Total hip replacement—Which type?

/ Adult

Total Hip Replacement-Which

Type?

G. C. Bannister

Introduction The surgeon offering total hip replacement is faced with a bewildering array of prostheses, the design rationale of which is generally unsupported by longterm clinical experience. Indeed. the surgeon who wished to emulate the results obtained by Charnley with his original flat-backed stem could not do so now as the implant has not been manufactured since the mid-1970s. The results of most prostheses that succeeded the original flat backed Charnley have. with the possible exception of the polished Exeter stem. produced less good results. As none of the stems currently on the market has remained unchanged for 15 years. surgeons have to choose their implants on the basis of the method of fixation and design.

Basis of Prosthetic Fixation All hip replacements secure fixation to bone by interference fit. This may be achieved with or without bone cement. Without cement, fixation depends on the tight apposition of metal or plastic to bone. As the femoral shaft is curved in the sagittal plane and most stems are straight. this fit is incongruent and interference occurs unevenly at points of contact. Cement does not bond chemically to bone, rather entering between the trabeculae enhancing the interference between the implant and bone by recruiting a greater surface area for the biological interface.

Basis of Prosthetic Design Prosthetic design perience stnrting

has evolved from empirical exwith Philip Wiles in 1938. who

Gordon C. Bannister RIChOrth. FRCS Ed (Orth), FRCS. Consultant Orthopaedic Surgeon. Southmead Hospital. Westbury-on-Trym. Bristol BSIO 5NB. UK. __-___ -__-

resurfaced both the acetabulum and femoral head with metal. McKee, in 1940. adopted the same approach, but only 50% of cases were successful. McKee employed F. R. Thompson’s prosthesis (1948) and a cobalt chrome socket which when secured to bone with polymethylmethacrylate. constituted the first viable hip replacement. Sir John Charnley’s first attempts at hip replacement involved resurfacing the head and acetabulum with teflon sheeting. This initially failed because of aseptic necrosis of the femoral head and foreign body reaction when the material was used as the acetabular component. Reduction in head size and the adoption, in 1962. of a high density polyethylene acetabular component resulted in the implant which forms the basis of modern designs. The Charnley produced consistent results. in part, because the prosthesis was not sold to any surgeon who had not attended an instructional workshop in its use. Surgical access by detaching the trochanter w;as the cause ofsome morbidity and in 1967 Muller developed the curved stem prosthesis. which could be implanted via a modification of Watson-Jones’ antero-lateral approach. The Charnley stem was straight in its distal ?/?, the Muller had a 32 mm head and the Charnley __ 77 mm

Complications The McKee-Farrar prosthesis implanted in its inventing centre of Norwich gave excellent results for 13 years. but produced a metallic sludge and was subject to early aseptic loosening elsewhere. It was abandoned in favour of metal on polyethylene prostheses of which the Charnley and curved stem Muller were the most widely used. The principal complication of the early first generation prostheses was stem fracture and aseptic loosening. Both of these related to the integrity

166 CURRENT ORTHOPAEDICS-ADULT of the supporting cement mantle. Weber and Charnley (1975)l and Chao and Coventry (1981)” both found stem fracture to be associated with inadequate proximal cement support. Beckenbaugh and Ilstrup ( 1978)3 noted increased radiographic loosening when the proximal cement mantle was less than 5 mm. The response to stem fracture was to change the material and design of the flat backed Charnley stem. Stainless steel was changed to Ortron and the prostheses was broadened, which made it more rigid, in 1976. The addition of a collar became fashionable in North America and cement technique was refined as a result of work carried out in Exeter” and Montreal.’ The bulkier, more rigid prostheses and change in cementing technique were designated ‘second generation’. Second generation technique involved cleaning bone. restricting the femoral medullary canal and pressurising cement. Harris from Boston published a series of hip replacements of mixed stem geometry, but standardised second generation cement technique after a mean of 3.3, 6.2 and 11.2 years.” Of 234 patients, 63 were lost after 3.3 years, 117 after 6.2 and 129 after 11.2. After 11.2 years the revision rate of the femoral stem for aseptic loosening was 3 %. 3 % of prostheses had migrated, 24% demonstrated a radiolucent line around 50 % of their cement-bone interface and 7 % exhibited endosteal bone lysis. These results are significantly better than the curved stem Muller performed in Cleveland, Ohio and in Scandinavia and the Charnley at the Mayo Clinic. The revision rates are comparable to those reported in the Charnley in Cape Town and slightly inferior to the Charnley at Wrightington and New York after 10 years and the Exeter in Exeter’ after 15 years. 7 of Harris’ cases demonstrated a radiolucent line at the cement bone interface after 3.3 years, but it appears they may have been lost to subsequent follow-up as only 2 cases presented after 6.2 years and there is a general agreement that once present, radiolucent lines persist. It is difficult to conceive that second generation cementing technique could be inferior to first, but there is evidence that changes in stem design were disadvantageous. Harris’ second generation stem was undoubtedly bulkier than the flat backed Charnley. The CAD, a broad valgus collared stem, fared worse than the flat backed Charnley at the Hospital for Special Surgery. New York8 and indeed there was a greater loosening rate amongst second generation Charnley stems in Cape Town.’ A stricter comparison of second generation cementing technique was undertaken in BostonlO when the T28 was compared when inserted by first and second generation cementing techniques. There was a higher incidence of migration, cement mantle inadequacy and radiolucent defects in first generation cases. The Exeter group examined the effects of second generation cementing technique on cup fixation’

recording a significantly lower incidence of cup migration over the first 11-13 years and a fourfold reduction in revision. It would appear that second generation cementing techniques do offer advantages, but that long-term these may be outweighed by stem geometry. The Exeter experience in acetabular component migration indicated that after 13 years migration rates were comparable in implants inserted using both first and second generation techniques, suggesting that cementing technique buys time. As 40% of patients are dead within 15 years, good cementing technique should delay the onset of aseptic loosening and reduce revision rates, but in younger patients it cannot be expected to compensate for unfavourable prosthetic design.

Cementless Hip Replacement Aseptic loosening, and in particular endosteal bone lysis, encouraged re-examination of cementless total hip replacement. In 1958 Peter Ring used an uncemented Moore stem with a polyester socket. The socket failed and in 1964 a conical threaded vitallium acetabular component was developed with instrumentation to position it exactly in the anatomical axis of the pelvis. Whilst initially the polyester cup had been the main source of failure, once the acetabular component had been developed, the Moore stem became the weak point. Some 33 % of Moore stems were revised compared with 7% of bulkier, longer Ring modification of it. The Ring prosthesis was satisfactory in 91% of cases after 5 years when implanted by the inventor. The results of the prosthesis in Ring’s hands compared favourably with any cemented series, but when implanted in independent centres only 75 % gave acceptable results. The same phenomenon was observed with the Lord Madreporic prosthesis, which had a roughened surface into which bone could grow. The Ring had the advantage of easy revision and results after revision of a loose Ring to cemented prosthesis were indistinguishable from primary arthroplasties. The problem with bone ingrowth was that it tended to take place at tht site of greatest stability, which was at the isthmus. Ring reported six cases of failure in which the prosthesis was sufficiently well-fixed that it could not be easily removed, yet was associated with massive proximal bone lysis and pain. More recent developments in cementless hip replacement have been to apply porous coating proximally in an attempt to ensure fixation at that site. For reasonable prospects of ingrowth close apposition of metal to bone is required, yet existing cementless hip arthroplasties can only derive stability from 3 point fixation because of the wide variation of intramedullary anatomy. Even the 8 sizes of the porous coated anatomic stem demonstrate discrepancies between stem and bone of between 0.8 to 2.2 mm and as the optimum pore size for bone ingrowth is 100 micrometres, this suggests that ‘off

TOT.AL HIP REPLACEMEhT-

the shelf’ prostheses used in current practice can expect only partial ingrowth at best. More recent designs emphasize these difficulties. The Harris-Galante stem was designed as part of a universal system for use in both cemented and uncemented form by two of the most prolific contributors to hip replacement literature in North America. After 5 bears. the uncemented stem required revision in 10 O.0. gave thigh pain in 28 91,. demonstrated proximal loosening in 33 “10 and exhibited calcar resorption of more than IO mm in 67 9 o.l’ The collar of the Harris-Galante stem prevented subsidence of the prosthesis into a more stable position. yet in the collarless porous coated anatomic such migration was associated with thigh pain, stabilisation did not occur and the incidence of symptoms increased with time. Porous coating increases the surface area for not only bone mprowth, but release of metal ions. The surf&es of these are oxidised and corrosion may occur with granuloma formation and bone destruction. This phenomenon occurs relatively more frequently in porous coated implants. but has been demonstrated in the smoothed surfaced Moore. Studying failure of the cementless Harris-Galante stem, Jast;, et al ( I991 ),I’ concluded that there was poor osseo-integration in the proximal metaphysis .lnd thal the prosthesis should not be used in heavy .tctive males. Whilst this conclusion is undoubtedly sound, it poses considerable difficulties in reconciling rhe place of this type of implant as young, active males .tre precisely the group of patients most likely to mount a biological response to implanted material and enhance its fixation. The difficulties in establishing design principles. the wide differences between inventor and independent practitioner and the continuing failure of tincemented stems, make them an unattractive proposition to the genera1 orthopaedic burgeon seeking reliable routine results. Cementless fixation is biomechanically more demanding in the femur than the acetabulum because the differential elasticit! of metal and bone dictates that movement betweelr the two Inevitably takes place.“’ the same basic principle to acetabular :Ippl>,ing fixation. there should be greater prospects of achieving osseo-integration because the length of the interface is \ ery much smaller. Charnley ( 1979)” originally perceived that cementless cup fixation would be feasible, although he had to abandon his metal backed polyethylene acetabular component when a significant proportic)n became loose.

Radiological

Failure

If surgeons cannot base their choice of prosthesis on clinical outcome in the form of revision rate. their only means can be by predictive factors, such as radiographic features. Such data are available usually and late anteroposterior radioon peri-operative gaphs. On peri-operative radiographs radiolucent

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lines between the cement and bone.’ ’ bat-us orientation’ and deficiency in the cement mantle all favour revision. The deficiency in the cement mantle in the region of the calcar femorale is strongly associated with stem fracture.’ There is Jmost no e\,idence to support the concept that collar~+tlcar contact enhances stem fixation. On late radmgraphs the appearance of lucent lines at the cemenr&bone interface. stem-cement dissociation ,.md distal migration of prosthetic components either at the prosthesis-cement or the cement -bone Interface “’ arr all associated with increased rates 01 revision. Progressive migration into varu\. calcm resorption” and endosteal bone 1~s~ are likewise strongly associated with increased re\ i4on rates. The vast majority of femoral component> perform best if they arc tixed stably at the time of surgery and remain in that position indetinitelq. Harris et ~1 ( 1982)” defined loosening as definite if there is ‘~1) prosthetic migration. probable. if a lucent line surrounds the cement-bone interface, and possible. iftherc i\ a lucenl line around 50 ‘,o of the cement- bone interface. This ix ;I useful classitication, but not unicersull> applicable. as it omits the important complication of cndosteal bone l\si> and is incompatible with the performancr of the polished double tapered Exeter slem. u-hich has an aseptic loosening rate of 2. I “;, at betMecn I8 and 20 yeaIs and the lowest incidence of lucent liner at the cement-bone mtcrface of an) prosthesi> thus fal reporlcd. Harris’ clahsitication is useful for all cemented femoral components other than the polished Exeter. although endosteal bone lysis is a severe complication that must be taken into account. The accumulated literature would suggest that re\,ision rates for aseptic loosening of femoral and acetabular components are comparable. Series of revision hip .trthroplasties indicate that aseptlc loosening of the itimoral component alone i\ by far the commoner cause. Radiographic examination has been less helpful in the study of the acetabular than the femoral componenl De Let and Charnley ( 1976)” (Fig, I : noted that demarcation appeared between the cenlcnt and bone in some 700,0 of prostheses. 7 I “,i oi which demonstrated the sign within the first heat- Of these lucent lines some 39% were progressi1.e and progression occurred most frequently when the lucent line kt’as in Zone 2 (Fig. 1). Some 9”,, of cemented cups migrated compared with 15 “,,o of uncemcnted. De Let and (‘harnley identified the aqsoci;llion of medical migration with a thin medial acetabular wall. but noted no clinical diflerences among41 patients with and without demarcation at the cement ~bonr interface. th,lt presThe Exeter group.’ using a technique surised cement onto clean cancellou\ bone demonstrated significant reductions in cup migration at 7 and I.7 year reviews. but not after IX 20 years. that cnmplctc radioHodgkinson et al” confirmed logical demarcation of the socket \~as
168 CURRENT ORTHOPAEDICS-ADULT

TYPE I

Fig. l-Classification

. :

!

of acetabular zones.

associated with the physical finding of a loose prosthesis at surgery as it would appear that both migration and demarcation are unfavourable radiological signs and that the technique and the prosthetic design minimises these is the one of choice.

geometry and orientation were in that order factors that correlated most closely with outcome that could be influenced by the surgeon. Factors most frequently associated with the outcome on late radiographs were subsidence at the cement-bone interface or varus migration, calcar resorption, radiolucent lines at the cement-bone interface, migration into varus and distal migration in that order. The surgeon choosing a prosthesis would be advised to adopt one that does not demonstrate these features, or does so minimally. A similar exercise comparing 242 acetabular revisions with 387 sockets that survived a minimum of 10 years demonstrated that the anatomical axis rotation was medialised in 79% of revisions compared with 9% of hips surviving for a mean of 14.2 years. The study included the hemispherical polyethylene Muller, the asymmetrical polyethylene Exeter and conical Ring acetabular components, all of which demonstrated exactly the same phenomenon. It would appear then from radiological studies that there is little difference between cemented and cementless acetabular prostheses, that prostheses demonstrating minimal number of progressive lucent lines and the least tendency to migrate should be chosen.

Choice of Prosthesis Comparisons of Surviving and Revised Hip Replacements The relative importance of operative technique and the host response to the strains imposed by long-term implantation of a prosthesis, gives some indication as to the importance of choice of total hip replacement. In an analysis of 107 revised and 214 cemented femoral stems surviving for 5 or more years,lg prosthetic types examined included the range of Muller hip replacements and the collarless polished double tapered Exeter stem. Cement mantle adequacy over the proximal l/3 of the prosthetic fixation, integrity of the cement-bone interface, prosthetic

Fig. 2-Subsidence

of Exeter stem. Note the valgus migration

The choice of prosthesis appears to relate to geometrical design in the femoral stem, but not the acetabulum. The cemented femoral stem appears to be more reliable than the uncemented. Acetabular fixation can be achieved equally well with and without cement. A good prosthesis implanted with poor surgical technique gives a poor result. The most favourable results of stem fixation reported in the literature are those of the Exeter, which is a polished collarless double taper, that migrates distally at the cementbone interface, but does so into valgus and not varus, appears to preserve proximal medial bone stock

and intact cement-bone

interface.

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ensure a suitable cement mantle are \ery rarely supplied. It would appear that the choice of acetabuar component depends on orientating the prosthesis as close to the anatomical axis of rotation as possible and ifcement is used in ensuring that it is compressed. There are theoretical disadvantages of’ porous coated prostheses in that osseo-integration is almost certainly incomplete and if there is movement and plastic debris under the component. granulomatous bone Iysis may occur and be extremely difficult to salvage as fixation of the cup makes removal difficult. Thus far this appears to be a largely theoretical consideration ;IS over-sized porous coated hemispherical ,tcetabular components have demonstrated excellent tisation and minimal complications. If a cemented cup is to be used it is essential that the bone be cleansed with pressurised lavage. bleeding minimised and cement pressurised as much as PO+ sible. This can be achieved both before prosthetic insertlon and enhanced by the use of an Ogee cup on prosthetic implantation. The Ogee cup ser\‘es not only to pressurise cement, but to prevent c’xces~~ve medialisation of the implant and offers the surgeon the best opportunity of producing a hip replacement with the longest possible life expectancy.

Fig. 3-Subsidence of Charnley round back stem. Note endosteal bone lysis around the tip of the prosthesis

(calcar) and shons minimal evidence of loosening at the cement-bone interface (Figs 2 and 3). The choice of prosthesis depends not only on stem geometry, but the reliability with which the surgeon can implant it. This depends on experience and instrumentation. No prosthetic system, at present. fulfils the criteria of producing good compression at the cement -bone interface, orientating itself in neutral or valgus and maintaining an even, intact cement mantle. The Muller straight stem is a bulky prosthesis designed to press fit into the femoral medullary cavity with close apposition of metal to bone. using cement as grouting to areas of poor contact. Instrumentation is excellent, orientation reliable. but there is a poor cement mantie and coupling between the rigid stem and relatively elastic bone is poor. Such a prosthetic design is more heavily reliant on the ability of the femur LU adapt to its change in mechanical environment and distal hypertrophy of the femoral shaft IS a frequent finding. This approach was adopted in part because of the results of Muller’s original prosthesis and because of the difficulty in orientating ;I narrow stem blind in the thick cement mantle. The use of spacers around a prosthesis ensures satisfactory prosthetic orientation and proximal occlusion. Using cement pressurisation instrumentation (Fowler et al 198X) improves prosthetic fixation. Although a valuable factor in the choice of hip replacement. spacers to

References study ot fractures oi I. Acbrr F A. Charnley J. A radiological acrvhc cement in relation to the stem of a femoral head prwthesis. J Bone Joint Surg 1975: 57B: 297 2. Ch.to E \I’ S. Coventry M B. Fracture of the femoral component after total hip replacement. J Bone Joint Surg 19x1: h3A: 1078 1094 Beckenbaugh R D. Ilstrup D M. Total hip .lrthiopl:lst> J Bone Joint Surg 1978: 60.4: 306 313 H,~Iawa M. Lee .4 J C, Ling R S M. Vangala S S. The shear strength of trabecular bone from the femur ,mtl jome factor\ attesting the shear strength of the cement bone interface. .41-ch Orthop Traumat Surg 1978: 92: 19 -31 5 Krause W R. Krug W. Miller J. Strength of the ccmcnt bone mterface. Clin Orthop 1982: 163: 290- 2Y9 6. Harris W H. McCarthy J C. O’Neill D .4. Femoral component I~~osemng usin g contemporary techntqucx ot femoral cement frwtion. J Bone Joint Surg !Y87. (,-I.\ lOhI- 1067 7. Fo\\Ier J L. Gie G A. Lee A J C. Ling R S hl. Experlcnce with the Exeter total hip replacement hince 1970 Orthop 01: N Am lY88: 19: 3: 477 489 8. Thomas B J. Salvati E A, Small R D. The ( 4D hip arrhroplssty. J Bone Joint Surg 19X6: 6RA h40 646 9. Dali D M. Lcarmonth I D. Miles A W. A compvrl~~rn of tirrt and second generation Charnley femoral stem\ in total hip replacement J Bone Joint Surg (B). in press IO. Roberts D W. Pass R, Kelley K. Radiographic comparl\on of .I Zrthop 19Xf*; I. 1 J-II -247 Il. Klni V H. Kim V F M Results ot‘ the Harrlh-Galantc wwntlesb hip prosthesls. J Bone Joint Sur? IW7. ‘-1B X3~b? II. Jasth M. Bragdon c‘ R. Malone? M’J. Hawr T. Harl-lb W H. Ingrintth of bone in failed fixation of porouh coat& femoral components. J Bone Joint Surg lY91; 73A: I??1 I.317 13. Charnley J. Kettlruell J. The elimination of \lip bct\\een proI 111~

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femoral head prosthesis in the Charnley total hip arthroplasty. Clin Orthop 1980; 147: 2622270 16. Paterson M. Fulford P, Denham R. Loosening of the femoral component after total hip replacement. J Bone Joint Surg 1986; 68B: 392-397 17. De Lee J G, Charnley J. Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 1976: 121: 20-32

J P. Shelley P. Wroblewski B M. The 18. Hodgkinson correlation between the roentgenographic appearance and operative findings at the boneecement junction of the socket in Charnley low friction arthroplasties. Clin Orthop 1988: 228: 105-109 19. Bannister G C. Mechanical failure in the femoral component in total hip replacement. Orthop Clin North Am 1988; 19(3): 5617576