· the operation of hip arthroplasty has had one of the greatest impacts in the history of surgery
· “Total hip replacement, indeed, might be the operation of the century” (Coventry, 1991)
· there have been a myriad of variations in design and technique
· rationalisation of these requires an understanding of the concepts behind design, fixation and failure of hip arthroplasty
· impossible to cover every aspect or every theory proposed
· important questions to answer
· what are the basic principles behind prosthesis design ?
· what is the role of cement and fixation ?
· what causes loosening ?
· in normal hip, forces on femoral head transmitted through cancellous and cortical bone
· produces stresses and strains in bone
· responsible for maintenance of bone density and trabecular patterns
· after hip replacement, femoral head force transmitted from stem to endosteal interface
· unnatural stresses experienced
· actual forces calculated using telemeterized system by Davey (1988) and Bergmann (1992)
· line of action of resultant force stays within region over upper half of head towards anterior aspect
· for level walking
· peak force is 2.5 - 3.5 BW
· angle of force is 20o to vertical
· force greater when lifting, running or jumping
· angle of force more anterior when rising from chair or climbing stairs
· can describe forces on femoral head by simple lever arms
· body weight depicted as load applied to lever arm extending from body’s centre of gravity to centre of femoral head
· abductor musculature acts on lever arm extending from lateral aspect of greater trochanter to centre of femoral head
· ratio of length of lever arms is BW:ABD = 2.5:1
· thus force of abductors must be 2.5 x BW to maintain pelvis level in single leg stance
· load on femoral head in stance phase is sum of forces created by abductors and body weight (ie. 3.5 x BW)
· force can be resolved into three components along Cartesian axis
FY force
· acts to impact stem down canal
· force carried by
· shear stresses along interface (cement-bone or stem coating-bone)
· wedging action of canal as it narrows
· collar (if present)
· there is varus moment of magnitude FY x offset of head from stem
· increases stem-bone force proximal-medial and distal-lateral
FZ force
· exerts lateral force and valgus moment on stem
· point reached down stem where net moment is zero
· varus moment produced by FY balanced out by valgus moment produced by FZ
FX force
· exerts torsion about long axis of stem
· magnitude depends on
· offset of femoral head from stem
· degree of hip flexion
· on normal walking, this is 2-3 BW x mm
· on ascending stairs, this is 50 BW x mm
· in 60 kg person, this is 35 Nm torque
Torsional forces
· can lead to
1. plastic deformation of femoral components
· seen in early Charnley stems manufactured from EN58J stainless steel (ductile alloy)
2. burnishing on matt-finished femoral components
· over medial half of posterior surface and lateral half of anterior surface
· produced by fretting of stem against cement
· seen in matt-finished Exeter stems in region of osteolysis
Proximal stress shielding
· if a substantial part of the load entering the stem bypasses the proximal femur, proximal stress-shielding occurs
· result is osteopaenia of the proximal femur
· most likely to occur where fixation occurs in the diaphysis
Excessive forces
· running and jumping associated with marked increases in forces applied to femoral head
· excess forces not able to be tolerated because
1. trabecular shock absorbers of normal hip sacrificed in arthroplasty
2. friction of arthroplasty is 70 times more than in normal hip
3. no such thing as impingement of neck on cup in normal hip
4. no protective or proprioceptive sensation in arthroplasty
· Charnley (1961) said “Objectives must be reasonable. Neither surgeons nor engineers will ever make an artificial joint which will last 30 years and at the same time during this period enable the patient to play football.”
Charnley’s principles
· integral part of Charnley concept is to
· shorten lever arm of body by deepening acetabulum (centralisation of femoral head)
· lengthen lever arm of abductor mechanism by reattaching greater trochanter laterally
· effect is to make lever arm ratio BW:ABD approach 1:1
· this theoretically decreases total load on hip by 30%
· modern practice
· avoids centralisation to preserve pelvic bone stock
· avoids trochanteric osteotomy to prevent reattachment problems
· minimises increasing abductor lever arm by increasing neck offset to reduce moment of bending and torsional forces
Valgus and varus positions
· valgus position of shaft
· decreases moment of bending
· decreases torsional force on stem
· increases axial loading of stem
· shortens lever arm of abduction
· places valgus strain on knee
· lengthens limb
· varus position of shaft does opposite
· mild degree of valgus preferable as varus leads to
· stem breakage
· stem loosening
· three types
· iron-based (stainless steel)
· cobalt-based
· titanium-based
· important properties are
· strength
· stiffness
· hardness
· corrosion
· biocompatibility
Strength
· strength of alloy depends on
· intrinsic strength
· manufacturing processes
Intrinsic strength
· metals can be strengthened by changing chemical composition
· called alloy strengthening
· strength of most metal alloys currently used is sufficient so that issue is largely technical or biomechanical
· exception is 316 stainless steel (early stem fractures)
· rule of thumb is
· cobalt alloy, titanium alloy, titanium, stainless steel
· 10, 8, 5, 5
Manufacturing processes
· poor manufacturing process can reduce strength
· by presence of voids and impurities
· can improve strength by work hardening
· cold working
· forging
· can adversely affect strength by surface treatments
· heat of sintering can produce voids
· application of beads, mesh or plasma spray may produce notch sensitivity
Stiffness
· depends on
· modulus of material
· geometric properties of implant
Modulus
· low modulus theoretically advantageous because it would
· reduce stress in component
· increase loading of bone
· reduce stress shielding
· high modulus theoretically advantageous because it would
· reduce stress in cement
· reduce cement failue
· rule of thumb is comparative modulus is
· cement, bone, titanium, cobalt
· 1, 10, 50, 100
Geometric
· geometric stiffness proportional to diameter4
Combined effect
· effect of modulus and geometric properties means 15 mm titanium stem as stiff as 13.5 mm Co-Cr stem
Hardness
· titanium has poor surface hardness
· can be improved by nitrogen ion implantation into surface
Corrosion
· titanium more active than chromium-cobalt thus forms passivation oxide layer more quickly
· but chromium-cobalt oxide layer more dense and more adhesive
· stainless steel most prone to corrosion
· should avoid contact between stainless steel and other metals (eg. wires around femoral stem)
· problem of contact between titanium and cobalt alloys more theoretical
Biocompatibility
Immunologic effects
· metallic ions can act as haptens and elicit type IV delayed hypersensitivity reactions
· nickel and chromium are biologically active
· titanium is bioinert
Neoplastic effects
· chromium, cobalt and nickel have carcinogenic potential
· epidemiological studies have shown increased risk of lymphoma and leukaemia in patients after total hip arthroplasty
· there are case reports of tumours in association with joint replacements
Stainless steel
· standard type is 316 type
· contains 18% chromium, 12-16% nickel, 2-3% molybdenum and balance iron
· no longer used routinely because of femoral component fracture with early designs
· problems with
· ultimate tensile strength
· corrosion
· newer alloys developed, eg. stainless steel 22-13-5 to overcome these problems
· chromium imparts corrosion resistance
· nickel facilitates manufacturing
· molybdenum imparts pitting corrosion resistance
· carbon is impurity (reflects grade)
Cobalt-based alloys
· typically contain 20-30% chromium, 5-10% molybdenum, 10-30% nickel and balance cobalt
· highest strength and stiffness of implant metals
· greater corrosion resistance than stainless steel but less than titanium
Titanium-based alloys
· titanium may be pure or combined with small amounts of aluminium and vanadium
· alloys have far superior mechanical properties to pure form
· alloys have advantages of being
· least stiff (minimises stress shielding)
· most corrosion resistant
· good strength, esp. to fatigue
· disadvantages are
· low ductility so prone to sudden failure
· poor wear characteristics so particulate metallic debris produced
· expensive
· processes used to increase surface hardness
· nitriding is formation of nitrogen compound on surface
· ion implantation is acceleration of nitrogen ions into surface to depth of 0.1 mm to create local strain on atomic level
· both harden surface
· broad class of materials
· contain metallic and nonmetallic elements
· bonded ionically in highly oxidised state
· ceramics used in orthopaedics
· alumina
· zirconia
· characteristics
· high compressive strength
· low tensile strength
· brittle
· poor crack resistance
· high surface hardness
· high surface wetability
· high surface tension
· theoretically advantageous because
· strong in compression
· low friction
· low wear
· no corrosion
· bioinert
· some disadvantages
· catastrophic failure possible
· do not tolerate impact or non-uniform loading
· difficult to manufacture concentric matching femoral heads and acetabular components
· cannot be articulated with metal or excessive wear of metal occurs
· still under evaluation for use in femoral heads
· zirconium stronger than alumina
· polymer consisting of long carbon chains
· characteristics
· tough
· ductile
· low friction
· low strength
· low hardness
· limited wear resistance
· thermoplastic
· may be produced by
· compression moulding
· machining
· sterilisation is problem
· cannot be autoclaved as causes softening and permanent degradation
· ethylene oxide sterilisation does not sterilise throughout
· high-dose radiation causes oxidisation
· usually sterilised by low-dose gamma radiation
· excellent material for articulating surface
· low friction
· main problem is particulate polyethylene debris
· criteria
1. must provide suitable range of movement
2. must transmit loads without
· pain
· bone destruction
· adverse bone remodelling
3. must subserve these functions for years
4. removable must be possible without major damage
Configuration
· curved or straight
· high rates of failure seen with curved stems
· straight instruments for canal preparation more precise
· most stems now straight
Fit and fill
· for cemented components
· initial stability achieved by matching shape of implant with shape of bone
· clinical drawbacks because
· complex shape of inside of femur precludes complete fill
· complete endosteal contact only possible with large components which are stiff and distally fixed and lead to stress shielding
Rigidity
· depends on
· elasticity of alloy
· disposition of alloy throughout stem
Cemented
· more rigid component theoretically shields cement from excessive loads
Cementless
· less stiff component theoretically transmits more forces to bone to reduce stress-shielding
Length
· no known optimum length
· should be “as long as necessary and as short as possible”
Collar
· major difference in opinion whether or not collar should be used
· Current Orthopaedics 1992
· Harris “Current data strongly support the concept that a collar is valuable”
· Ling “The use of a collar may have a number of deleterious side effects”
· potential functions of collar
· transmit load to proximal femur to prevent stress shielding
· transmit load to proximal cement column (if cemented)
· form of stop to indicate where should stop pushing stem
Collarless school
· collarless prosthesis can transmit loads from femoral head to proximal femur
· collars can not be made to consistently maintain collar-calcar contact
· collar is detrimental
· fretting between undersurface of collar and bone and/or cement produces debris
· during subsidence, collar acts as calcar pivot and stem tilts into varus
· loading of cut surface of femoral neck is not physiological and no trabeculae to transmit damped loads to proximal femur
Collared school
· presence of collar is advantageous because it
· helps to pressurise cement
· decreases micromotion in gait
· increases load transfer to proximal medial bone
· reduces strain in cement in proximal medial region of cement mantle
· careful preparation of calcar can increase calcar-collar contact
· low loosening rates seen with collared implant
Surface finish for cemented stem
· major difference of opinion with regard to whether or not stem should be bonded to the cement
· theoretical analyses provide conflicting opinions
· Clinical Orthopaedics 1992
· Harris “increasing the fatigue life of the cement-metal interface...(is) distinctly valuable”
· Ling “it is unwise to design prostheses that rely heavily on the presence of a good (cement-stem) bond”
· mechanical bond between stem and cement only possible by mechanical interlock between asperities on matte stem and cement
· precoating stem with cement enhances strength of interlock
· smooth stem has surface roughness of 0.01 um
· rough stem has surface roughness of 0.5 to 5 um
Polished school
· facts
· cement weak in tension
· cement undergoes creep
· movement beween matte stem and cement leads to production of debris
· stem-cement bond may ultimately fail
· polished stem prevents bond between stem and cement
· results are
· does not produce tensile forces in cement
· protects cement-bone interface
· takes advantage of creep of cement by stem subsidence and taper lock
· avoids production of debris
· does not rely on cement-stem bond for rigidity
Matte/precoated school
· autopsy studies have shown excellent bone-cement osteointegration with intact cement-metal interface
· extrapolated that not essential for cement-metal interface to slip to protect cement-bone interface
· significant subsidence seen with polished stems explained by creep may be due to cement failure
· extrapolated that lack of cement-metal interface may predispose to cement failure
· debonding of cement-metal interface leads to high stresses at junction of bonded-debonded interface
· precoating stem with PMMA strengthens metal-cement interface
Surface enhancement (non-cemented)
Porous coating
· concept of biological attachment of implants directly to skeleton
· porous coating allows growth of bone directly into pores
· pore diameter of 100um and 500um is optimal
· titanium shows the best porous ingrowth
· coating can be applied by
1. cast surface
· grooved, beaded or pebbled surface during fabrication
· surface has same properties of base material
· cannot create adequate porosity
2. sintering
· beads placed on surface of base material and heated
· bonds produced between beads and surface
· heating can decrease strength of base material
3. plasma spraying
· stream of particles sprayed on surface in protective high temperature gas stream
· particles melt and weld to implant on contact
· produces minimal heat related damage to base material
4. preformed structures
· pads of mesh welded to surface of base material
· shorter periods of high temperature required
· difficult to contour mesh to complex designs
Hydroxyapatite coating
· bioactive polymer with free calcium and phosphate on surface
· bone forms chemical bond with coating
· encourages bone ingrowth into metal substrate
· problems are
· debonding of hydroxyapatite from base material
· absorption of coating by osteoclast-like cells
Stem taper (cemented)
· taper necessary because of anatomical constraints of inside of proximal femur
· a few prostheses use inherent properties of taper as part of mechanism for fixation and load transmission
· in engineering practice, taper is efficient method of transmitting axial and torsional forces
Head size
· range is 22 mm to 32 mm
· features of large head
· lower dislocation rate
· greater friction
· greater volumetric wear
· features of small head
· lower friction
· higher dislocation rate
· greater linear wear
· 26 mm or 28 mm head seems to be good compromise
Offset, neck length and neck angle
· variables all related
· offset is horizontal distance between midline axis of stem and centre of femoral head
· affects lever arm of abductors
· alteration of neck length or neck angle can effectively alter offset
· misorientation of stem can alter functional offset
Modularity
Morse taper
· extensive use in industry to provide fricional rotator coupling
· consist of
· trunion (male portion)
· bore (female portion)
· tapers vary in
· diameter
· angle (from 2o to 8o)
· used to reliably join modular components
· not standardised in orthopaedic industry
· may appear to be compatible but may be completely incompatible
· longevity of connection may be affected by
· surface finish
· composition
· design tolerances
· presence of blood degradation products
· may be problems with
· fretting
· fretting corrosion
· concern regarding use of cobalt-chromium alloy for bore in contact with trunion of titanium-alloy or cobalt-chromium alloy
· esp. cobalt-chromium head on titanium stem
· unknown whether use of ceramic heads better
· ceramic heads not susceptible to corrosion
· metal male portion may still undergo corrosion
· problem with ceramic heads is hoop stresses
· may lead to catastrophic failure
· risk may be minimised by
· use of zirconia rather than alumina
· matched bores and trunions
· minimum bore size
Modularity of femoral head
· advantages
· head size and neck length (and leg length) can be altered for particular patient
· new femoral head may be fitted to sound stem in revision surgery
· disadvantages
· corrosion
· production of debris
· simultaneous variation of length and offset
Thickness and shape
· polyethylene should be at least 8 mm thick to avoid
· risk of fracture
· excessive transfer of force to cement (cemented)
· polyethylene should be symmetrical because eccentric sockets
· produce turning moment over and above that created by frictional torque
· have increased stress distribution if misorientated
Metal backing
Cemented
· introduced to provide option for exchange of worn polyethylene liner in revision surgery
· theoretical evidence that metal backing may improve absolute stress level and distribution of stress in cement and bone
· provides potential source of fretting and debris formation
· clinically associated with increased incidence of loosening
· metal backing increases price
Noncemented
· polyethylene should not come into contact with bone where there is possibility of micro-movement
· press-fit polyethylene cups developed
· good initial results
· late massive granuloma formation and catastrophic failure
· metal backed components necessary
· may be secured to bone by
· press-fit shell
· threaded shell
· threaded shells show problems with
· migration
· loss of fixation
Congruence
· between liner and shell
· inadequate congruence may lead to
· excessive stress and fracture of UHMWPE
· excessive motion and wear of UHMWPE
Extended lips
· on posterolateral wall of socket
· designed to improve stability of head within socket
· may cause impingement in full flexion and abduction
· torque thus produced may contribute to loosening
· probably does not conver real benefit to stability
Pressurisation flanges (cemented)
· two purposes
· increase cement pressurisation
· inprove accuracy of cup in acetabulum
Screws and spikes (uncemented)
· noncemented metal liner uses press-fit for initial bone-metal fixation
· screws and spikes used to augment fixation
Screws
· clearly improve initial fixation
· can pose hazard to neurovascular structures
· produce problems later
· subsidence of cup leads to screw protrusion and wear of polyethylene liner
· screw-liner interface is source of fretting
· unfilled holes for screw fixation allow pathway of polyethylene particulate debris from liner-polyethylene interface into bone-liner interface
· polyethylene may creep into unfilled holes and lead to early failure of liner
Spikes
· may produce initial inability to obtain
· proper alignment
· adequate seating
· satisfactory function requires adequate fixation
· fundamental facts about fixation
· no such thing
· always some movement
· two types of movement
1. reversible displacement
· recoverable movement between loaded and unloaded statesof implant
· may be as much as 40 um (human hair)
2. migration
· relative movement over time that is not recoverable
· depends on establishment of initial mechanical interlock between implant (including cement if applicable) and bone
· for initial fixation is on surgeon at time of operation
· determines amount of movement between implant and bone
· amount of movement determines type of junctional tissue between implant and bone
· inevitably produced in peri-implant zone during preparation
· at conclusion of operation, implant fixed to dead bone
· induced by zone of bone necrosis
· first step in production of junctional tissue
· junctional tissue may be
· direct contact between living bone and implant (osseointegration)
· fibrous tissue between bone and implant
· both may be present
· osseointegration is mechanically and biologically superior
· of peri-implant necrosis important
· controlled by movement in early post-operative period
· bony healing only occurs if amplitude less than 50-100 um
· continue after insertion of implant
· mechanical stress is major stimulus for maintenance of skeletal mass
· bone continues to turn over
· factors that influence turnover may influence continuing implant fixation
· to think “cement vs cementless” “may well miss the essence of the problem, because both cemented and uncemented implants have been shown to be anchored in bone, provided proper healing conditions prevail
· cement generates controversy
· ten-fold difference between best and worst published figures for cemented femoral loosening
· indicates that failures of cemented arthroplasty may be blamed on cement
· “cement disease” emerged in 1987
· cement accused of
· killing bone chemically and thermally
· inevitably producing fibrous tissue
· being not strong enough and too brittle
· causing cardiovascular collapse
· attracting macrophages
· interfering with leukocyte function
· triggered by release of studies showing poor results of
· Charnley’s cemented cups at 10 years
· cemented arthroplasty in younger patients
· presumed that cement lead to osteolysis associated with loosening
· move towards development of cemented prostheses
· theoretical advantages
· no cement to produce osteolysis
· direct ingrowth of bone into implant to transmit forces
· problems seen
· thigh pain
· osteolysis in absence of cement
· inadequate bone ingrowth because of difficulty of achieving initial interlock to prevent early interface movement
· refinement of cementing techniques capable of improving results
· primarily with femoral component
· less satisfactory outcomes in acetabulum
· development of concept of “hybrid hip”
· cemented femoral component and uncemented acetabular component
· need longer followup of uncemented cups (at least 10 years)
· referred to as a “grout”
· has two functions
· fixation of implant to bone
· transmisson of loads to femur and pelvis
· initial fixation depends on mechanical interlock between bone and cement
· unique feature of cement is its ability to take detailed cast of endosteal surface
· concept of perfectly customised implant
· is self-curing poly methyl methacrylate
· supplied in
· 40g pack of powdered polymer granules
· 20 g phial of liquid monomeric methyl ester of methacrylic acid
· powder contains
· radio-opacifier (zirconium oxide or barium sulphate)
· benzoyl peroxide as activator to free-radical-assisted polymerisation reaction
· fluid contains
· teritiary amine as initiator
· ascorbic acid as inhibitor to prevent spontaneous polymerisation of monomer
· powder and fluid mixed together
· produces dough
· monomer takes polymer into solution
· benzoyl peroxide causes polymerisation of monomer
· viscosity of dough increases
· heat given off (exothermic reaction)
· dough changes from fluid to solid
· variable temperature-sensitive time
· amorphous polymer which is
· weak in tension
· strong in compression
· elastic modulus between that of cancellous and cortical bone
· no adhesive properties
· strength affected by
· laminations
· entrapped blood
· attempts to improve tensile strength have been attempted
· vacuum mixing and centrifugation
· increased laboratory tensile strength
· unclear effect on longevity of cemented hip arthroplasty
· has viscoelastic properties at body temperature
· this confers on it a number of properties
· load spreading
· shock-absorbing
· decoupling
· monomer contracts when it polymerises
· contracts 20%
· monomer makes up 1/3 of cement
· leads to 6-7% contraction of cement
· rise in temperature causes thermal expansion
· of cement itself
· of air or vapour in voids in dough
· overall effect is small overall shrinkage
Local and systemic toxicity
Local
· monomer cytotoxic experimentally
· shown that chemical trauma of cement adds nothing to mechanical and vascular trauma of preparation of implant site
Systemic
· monomer implicated in adverse cardiovascular effects
· esp. hypotension
· may be due to embolisation of fat and inflammatory mediators into circulation during increased intramedullary pressure
Heat production
· implicated in thermal necrosis at bone-implant interface
· shown that heat of polymerisation incapable of producing significant rise in temperature locally at interface
· due to volume of cement and rate of heat dissipation
Tissue reaction
· thought that histological reaction to cement was formation of fibrous tissue
· retrieval studies have shown direct contact between bone and cement in well-fixed specimens many years postoperatively
· preservation of some cancellous bone
· to allow interlock
· achievement of femoral pressurisation
· into closed space
· with proximal pressurisation
· creation of full bony cover for acetabulum
· avoidance of contamination of interface by bleeding
· delayed component insertion when viscosity high enough to stop bleeding
· relies on creation of an optimum press-fit
· achieved by
· careful preparation of bone
· accurate sizing and alignment of prosthesis
· initial stability vital because micromotion causes failure of ingrowth
· porous coated surfaces allow direct bone ingrowth into pores
· experimentally, ingrowth starts at 2 weeks and is maximal at 12 months
· problems of porous coatings are
· debonding of coatings with loss of stability and production of wear particles
· weakening of implant due to process of porous coating
· variable degree of ingrowth
· osseointegration enhanced with hydroxyapatite
· similar problems of debonding
· unique to cementless hips
· cause unclear
· may be related to
· instability of interface
· stiffness of implant
· loosening defined as failure of adequate fixation rather than the presence of any movement
· the amount of movement that distinguishes loose from not loose is 40-500 um
· occurs when stresses applied to junctional tissues and supporting bone persistently exceed their strength
· early loosening due to
· defective primary surgery (failure to achieve fixation)
· gross overload
· infection
· late loosening due to osteolysis
· loss of material from solid surfaces as a result of mechanical action
Mode 1
· two primary bearing surfaces rubbed together as intended
· seen in femoral head and inside of acetabular socket
· least spectacular but probably most important
Mode 2
· primary bearing surface rubbed against secondary non-bearing surface
· seen if femoral component of knee prosthesis penetrates polyethylene tibial plateau and contacts underlying metal tray
Mode 3
· third-body particles become entrapped between primary bearing surfaces
· eg. PMMA fragments, bone chips, metal fragments
· may cause
· direct abrasive wear
· roughening of bearing surfaces with increased rate of wear
Mode 4
· two secondary non-bearing surfaces rub together in manner not intended
· may include screw-shell fretting, shell-socket fretting, neck-socket impingement, stem-cement fretting, porous coating-bone fretting
Adhesive wear
· results when interatomic forces between wear couple become greater than intrinsic forces between molecules of bulk material
· material thus adheres to opposite surface of wear couple
· primarily affects UHMWPE
· transferred to harder surface
· subsequently shed into joint space
· can also affect passivated layer of oxide on alloy
· oxide transferred to UHMWPE
· becomes free oxide polishing powder
Abrasive wear
· analogous to production of sawdust on surface of wood by sandpaper
· soft surface abraded by rougher harder surface
· abrasive wear
· proportional to surface roughness
· inversely proportional to surface hardness
· loss of substance called two-body wear
· when third substance interposed between two surfaces, called three-body wear
Fatigue wear
· results from repeated loading of contacting surfaces
· cyclic stresses initiate and propagate surface cracks
Fretting wear
· loss of material from contacting surfaces as they undergo oscillatory tangential displacement of small magnitude
· result of adhesion and/or abrasive wear
· every coating and every surface is a potential source of debris
Polyethylene wear
· acetabular cups undergo adhesive and abrasive wear
· linear wear estimated to be 0.1 mm/year
Wear of metallic articulating surfaces
· passivation layer has low shear strength
· oxide particles may be released
· titanium oxide more susceptible than chromium oxide
· release of oxide particles can
· interact with polymer and accelerate polyethylene adhesive wear
· form irregular metallic surface and facilitate two-body polyethylene abrasive wear
· clump and facilitate three-body polyethylene abrasive wear
Wear of stems
· due to fretting of
· stem-cement interface
· porous coating-bone interface
Corrosion
· see above
· leads to loosening
· results from particle-induced bone resorption
· three important factors
· generation of wear debris
· access of debris to implant-bone interface
· biological reaction to wear debris
· as described above
· sources are
· polyethylene articulation (most important)
· modularity
· other, esp. abrasion at metal-cement interface (cemented) and at metal-bone interface (uncemented)
· majority of particles are < 1um in size
· concept of effective joint space
· all periprosthetic regions accessible to joint fluid and therefore to wear debris
· access can be via
· partial proximal porous coating
· cement mantle defect
· interface membrane develops
· is classic foreign body granuloma
· polyethylene, metal, cement debris found in cells within membrane surrounding loose implants
· cells consist of
1. macrophages (50-80%)
· scavenger cells
· phagocytose particles <10um
· release cytokines after phagocytosis
· cytokines initate bone lysis
· most common cytokines are prostaglendin E2 and interleukin-1
2. fibroblasts (10-30%)
· reparative cells
· produce collagenous matrix to wall off granulam
3. giant cells (0-15%%)
· coalescence of macrophages
· surround and phagocytose larger particles
· not very active wrt. cytokine production
4. endothelial cells (5-10%)
· vascular cells
· role unclear
5. T lymphocytes (0-10%)
· may modulate response
· role unclear
6. osteoclasts (5%)
· decalcify bone
· activated by interleukin 1
· proposed pathogenesis is
· production of wear particles
· phagocytosis and cytokine production
· osteoclast activation and bone resorption
· increased implant micromotion and wear
· increased production of wear particles
· main problem of hip replacement is loosening from osteolysis
· to overcome this problem, must understand concepts of
· forces in hip arthroplasty
· characteristics of biomaterials
· aspects of prosthesis design
· implant fixation
· wear
· best current combination may be
· cemented polished collarless stem
· zirconia head
· uncemented press-fit cup
Brittle
· sustains little or no plastic deformation prior to fracture
· opposite to ductile
Centre of gravity
· the point at which the mass of an object is thought to be concentrated
· the point at which any object is balanced exactly
Cold working
· mechanical deformation of material beyond its yield stress
· result is
· improvement in yield and ultimate strength
· reduction of ductility
Component of a force
· portion of a force acting in a particular direction
Corrosion
· release of ions and compounds as a result of chemical action
Uniform
· corrosion of surface of metal
Galvanic
· when two different metals placed in contact in electrolyte
· electrons move from base to noble metal
· base metal becomes anodic and more susceptible to corrosion
· noble metal becomes cathodic and more resistant to corrosion
Stress
· tensile side of metal is anodic and prone to corrosion
Crevice
· surface defect in form of crevice or crack leads to relative oxygen depletion
· defect becomes anodic and corrosion facilitated
Pitting
· surface defect in form of pit or impurity leads to similar anodic degradation
Fretting
· abrasive wear accompanied by corrosion
· protective oxide layer removed by abrasion and corrosion can occur
Creep
· plastic deformation at stresses below yield point
· deformation is time- and temperature-dependent
Ductile
· may be deformed permanently without fracture
· ie. can be drawn into wire or rolled into sheet
Forging
· gradually shaping a cast piece of alloy into final shape by successive compression between mating negative dies
· can be hot or cold
· produces
· increased strength
· reduction in grain size
Fretting
· loss of material from contacting surfaces as they undergo oscillatory tangential displacement of small magnitude
Hardness
· ability of material to resist plastic deformation at its surface
· not able to infer hardness from other properties
· because materials have different properties at their surface
· measured by pressing indentometer with known load into surface and measuring resulting impression
Modulus of elasticity
· syn. Young’s modulus
· measure of rigidity or stiffness
· calculated by dividing load (stress) by amount of deflection (strain)
· high modulus is stiff; low modulus is pliable
Moment of bending
· measure of bending intensity created by a force
· calculated by multiplying force by its lever arm
Passivation
· formation of insoluble salt (usually oxide) on metals
· oxide layer inhibits metal egress and thus inhibits corrosion
Plastic deformation
· deformation caused by shear stress
· results in permanent changes in shape of sold
Stiffness
· resistance to strain (deformation)
Strain
· force per unit area
Strength
· maximum resistance to stress before failure
Yield strength
· stress necessary to cause plastic deformation
Fatigue strength
· stress that with repeated applications will cause breakage
Ultimate tensile stength
· maximum stress a material can withstand before breaking
Stress
· amount of deformation (percent elongation) compared with original dimension
Torque
· twisting moment
· force times lever arm
Torsion
· forces applied tending to rotate an object about its long axis
· ie. twisting
Toughness
· energy necessary for fracture
· area under stress-strain curve
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