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DISSERTATION REPORT ON

BIOMECHANICAL STUDY & DEVELOPMENT OFPARAMETRIC CAD MODEL FOR KNEE IMPLANT

SUBMITTED IN

PARTIAL FULFILLMENT FOR THE AWARD OF THE DEGREE OF

MASTERS OF TECHNOLOGY

IN

MECHANICAL ENGINEERING(CAD/CAM)

BY

RONAK R SHAH(P12CC002)

UNDER THE SUPERVISION OFDr. H. J. NAGARSHETH

2013 2014

MECHANICAL ENGINEERING DEPARTMENT

SARDAR VALLABHBHAI NATIONAL INSTITUTE OFTECHNOLOGY, SURAT,

GUJARAT, INDIA - 395007


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MECHANICAL ENGINEERING DEPARTMENT

SARDAR VALLABHBHAI NATIONAL INSTITUTEOF TECHNOLOGY, SURAT

CERTIFICATE

Date:

This is to certify that the dissertation report entitled Development of parametric

CAD model for customized knee implant submitted by RONAK R SHAH

(Admission No. P12CC002) in partial fulfillment of the requirements for the award

of the degree of Master of Technology in Mechanical Engineering (CAD/CAM)

during the academic year 2013-2014 at Sardar Vallabhbhai National Institute of

Technology, Surat . The thesis is record of his own work carried out under the

guidance of Dr. H. J. Nagarsheth. The matter embodied in the dissertation report has

not been submitted to any other University or Institution for award of any degree or

diploma.

Place: SVNIT, Surat

Dr. A. A. ShaikhAssociate Professor &

PG-Incharge (CAD/CAM)Mechanical Department,

SVNIT

Dr. H. J. NagarshethThesis SupervisorProfessor & Head

Mechanical Department,SVNIT


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DECLARATION

This work or any part thereof has not previously been presented in any form to the

University or to any other body whether for the purposes of assessment, publication orfor any other purpose (unless otherwise indicated). Save for any express

acknowledgments, references and/or bibliographies cited in the work, I confirm that

the intellectual content of the work is the result of my own efforts and of no other

person.

It is to be noted that Ronak Shah is to be identified as the author of this M.Tech

dissertation report. All rights reserved to author and the University.

RONAK SHAHAdmission no: P12CC002

M.Tech CAD/CAMSVNIT, Surat


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DISSERTATION APPROVAL CERTIFICATE

This is to certify that the dissertation report entitled DEVELOPMENT OF

PARAMETRIC CAD MODEL FOR CUSTOMIZED KNEE IMPLANTS submitted

by RONAK R SHAH (Admission No. P12CC002) in partial fulfillment of the

requirements for the award of the degree of Master of Technology in Mechanical

Engineering (CAD/CAM) during the year 2013 -2014 of the Sardar Vallabhbhai

National Institute of Technology, Surat is hereby approved for the award of the

degree.

Examiners:

1

2

3

Date:

Place: SVNIT, Surat


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i

ACKNOWLEDGMENT

I wish my sincere gratitude to all the individuals who have assisted me for the

research work. For most, I would sincerely express my gratitude to my research guide

Dr. H. J. Nagarsheth (Professor, SVNIT) for his supervision, invaluable help,

motivation and permitting me to select present topic as my dissertation work.

Secondly, I would thank Dr. Bharat Mody (Director and Chief Arthroplasty Surgeon,

Welcare Hopsital) for his guidance in teaching me knee anatomy, sharing his

experience on knee implant and lecture notes and providing the knee implants for the

research work. I would acknowledge guidance of Dr. A. A. Shaikh in permitting me

to use reverse engineering laboratory and helping me in reverse engineering of

implants. I thank Dr. D. P. Vakhariya and Mr. Anil Mahto for their indirect help and

motivation.

The help by the PhD. Students of the department and all friends is unforgettable for

their encouragement and friendship. I personally thank Sachin Gupta and Ragerman

P. for their help during the project. I appreciate my family for their moral support,

understanding and bearing me for not able to spend time with them. At last I would

like to remember Dr. A.P.J Abdul Kalam, who inspired me to work in the field of

biomechanics during my schoolings. Lastly, I would always thank the almighty for all

source of energy he is providing me.

RONAK SHAHSVNIT, SURAT


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ABSTRACT

The knee is the synovial and highly stressed joint of the human body. It is subjected tomultiple modes of failures such as diseases, wear, creep, fatigue and fracture or their

combination. The failure of knee bones causes severe pain and restriction in knee

motion. The total knee arthroplasty is surgical method of restoring the articulating

surfaces of knee to help the patient restore its knee functions using artificial

components called prosthesis. The designing of knee prosthesis requires the

knowledge of knee anatomy, kinematics and factors affecting the performance of

prosthesis in addition to mechanical engineering principles. The issues with the present knee prostheses that limit its longevity and performance are rapid wear of

polyethylene insert, loosening of the joint, non-customized joint and post

manufacturing issues.

The objective of present study is to develop a CAD model facilitating development of

customized knee implants. This was done by reverse engineering (RE) the knee

implant procured from orthopedic surgeon. The point cloud data of the prosthetic

components was collected using needle scanner. The point cloud data were then

imported in RE software Geomagic Studio 2013, where the polygonal and surface

model were developed according to the RE procedure. The software provided the

output inform of surface model and extracting various features of which the model is

created. These data were recorded in excel sheet. The various unknown dimensions of

the model were obtained by solving for the unknowns using vector theory and

compared them by physically measuring them with Vernier calipers. The final

parametric CAD model was deve loped in Creo Parametric 2.0, whose dimensions

can be varied to satisfy specific requirements of individual patients.

Keywords: Articulating Surfaces, Knee Prosthesis, Parametric CAD Model, PointCloud Data, Reverse Engineering.


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iii

CONTENTS

ACKNOWLEDGMENT ........................................................................... i

ABSTRACT ...............................................................................................ii

CONTENTS ............................................................................................ iii

LIST OF FIGURES ................................................................................vii

LIST OF TABLES ................................................................................... ix

GLOSSARY .............................................................................................. x

ABBREVIATIONS .................................................................................xii

1 INTRODUCTION ...................................................................... 1-1

1.1 INTRODUCTION ............................................................................. 1-1

1.2 PROSTHESIS FAILURE MODES ................................................... 1-2

1.2.1 ASEPTIC LOOSENING ........................................................ 1-2

1.2.2 RAPID WEAR ....................................................................... 1-3

1.2.3 INSTABILITY AND MISALIGNMENT ............................. 1-4

1.2.4 INFECTION ........................................................................... 1-5

1.3 ISSUES WITH STANDARD IMPLANT SIZES.............................. 1-5

1.4 AIM OF STUDY ............................................................................... 1-6

1.5 OUTCOME OF RESEARCH WORK............................................... 1-7

1.6 ORGANIZATION OF THE THESIS ................................................ 1-7

2 KNEE JOINT ............................................................................. 2-1

2.1 KNEE ANATOMY .......................................................................... 2-1

2.1.1 BONES ................................................................................... 2-2

2.1.2 LIGAMENTS ......................................................................... 2-3

2.1.3 CARTILAGES AND SOFT TISSUES .................................. 2-5

2.2 BONES STRUCTURE AND PROPERTIES .................................... 2-6


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2.3 SOFT TISSUES STRUCTURE AND PROPERTIES ...................... 2-7

2.3.1 ARTICULAR CARTILAGES ............................................... 2-7

2.3.2 LIGAMENTS ......................................................................... 2-8

2.4 KNEE JOINT ARTICULATION MECHANICS ............................. 2-9

2.4.1 GEOMETRY OF ARTICULATING SURFACES ............... 2-9

2.4.2 RANGE OF MOTION ......................................................... 2-10

2.5 LOADS ON KNEE JOINT .............................................................. 2-11

3 KNEE IMPLANTS .................................................................. 3-14

3.1 HISTORY OF KNEE IMPLANT .................................................... 3-14

3.2 KNEE IMPLANTS CLASSIFICATION ........................................ 3-16

3.2.1 BASED ON RELATIVE MOTION .................................... 3-17

3.2.2 BASED ON DESIGN .......................................................... 3-18

3.2.3 BASED ON IMPLANT ASSEMBLY ................................. 3-19

3.2.4 BASED ON SURGICAL REQUIREMENTS ..................... 3-20

3.2.5 BASED ON IMPLANTATION METHOD ......................... 3-21

3.3 KNEE PROSTHESES REQUIREMENTS ..................................... 3-21

3.3.1 DESIGN REQUIREMENTS ............................................... 3-21

3.3.2 MATERIAL PROPERTIES REQUIREMENT ................... 3-22

3.3.3 MANUFACTURING REQUIREMENTS ........................... 3-22

3.4 IMPLANT MATERIALS ................................................................ 3-23

4 LITERATURE STUDY ............................................................. 4-1

4.1 DESIGN AND MODELING OF KNEE PROSTHESIS................... 4-1

4.2 BIOMECHANCIS OF KNEE ........................................................... 4-6

4.3 EXPERIMENTATION AND SIMULATION .................................. 4-7

4.4 SUMMARY OF LITERATURE STUDY ....................................... 4-10

4.5 OBJECTIVES BASED ON LITERATURE STUDY ..................... 4-10

5 BASICS OF REVERSE ENGINEERING ............................... 5-1


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5.1 DEFINITION ..................................................................................... 5-1

5.2 ADVANTAGES AND LIMITATIONS OF RE................................ 5-1

5.2.1 ADVANTAGES OF RE ........................................................ 5-1

5.2.2 LIMITATIONS OF RE .......................................................... 5-2

5.3 SCANNER DEVICES ....................................................................... 5-2

5.3.1 CONTACT TYPE SCANNERS ............................................ 5-2

5.3.2 NON-CONTACT TYPE SCANNERS .................................. 5-3

5.4 REVERSE ENGINEERING PROCEDURE ..................................... 5-6

5.4.1 POINT PHASE ...................................................................... 5-6

5.4.2 POLYGONAL PHASE .......................................................... 5-8

5.4.3 CURVE PHASE ..................................................................... 5-9

5.4.4 NURBS SURFACE PHASE ................................................ 5-10

6 PROCESSING OF POINT CLOUD DATA ............................ 6-1

6.1 SCANNING OF KNEE IMPLANT COMPONENTS ...................... 6-1

6.2 PROCESSING OF POINT CLOUD DATA ..................................... 6-3

6.2.1 Step 1: POINT PROCESSING .............................................. 6-3

6.2.2 Step 2: POLYGONAL PHASE.............................................. 6-5

6.2.3 Step 3: CURVES EXTRACTION ......................................... 6-7

6.2.4 Step 4: FEATURE DETECTION AND EXTRACTION ...... 6-8

7 DEVELOPMENT OF CAD MODEL ...................................... 7-1

7.1 METHODOLOGY ADOPTED ......................................................... 7-1

7.1.1 VECTOR THEORY ............................................................... 7-2

7.2 TIBIAL COMPONENT..................................................................... 7-2

7.2.1 SECTIONAL PLANE METHODS ....................................... 7-2

7.2.2 DIMENSIONS BY VECTORS ............................................. 7-6

7.3 FEMORAL COMPONENT............................................................. 7-12

7.3.1 SECTIONAL PLANE METHOD ........................................ 7-12


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7.3.2 DIMENSIONS BY VECTORS ........................................... 7-14

7.3.3 IMAGE PROCESSING METHOD ..................................... 7-17

RESULTS AND CONCLUSION ............................................................ 1

RESULTS .......................................................................................................... 1

CONCLUSION .................................................................................................. 1

FUTURE WORK ...................................................................................... 3

REFERENCES ......................................................................................... 4


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LIST OF FIGURES

Figure 1.1: Femoral component retrieved from a subject suffering from asepticloosening .................................................................................................................... 1-2

Figure 1.2: Tibial Insert retrieved indicating rapid wear and thinning of edges ........ 1-3

Figure 2.1: Essentials of knee anatomy ..................................................................... 2-1

Figure 2.2: Distal femur geometry ............................................................................. 2-9

Figure 2.3: Tibial plateau geometry ......................................................................... 2-10

Figure 2.4: DOF for knee joint ................................................................................ 2-11

Figure 2.5: Knee Joint Kinematics and representation of forces and moments actingon knee. .................................................................................................................... 2-12

Figure 3.1: Platt-Peppler's distal femur implant design ........................................... 3-14

Figure 3.2: Waldius hinge design ............................................................................ 3-14

Figure 3.3: Uni-Conylar Implant (1973) .................................................................. 3-15

Figure 3.4: Modified Smith-Peterson implant ......................................................... 3-15

Figure 3.5: Bi-condylar Implants ............................................................................. 3-15

Figure 3.6: Total condylar implants ......................................................................... 3-16

Figure 4.1 Mathematical models representing tibiofemoral joint .............................. 4-3

Figure 4.2: Lower Limb Model for probabilistic FE analyses................................... 4-8

Figure 4.3: Axial displacement of tibia and femur 80-90kg BW .............................. 4-9

Figure 5.1: Active illumination stereo system ........................................................... 5-4

Figure 5.2: Structured light illumination scanning principle ..................................... 5-4

Figure 5.3: Phases of RE process ............................................................................... 5-6

Figure 6.1: Tibial component and fixture .................................................................. 6-1

Figure 6.2: Needle scanner settings window ............................................................. 6-2

Figure 6.3: Femoral component on needle scanner ................................................... 6-2

Figure 6.4: Point cloud data of the knee prosthetic components imported in RE

software ...................................................................................................................... 6-4

Figure 6.5: Point Processing tools on the software .................................................... 6-4

Figure 6.6: Automatic polygonal model of the tibial and femoral component .......... 6-5

Figure 6.7: Polygon model processing tools in Geomagic Studio ............................. 6-5

Figure 6.8: Final accurate polygon model of the knee prosthetic components ......... 6-6
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Figure 6.9: Cross sectional curves created on the polygonal models of femoral and

tibial components ....................................................................................................... 6-7

Figure 6.10: Feature detection tools in Geomagic Studio .......................................... 6-8

Figure 6.11: Region segmentation on tibial facet model ........................................... 6-9

Figure 6.12: Region segmentation on femoral facet model ..................................... 6-10

Figure 6.13: Deviation contour of automatically detected features for tibial

components .............................................................................................................. 6-10

Figure 6.14: Deviation contour of automatically detected features for femoral

components .............................................................................................................. 6-13

Figure 7.1: Location of different sectional planes on tibial polygonal model ........... 7-3

Figure 7.2: Parameters of tibial insert in sagittal plane ............................................. 7-4

Figure 7.3: Insert parameters in transverse plane ...................................................... 7-5

Figure 7.4: Insert parameters in coronal plane........................................................... 7-6

Figure 7.5: CAD model of tibial insert developed in Creo Parametric 2.0 ............. 7-11

Figure 7.6: Section planes for femoral component .................................................. 7-12

Figure 7.7: Various parameters identified in sagittal plane for femoral component 7-13

Figure 7.8: Femoral component image perpendicular to sagittal plane ................... 7-17

Figure 7.9: Edited image of the femoral component ............................................... 7-18

Figure 7.10: Edge detected of the femoral component articulating surface ............ 7-18

Figure 7.11: CAD model of femoral component developed in Creo Parametric .... 7-19
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LIST OF TABLES

Table 2.1: Function of knee ligaments ...................................................................... 2-4 Table 2.2: Functions of different muscles controlling knee motion .......................... 2-6

Table 2.3: DOF for tibiofemoral joint...................................................................... 2-10

Table 4.1: Magnitude of peak force and moment on knee implant .......................... 4-7

Table 4.2: Result discussion of FE analysis of knee joint using ANSYS ................. 4-9

Table 6.1 Needle scan summary for knee implant components ................................ 6-3

Table 6.2: Summary of point cloud processing ......................................................... 6-4

Table 6.3: Summary of polygon processing .............................................................. 6-6 Table 6.4: Automatic segmentation settings summary .............................................. 6-9

Table 6.5: Sphere features extraction details of tibial part ........................................ 6-9

Table 6.6: Plane features extraction details for tibial component ............................ 6-11

Table 6.7: Sphere features extraction details for femoral component ..................... 6-11

Table 6.8: Cylinder features extraction for femoral component .............................. 6-12

Table 6.9: Plane features extraction for femoral component ................................... 6-12

Table 7.1: Dimension values from sagittal plane sections ......................................... 7-4

Table 7.2: Dimension values from transverse plane sections .................................... 7-5

Table 7.3: Dimension values from coronal planes sections ....................................... 7-6

Table 7.4: Dimensions values of different parameters in sagittal plane .................. 7-13

Table 7.5: Coordinates of pixels of the edge detected ............................................. 7-19

Table 8.1: List of various variables and their values for the present CAD model of

prosthesis components ................................................................................................... 1


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GLOSSARY

Anatomy Study of structures generally macroscopically.

Anterior/PosteriorDescribe relative position of structure to the front or

back of the body respectively.

Cartilage

Its an avascular form of connective tissue consisting of

extracellular fibers embedded in matrix that contains

cells localized in small cavities.

Coronal PlaneVertical plane dividing the body in anterior and

posterior parts.

In vitro Outside the body of living organism

In vivo Inside the body of living organism

Medial/Lateral

Describe relative position of structures to the medial

sagittal plane. Medial position is near to medial sagittal

plane while lateral is away/farther to the medial sagittal

plane.

Prosthesis Replacement of a body part for corrective action.

Proximal/Distal

Describes the position with reference to being closer to

or farther from a structures origin. Distal position

occur farther away toward the end of limbs while

proximal occur closer to and toward the origin of the

limb.

Sagittal PlaneVertical Plane perpendicular to coronal plane, dividing

the body parts into left and right parts.

Sesamoid boneSmall round bones formed in a tendon where it

passes over a joint.


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Synovial joints

Type of joints between skeleton components where the

elements involved are separated by narrow articular

cavity and cartilage layer covers articulating surfaces.

Transverse/Horizont

al Plane

Plane perpendicular to both coronal and sagittal plane

such that body is divided into superior and inferior

parts.

Trochlear GrooveConcave groove on the femur anterior distal end to

accommodate patella.


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ABBREVIATIONS

ACL Anterior Cruciate Ligaments

AL Anterolateral

AM Anteromedial

AP Anterior-Posterior

DOF Degree Of Freedom

FEA Finite Elemental Analysis

LCL Lateral Collateral Ligaments

MCL Medial Collateral Ligaments

PCL Posterior Cruciate Ligaments

PE Polyethylene

PL Posterolateral

PM Posteromedial

RE Reverse Engineering

ROM Range Of Motion

TKR/A Total Knee Replacement/Arthroplasty

UHMWPE Ultra High Molecular Weight Polyethylene


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1 INTRODUCTION

The chapter defines biomechanics and introduces to knee prosthesis. The various

failure modes of the knee prosthesis are explained and why there is a need for the

development of the customized knee implant.

1.1 INTRODUCTION

Biomechanics is the study of structure and functions of various biological systems

such as plants, animals, organs, etc. with mechanical perspective. Biomechanical

study includes various specializations such as ergonomics, musculoskeletal and

orthopedics, implants development, sports, kinesiology and human factors &

occupational biomechanics. The study includes multidisciplinary subjects such as

Newtonian mechanics, material sciences, mechanism analysis, structural analysis,

kinematics and dynamics of the system and many other aspects of mechanical

engineering.

Knee is the most complex and synovial joint among all the joints in human body. It is

subjected to maximum stress, impact and wear. Knee joint comprises of three bones

namely, femur (thigh bone), patella (knee cap) and tibia (lower limb). The knee injury

or disease (arthritis) may result in restriction of knee kinematics. The knee kinematics

can be restored by replacing natural articulating surfaces with the artificial prosthesis

through surgical procedure called arthroplasty. There is increase in the patients

suffering from knee arthritis and getting arthroplasty surgery. It is about 440,000 TKR

carried out worldwide in 2005 survey and 35,000 revision surgeries yearly [1] . The

main cause of failure of implants is the design of articulating surfaces that restores the

natural articulating bone surfaces to maintain the knee kinematics. The present era of

21 st century is focused on developing technology for customized/tailor-made knee

prosthesis. This can be achieved with the integration of CAD and CAM technologies.

The present work focuses on development of solid CAD model for development of

customized knee implant to facilitate the specific requirements of individuals. A

detailed study of knee anatomy and biomechanics of knee was carried out to

understand the functioning of different components.


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1.2 PROSTHESIS FAILURE MODES

This section describes most common failure modes of knee implants which includes

aseptic loosening, wear, instable or misalignment and infection. It was observed that

50% of early revision total knee arthroplasty were related to instability, misalignmentor malposition, or failure due to fixation loss.

1.2.1 ASEPTIC LOOSENING

The aseptic loosening is the failure mode occurring at the bone-implant interface in

absence of infection or allergic reaction. The joint loosening occurs at the joint

interface of all components of prosthesis. The aseptic loosening is observed 10-20

years post implantation surgery [2] .The aseptic loosening is slow and difficult to

detect until the gap at the bone-implant is visible.

Failure mechanism:

Wear debris present in the vicinity of the prosthetic components majorly sized less

than 5 m which causes inflammatory reactions. The inflammatory reactions are

caused accumulation of macrophages or recruitment of lymphocytes by immune

system. The reaction results in loss of bone due bone resorbing i.e. eating the bone

cells.

Figure 1.1: Femoral component retrieved from asubject suffering from aseptic loosening


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Cause of failure:

Wear debris of prosthetic components and bones present between the

articulating surfaces release fine particles that are released to tissues. The wear

debris are due to wear of implant components, improper cleaning of bone

particles left post sectioning and cement particles.

Inadequate initial fixation and mechanical fixation loss over period can lead to

formation of debris either of cement, bone or implant particles.

Biological loss caused by particle-induced osteolysis around the implant

which causes the immune system to generate cytokines which stops the bone

formation and healing process [2] .

1.2.2 RAPID WEAR

The rapid wear of PE components causes loosening of joint i.e. establishing no

contact between the components during kinematics. This results in unbalanced

kinematics causing severe joint pain and restriction in ROM.

Failure mode :

The poor surface finish of the articulating surfaces increases the friction

between the articulating surfaces causing wear of PE components.

Micro motion between the non-articulating surfaces of the prosthetic

components.

If the contact pressure between the articulating surfaces is increased by any

means, increases the localized stress concentration at the contact point. The

Figure 1.2: Tibial Insert retrieved indicating rapid wearand thinning of edges


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localized stress is cyclic and thus causes fatigue failure by localized failure of

PE components i.e. erosion.

Cause of failure:

Shape approximation of articulating surface to simple geometry changes the

kinematics of knee joint.

Increase in contact pressure causes increase in wear, which is proved in

various research papers [3] [6] . The increase in contact pressure can be either

by improper balancing of ligaments or selection of joint size.

Micro motion between the tibial insert and tibial metallic tray in modular

design implants leads to PE wear debris [7] .

Poor surface finish i.e. the articulating surfaces are not smooth and polished as

per the requirement which reduces friction and wear.

The material properties of the PE might not be as per standard or the cross-

linked bonds might get weak during the manufacturing process and

sterilization.

1.2.3 INSTABILITY AND MISALIGNMENT

The instability of knee implant is related to load distribution between medial andlateral regions of the components. Misalignment of implant is related to eccentricity

between the anatomical axis and implant axis.

Causes:

The instability of the knee implants is caused due to improper design of

implant components, inaccurate bone resectioning, ligaments tensioning fault

and misalignment. The instability at later stages is also caused due to wearingof components.

Misalignment of knee implants is caused due to inappropriate surgical method,

inaccuracy in manufacturing, selection of wrong implant, loosening of joint,

mechanical fixation error and anatomical defects of the patient.


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Results:

The instability and misalignment are inseparable factors. The instability can

cause misalignment and vice versa. It results in severe pain and immobility.

Change in knee joint kinematics is caused due to either imbalance of ligament

tension, design change or misalignment of joint which changes the direction

and magnitude of forces and constraints which guide the joint motion.

Increase in wear is result of improper contact pressure and change in

kinematics of the joint.

In severe case, the ligaments get damaged especially collateral ligaments

which control the kinematics of joint and connect the two long bones.

1.2.4 INFECTION

Post-surgery infection occurs either due to wound or immune system not accepting

the implantation or allergic reaction due to implant material. This is biological cause

of failure.

Cause of failure:

Infections are caused due to metabolism of bacteria. If the bacterial micro-

organism get access to the implant location, they reproduce and cause

infection.

Some patients are allergic to the metallic ions such as V and Ni present in the

metallic implant components causes allergic reaction with the body tissues.

Results:

The infection causes fever, no healing of the wound, joint stiffness and increase in

pain.

1.3 ISSUES WITH STANDARD IMPLANT SIZES

Till date the implant manufactures develop implant of limited range of standard sizes.

Most of the implant manufacturers are either Americans or Germans and so their

implant standards are decided by studying the anatomy of mass in those regions. The

anatomy of patients from other regions of the world is different from them and

anatomy differs with individuals. The implant standard sizes does not satisfy the


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individual anatomical need such as stem length, articulating surface shape, overall

geometry and so on.

The surgeons need to make optimal selection of the implant size which is very likely

experienced based decision making from the available range of sizes. The wrong

selection includes following performance issues:

1. If the implant is oversized , the bone resectioning is large i.e. the amount of

bone removed is more. The ligaments get over-tensed and contact pressure

between the articulating surfaces reduces. Thus joint laxity increases though

the stress and wear reduces which affects the stability. The large size implant

increases the weight of the leg.

2. If the implant is undersized , the bone resectioning is less i.e. bone

preservation. The ligaments can be properly tensed but the range of motion

reduces. The contact pressure between the articulating surfaces increases

which increases the wear of PE i.e. implant failure but the joint stability is

achieved.

The above mentioned problem can be solved by developing customized knee implant.

The customized knee implant will develop the implant of the required size and

anatomical shape of the patient and will also consider some special requirements

suggested by the surgeon. This can be achieved by a parametric solid CAD model is

that will permit the design dimension to be varied. The CAD model can then be easily

imported to advance CNC controller to control the cutting path program to

manufacture precise components with high degree of surface finish.

1.4 AIM OF STUDY

To identify the various problems surgeons and patients complain about the

performance of knee implant, various surgeons specialized in knee arthroplasty were

interviewed. It was concluded from the study that knee implant longevity is less,

ROM is less than natural knee and its cost in India is very high. Also it is sometimes

difficult to select the implant size for the patient. To investigate the problems with the

knee implants, Dr. Bharat Mody provided a knee implant retrieved from a subject

suffering from joint loosening for the research work. The prosthesis had femoral

metallic component and tibial uni-body component.


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The present study and dissertation work focuses on following aspects of knee

prosthesis:

1. Development of CAD model using reverse engineering technology.

2. Identify the design parameters of the retrieved knee prosthesis.3. Comment on the design issues of retrieved implant.

4. Suggest a methodology to develop customized knee implants.

1.5 OUTCOME OF RESEARCH WORK

The CAD model of the knee implant will be developed which will permit change of

dimensions for large number of variables which will satisfy the needs of specific

individual. The CAD model can be converted into .iges format which can be

imported to all CNC cutting tool programming and also RP machines to manufacture

the components.

1.6 ORGANIZATION OF THE THESIS

The thesis is organized in different chapters as described below:

Chapter 1, Introduction ; introduces to the biomechanics. The chapter highlights the

common failure modes of implant and need of customize implants.

Chapter 2, Knee Joint ; describes human knee anatomy and the various mechanical

aspects of the natural knee joint like bone and ligament structure and their properties,

joint mechanism, DOF and loads acting on the joint.

Chapter 3, Literature Study ; discusses research work done on developing knee

implant, knee implant failure and testing and biomechanics of knee joint.

Chapter 4, Knee Implant; describes the knee implants classification and design and its

requirements.

Chapter 5, Basics of Reverse Engineering ; explains in detail reverse engineering. The

chapter includes RE process, advantages and limitation of RE and description of

different scanning methods.

Chapter 6, Processing of Point cloud data ; has the details of experiment performed to

scan the prosthetic components to obtain the point cloud data using needle scanner.


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The detailed explanation of each step for converting the point cloud data to the

required polygonal mode and data extraction using RE software is mentioned.

Chapter 7, Development of CAD model ; contains the detailed procedure of deriving

the dimension of various design variables using different methods.


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2 KNEE JOINT

2.1 KNEE ANATOMY [8]

Knee joint is formed by three bones, cartilages and stabilizing ligaments. To design

the knee implant, it is essential to study the functions of each of these components.

The Figure 2.1 shows the anatomical structure of the knee joint.

The knee joint comprises of three bones femur, tibia and patella which articulate

relative to each other forming the joint. The femur and tibia form tibiofemoral joint

and femur and patella forms patellofemoral joint. The former joint is very essential asit dominates the knee kinematics while the later joint becomes the stabilizer and

controller. Femur and tibial articulating surfaces are covered with the cartilages to

prevent wear of bones and reduce the friction between them. The various elements of

knee are described in detail in following sections.

Figure 2.1: Essentials of knee anatomy[Website: www.webmd.com]


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2.1.1 BONES

Femur (thigh bone)

Femur is the strongest and longest bone. Its shaft, almost cylindrical in most of its

length and bowed forward, has a proximal round, articular head projecting mainlymedially on its short neck, which is a medial curvature of the proximal shaft. The

distal extremity is more massive and is a double 'knuckle' (condyle) that articulates

with the tibia. The shaft of femur is bowed forward and has an oblique course from

the neck of the femur to the distal end. The shaft is narrowest centrally, expanding a

little proximally, particularly towards its distal end. The distal end of the femur is

widely expanded as a bearing surface for transmission of weight to the tibia. It has

two massive condyles, which are partly articular. Anteriorly the condyles unite andcontinue into the shaft; posteriorly they are separated by a deep intercondylar fossa

and project beyond the plane of the popliteal surface. The walls of intercondylar fossa

serves location for attachment of the cruciate ligaments. The wall formed by lateral

surface of medial condyle is used for attachment of posterior cruciate ligament and

the wall formed by medial surface of lateral condyle is used for attachment of anterior

cruciate ligaments. The lateral condyle is bigger anteroposteriorly compared to medial

condyle. The epicondyles serve attachment of the collateral ligaments laterally and

medially respectively.

The patellar surface extends anteriorly on both condyles, especially the lateral. It is

transversely concave, vertically convex and grooved for the posterior patellar surface.

The trochlear groove helps to stabilize the patella. An abnormally shallow groove

predisposes to instability

Patella (knee cap)

The patella is the largest sesamoid bone. It is embedded in the tendon of quadriceps

femoris, anterior to the knee joint. The patella is flat, distally triangular, proximally

curved, and has anterior and posterior surfaces, three borders and an apex. In the

living, its distal apex is a little proximal to the line of the knee joint when standing. It

is triangular with its apex pointed inferiorly for the attachment to the patellar

ligament; broad and thick base for attachment of the quadriceps femoris muscles from

above and its posterior surface articulates with femur. The shape of the patella can

vary: certain configurations are associated with patellar instability.


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Tibia (shine bone):

Tibia lies medial to fibula and is exceeded in length only by the femur. Its shaft is

triangular in section and has expanded ends: a strong medial malleolus projects

distally from the smaller distal end. The anterior border is sharp and curves medially

towards the medial malleolus. Together with the medial and lateral borders it defines

the three surfaces of the bone. The exact shape and orientation of these surfaces vary

from individual to individual.

The proximal end has expanded structure having bearing surface to carrying body

weight transmitted by femur. It has massive medial and lateral condyles, an

intercondylar area and a tibial tuberosity. The tibial condyles overhang the proximal

posterior surface of the shaft. The anterior condylar surfaces are continuous with a

large triangular area where the apex is distal and formed by the tibial tuberosity, and

the lateral edge is a sharp ridge between the lateral condyle and lateral surface of the

shaft.

2.1.2 LIGAMENTS

The knee bones are connected firmly and stabilized kinematics is maintained by the

four essential ligaments. These ligaments are divided as cruciate and collateral

ligaments.

Cruciate Ligaments

The cruciate ligaments are very strong and are located a little posterior to the articular

center. Synovial membrane almost surrounds the ligaments but is reflected posteriorly

from the posterior cruciate to adjoining parts of the capsule. The intercondylar part of

the posterior region of the fibrous capsule has no synovial covering.

Anterior cruciate ligament (ACL) is attached to the anterior intercondylar area

of the tibia and ascends posterolaterally to posteromedial aspect of the lateral

femoral condyle. Its average length is 38 mm, and average width is 11 mm. It

is formed of two, or possibly three, functional bundles.

The posterior cruciate ligament (PCL) is thicker and stronger than the anterior

cruciate ligament. The posterior cruciate ligament is attached to the lateral

surface of the medial femoral condyle and extends up onto the anterior part of


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the roof of the intercondylar notch, where its attachment is extensive in the

anteroposterior direction. Its average length is 38 mm and average width is 13

mm.

Collateral Ligaments

The collateral ligaments are connected on the either side of the joint and stabilize the

hinge-like motion. The collateral ligaments stabilize the knee joint and connect the

two long bones. They prevent side to side displacement of the ligaments.

Lateral Collateral Ligament (LCL) is strong, round fibrous threads located

between lateral epicondyle of the femur and lateral side of the head of fibula.

It is about 8-9 cm long with insertions of the gracillis, sartorius, andsemitendinosus. The ligament fails due to excessive adduction or twisting.

Medial Collateral Ligament (MCL) is broad, flat membranous band. It is

attached proximally to the medial epicondyle of femur immediately below the

adductor tubercle; below to the medial condyle of the tibia and medial surface

of its body. It resists forces that would push the knee medially, which would

otherwise produce valgus deformity. It is about 10 cm long, inserted 2.5 cm

below the tibial condyle.

Table 2.1: Function of knee ligaments [9]Ligament Function

Anterior Cruciate

Resists anterior motion of the tibia on fixedfemur and extremes of knee extension.

Resist varus displacement at 0 degrees offlexion.

Posterior Cruciate Resists posterior motion of the tibia on a fixed

femur and extremes of knee flexion.

Resist varus displacement at 0 degrees of flexion

Lateral Collateral

Resists varus displacement at 30 degrees offlexion.

Resists posterolateral rotatory displacement withflexion that is less than approximately 50degrees.

Medial Collateral Resists valgus angulation. Works in concert with ACL to provide restraint

to axial rotation.


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2.1.3 CARTILAGES AND SOFT TISSUES

Menisci

Menisci are semilunar cartilages. They are crescentic laminae deepening the

articulation of the tibial surfaces that receive the femur. Their peripheral attached

borders are thick and convex, their free borders thin and concave. Thus the cross

section of menisci can be approximated to wedge shaped. The proximal surfaces are

smooth and concave and in contact with the articular cartilage on the femoral

condyles. The distal surfaces are smooth and flat, resting on the tibial articular

cartilage. Each covers approximately two-thirds of its tibial articular surface. The

medial meniscus, broader posteriorly, is almost a semicircle in shape. The lateral

meniscus forms approximately four-fifths of a circle, and covers a larger area than the

medial meniscus. Its breadth, except that of the short tapering horns, is uniform.

The main functions of menisci is to spread load by increasing the congruity of the

articulation, give stability by their physical presence and as providers of

proprioceptive feedback, probably assist lubrication, and may cushion extremes of

flexion and extension.

Quadriceps:

Quadriceps muscle consists of three vastus muscles i.e. vastus medialis, vastus

intermedius and vastus lateralis and the rectus femoris muscle. Quadriceps houses

patella and stabilizes it position during knee motion. Patellar ligaments are

functionally continuation of the quadriceps muscles below the patella. They are

attached to apex margins of the patella and below tuberosity. The different quadriceps

muscles include Sartorius, gracilius and semitendinosus muscles.


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Table 2.2: Functions of different muscles controlling knee motion [10]

QuadricepsMuscles Function

Vastus medialis Extends the leg at the knee joint

Vastus intermedius Extends the leg at the knee joint

Vastus lateralis Extends the leg at the knee joint

Rectus femorisFlexes the thigh at the hip joint andextends the leg at the knee joint

Gracilis Adducts thigh at hip joint and flexesleg at knee joint

Biceps FemorisFlexes leg at knee joint; extends andlaterally rotates thigh at hip joint andlaterally rotates leg at knee joint

SemitendinousFlexes leg at knee joint; mediallyrotates thigh at hip joint and leg atknee joint

SemimembranosusFlexes leg at knee joint and extendsthigh at hip joint; medially rotatesthigh at hip joint and leg at knee joint

2.2 BONES STRUCTURE AND PROPERTIES

Bones are complex composite structures and thus their properties are non-linear

isotropic in nature. The structure is divided in four layers viz. tropocollagen,

ultrastructure, microstructure and macrostructure. The smallest and first unit is

tropocol lagen molecul e , approximately 1.5 by 280 nm size, made up of three

individual left-handed helical polypeptide (alpha) chains coiled into a right handedtriple helix and associated with the apatite crystallites (Ap.). The crystallites appear to

be about 42060 nm in size. Second level is ultrastructural where the collagen and

Ap are intimately associated and assembled into a microfibrilar composite, several of

which are then assembled into fibers from approximately 3 to 5 m thick. In third

level microstructural , the fibers are arranged either randomly (woven bone) or

organized into concentric lamellar groups (osteons) or linear lamellar groups

(plexiformbone). Dense bones are found in shaft kind long bones called corti cal bone

while porous structure is found at the articulating ends called cancellous bone .


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Finally the macrostructural level where the whole bone is constructed of osteons and

portions of older, partially destroyed osteons (called interstitial lamellae).

Human bone is a viscoelastic material but cortical bone is approximated as

anisotropic, linear elastic material following Hookes law. The tensor property of the

cortical bone is represented in equation(1).

i ij jC [11] (1)

Where,

i, j = 1 to 6

i = stress tensor,

C i j Stiffness co-efficient (elastic constant) and

j = strain tensor

The stiffness matrix [C ij] for transverse isotropic material of bone is given byequation(2)

11 12 13

12 22 23

13 23 33

44

44

66

0 0 0

0 0 0

0 0 0[ ]0 0 0 0 0

0 0 0 0 0

0 0 0 0 0

ij

C C C

C C C

C C C C C

C

C

[11] (2)

2.3 SOFT TISSUES STRUCTURE AND PROPERTIES

Biological soft tissues are nonlinear, anisotropic, fibrous composites. The structure

and material properties of tissues vary to accommodate different tissue functions.

2.3.1 ARTICULAR CARTILAGES

Articular cartilage is found at the ends of bones, where it serves as a shock absorber

and lubricant between bones. It is best described as a hydrated proteoglycan gel

supported by a sparse population of chondrocytes, and its composition and properties

vary dramatically over its 1- to 2-mm thickness. The overall structure of articular

cartilage is analogous to a jelly-filled balloon. The proteoglycan (PG) rich middle

zone is osmotically pressurized, with fluid restrained from exiting the tissue by the


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dense collagen network of the superficial zone and the calcified structure of the deep

bone. The interaction between the mechanical loading forces and osmotic forces

yields the complex material properties of articular cartilage.

The behavior of cartilage is highly viscoelastic. A compressive load applied to

articular cartilage drives the positively charged fluid phase through the densely

intermeshed and negatively charged solid phase while deforming the elastic PG-

collagen structure. The mobility of the fluid phase is relatively low, and, for rapid

changes in load, cartilage responds nearly as uniform linear elastic solid with a

Youngs modulus of approximately 6 MPa. (Cross referencing - Carter and Wong,

2003) . For low loading rates, the stiffness/Youngs modulus is given as:

0.3660 (1 ) E E [11] (3)

Where,

E 0= 3.0 MPa and

= stress on tissues in MPa

2.3.2 LIGAMENTS

Ligaments are long strands tissues parallel to the axis of loading. They are composed

largely of, but contain very little of the PGs that give cartilage its unique mechanical

properties. The collagen fibrils may be hollow tubes, combine in a hierarchical

structure, with the 20 40 nm fibrils being bundled into 0.2 12 m fibers. These fibers

are birefringent under polarized light, reflecting an underlying wave or crimp

structure with a periodicity between 20 and 100 m. The fibers are bundled into

fascicles, supported by fibroblasts or tenocytes, and surrounded by a fascicular

membrane. Finally, multiple fascicles are bundled into a complete tendon or ligament

encased in a reticular membrane.

As the collagen fibrils are significantly crimped, initial loading acts to straighten these

fibrils. At higher loads, the fibrils lengthen. Tendons have nearly linear properties

from about 3% strain until ultimate strain, which ranges from 9 to 10%. The tangent

modulus in this linear region is approximately 1.5 GPa. Ultimate tensile stress


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reported for tendons is approximately 100 MPa. Normally they operate in range of 5

to 10 MPa with factor of safety of 10.

2.4 KNEE JOINT ARTICULATION MECHANICS

Joint articulation mechanics is essential to study to design joint prosthesis for

restoring the articulating functions. Knee joint comprises of tibiofemoral articulation

and patellofemoral articulation.

2.4.1 GEOMETRY OF ARTICULATING SURFACES

The shape of the posterior femoral condyles may be approximated by spherical

surfaces. The approximation of femoral condyles is not yet finalized as its shape

changes with person designing joint. The most common approximations are single

sphere, multi-sphere and elliptical.

The tibial plateau has greater width than the femoral condyle distances. The medial

and lateral condyles are concave and convex surfaces as dominating feature. The

tibial plateaus have the shape complimentary to femoral condyles shape. The axis of

rotation of the condyles is perpendicular to the sagittal plane and intersects the

femoral condyles.

Figure 2.2: Distal femur geometry [11]


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Patella makes gliding/rolling motion along the femoral articulating surface. During

the entire flexion range, the gliding motion is clockwise whereas rolling motion is

counter-clockwise between 0 to 90 and clockwise for remaining range of 90 to

120.

2.4.2 RANGE OF MOTION

The knee joint is considered to have all six degrees of freedom (DOF) i.e. 3

translations and 3 rotations. Among all these 2 translations and 2 rotations have very

small range and so mostly neglected. The Figure 2.3 shows different DOF of a knee

joint. Patella is constrained by quadriceps and trochlear groove of the femur. The

range of motion for each DOF of the knee joint is summarized in the Table 2.3.

Table 2.3: DOF for tibiofemoral joint

Translation Motion RangeValue

Rotational Motion RangeValue

Anterior-Posterior(AP) translationduring extension andflexion.

5-10mm Extension-Flexion alsocalled Range of motion(ROM).

120- 150

Medial-Lateral (ML)translation.

1-2mm Varus-Valgus rotation alsoknown asabduction/adduction.

Proximal-Distal(PD) translation.

Internal-external rotation(IER).

0-30 IR and0-45 ER 90 flexion.

Figure 2.3: Tibial plateau geometry [11]


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The AP translation is observed during flexion and extension of the joint. This

indicates that the joint does not behave ideally as hinged joint and the condylar shape

cannot be completely spherical. The ML and PD translations are minute and not

clearly detected. ML translation occurs due to misalignment of the bones and

unbalanced tensions in the ligaments. PD translations are observed during impact

loading and excessive bone occur has occurred.

Flexion-Extension is the dominating DOF with highest range of 0-120 for average

persons normal activities. Extension is considered when the femur and tibial axis are

collinear or parallel i.e. straight leg condition. As flexion angle increases, the stress in

knee ligaments increases. The IER is rotation of the bones about their axis and it is

observed at 90 flexion. This can be considered as twisting of leg about knee joint.

The varus-valgus rotation is not significant and depends upon the anatomy of an

individual person.

2.5 LOADS ON KNEE JOINT

Knee joint is subjected to the highest impact loads. The load acting is function of

factors which include body weight (BW) of the person, type of ground and the contact

interface between ground and foot. The forces acting on the knee joint can be

classified as external and internal forces. The external forces are due to the human

activities and interaction with the ground while the internal forces are due toconstraints by the ligaments and tissues. The Q angle of the knee has significant

Figure 2.4: DOF for knee joint

[Website: http://ajs.sagepub.com]


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influence on the load as it changes the components of ground reaction and BW acting

on the knee.

The impact load on knee during running and other impactful activities reaches

maximum value of 5 to 8 times BW during the take-off period of the gait cycle [12] .

For normal walking and stair climbing activities, the load acting on knee is 2-3 times

BW [8], [12], [13] . The load transfer across the knee bones occur through the soft

interface tissue menisci. It participate in load sharing by creating a better confirming

joint between the distal femur and proximal tibial surfaces through its wedged shape

structure. Menisci participate in damping of the impact load before transferring the

load from one joint to other.

The friction force acts between the femoral and tibial articulating surfaces due to the

sliding action. This frictional force acts in horizontal direction. The other internal

forces acting on the joint are restraining forces by the ligaments and soft tissues. The

primary restraining force is applied by the cruciate and collateral ligaments while

secondary restraining force is applied by menisci and meniscofemoral ligaments.

Figure 2.5: Knee Joint Kinematics and representation of forces and momentsacting on knee.

[Website: ajs.sagepub.com]


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The patellofemoral joint is also heavily loaded especially when the knee is flexed and

the load bearing activities is carried out. This joint load ranges from 3 to 5 times

BW [8] . This large load is because of the quadriceps tissues that assures joint between

the femur and patellar surfaces.


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3 KNEE IMPLANTS

The chapter summarizes the evolution of knee implants, the classification and

requirements to be fulfilled for development of the knee implants based on theimplant selection lecture delivered by Dr. Bharat Mody.

3.1 HISTORY OF KNEE IMPLANT

Platt- Pepplers Mold Arthroplasty design for the distal femur was used in

1938 by Smith Peterson.

McKeever developed single metallic component for replacement of tibial

plateau was developed in late 1950s [14] .

In 1951, Waldius hinged design made of ceramic was used as knee implant.

Later in 1958, the ceramic was replaced by Co-Cr alloy and continued to be

used till early 1970s.

In 1953, the Smitt-Peterson design was modified. It included femur stem and

patella groove on the femoral component.

Figure 3.1: Platt-Peppler's distal

femur implant design

Figure 3.2: Waldius hinge design


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1970s was the new era of knee implant development. Many new implant

designs were developed with increase in complicated shape and various

implanting techniques that covered broad range of knee diseases. The new

designs include uni-condylar implants, bi-condylar implants and total condylar

implant. Also cruciate retaining and sacrificing joint designs were developed.

After 1975, the knee implant designs were focused to make the condyles shape

like the natural epicondyles. The cement-less implant design was designed.

The advancement in total knee condylar implant was development of mobile

bearing implants, where the PE wear was reduced and for revision surgery

insert can be replaced without bone resectioning.

Figure 3.4: Modified Smith-Peterson implant

Figure 3.3: Uni-Conylar Implant(1973)

Figure 3.5: Bi-condylar Implants


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3.2 KNEE IMPLANTS CLASSIFICATION

The knee implants are classified based on various factors listed below:

Based on relative motion

The femoral and tibial components of the implant have relative motion

between them. There are three types of implants viz. fixed bearing, mobile

bearing and medial pivot implants (or rotating platform).

Based on implant design

The implants are designed with the cruciate ligaments with and withoutsacrificing. There is no solid argument to confirm which type of implant is

more stable and long lasting.

Based on implant assembly

The implants are designed based on assembly consideration as modular and

uni-body tibial tray implant designs.

Based on surgical requirements

The implants are designed for different knee diseases requirement. It includesuni-condylar, bi-condylar and total condylar implants.

Based on implantation method

The knee prosthesis implantation procedure includes cemented and cement-

less. The cemented implants use PMMA) cement i.e. function as grout. Non

cemented implants are press fits and it allows bone to grow into the porous

surfaces on the backside of these devices.

Figure 3.6: Total condylar implants


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3.2.1 BASED ON RELATIVE MOTION

1. Fixed bearing implants

The fixed bearing implants are most common and cheap. The PE insert is fixed firmly

within the tibia or to the metal platform base which is fixed to the tibia. The femoralcomponent rolls on the PE insert. The femoral shape is not perfectly confirming the

condyles shape on the tibial insert. These type of implants are most suitable to old

aged patients.

Advantages:

Range of motion is large.

Less issue in alignment. Least costly.

Disadvantages:

Not suitable for patients with obesity and patients involved in heavy load

carrying activities.

The life of implant is small due to quick wear.

2. Mobile bearing implant

The mobile bearing implant is advance version of fixed bearing implant. The PE

insert can rotate short distances inside the metal tibial tray. It provides large range of

motion. The femoral condyles shape is congruent to the condyles shape on the tibial

insert over the entire range of motion. The femoral component rolls on the opposite

surface of the insert. It is suitable for young and active patients.

Advantages:

Long life of the implant as wear is less. Suitable for patients with obesity and involved in heavy weight carrying. Large flexion angle compared to fixed bearing implant and allows small

lateral movement.


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Disadvantages:

Costly implants. The knee stability after implantation is highly dependent on ligaments.

Shear force increases at the implant-bone interface.

3. Medial pivot implant

The medial pivot implant resembles the natural knee kinematics. The implant permits

internal rotation, twisting, bending and flexion without compromising the stability.

During flexion, lateral side rolls back while medial side rotates in its place.

Advantages:

No kinematic changes of the knee after implantation and can perform all

activities that were done with the natural knee.

Knee soft tissues do not imbalances and reduce the stability of the joint. Preserves bone by reducing the amount of resectioning.

Disadvantages:

Complicated design. Difficult surgical procedure. The costliest implant among all the previous mentioned.

3.2.2 BASED ON DESIGN

The knee prostheses are implanted on resectioning the knee bones. The implants are

designed to retain the cruciate ligaments or not. The ACL are mostly sacrificed for

placement of tibial insert/tray. The implants are mostly PCL retaining and PCLsacrificing. There is no clear indication of different performance of the PCL retaining

or stabilizing implants.

1. PCL retaining implants

In PCL retaining, the femoral and tibial components have slots to accommodate the

ligaments and PE insert has central flat surface. The contact surfaces are reduced due

to slot and increases the contact pressure [14] . The quadriceps efficiency is not

compromised but reduction in roll-back of femur on full extension or flexion.


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2. PCL stabilizing implants

In PCL sacrificing, a wedged type raised post is present on the PE insert. It guides the

femoral component and restricts its motion so as to compensate the loss of PCL. As

there are not slots, the contact surface is large and thus reduced contact pressure [14] .

The implants though allows AP translation during full flexion and extension, it is

compromised with the quadriceps efficiency. Patients with these implants have

difficulty in activities involving large flexion angle like rising from chair or sitting

with legs folded.

3.2.3 BASED ON IMPLANT ASSEMBLY

The implants are design considering the assembly technique of the implant

components.

1. Uni-body tibial tray:The implant tibial tray is made entirely of PE material as a single unit. The proximal

end of tibial component has the articulating surface replicating the tibial condyles

while the distal end has stem which is fixed in the cavity of tibia and forms the bone-

implant interfacing surface.

Advantages:

Less number of components. Micro-motion between the insert and tibial tray is eliminated. Low cost

Disadvantages:

In case of revision surgery due to PE wear, the entire implant needs to be

replaced. Issues with proper fixation of PE insert with tibia. In course of time, the implant gets misaligned.

2. Modular tibial tray:

The modular tibial tray design implant has metallic tibial tray that replicates tibial

plateau and PE insert is press fitted between the femoral component and tibial tray to

replicate the function of meniscus.


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Advantages:

Allowing surgeon with intraoperative choice with regards to component

thickness and implant constraint.

In case of revision surgery due to wear of PE insert, the insert is replaced with

new one with considerable thickness without altering the bone fixations.

The metallic components are fixed with screws to bones, thus misalignment

with time due to fatigue failure is avoided.

Disadvantages:

The underlying superior surface of the metallic tray creates an unintentional

bearing surface causing micro-motion which is source of PE wear debris.

Failure rates due to wear and loose fitting with the tibial tray increases

compared to uni-body design.

The holes for screw expose the wear debris to the fixation interfaces which

will cause osteolysis i.e. loosening of fixation bond.

3.2.4 BASED ON SURGICAL REQUIREMENTS

The implants are designed to meet the specific surgical requirements for the treatment

of the knee diseases.

1. Uni-condylar Implant

The uni-condylar implants are designed for replacement of single femoral condylar

surface and corresponding tibial condylar region. The uni-condylar implant offers

advantages such as bone and cruciate ligaments preservation, range of motion is

increased and maintains natural kinematics. The drawbacks of such implants areincreased wear, strict patient requirements, difficult to align and perform arthroplasty

and disease still propagate. (Figure 3.3)

2. Bi/duo-condylar implant

The bi-condylar implant replaces both condylar surfaces of the femur, entire tibial

plateau but not patella. The advantages of these implants are propagation of disease is

stopped, easy to align and do implant fixation. But the main drawback is kinematics

of patellofemoral joint changes. (Figure 3.5)


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3. Total condylar implant

The total condylar implant replaces condylar surfaces of femur, entire tibial plateau

and patellar component. The designing these implant is complicated as kinematics of

tibiofemoral and patellofemoral is to be restored. The remodeling of ligaments helps

to stabilize the knee post arthroplasty. (Figure 3.6)

3.2.5 BASED ON IMPLANTATION METHOD

The implants fixation with the bone is done either using cement or without cement.

Cemented implant

The cemented implant uses special ceramic PMMA cement which functions as grout

when poured into the porous area between the bone-implant interfaces. The cement

helps in building initial strength and fight with infections as it dissolves antibiotics in

it. This is the most common implant design and not suitable with patients involved in

large activities.

Cement-less implant

The cement-less implant design is such that the implant fixation occurs due to bone

growth into the surface of implant. The backside of the implant is coated with porous

material or textured to permit bone growth and proper bonding with the implant

components. Initial fixation is by using screws or pegs. The main advantage of this

implant is elimination of osteolysis cause. Recovery takes longer time and not suitable

for patients suffering from osteoporosis.

3.3 KNEE PROSTHESES REQUIREMENTS

The various requirements of design of knee prostheses are divided into threecategories, design, material properties and manufacturing requirements.

3.3.1 DESIGN REQUIREMENTS

1. The knee implants should not alter the kinematics of natural knee.

2. The implant should permit activities involving high flexion angles without

pain or abnormality.

3. The tibiofemoral joint should permit AP translation and small amount ofinternal rotation along with the flexion-extension.


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4. The tibial component should guide and stabilize the femoral component in

case of posterior stabilizing implant.

5. Insert bearing surface should be confirming to femoral articulating surfaces.

This will reduce contact pressure and help to stabilize kinematics.

6. There should be access to natural lubrication by synovial fluid at the mating

articulating surfaces and escape of wear debris and other abrasive particles

that accumulate between them.

7. The design of implants should have proper fixation methods and easy to align

the components to anatomical axis.

3.3.2 MATERIAL PROPERTIES REQUIREMENT

1. The material of implant should be biocompatible. It means that the implant

materials should not participate in any kind of reaction with the body tissues.

2. Material should allow tissue regeneration and bone growth over the implant

for better fixation and stability.

3. The material should have appropriate combination of tensile strength,

compressive strength, wear resistance (or harness) and fatigue strength. The

ASTM standard F2083-12 helps in selection of correct material for implants.

4. The density of material should be similar to the bone so that it does not

increase the weight of the component.

5.

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