osteolysis

Introduction

Definition

Osteolysis

· dissolution of bone

· pathological process causing loss of bone adjacent to prosthesis

· may be linear or focal

Loosening

· failure of adequate fixation

· early loosening due to

· defective primary surgery (failure to achieve fixation)

· gross overload

· infection

· late loosening due to osteolysis

significance

· loosening and periprosthetic bone loss currently most important complications of THR

· INFECTION
ß

· STEM BREAKAGE
ß

· LOOSENING / LOSS OF BONE STOCK

· osteolysis is central mechanism for this

· combination of mechanical and biological factors

· WEAR (MECHANICAL)
ß

· OSTEOLYSIS (BIOLOGICAL)
ß

· LOOSENING

history

Teflon

· osteolysis was first of major problems of THR

· first design of Charnley in 1958 involved PFE (polytetrafluoroethylene, Teflon) acetabulum

· very low coefficient of friction with stainless steel

· spectacular early results

· disastrous complications

· failure rate 95% in 2 yrs

· abandoned in 1961

· failure due to

· poor wearing properties of PFTE

· response of body to PFTE wear particles

· next step was to improve wear characteristics of Teflon

· filled with glass fibre

· enhanced wear by factor of 20 in vivo

· poor results in vitro

· wore just as quickly

· filler acted as abrasive

UMWPE

· introduced by Charnley in 1962

· principle was to find material with

· low coefficient of friction

· high wear resistance

· UMWPE had

· very high wear resistance

· low coefficient of friction (but 5 times higher than PFTE)

· decreased coefficient of friction under high stress

· capable of boundary lubrication by synovial fluid

· lysis reported by Charnley in 1968

· attributed to culture-negative sepsis

Particles and macrophages

· association between osteolysis and particle wear and macrophages first made by Harris in 1976

· extensive localized bone resorption within femur after 4 THRs

· amount and location of resorption suggested the presence of infection or tumor but no evidence of either condition

· a reoperation the femoral components were not rigidly fixed but were only slightly loose

· histologically sheets of macrophages, a few giant cells, and multiple small fragments of a birefringent material, but no inflammatory cells

· benign, non-inflammatory, adverse tissue response can occur in relation to the femoral components of total hip replacements

Cement disease

· osteolysis attributed to cement by Hungerford and Jones in 1987

· bony environment surrounding loosened cemented prosthesis is abnormal pathologic condition which will progress to total failure

· biomaterial properties of cement used for fixation contribute to the pathologic state

· "cement disease" does exist

· cement satisfactory for elderly patients with low activity levels and relatively short life expectancies

· unlikely that methyl methacrylate can be rendered satisfactory in the long run for the young, the active, or the overweight patient

· elimination of "cement disease" can only occur with the elimination of cement

· alternatives include development of prostheses with satisfactory surfaces for either press-fit or biologic ingrowth

Cementless prostheses

· major innovation of 1980s was development of cementless prostheses

· osteolysis associated with cementless components first reported by Maloney / Jasty / Harris in 1990

· focal femoral osteolysis after THR without cement

· histological specimens contained focal aggregates of macrophages with particulate polyethylene and metallic debris

· osteolysis not identified less than two years postoperatively

· in most patients, osteolysis appeared after three years

· femoral osteolysis can occur around uncemented components

Particle disease

· current thinking

· mechanical wear Þ particles Þ biological osteolysis

Basis of osteolysis

Mechanism of wear

Definition

· loss of material from solid surfaces as a result of mechanical action

Modes of wear

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 penetrates polyethylene liner and contacts underlying metal shell

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

Types of wear

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

· occurs with aging UHMWPE

Fretting wear

· loss of material from contacting surfaces as they undergo oscillatory tangential displacement of small magnitude

· result of adhesion and/or abrasive wear

Wear of biomaterials

· 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

· 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

Other sources of debris

· Morse taper

· silicates used in manufacture of UHMWPE

· surgical instruments and wires

Biology of osteolysis

Implant-bone interface

· investigated with autopsy studies

Stable implants

· Kwong & Maloney (1989) harvested femora after THR

· tested mechanical motion of bone under load

· some implants showed

· secure fixation (no micromotion)

· radiolucent lines in x-ray

· intimate contact between cement and endosteal bone

· radiolucent lines

· not due to to formation of fibrous tissue

· represented remodelling of cement-bone interface

· second medullary canal developed between inner and outer cortices

Loose implants

· Goldring (1986) studied histology radiolucent areas in loose components

· histology of tissue surrounding the loosened components revealed presence of synovial-like lining adjacent to cement

· tissue heavily infiltrated with particulate cement and polyethylene

· associated with a foreign body-type giant cell reaction

· postulated that this tissue response may be responsible for the bone lysis associated with loosening

· tissue culture medium of fragments from membrane showed

· high prostaglandin E2 levels

· enhanced bone resorbing activity

· similar membrane found in cemented and cementless components

Membrane

· membrane has features of foreign-body granuloma

· macrophages

· fibroblasts

· giant cells

· few inflammatory cells (incl. lymphocytes)

· forms in response to material resistant to degradation or removal

· Kim (1993) found that membrane somewhat different depending on type of implant

· in cementless membranes

· more metallic debris (more exposed metal)

· more macrophages (metallic particles smaller)

· in cemented membranes

· more polymeric debris (cement and polyethylene)

· more giant cells (polymeric particles larger)

· Schmalzried (1992) found membrane at great distances from articular surface

· particulate-laden macrophages found in areas of linear and lytic bone loss

· particles of polyethylene and metal

· areas as far away as distal tip of prosthesis

· thought to be due to access of joint fluid to distal periprosthetic region

Animal studies

· inertness relative

· even materials inert in bulk form can elicit inflammatory reaction

· bulk material well tolerated

· no inflammation secondary to bulk PMMA implant

· bone can grow into PMMA

· foreign-body reaction to particulate PMMA

· metal can induce inflammatory response

· Cr-Co particles causes activation of macrophages

· small particles induce more inflammation

· particles < 20 microns induce greatest response

· macrophages stimulate bone resorption when they phagocytose particles

· macrophages activated by phagocytosis of inflammatory particles

· activated macrophages stimulate osteolysis

· immune system does not play important role

· no effect of absence of T and B cells

Osteolysis

Creation of particles

· particles produced by variety of means

· mainly secondary to abrasive wear

· UHMWPE probably most important

· common to cemented and uncemented prostheses

· small particles most damaging

· particles > 10 microns cannot be injested by macrophages

Migration of particles

· debris dispersed in joint fluid

· limits of joint fluid determined by intimacy of contact between prosthesis and bone

· joint fluid follows path of least resistance

· areas that joint fluid reaches are part of joint space

· termed effective joint space

· flow of fluid produced by increased pressure in joint space

· intact barrier at interface of metal and cement may retard access of particles

· access permitted by

· defect in cement mantle

· incomplete proximal porous coating or ingrowth

· particles can also migrate out of periprosthetic space

· particles of metal and cement can contribute to wear

Phagocytosis of particles

· debris phagocytosed by macrophages

· larger particles phagocytosed by confluence of macrophages (giant cells)

· macrophages unable to digest particles

· respond by releasing cytokines

· activation of macrophages is function of number, type and size of particles

· particles < 10 microns induce most activation

Activation of macrophages

· macrophages release

· prostaglandin E2

· interleukin-1

· platelet-derived growth factor

· tumour necrosis factor -alpha

· collagenase

· PG E2 induces osteoclastic activity

· IL-1 induces production of collagenase and prostaglandins

· PDGF induces fibrous tissue formation

· TNF alpha activates osteoclasts

· collagenase removes osteoid layer from calcific bone and prepares surface for osteoclastic attachment

· IL-1 probably plays central role

Result

· end-result is

· formation of interface membrane

· containing activated macrophages

· releasing cytokines capable of causing bone lysis

Spectrum

· aseptic loosening and focal osteolysis are continuum of biological response to wear debris

· loosening can occur without focal osteolysis

· focal osteolysis can occur in stable implants

Incidence of osteolysis

cemented prostheses

Femoral component

Cementing technique

First-generation

· cement finger-packed

· no medullary plug or cement gun

Second-generation

· medullary plug

· low-viscosity cement

· introduction with cement gun

Third-generation

· cement pressurisation

· reduction of cement porosity

· centralisation of stem

Effect

· studies vary significantly

· trend observed

· Sutherland (1982) followed 100 THRs after 10 yrs using first-generation technique

· incidence of aseptic loosening was 40%

· Mulroy (1990) followed 105 hips for minimum of 10 yrs using second-generation technique

· incidence of femoral loosening was 3%

· Poss (1993) compared same stem but different cementing techniques

· incidence of femoral loosening was

· 24% at 4 yrs with first-generation technique

· 0% at 8 yrs with second-generation technique

Cement mantle

· cement mantle reviewed by Maloney (1990)

· area of osteolysis usually corresponded to either a defect in the cement mantle or an area of very thin cement

· microscopic focal cement fracture found in area of lysis

· considered that local fragmentation was stimulus for local osteolysis in an otherwise stable cemented femoral component

· Anthony / Ling (1990) reported effect of defect in cement mantle

· 4 cases of localised endosteal bone lysis in the femur occurring with cemented femoral components not obviously loose on x-ray

· area of lysis shown at operation to be related directly to region with local defect in the cement mantle surrounding the stem

· defects provide route through which the contents of the joint cavity may reach the endosteal surface of the femur

· subsequently leads to localised bone lysis and later to frank loosening

Acetabulum

Mechanism

· Schmalzreid (1992) looked at acetabular osteolysis in autopsy specimens

· begins circumferentially at intraarticular margin

· progresses toward the dome of the implant

· leading edge of membrane is a transition zone from regions of membrane interposition between the cement and the bone to regions of intimate cement-bone contact

· mechanical stability of implant determined by extent of bone resorption and membrane formation at cement-bone interface

· progressive bone resorption result of the macrophage inflammatory response

· no evidence in support of a mechanical basis for failure of

· Garcia-Cimbrelo (1992) reviewed 680 LFAs after minimum 12 yrs

· total cumulative probability of loosening of 19%

· early loosening associated with

· deficient structure of the bone of the acetabulum

· a previous congenital dislocation of the hip

· acetabular fracture

· acetabular protrusion

· late loosening was associated with the depth of acetabular wear

Metal backing

· metal backing initially designed to allow replacement of the polyethylene liner

· became universally accepted as a means of decreasing peak stresses at the interfaces of bone and cement

· Cates et al (1992) examined radiographic polyethylene wear in 233 cemented THRs with metal-backed or non-metal-backed cup

· mean linear wear rate was 0.11 mm/yr in metal-backed sockets and 0.08 mm/yr in non-metal-backed sockets

· mean volumetric wear rate was 66.2 mm3/yr in the metal-backed sockets and 48.2 mm3/yr in the polyethylene sockets

· addition of metal backing to a cemented acetabular cup resulted in a 37% increase in mean polyethylene wear rates

· explained higher failure rate of cemented metal-backed cups

· advocated use of an all-polyethylene cup in cemented THR

Cementing technique

· incidence of acetabular loosening has been only slightly influenced by improvements in cementing techniques

· difficult to adequately pressurise acetabulum

cementless prostheses

Femoral component

· minimum 4-yr followup required to identify effect

· lysis rarely appears before 2 yrs

· AML prosthesis is typical

· initially no lysis reported

· at 5 yrs, 22% incidence of lysis

· at 8 yrs, incidence of 42%

· comparable figures for other prostheses

· Goetz (1994) performed prospective study

· cemented vs cementless stem

· incidence of lysis at 4 yrs was 0% in cemented stem and 29% in cementless stem

· thought that cement seals off femur and delays ingress of particulate polyethylene matter

· may be increased metallic debris with cementless stem

Acetabulum

· high incidence of lysis reported

· Kim reviewed AML cups at 7 yrs

· found 36% incidence of acetabular lysis

Backing

· Maloney (1992) looked at all-polyethylene uncemented cups

· early loosening and osteolysis in most cases

· conclusion that uncemented, nonmetal-backed polyethylene acetabular components should not be used

Fixation

· Schmalzreid 1994 followed 122 press-fit cups for 4 yrs

· no loosening or revision

· better than previous screw fixation

Holes

· Maloney (1993) reviewed 14 cases of osteolysis in uncemented cups

· 11 had unfilled holes in metal shell

· thought that may have acted as a conduit through which wear debris could gain access to the implant-bone

Interface

· Schmalzried (1994) studied liner-metal interface

· empty secrew holes contained granulomatous tissue and polyethylene and metal debris

· back surfaces of the PE liners showed surface deformation, burnishing, and embedded metal debris

· screws demonstrated fretting at the base of the head and on the proximal shaft

· concluded that non-articular modular junctions create new interfaces for the generation of particulate debris, which may cause granulomatous reaction

Thickness

· Amstutz (1992) reported 10 cases of catastrophic failure of the polyethylene liner

· failure occurred as a result of either 'wearthrough' to the metal backing, liner fracture or a combination of both

· seen only in cups with polyethylene thickness <5 mm

modularity

· Urban (1994) studied migration of corrosion products from modular prostheses

· identified corrosion at tapered interface between head and neck

· found corrosion products at articular surface and along membranes at bone-implant interface

· found granulomatous response to corrosion product

Prevention of osteolysis

femur

· second- or third- generation cementing techniques

· distal plug

· cement gun

· centralisation

· thick complete cement mantle

· at least 2.5 mm

· minimise modularity

· consider monobloc design

acetabulum

· decision more difficult because of lack of long-term data

· either

· cemented all-polyethylene cup

· cementless metal-backed cup

· use polyethylene that is

· thick (> 6 mm)

· hemispherical

· free of defects

· avoid 32 mm head

· thinner liner

· greater volumetric wear

· use head with good wearing characteristics

· avoid titanium head

· consider ceramic head

· if uncemented, consider

· press-fit

· congruous liner-shell interface

· stable liner

· avoidance of screws and screw-holes

·