.1.1 Initial Product Design Specification (PDS)
A full PDS was developed for a prototype device aimed at use in clinical trials but for
the purpose of this project, which will only be tested in laboratory conditions, the
original PDS was condensed down. This was carried out in order to focus on the
fundamental design requirements and to allow more creative freedom for concept
development. The PDS used for the project at this stage is shown below.
1. Performance
1.1 Must hold head taper axially aligned with neck taper axis
1.2 Must deliver impact axially aligned with neck taper axis
1.3 Must deliver adjustable impact from 4KN - 6KN to +/- 0.5KN
1.4 Must isolate impact to head-neck taper junction
2. Customer
2 1 Must be able to use with varying head size
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An initial sketch was made at this point to record any design ideas that had come to
mind so far. This initial sketch is shown in Figure 32 as follows.
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5.1.3 Radial Thinking
Radial thinking was used to expand on different concepts and help to develop and
record different concept ideas. Each of the four key objectives, A to D, were listed in
a bubble in the middle of a blank page and ideas stemmed outwards from this
starting point. Two examples of this exercise are shown as follows. Figures 33
shows the development of Objective B, and Figure 34 shows the development of
Objective C.
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Figure 34: Development of Objective C
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(A1). Three Prong Flexible Support and Centring Cone
Figure 35: Three Prong Flexible Support and Centring Cone Sketch
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(B). Holding impactor axially aligned with neck taper axis
*DATUM* Surgeon holding impactor aligned by hand
(B1). Bent Hex-Rod
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(B3). Split Cup/Split Mould
Figure 39: Split Cup/Split Mould Sketch
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(C2). Slide Hammer - To Charge Spring
Figure 41: Slide Hammer - To Charge Spring Sketch
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(C4). Electro-Magnets (Solenoid Actuator)
Figure 43: Electro-Magnets Sketch
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(C6). Screw Mechanism - To Charge Spring
Figure 45: Screw Mechanism - To Charge Spring Sketch
(C7) Lever To Charge Spring
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(C8). Pneumatic Piston
Figure 47: Pneumatic Piston Sketch
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(D2). Mechanism to Hook around the Lips at Base
Figure 49: Mechanism to Hook around the Lips at Base Sketch
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The result of this exercise is the selection of concepts to address each of the four
key performance requirements. This exercise was carried out using an excel spread
sheet and is shown on the following page in Table 6. The highest scoring concept
from each of the four sections was highlighted in yellow.
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As the outcome from Table 6 has shown, the following sub-system concepts have
been chosen and are listed as follows.
(A1) 3 Prong Flexible Support + Centring Cone
(B1) Bent Hex-Rod
(C8) Pneumatic Piston
(D1) Rod Inserted In Stem Hole
Since the main function of the device is key requirement C (Must deliver adjustable
impact from 4KN to 6KN, to +/- 0.5KN) and the second and third ranked ideas in
this category, which were Electro-Magnets (C4) and Lever to charge spring (C7),
also scored relatively high, all of the top three designs in this category will be looked
into in more detail before finally settling on a single concept. It is also worth
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in the design. This investigation was performed by looking into existing products that
were applying these three powering methods, and checking the suitability of each
method for the device in this project. These are examined as follows, starting with
the Pneumatic approach.
Pneumatic Piston
One of the best products to look at to examine the functionality of the different
methods of powering a device that provides an impact is the Nail Gun. A pneumatic
Nail Gun is shown in Figure 50 [49], below.
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can become quite complex and expensive when designing and manufacturing. This
project is trying to provide the simplest possible powering method, so for this reason
the pneumatic approach does not seem to be the best fit.
Electro magnets
Electro magnets, or specifically in this case electro solenoids, can be used to
electrically initiate magnetic fields, which can propel objects to create an impact.
This type of system is explained once again using a nail gun example in Figure 51
[49] below.
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that may be close to the device when in use. This also, much like the pneumatic
option, this greatly increases the complexity in the design and means that the device
will have to abide by more constraining standards during the design process. This
will add difficulty and complexity to the future work on a clinical device, and thus
rules out this technology as a possible option.
Lever to Charge Spring
The final option for powering the device is a charged spring, which is compressed
using mechanical advantage, such as a lever system. An electrically powered
mechanical spring system is shown in Figure 52 below, again using an example of
an electric powered nail gun.
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more sustainable and reliable with no dependency on other inputs such as electricity
and compressed air. The surgeon could use their energy to provide the work
required to compress/charge the spring could be made easier with a leverage
system. An example of the employment of this sort of powering mechanism is
demonstrated in a simple can-crushing device as shown in Figure 53 below.
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5.1.7 Development of Final Powering Concept
Now that the method which would power the device had been chosen, the next step
was to develop the design in more detail to prove that it could actually work.
There were three main design objectives that had to be met for the device to be able
to function. The first was to confirm the final leverage method to compress the
spring. The second was to come up with a way in which the device could deliver an
adjustable impact (which had not been focussed on previously). Finally, the third
objective was to design a trigger/release mechanism to actuate or initiate the
impact.
Mechanical Leverage System
This stage involved more sketching and research, but this time just focussed on
leveraging systems. Items such as hand-operated juicers, in which they compress
the fruit to extract juice were seen as applicable to the design of the device A
M h d l i i d d l f f h
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More can-crusher products were also investigated, and an example of one of the
sketches trying to use this approach is shown in Figure 55 below.
Figure 55: Adapted Can Crusher Sketch
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Figure 56: Corkscrew Lever System Sketch
This sort of mechanism could be attached onto the end of the device, and the
surgeon could use both hands to push the levers down to the sides of the device.
This would compress a spring that can be held in place by a locking mechanism and
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Figure 57: Twisting Adjustment and Release Concept Sketch
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Figure 59: Spring Compression adjustment System Sketch
Trigger/Release Mechanism
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Figure 60: Slotted Trigger Release Mechanism Sketch
Selection of the Concept for Objective C
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Selection of the Concept for Objective C
The final selection of the concept for the device is shown in Figure 63 below. This
incorporates the corkscrew method of leverage seen in Figure 56 previously,
positioned out of sight to the left of the lower sketch within Figure 63 below. It also
incorporates an adjustable force mechanism behind the spring and a trigger
mechanism as shown.
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Figure 64: Tubular Casing Surrounding Spring Sketch
5.1.8 Mechanical Feasibility of Chosen Concept
Before progressing further, the leverage mechanism must be validated theoretically
to ensure that it could realistically function and be used by a surgeon. A rough
sketch of the device moving through the charging motion is shown in Figure 65
The leverage force has been simplified and marked out on the Figure 56 which is
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The leverage force has been simplified and marked out on the Figure 56 which is
shown again below and renamed Figure 66 for clarity.
Figure 66: Force Balancing Free Body Diagram
Using static force balancing analysis, Equation 1 can be derived and is as shown
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Using static force balancing analysis, Equation 1 can be derived and is as shown
below.
=
(1)
Substituting in the values;
2 =(6 10) 0.01
0.3
Solving for F2;
2 = 200
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Substitute in known values;
100 = (9.81)
Rearrange and solve for m;
= 10.2
Where;Kg = Kilogram
This means that a 10.2Kg weight could be hung on one side, and it can be
Where;
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F= Impact Force
t= Impact duration (time over which impact occurs)
m= Mass
v= change in velocity
The force used in this equation is 6KN, since it is the highest end of the scale.
The mass used for this calculation was based on the average mass of an
orthopaedic mallet, which was found to be approx. 0.4Kg (kilograms).
Since the impact duration is unknown, a previously-recorded impulse value was
taken from a PhD student at the University of Bath who had carried out a study on
impacts and had acquired this data for 2KN, 4KN and 6KN impact forces. It should
be noted that the tip material used during these tests is unknown, and this may have
a large effect on the testing results. The impulse recorded for 6KN was 11.1Ns
(newtons per second)
Now that the initial velocity has been found, this can then be used to find the kinetic
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energy (KE) at the point of impact. This can be found using Equation 4.
=
(4)
Substituting in the known values;
=1
2(0.4)(27.75)
Solving for KE;
Since both k and x are both unknown at this point, it is decided to use a spring
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displacement (x) of 60mm (millimetres) or 0.06m (meters). This has been chosen as
this displacement should leave enough room for sensitive adjustments to be made
to the spring later in the design process.
Substituting these values into Equation 5;
154.0125 =1
2(0.06)
Rearranging and solving for k;
= 85562.5/
had to be proven experimentally first. This was to be proven using a testing rig
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designed purely for this purpose.
The first step in this process was to size a spring. The k value found in the last
section and the displacement of approx. 60mm was used to source a spring from
Lee Springs. The specifications for this spring are shown in Appendix 3.
It was also decided to use three magnitudes of impact force during testing. This was
done so that a line could be graphed through the three averaged points on an
impact impulse vs. spring compression displacement chart to show the predictability
and repeatability of the spring method. It could also be used as an opportunity to
explore the effects of different tip material on the impactor, and what effect they
have on the process.
To simplify the design, an Instron Impact Loading machine (30KN max load), as
shown in Figure 67 below, would be used to compress the spring to the required
The key functions of the testing rig were to allow the Instron to compress the spring,
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to hold the spring in its compressed state and then to allow the spring to be
released over a load cell in order to measure the impact force administered.
One of the early sketches in the concept development stage for this rig is shown in
Figure 68 below.
Figure 68 includes a lever release mechanism, which is actuated by twisting a collar
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fixed on the outside of the top cylinder (illustrated in the top left of the Figure). It also
features a threaded shaft with a threaded stopper disc that can be adjusted to allow
varied spring compression displacements. The design was refined further to try to
reduce its complexity, so as to save time during the detailed design phase and
manufacture. Figure 69, below, shows the refined version of the concept for the
testing rig.
The device could be removed from a solid plate base on which a load cell would be
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secured, and then mounted using a special jig. This would be done so as to allow
the Instron to push down the body of the device with the impactor tip placed on a
fixed spigot so that the spring could be compressed inside.
Various trigger mechanisms were generated, with the final idea being a side-acting
lever. This lever would then fit into one of three specifically laid out slots that were
designed to allow the spring to be held in a compressed state under 6KN, 4KN and
just over 2KN of load in the Instron. Figure 70 below, shows the first concept,
followed by Figure 71, which shows the next development, and then finally Figure
72 showing the simplified lever release mechanism.
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Figure 71: Lever Design Development
5.2 Detailed Design
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The Test Rig concept could now be developed further on SolidEdge. Stress
calculations and considerations could also be made for the manufacture of the
device, and could be followed by, the provision of draft drawings to the Machine
Shop and the manufacture of the testing rig.
5.2.1 Solid Modelling
The design had three specific compression displacement slots, slotted into an
internal impactor rod (visible in Figure 73 below protruding from the top of the
device), which was propelled by the release of the compressed spring. An isometric
view of the finished model is shown in Figure 73 below, with fasteners removed for
clarity. The assembled model is shown at the 6KN load setting.
5.2.3 Draft Drawings
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Once the final solid models had been completed a full set of draft drawings were
produced for the design. These drawing are listed in Appendix 4 and can also be
found towards the end of the report.
5.2.4 Manufacturing
The SolidEdge solid modelling program allowed the interaction between the parts to
become visible, and the manufacturing methods could be taken into consideration.
The Testing Rig was manufactured in a machine shop based in the University of
Bath and so the development of the design on SolidEdge was reviewed in stages
with the Machine Shop Technician that would be manufacturing the Rig. This helped
to simplify the manufacturing stage, as the design was customised to make use of
the most easily-available materials that were currently in stock The device was
6. TESTING
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This section of the report covers the calibration of the load cell and the testing
carried out on the Rig. The Results from this testing will be discussed in the next
Chapter.
6.1 Calibration of the Load Cell
The first step was to calibrate the Load Cell, which would be used to measure the
impact loading during the Rig testing. This was performed to ensure that the load
cell was fully functional, and also to establish a scale factor so that the output, when
Rig testing, could be provided in newton instead of in volts, which is what the load
cell measures.
An Instron loading machine (max loading of 30KN) was programmed to descend
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Figure 74: First Calibration Test plot of Voltage Vs. Time
This Data was then plotted and a trend line added across the average points so that
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the slope, and hence the scale factor, could be established. This chart is shown in
Figure 75 below.
y = -2678.5x + 13.413
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
-4.0000 -3.0000 -2.0000 -1.0000 0.0000
Load Vs. Voltage
Load Vs. Voltage
Linear (Load Vs. Voltage)
6.2.1 Procedure
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The testing involved measuring and recording several variables. These variables
are listed as follows.
1. Final displacement of spring during spring compression on Instron
2. Final load recorded during spring compression on Instron
3. Impact data recorded via LabView
Variable 1 and 2 were read directly from the computer monitor hooked up to the
Instron. The LabView data was analysed later using Matlab.
The Testing procedure involved several steps. The first step involved slotting the
required material tip into the hole at the tip of the impactor rod and then holding it in
place with the grub screw. The next step was to mount the Spring Section of the
Testing Rig onto the test fixtures, which had been designed and manufactured as a
part of this project. They held the rig in a safe and secure position during the
Figure 77, as follows, shows the Spring Section of the Device fixed securely in the
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mounting points.
Figure 77: Spring Section in mounting points, close up view of top, close up view of bottom
ger
The top fixture was then lowered to the point where the device was securely in
g-clamps. Figure 78, as follows, shows the base plate mounted to the table with and
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without the rubber base and finally with the spring section sitting top of the base
structure prior to the assembly nuts being screwed attached.
accessed by copying the following file link into the address bar on an internet
b
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browser.
https://www.dropbox.com/s/6o668aoik0xtirz/video-2013-08-21-12-46-46.mp4
6.2.2 Data recorded
The materials were changed over after each test. The material testing order was
steel, nylon, PVC rubber and then back to steel again, which started off the next
round of testing. This allowed the rubber time to return to its original shape and
elasticity, as it temporarily deformed following impaction.
Figure 79, as follows, displays the displacement and load data which was recorded
during the Rig testing on an excel spread sheet. This figure also contains notes that
were taken to record changes in material tip samples following the failure of a
material tip during a test
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The impact data recorded using LabView was extracted from the text file output
sing Matlab and then inserted into an e cel spread sheet for anal sis The
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using Matlab and then inserted into an excel spread sheet for analysis. The
parameters analysed for each impact test were as follows.
1. Peak Force recorded during impact in Newtons
2. Duration of Max Peak in Seconds
3. Duration of impact
These were extracted from graphs generated on Matlab and then fed into the excel
spread sheet. Figure 80, below illustrates the 3 parameters recorded from these
graphs.
vibration damping. The rubber can be considered as a representation of the soft
tissue in a patient as it absorbs some of the impact force from the Testing Rig
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tissue in a patient as it absorbs some of the impact force from the Testing Rig.
However, it must be noted that an error occurred and the steel test wrote over the
Nylon test, hence why they both have the same data in the single round of testing
without the rubber base. The analysis of this data is covered in the next section of
the report.
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Figure 82: Extrapolated Load Cell Data, 4KN and 6KN
The extrapolated load cell data for the 4KN and 6KN loads are shown in Figure 82
as follows.
7. RESULTS AND DISCUSSION
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This section of the report covers the analysis and discussion of the Instron and Test
Rig Data acquired in the previous chapter and also an evaluation of the Testing Rig.
7.1 Evaluation of Instron Data
During the testing phase, some notes were made on the spread sheet where the
Instron data was being recorded. For example, as can be noted from the comments
on Figure 79 shown previously, both PVC and Rubber failed on the first round of
testing at 4KN of loading on the Instron. New replacement tips for both materials
failed after Round 2 (their first impacts) at 4KN loading. Figure 83 below shows
images of the PVC and Rubber Tip materials which were destroyed on Round 1 of
It was also clear from the average max displacement figures shown previously in
Figure 79 that the Rubber Tip Sample compressed during loading on the Instron to
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Figure 79 that the Rubber Tip Sample compressed during loading on the Instron to
approx. two thirds of its original thickness. This pre-deformation may have reduced
the ability of the material to lower the impulse force (as it would normally prolong the
impact duration of the peak force as seen from the load cell data).
There was some error present in the inconsistency at the point at which the load
and the displacement were reset to zero on the Instron before the spring was
compressed. There was a learning curve during the experimentation and it was
discovered towards the end of the testing that the best time to reset the load and
displacement readings to zero, before compressing the spring, was to watch to see
the top of the Impactor rod move in relation to the devices outer structure. This way
it was sure to be the point at which the spring was about to experience actual
compression, as opposed to the rig just settling out. However, even taking this error
extremely large effect on the duration of the max peak which in turn affects the
resulting calculation of the impulse force. It is important to note that when using
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resulting calculation of the impulse force. It is important to note that when using
these original impulse values the tip material used in the study in which the impulses
were recorded, was not provided to this project. This would explain to some degree
the significantly high impact forces recorded, especially with steel, which in many
cases exceeded the measurement band of the Load Cell. As per the specifications
on the Load Cell it has a max measurement capacity of 5,000 lbs (or 2267.69 Kg, or
22,246 N). The Load Cell can only measure up to 10 Volts which works out as
approximately 26,785 N, when referring back to the scale factor. It is clear that the
bandwidth of the Load Cell could not cope with the large magnitudes of force being
delivered. This brings into question the remaining results as the forces being
recorded were far greater than what was to be expected. When the Steel testing
was performed for the 6KN loading it was deemed that the load cell had been
cell data) due to the extremely sudden momentum change, as it bounces off the
load cell.
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oad ce
7.3 Evaluation of Testing Rig
Despite the greater peak forces than expected, the Testing Rig performed extremely
well given that the load cell failed before the Testing Rig. It functioned safely under
the loads and was quick and simple to use. The only improvements for future use
would be shortening the four threaded studs holding the spring section to the base
plate as this could be a little time consuming screwing the nuts on and off the long
sections when assembling and disassembling the rig during tests. The edge of the
release lever that fitted into the slots came under great deal of stress and future
designs may benefit from heat treating to harden the edge to reduce wear when
i th l d l d
8. CONCLUSIONS
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This section looks at the overall outcome from the different sections of this project
and their contribution to the overall aim of designing and developing a medical
device to improve the assembly of head/neck taper junctions in MTHRs.
To start with, a significant literature review was carried out for this project which
resulted in the definition of fundamental design requirements for the proposed
device. These requirements were based on an analysis of the most relevant and
credible sources available.
The user needs and design requirements developed in the next stage of the project
were of significant value, particularly the Surgeons Survey and the PDS. The
Surgeons Survey, although the feedback came late in the projects life it is still of
great value to this project and other studies in the region of MTHR taper junction
l i Th S S th d l t i f ti f
The development and manufacture of the Testing Rig has produced a finished
device which can be reused for future testing or can be manufactured again on a
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g g
smaller scale to suit a smaller spring size. All of the manufacturing draft drawings
are contained in the Appendix and only need to have the dimensions scaled down
and another smaller version could easily be produced.
The data acquired from the testing phase has shown the dramatic influence that the
tip material has on the impulse force. It has also uncovered improvements for future
testing, such as impacting a head/neck assembly fixed to a load cell and using a
smaller spring size, followed by comparing the assembly and disassembly data with
specimens assembled using a mallet and impactor.
Overall, this project has produced a detailed final design concept which can meet
the design requirements established in this project. It has also produced a testing rig
capable of aiding the laboratory development of this design concept. The feasibility
10. REFERENCES
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11. APPENDICES
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APPENDIX: 1
SURGEONS SURVEY RESPONCE
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APPENDIX: 2
PDS [10]
1. Performance
1.1 Must axially aligned seating of head on neck taper axis prior to impaction
1.2 Impact must be delivered in axial alignment with neck taper axis
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p g p
1.3 Deliver impact force of between 4KN and 6KN, adjustable to 0.5KN
1.4 Must try to isolate majority of impact to head/neck taper junction
2. Customer
2.1 Must be able to facilitate a range of different head sizes (Determine using Surgeons Survey)
2.2 Must be able to fit into the exposed cavity in the patient created by the incision (Surgeons
Survey will determine the exact figures)
3. Medical Standards
3.1 Must comply with ISO13485:2003/AC:2007, has been