5.1 Joints of the Body 5.1 Joints of the Body
Transcription
5.1 Joints of the Body 5.1 Joints of the Body
5.1 Joints of the Body • Synovial joints – Diarthrodial joint (large motion) – Stabilized and constrained by a fibrous joint capsule, which may contain connective ligaments. – The Th articulating ti l ti surfaces f off th the b bones are covered d with ith a thi thin llayer off articular ti l cartilage – Contains highly viscous synovial fluid, which is secreted by a thin layer of synovial cells (synovium) – Extremely low friction coefficient (μs <0.01, μk<0.003): Table 1.1 • Classification of synovial joints 1. Ball and socket joint: shoulder, hipÆ ball(head of the femur) and the socket (acetabulum), kinematic constraints 2 Bicondylar joint: two curved condyles articulate against relatively flat surfaces 2. on the mating bone, unstable, ex) knee 3. Multiple bone joint: multiple bones are closely fitted and bound together by ligaments Combined motion makes large movement ligaments. movement, ex) wrist wrist, ankle 94 5.1 Joints of the Body • Synovial joints 1. Ball and socket joints Hip Joint Conforming contact b/w the femoral head and acetabulum (substantial ( b t ti l kinematic ki ti constraint) t i t) Shoulder Joint Non-conforming contact b/w the humeral head and the socket of glenoid (little kinematic constraint Åaffected by muscle and soft tissues 95 5.1 Joints of the Body • Synovial joints 1. Ball and socket joints - shoulder Bones & articular g cartilage + ligaments + capsules (synovial joint) + muscles 96 5.1 Joints of the Body • Synovial joints 2. - Bicondylar joints Two pairs of articulating surfaces Relatively flat articulating surfaces Little kinematics constraints Greater range of motion in one plane Unequal loads, may resist moment with little activity of muscles or tissues - Ex) knee joint 97 5.1 Joints of the Body BIOME LAB • Synovial joints 3. Multiple bone joints: - Between ball socket and bicondylar joints - Fitted and bound together by ligaments - Combined motion makes large g movement Ex) Wrist: 8 carpal bones Ankle: 7 tarsal bones wrist 98 5.2 Joint Stability Joint stability the ability of a joint to maintain an appropriate functional position throughout its range of motion. The characteristics of stable synovial joints 1. 2. 3 3. Joint contact occurs between surfaces, both of which are covered with articular cartilage There exists a unique position of equilibrium for any loading. Small changes in either the magnitude or direction of the functional load do not lead to large changes in the position of joint contact. 99 Ankle Sprain 발목 염좌 비디오 5.2 Joint Stability Main mechanism for joint stability 1) Contact at the articular surfaces • Primary mechanism of stability • Passive mechanism • Varying extent in all articulating diarthrodial joints – Example » Hip joint is very stable due to the geometric constraints » Shoulder joint is lesser stable » Contact forces are perpendicular to articulating surface 100 5.2 Joint Stability • Ex. Contact points and joint loading • • • Known: magnitude and orientation of GRF, orientation of muscle and contact force Unknown: magnitude of muscle and contact force FM 2 > FM 1 Answer>> – JJoint i t contact t t iis much h greater t when h it acts t att point 2 than at point 1 as is the muscle force Thi strong This t association i ti b between t th the llocation ti of the joint contact force and the magnitude of the muscle force This is a key attribute that needs to be incorporated into design of artificial joints; an artificial joint should be designed in a way that th t a llarge contact t t fforce should h ld nott occur to prevent wear Neuromuscular system has been tuned for equilibrium ilib i Æ Disruption Di ti h harm th the jjoint i t – – – J 2 > J1 5.2 Joint Stability 2) Muscle action • Voluntarily induced by active muscle contraction • Equilibrium can be achieved and the point of contact controlled by control of the muscle forces 3) Co-contraction of agonist and antagonist • Co-contraction increases stability by increased compression • Particularly important for frontal plane • No protection to acute traumatic situation due to response delay (~20ms) 4) Ligament • • • • Passive Resistant to abnormal range of motion. Soft at normal range. Important balance b/w laxity and allowable joint motion Imbalance by trauma trauma, disorder disorder, replacement affect stability of joint 102 5.2 Joint Stability • Stability is extremely important for the design of total joint replacement - Integrity of arthroplasticity - Correct alignment necessary • Medially directed force (or varus /adduction moment) in the knee joint - Equilibrium by two supporting elements - contact at curved medial tibial plateau generate lateral force - Stretch of lateral collateral ligament (fibula collateral ll t l liligament) t) • Medial contact force is higher than lateral contact force during gait – more disease on medial side 103 5.3 Joint Replacement BIOME LAB Joint Replacement – – A surgical procedure where articulating surfaces are replaced to restore function to joints that have become damaged and painful Most common reason for joint replacement is Arthritis (osteoarthritis and rheumatoid arthritis Osteoarthritis – – – – – Affects Aff t iisolated l t d jjoints i t such h as hip, hi kknee, or elbow lb Causes • Normal wear and tear during lifetime • Secondary to joint trauma (sports injuries) Destruction of the articular cartilage The damage is restricted to the articular surface • The bone tissue is generally normal • Joint is provided good support for an implant. g is still irreparable, p , but considerable research ongoing g g Articular cartilage 104 5.3 Joint Replacement Current Techniques –Articular cartilage repair Mosaicplasty p y Autologous g Chondrocyte y Implantation p Microfracture Rheumatoid arthritis – – – – Systemic disease that often affects multiple joints Damages the articulating surfaces, bone tissue and soft tissue Deformities and loss of function Some types of fixation may be difficult to achieve in the rheumatoid patient, and the prosthesis may have to provide functional constraints that are unnecessary for patient with osteoarthritis osteoarthritis. 105 5.3 Joint Replacement • 품목번호 1 3010.01 0 B3010.01~02 인공발목관절 2 B3020.01~02 인공팔꿈치관절 3 B3030.01~02 인공손가락관절 4 B3040.01~02 인공엉덩이관절 5 B3050.01~02 인공무릎관절 6 B3060.01~02 인공어깨관절 7 B3070.01~02 B3070 01 02 인공발가락관절 품목명 8 B3080.01~02 인공손목관절 9 B3160.05~06 인공추간판 인공어깨관절 Shoulder 인공팔꿈치관절 Elbow 인공추간판 Disc 인공손목관절 Wrist Usually, both surfaces of a joint pair are replaced Exception: hemiarthroplasty of the hip ~ natural socket in th pelvis l i iis retained t i d the 인공엉덩이관절 Hip 인공손가락관절 Finger 인공무릎관절 Knee 인공발목관절 Ankle 인공발가락관절 Toe Artificial Joints (world market) (Unit: Million Dollar) Item 2001 2005 CAGR(%) K Knee 2 219 2,219 4 100 4,100 17 2 17.2 Hip 2,437 4,600 16.7 Sum 4,656 8,700 16.9 Artificial Joints (world market overview) 20,000 Million Do ollar • No. BIOME LAB World: 16% % increase p per year Korea: 12% increase 15,000 10,000 5,000 0 2005년 2006년 2007년 2008년 2009년 2010년 인공무릎관절 인공엉덩이관절 4100 4551 5052 5607 6224 6909 인공무릎관절 인공엉덩이관절 4,600 5336 6190 7180 8329 9662 2002년 Biomet, Merrill Lynch, “Global Industry Analysts” 2005년 Stryker, “Market Overview” 2011년 KISTI “Artificial Joints World Market Overview” Biomechanical Test Criteria for Artificial Joints Design Failure, Fatigue failure (내구성) Osteolysis (골용해, 골흡수) Coating (표면처리) Range of motion (운동범위) Design Successful surgery: 1.Artificial joint 2.Clinician 3.Patient DesignBearing Surface Wear (마모) 5.3 Joint Replacement Successful total replacement designs involve both functional and structural considerations. – – – – – – Normall range off motion N ti The strength of the prosthetic components Damage to the articulating surfaces The strength of the interfaces b/w the implant and the bone The potential adaptation of the bone due to altered loading Implanted components must be biocompatible 109 5.3 Joint Replacement Function • The function of joint involves force transmission coupled with kinematics • The motion of a natural joint is determined by the geometry of the joint surfaces and others – – The ligament ligament, joint capsule capsule, soft tissue structure muscle force Prosthesis substitutes the function that was previously supported by ACL, articular cartilage, and/or PCL • To design the articulating surfaces, joint loads must be known • Difficult to get precise values of joint contact forces because the system is indeterminate. • The kinematics and the joint forces are coupled coupled. – An interesting case in point is the knee, which is constrained by soft tissue structures and substantial laxity. 110 5.3 Joint Replacement • The kinematics created during total joint replacement surgery can influence the joint loads. – – – The location of the femoral head w w.r.t r t the femur The location of the acetabulum w.r.t the pelvis The relative motion between the femur and pelvis can be altered, both the magnitude and direction of hip joint force may be affected affected. • The distance between the origin and insertion of a muscle may be lengthened or shortened when the geometry of the system is changed changed. • This cause changes in muscle force capabilities because of the forcelength relationship. 111 5.3 Joint Replacement Structure • Total joint replacement fall into two classes 1) Replace only the articulating surface 2) Replace the surface and a substructure of a bone 1) Surface replacement – • Acetabular cup(hip), glenoid(shoulder), femoral and tibial components The goal is to transfer the joint loads to the underlying cancellous bone similar to the way it is done in the normal joint 112 5.3 Joint Replacement Structure 2) Surface and substructure of bone replacement – – – Femoral components for artificial hips Humeral components for shoulder replacement Loose hinges for elbow prostheses Femoral components • Humeral components Elbow components Components off artificial are fi fixed bone with (bone C ifi i l jjoints i d to b i h either i h PMMA (b cement) or cementless technique. 113 5.3 Joint Replacement • Thin layer of PMMA can be used to precoat portion of the fixation stem – – – • Only a portion of the stem is bonded to the cement The cement and precoat stem creates strong interface The transition between the bonded and unbonded interface regions may be a region of stress concentration Without precoating, the cement fills the space between the implant and the bone – – – The strengths Th t th off th the iinterface t f d depends d upon mechanical h i l iinterlock t l k At the cement-bone interface, the PMMA interlocks with the roughness of the bone At the cement cement-implant implant interface, interface grooves and undercuts or roughened surface are often employed 114 5.3 Joint Replacement • The disadvantage of PMMA is low tensile fatigue strength. – – – • The bone-implant system must be designed so that stresses in the cement and the interfaces are within safe limits Cement is the weak link in the bone-cement-implant system New method: Cementless fixation • Press-fit prosthesis • Biologic fixation Biologic fixation – – – Coat portions of the prosthesis with a porous layer Layers of metal beads or mesh sintered Ingrowth of the bone into the porous layer provides l long-term t fi fixation ti – Trabecular Metal (Zimmer) 115 5.3 Joint Replacement The Composite structure: Load transfer – – – – The new composite: consist of cortical bone, cancellous bone, implant, cement etc cement, Important to understand load transfer b/w components Sufficient strength & Biocompatibility Fatigue problem for polymers B Bone adaptation d t ti – – Bone remodels with environment loading changes Implants share the load and the bone remodels. The bone remodels to meet the requirements of new loading condition E.g.) parallel load sharing : vicious cycle Implant carries loadingÆ stiffness of bone decreases Æ relative stiffness of implant increases Æ load carried by bone further decreases Æfurther decrease in bone stiffness 116 5.3 Joint Replacement Articulating Surfaces – – – – – Surface damage due to contact force is of concern The small particles, debris accumulates, the biological response can lead to i f ti infection and d lloosening i The mechanism still unclear Reduce stress, better material to resist wear Æ design problem Experiments- different designs Fig S Fi Surface f d damage off a polyethylene l th l tibial component Modular Components – Modularity provides flexibility in surgery Custom Design – Custom designed implant-correct congenital problems problems, replace failed prosthesis, or treat patient with bone tumors 117 5.3 Joint Replacement Design of bone-implant system z g Performance ((in p Factors Influencing particular,, structural p performance)) 1) Implant, 2) Patient, 3) Surgical factors – Surgical instrumentation to aid surgeon to get consistent position and orientation and good fixation Æ Engineer’s role – Variables associated with p ((material,, the implant geometry, manufacturing, etc) Variations by experience of surgeon Variation in patients – – Æ Robust-insensitive design to variations Figure: Factors influencing the design of a bone118 implant system 5.3 Joint Replacement Evaluation of Implant Performance – Long-term follow-up studies, using statistical analysis – Survival analysis: probability of survival in some time post surgery. Little information of design features on performance – Retrieval analysis: examination of device during revision surgery or autopsy. Best source for evaluating in vivo performance, Failure mode identified. – Preclinical evaluation before new design usage: in vitro experiments, simulation studies,, in vivo animal studies,, and in vivo human studies – New designs are tired by a few skilled surgeons and multicenter trials before general usages. FDA procedures 119 5.4 Structural Analysis: Bone-implant BIOME LAB Beam theory – – – To determine the stresses in the structure as a function of geometry, loads, and material properties of the structure One-material beam or composite beam Plane sections before loading remain plane after loading: axial loading Æ strain is constant over the cross section bending Æ strain varies linearly from the neutral axis Fig. 5.2 Proximal femur with prosthesis Æ composite beam: fixation stem, cement (interface layer), bone Fig. 5.1 Long bone with a fracture fixation plate Æ composite beam 5.4.1 Symmetric Beams 1) One-Material Beams: Axial Loading Fig. 5.4 Equilibrium P = Fig. 5.3 Deformation of axially loaded beam ε x = lim PQ → 0 P *Q * − P Q PQ σ x = Eε x S in ce ε x is co n stan t (C ), σ x = C E ∫ σx = A σ xdA = P A ∫ A C E dA = C EA : ax ial stress CE = P A ΔL P E = L A PL ΔL = AE 121 5.4.1 Symmetric Beams 2) One-Material Beams: Bending Fig. 5.6 Equilibrium Fig. 5.3 Deformation of a beam in pure bending M ( ρ − t)dθ − ρ dθ t = − ρ dθ ρ ρ : rad iu s o f cu rvatu re o f th e n eu tral ax is z = − ∫ σ x td A = A E ρ ∫ A t 2 dA Mt I A t n etu ral ax is, fo r p u re b an d in g ε x = lim σx = − σ x = Eε x P = − ∫ σ xdA = 0 PQ → 0 A 122 5.4.1 Symmetric Beams 3) Three-Material Beams: Axial loading 3 component: boneB , cement (interface layer)C, fixation stemP εx = C εx = − co n stan t (ax ial lo ad in g ) t ρ (b en d in g ) σ i = E iε i From Equilibrium condition P = ∫ AB σ xdA + ∫ AC σ xdA + ∫ AP σ xdA P = C ( E B A B + E C AC + E P A P ) σj = Figure Three-material beam consisting of bone, cement, and prosthesis E jP E B A B + E C AC + E P A P F o r n -d im en sio n al b eam , σj = Ej P n ∑E i =1 i Ai W h ere, P to tal lo ad actin g o n th e cro ss sectio n 123 5.4.1 Symmetric Beams 4) Three-Material Beams: Bending 3 component: boneB , cement (interface layer)C, fixation stemP M = − ∫ σ x td A − z AB ∫ AC σ x td A − ∫ AP σ x td A T h e resu ltan t fo rce at cro ss sectio n is zero fo r p u re b an d in g P = ∫ M = z σj = AB σ xdA + 1 ρ ∫ AC σ xdA + ∫ AP σ xdA = 0 (E B I B + EC IC + E P I P ) − E jMt E B I B + EC IC + E P I P The location of the neutral axis for a three-material beam yˆ = y B E B A B + y C E C AC + y P E P A P E B A B + E C AC + E P A P y is th e lo catio n o f th e cen tro id al ax is o f th e i th m aterial 124 5.4.1 Symmetric Beams EX. Two-material beam Consider a two-material beam consisting of bone1 and stainless steel2 subject to axial loading P = 1 0 0 0 N E1 = 1 7 G P a E2 = 200G Pa A 1 =560m m 2 A 2 =50m m 2 What is the portion of the load carried by component 1 (bone)? Sol> 17 σ1 = S tress in co m p o n en t 1, 1 0 0 0 = 0 .8 7 M P a 1 7 (5 6 0 ) + 2 0 0 (5 0 ) N o te th at th e stress in co m p o n en t 1 can b e w ritten as P σ1 = 1 A1 F ro m w h ich w e see th at P1 E P , = 2 1 A1 ∑ E i Ai F ro m σ j = Ej ∑E i =1 i =1 an d th en o b tain P1 = P E 1 A1 2 ∑E i =1 i Ai = P n 1 7 (5 6 0 ) = 0 .4 9 1 .9 5 2 × 1 0 4 125 i Ai 5.4.2 Case Studies: Compression analysis of the vertebral body – load sharing between cortical and trabecular bone • Engineered structures: uniform materials, mathematically well defined shapes (e.g. planes of symmetry) • Bones are not engineered structures – Irregular shape Å complex adaptation & genetic programming – Heterogeneity across the population – How to generalize results from a single bone to entire population? • Modeling issue – Balancing the need for fidelity with the available resources • Osteoporosis – Geometric and material properties – Radiographic diagnosis, preventive action or treatment Fig 5.12 T-10 vertebral body Left) 54 yr-old, Right) 82 yr-old 126 5.4.2 Case Studies: Compression analysis of the vertebral body – load sharing between cortical and trabecular bone Spine fracture The most prevalent in osteoporotic fractures, 700,000 annually in the U.S. By a permanent deformity (not by a complete separation of bony fragments) • The etiology of spine fractures – Wedge fractures • isolated overload • cumulative damage • longer-term creep or fatigue • the relative role of the cortical shell versus trabecular centrum i lload in d sharing h i Æ which hi h b bony compartment t t needs d tto b be monitored it dd during i screening for osteoporosis or targeted for drug treatment 127 Fig 5.13 Basis for classification Wedge fracture is the most common in the elderly. Biconcave and crush fractures are more common in younger individuals (due to high-speed high speed impact) Fig g 5.14 Vertebral body, y 60μm μ resolution. The shell is a fused set of trabeculae than a continuous thick cortex 128 How much load does the cortical shell carry? ÆSimple model :Two spring in parallel (cortical bone + trabecular bone) - Cross section: circular - From direct measurement (Thickness of cortical bone: 1/3~1/2 mm) - Due to dimension, material properties are uncertain (different from uniaxial test using large and uniform specimen) – Fig g 5.15 Circular cylindrical y model of the vertebral body. Uniform deformation of δ Uniaxial compressive behavior of the vertebral body • • • • Uniform displacement δ The strain ε=δ/H Compressive force P = Pc+Pt A Assuming i lilinear elastic l ti b behavior h i 129 Pc E c Ac = P E c A c + E t At w h ere E i : elastic m o d u lu s Ai : cro ss-sectio n al area • The role of the trabecular density (ρ) , cortical shell thickness (t), cortical shell elastic modulus (Ec) A ssu m e th at E t = m ρ Pc 1 = = E t At P 1+ 1+ E c Ac Ec 1 mρ At = {(1 + 2 t / D ) − 1} 2 T yp ically , t ≈ 0 .3 5 m m , D ≈ 3 0 m m Pc = P 1+ Fig g 5.15 Circular cylindrical y model of the vertebral body. Uniform deformation of δ Ac = π 4 π 4 D2 ( D + 2t )2 − π 4 D2 1 mρ 0 .0 5 E c 130 Pc = P 1+ 1 mρ 0 .0 05 Ec ca se 1 . E c ≈ 1 7 0 0 0 M P a , m ρ ≈ 3 0 0 M P a m ρ / E c = 0 .0 1 8 (E t /E c ≈ 0 .0 1 8) Pc / P ≈ 7 4 p ercen t ca se 2 . m ρ / E c = 0 .55 (E t /E c ≈ 0 .5) 5) Pc / P ≈ 9 p ercen t Fig g 5.15 Circular cylindrical y model of the vertebral body. Uniform deformation of δ The relative stiffness of the cortical to trabecular bone strongly affect the mechanical behavior of the vertebral body. 131 5.5 Total Hip Replacement BIOME LAB Goal of joint replacement 1) Relieve pain, 2) Improve function, 3) Longevity Hip: a ball and socket joint - Provide 3 rotation (flexion-extension, abduction-adduction, internal-external rotations of the femur w w.r.t r t the pelvis) and transmitting loads - Component fixed by bone cement or bone ingrowth/ongrowth to the prosthesis via porous or coating - Common structural failure ① ② ③ fracture of the fixation stem in the central zone interface failure in the proximal/distal zones bone resorption due to stress-shielding • Functional and structural issues of THR - How changing the centers of the articulating surfaces w.r.t the pelvis and the femur affects the forces transmitted across the joint Approach to study THR can be applied to other joints - 132 5.5 Total Hip Replacement 5.5 Total Hip Replacement Kinematics and Loads – – – – – THR alters kinematics of the proximal femur to the pelvis. Th goall iis to position The ii the h components to retain i the h normall kinematic and structural relationship. Muscle to operate at the same point on its length-tension curve and maintain normal muscle l excursion i Reconstruction of normal anatomical geometry Neck of femoral component may impinge on the rim of the acetabular component For patient with abnormal joint due to disease or damage, new-positioning is necessary 134 5.5 Total Hip Replacement • The stiffest path carries the greatest load. Dense cancellous bone in the proximal femur – – The center Th t region: i : apparentt d density it 2 ti times, elastic modulus 4 times A stiff inner core and outer layer Δ C =Δ O = PC L P L = O E C AC EO AO the length and areas are the same, Fig 9.2 A dense region of cancellous bone transfer load from the surface of the head to the dense cortical bone at the calcar PC E C =4 = PO EO The stiff core carries 80% of the total load. - The line of action of joint contact force lies along the region of dense cancellous bone Fig 9.3 Cross-section of a 135 hollow cylinder with a stiff core 5.6 Total Knee Replacement • • • • • • BIOME LAB In the knee, role of soft tissues is very important, it is less-conforming joint and constrained by the soft tissues 3 component: femur 3-component: femur, tibia tibia, and patella The motion of patella is guided by the trochlear groove and the action of the quadriceps M i l ACL and Mainly d PCL CL provide id anterior-posterior i i constraint. i MCL and d LCL provide id varus-valgus constrains. ACL may provide tibial internal rotation and valgus rotation constraints Damage to the soft tissue may cause secondary injuries to other joint structure and alter joint kinematics 136 5.6.1 Knee Function 1) kinematics • • • • • • 6 DOF motion, primary motion is flexion-extension Hyperextension ~ 5°, Full flexion ~ 160° Tibial axial rotation is least in full extension, greater in flexion Valgus-varus Valgus varus rotation may produce lift-off. lift off. Valgus-Varus Valgus Varus rotation is constrained by collateral ligament and muscle contraction Anterior-posterior translation is constrained by cruciate ligament and the geometry g y of condyles, y ,p plateaus,, and menisci Injury results in loss of ligament function - ACL rupture: anterior tibial translation increases - PCL rupture: posterior tibial translation increases, which reduce the quad moment arm and decrease the range of motion of the knee 137 5.6.1 Knee Function 2) Loads and Moments • • • • Femoral-tibial contact force ~ several times of body weight during functional activities Muscle forces, cruciate and collateral ligaments, menisci provide stability Sliding and rolling-tibiofemoral motion Patella-femoral contact 138 5.6.2 Knee Structure • Load transfer from the joint surfaces to the cortical bone in the proximal tibia has been studied the most, because tibial loosening of prosthetic components has been a big problem • Figure 10.6 The cancellous bone texture is oriented in the proximal-distal direction. The density seems to decrease from proximal to distal • Proximal region: load is mostly carried by cancellous bone • From proximal to distal, the cancellous bone becomes less dense and the outer shell becomes thicker(stiffer). As the cancellous bone disappear, outer shell (di (diaphysis) h i ) carries i th the entire ti lload. d CrossFig 10.6 Cross sectional view of the proximal tibia. Regions of dense cancellous bone transfer the condylar forces distally to the cortical shell 139 5.6.3 Knee Replacement • • • • • • TKR is more complex than THR, because soft tissues provide most of kinematic constraints. Prosthetic component provides substitute function for the ligaments that are lost in surgery Reconstruction of normal range of motion and geometry Replace only surface (distal femur and proximal tibia) Canlellous bone in the proximal tibia and surface damage in polyethylene component Hi h contact stress and High dd debris b i iincrease risk i k off llate lloosening i 140 5.6.3 Knee Replacement 1) Types of Replacements • Unicondylar replacement (lateral or medial replacement only) • Bycondylar replacement 1. Unconstrained type (mobile-bearing) 2. Semi-constrained type - PCL substituting design (more consistent performance) - PCL retaining design (mostly ( flatter tibial surface, less conforming contact, better kinematics) 3. Constrained type - Single hinge axis Æ not common: the axis of rotation moves during g increases flexion. Fixed axis takes all load. Loosening 141 5.6.3 Knee Replacement 2) Function • • Joint motion controlled by joint load, articulating surfaces, soft tissues Symmetric or unsymmetric design • 1. Two types of contacts The surfaces of the femoral and tibial components are both toroidal The lateral-medial geometry of the femoral and tibial articulating surfaces is highly conforming 2. • - sym unsym Lift-off Varus or valgus moment S iff articulating Stiff i l i surface f Flat surface ~ extreme edge loading, increase risk of failure and wear on polyethylene 142 5.6.3 Knee Replacement 3) Structure • • • • - Compressive force on femoral component along the femoral axis Force on the tibia moves w.r.t. the tibial plateau during flexion-extension. More loosening in tibial component. j structural p problems Three major 1. component failure (design problem, stress concentration) 2. fixation failure (porous coating degrades fatigue strength) 3 surface damage 3. Most common failure: crushing the cancellous bone under tray Stiffest and strongest subcondral cancellous bone is removed R Remaining i i cancellous ll b bone must carry lload. d E Endosteal d lb bone shell h ll in this region is thin. The amount of bone resected should be minimized 143 5.6.3 Knee Replacement 3) Structure (continued) • Tibial tray with a central large peg or smaller fixation peg beneath the medial and lateral plateau • yp y y y y y polyethylene or p polyethylene with metal tray Entirely • Fixation on the tibia 1 PMMA – bone cement 1. 2. Porous coating-cementless 144 5.6.3 Knee Replacement BIOME LAB 4) Long-Term Clinical Evaluation • • Survival analysis – success rate Retrieval analysis g y – revision surgery - relationship between design and failure - failure mode analysis with particular reasons Total knee design issues (1~3:structural) 1. 2. 3. 4. 5. Kinematics Contact stress-wear Fixation Ligament tension Patient Function 145