design and development of a medical device to improve the assembly of head/neck taper junctions in...

7/27/2019 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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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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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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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.


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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21. Fessler, H. and D.C. Fricker, A study of stresses in alumina universal heads offemoral prostheses. ARCHIVE: Proceedings of the Institution of MechanicalEngineers, Part H: Journal of Engineering in Medicine 1989-1996 (vols 203-210),1989. 203(18): p. 15-34.

22. Shareef, N. and D. Levine, Effect of manufacturing tolerances on the micromotion atthe Morse taper interface in modular hip implants using the finite element technique.Biomaterials, 1996. 17(6): p. 623-30.

23. Lieberman, J.R., et al., An analysis of the head-neck taper interface in retrieved hipprostheses. Clin Orthop Relat Res, 1994(300): p. 162-7.

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35. Controlled Force Mallet. 24/12/1992.

36. Controlled Force Impacting Device. 31/03/2006.

37. Handling Device for hip Prosthesis implant. 2/3/1988.

38. Hip Joint Prosthesis and Fitting Tool. 24/10/1997.

39. Device for handling ball heads of joint prostheses. 2003.

40. Femoral Head Holder and Impaction Instrument and Method of use. 2003.

41. Method of applying a femoral head resurfacing prosthesis. 2007.

42. Geraet zum Eindruecken von Heftzwecken. 1952.

43 Hand-held nail gun has helical driving spring made of tubular spring steel 2007

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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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

design and development of a medical device to improve the assembly of head/neck taper junctions in...

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