The John Charnley Award

THE JOHN CHARNLEY AWARD

A Study of Implant Failure in Metal-on-MetalSurface Arthroplasties

Pat Campbell, PhD*,†,‡; Paul E. Beaulé, MD§; Edward Ebramzadeh, PhD†,‡;Michel LeDuff, MA*; Koen De Smet, MD�; Zhen Lu, PhD†,‡; and Harlan C. Amstutz, MD*

The reintroduction of surface arthroplasty of the hip withmetal-on-metal bearings has the potential to eliminate orsubstantially reduce long-term wear-induced osteolysis asthe major cause of failure. To determine important modesof failure, implant retrieval analysis was done on 98 failedsurface arthroplasty components from different manufac-turers. Analysis involved sectioning the components, mea-suring cement mantle thickness and the depth of penetra-tion, histopathology, and measurement of the bearingwear. A finite element model was constructed to examinecement thermal necrosis. Femoral neck fracture and femo-ral loosening were the main causes of failure. The finiteelement model showed thermal necrosis could occur whencysts were filled with cement. Histologic observationsverified necrosis of interfacial bone, although adaptive re-modeling was commonly seen. The amount of cement var-ied considerably with implant type, and failure mode andwas greater in loosened components. Although implantfailure is multifactorial, these observations should be acause for concern in current cementing techniques in

which controlling mantle thickness and extent of penetra-tion may be difficult. Optimizing cement technique toavoid leaving the component proud, and to avoid extensivecementation of the femoral head, may be important inreducing some modes of failure.

With the application of metal-on-metal bearings, sur-face arthroplasty is again being performed in a growingnumber of centers worldwide. Although relatively fewprocedures have been performed in the United States,thousands of surface arthroplasty components have beenimplanted in Europe and Australia. We anticipate theproblems faced by the first generation of metal-polyethylene surface arthroplasties, primarily related todebris-induced osteolysis caused by polyethylenewear,3,5,6,23 can be overcome by the current generation oflow wearing metal-on-metal surface arthroplasties.

Short-term clinical followup reports of metal-on-metalsurface arthroplasties have been encouraging,4,15–17 al-though femoral neck fractures4,6,33,34 and femoral loosen-ing2 have been identified as causes of failure. Risk factorsin surface arthroplasty highlight the importance of patientselection criteria and good bone quality for implant sur-vival.11 Currently, the role of femoral head vascularity inimplant durability is controversial; some surgeons are con-cerned the posterior surgical approach sacrifices the im-portant extraosseous blood supply to the femoralhead,8,10,32 whereas others believe adequate blood supplywill be provided intraosseously.21 Although the reducedwear of metal-on-metal bearings is well recognized, therehave been concerns that heat-treating the components aftercasting can lead to higher wear, possibly sufficient tocause osteolysis.15 The unknown long-term consequencesof metal wear debris are also a concern.26,28 Despite theseconcerns, the conservative nature of surface arthroplastyand the restoration of a high degree of function, includingthe ability to return to sports, make this surgery appealing

From the *Joint Replacement Institute at Orthopaedic Hospital, Los Angeles,CA; †UCLA David Geffen School of Medicine, Los Angeles, CA; ‡J. VernonLuck Research Center at Orthopaedic Hospital, Los Angeles, CA; §Divisionof Orthopedic Surgery/Chirurgie Orthopédique, University of Ottawa,Canada; and �ANCA Clinic, Heusden, Belgium.The institution of one or more of the authors (PC, PEB, KDS, HCA) hasreceived funding from Wright Medical Technology; PEB, KDS, HCA areconsultants for Wright Medical Technology. One or more of the authors hasreceived funding from the William G. McGowan Charitable Fund (PC, PEB,MLD, HCA), the Orthopaedic Hospital Foundation (PC, PEB, MLD, EE,ZL, HCA) and Wright Medical Technology, USA (PC, PEB, KDS, HCA).Each author certifies that his or her institution has approved the humanprotocol for this investigation and that all investigations were conducted inconformity with ethical principles of research, and that informed consent forparticipation in the study was obtained.Correspondence to: Pat Campbell, PhD, Orthopaedic Hospital, 2400 S.Flower Street, Los Angeles, CA 90007. Phone: 213-742-1134; Fax: 213-744-1175; E-mail: [email protected]: 10.1097/01.blo.0000238777.34939.82

CLINICAL ORTHOPAEDICS AND RELATED RESEARCHNumber 000, pp. 000–000© 2006 Lippincott Williams & Wilkins

1

Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.

to young, active patients. In our experience, patients arewilling to travel long distances to specialty, high-volumecenters, often at their own expense, for this surgical op-tion.

With the introduction of any new device there will oftenbe a learning curve as surgeons gain experience and un-derstand the limitations and the factors involved in clinicalsuccess and failure. This occurred with the introduction ofcementless fixation and it will likely be no different forsurface arthroplasty of the hip, which is recognized as amore technically demanding operation than standard totalhip replacement. Before the orthopaedic communitymoves forward with the widespread use of surface arthro-plasty, it is paramount we identify mechanisms of failurenot fully understood in the previous metal-polyethylenesurface arthroplasty experience.

Despite many procedures performed over the past sev-eral years, failure analysis reports of revised metal-on-metal surface arthroplasty specimens are limited. We ex-amined the failure mechanisms in metal-on-metal surfacearthroplasty components submitted to our laboratoriesover the past 8 years. We compared the findings withfailure modes identified in the metal-polyethylene surfacearthroplasty era to examine the influence of removing theproblem of wear-debris induced osteolysis as a primarymode of failure. The primary goal of these analyses was tounderstand which failures may be preventable through op-timized patient selection and surgical techniques beforethe widespread reintroduction of surface arthroplasty. Sec-ond, we looked for new complications that were unique tothis generation of metal-on-metal surface arthroplasty.

MATERIALS AND METHODS

One hundred forty specimens from four metal-on-metal surfacearthroplasty designs were submitted to our implant retrievallaboratory from 1997 to 2005. Forty-two of the revised compo-nents had insufficient clinical information or lacked a femoralcomponent that was suitable for inclusion in the analysis offailure mechanisms, leaving 98 for analysis. There were 50 maleand 48 female patients with average ages of 49 years (range33–66 years) and 51 years (range 24–78 years), respectively. Theimplants have been described in detail elsewhere.2,9,15,16,29 Toassess failure modes of the various designs, the retrieved im-plants were studied by type and we summarized the design fea-tures of these components (Table 1). We examined 58 Con-serve� Plus (Wright Medical Technology, Arlington, TN), 23McMinn (Corin, Cirencester, United Kingdom), 13 BirminghamHip Replacement (Smith & Nephew, Memphis, TN), and fourCormet 2000 (Corin) components. All femoral components werecemented, and acetabular components were noncemented, ex-cept for most of the McMinn components. The implants wereinserted using a posterior approach. Most of the 40 patients fromour institution had intraoperative photographs of the femoralhead taken before and after surgical preparation for implantation. T

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Clinical Orthopaedicsand Related Research2 Campbell et al


At revision, the femoral components were resected with aportion of the femoral neck where possible, and were immedi-ately fixed in buffered formalin. In 49 revisions, the acetabularcomponents also were removed. The components were in-spected, photographed, and measured for wear depth and clear-ance using a coordinate measuring machine. Initially, femoralcomponents were sectioned into a variable number of sections toallow inspection of the cement-bone interfaces and access tosamples of the bone from various locations for decalcified his-tologic analysis. Later, a more systematic sectioning protocolwas followed to facilitate cement interface analysis on a sub-group of cases (see below). For each of these 98 cases, a modeof failure was determined based on our previously applied cri-teria.5

Cement fixation and bone histology were quantitatively ana-lyzed in a group of 45 failed metal-on-metal surface arthroplastyimplants, including 24 that had failed because of femoral neckfracture or loosening and 21 that failed because of other causes.All 45 implants were sectioned by cutting a 2-mm thick coronalsection from the middle of the metaphyseal stem and from themiddle of each resulting portion to yield three 2-mm thick slicesfrom the anterior, middle, and posterior portions of the femoralhead. Because only specimens with an intact cement-bone inter-face were suitable for this analysis, implants that had loosened tothe point of disassociation of the bone from the cement could notbe included. The sections were radiographed and photographed.The following analyses were then performed.

The thickness of the cement mantle (defined as the cementlayer between the metal and the outer edge of the preparedsurface of the bone) and the depth of cement penetration intocancellous bone (ie, the interdigitation of cement into cancellousbone starting from the outer edge of the prepared bone) weremeasured in a blinded manner in 11 BHR implants, 22 Con-serve� Plus implants, 11 McMinns implants, and one Cormetimplant. The cut section photographs and microradiographs werescanned into a computer, calibrated to within 0.1 mm, and animage analysis software program (MetaMorph� Version 4.6,Universal Imaging Corp, Downingtown, PA) was used to mea-sure the thickness and area of the cement mantle and the depthand area of penetration of cement into the cancellous bone in 10sites across the entire section. The data were averaged to givecement mantle thickness, penetration depth, total mantle andpenetration areas, and the percentage of the femoral head cross-section within the component occupied by cement (combiningthe areas of the cement mantle, penetration and cement-filledfixation pegs or cysts) in each of the anterior, middle, and pos-terior sections.

Specimens from all 98 femoral components were examinedhistologically and the general appearance of the bone and mar-row was used to determine if the failure was related to osteone-crosis, fracture, cement interface loosening or infection usingstandard histopathologic criteria.30 The bone sections were care-fully removed from the sectioned metal shell and divided intotwo or three smaller pieces that included the cement. The pieceswere photographed to preserve orientation, then decalcified andembedded in paraffin for routine sectioning and staining withhematoxylin and eosin. Because this involved paraffin embed-

ding, the bone cement was removed, but the bone within thecement and the interfacial soft tissues were preserved.

The presence of interfacial membranes and bone necrosis wasrecorded for each of the 45 cases used for cement analysis.Semiquantitative histologic analysis of features at the proximalcement interface, the middle of the head and the component-femoral neck edges was carried out in a blinded fashion on 25 ofthese sections, using a modification of the scheme used byHowie et al.24 Bone viability, bone formation, marrow viability,and interface membrane formation were rated as none, low if thefeature occupied less than 10% of the 4x field of view, moderateif it occupied 10% to 50%, and high if more than 50% of the fieldof view showed the feature. Bone viability was judged by thepresence of osteocyte nuclei in most lacunae of the bone trabec-ulae. Image analysis software (Image One, West Chester, PA)was used to position a 100-point grid over the field of view. Theproportion of each feature was calculated by recording whichfeature lay at the intersecting points of the grid. The thickness ofinterfacial membranes was measured in 10 places using a cali-brated caliper function.

The clinical and radiographic histories of each case werereviewed together with the results of the above analyses to de-termine the factors associated with their failures.

Statistical analysis was done using SPSS version 11.5 ana-lytic software (SPSS, Inc, Chicago, IL). Means and standarddeviations of all continuous variables were calculated for eachsubcategory of failure mode, implant type, etc., and then werecompared using student’s t tests. When there were three or morecategories, analysis of variance was used. For categorical vari-ables, such as gender or implant type, chi square analysis wasused to compare the ratio of failures in each subcategory. Lo-gistic regression analysis then was used to assess the relativeeffects of each independent variable (when all variables wereconsidered in the same analysis) on the risk of failure (looseningor fracture) compared with survival until revision.

One hundred five components out of the 140 in our archivedcollection were measured for wear prior to the destructive sec-tioning procedure, which was not always performed in the samefashion. Consequently, only 57 components used in the failureanalyses above were measured. Twenty-seven specimens wererevisions of one side only, and 39 pairs were available for theanalysis of clearance and wear. Revisions were performed fromless than 1 month to 110 months. Wear depth was measured witha coordinate measuring machine (BMT 504, Mitotoyo, Aurora,IL) at 300 to 400 points over the surface of the implant. Theoriginal clearance between the ball and cup was calculated fromthis data. Linear regression analyses were performed to examinecorrelations between clinical and implant factors with weardepth.

The cement thickness analysis led to the observation that,despite being designed to have a 1-mm or less space for a cementmantle within the femoral head, the thickness of this layer oftenwas much greater. Cement penetration was variable, often ex-tending well into the cancellous bone, and femoral head cystswere almost always filled with cement. A finite element modelwas constructed using MSC.PATRAN modeling software (MSCSoftware Corp, Santa Ana, CA) and analyzed using ABAQUS

Number 000Month 2006 Metal-on-Metal Failure Analysis 3


software (ABAQUS, Inc, Pawtucket, RI) to evaluate the tem-perature within the bone adjacent to the cement. The three-dimensional finite element model had a metal shell with a di-ameter of 46 mm and a maximum thickness of 7 mm at the apex.A bone cement layer 1.5-mm thick was chosen. To simplify thecalculations, the cross-sections of the cancellous and corticalbone beyond the extent of the metal shell were modeled ascircular. This basic model was meshed with eight-node heattransfer elements. The femoral shell was represented by 1734elements, the cement layer by 1734 elements, the reamed can-cellous bone by 5340 elements, and the cortical shell by 600elements. The stem-cement and bone-cement interfaces weremodeled as completely bonded. The thermal parameter analyseswere performed on five configurations of the cement layer: (1)no cement penetration into the cancellous bone (ie, the basicmodel); (2) 1.5 mm penetration; (3) 6 mm penetration; (4) 6 mmpenetration plus a cement-filled head cyst of 1 cm3; and (5) 6mm penetration plus a cement-filled head cyst of 2 cm3. Thedepth of cement penetration and the presence of a cement-filledcyst were modeled by changing the properties of the correspond-ing elements of the basic model from those of cancellous bone tothose of cement. Consequently, the overall mesh density anddistribution were the same for the five configurations of thecement layers.

The thermal properties of metal, bone cement, cancellousbone, and cortical bone were based on previous publications.25,27

The heat dissipation from the surfaces of the metal shell andcortical bone was modeled by applying convection with an am-bient temperature of 23°C. The analysis combined transient heattransfer analysis of the finite element model and the calculationsoutside the finite element model for the heat generation duringcement polymerization. The amount of heat generated in thecement layer was calculated using the formulas developed forSurgical Simplex cement by Baliga et al:7

� = ��0

tSdt� �Qtot

where � is the degree of polymerization, S is the rate of heatgeneration per unit volume and Qtot is the total amount of heatper unit volume, which is 1.55 × 108 J/m3.

The thermal analysis was performed in a series of time steps.The initial rate of the polymerization was taken as 10−5, assuggested by Baliga et al,7 which led to a heat flux of 35.4 W/m3

for all of the cement elements, and a duration of 43.8 seconds forthe first step, using a user-complied program. These initial val-ues were put into the finite element model to run the first step ofthe analysis. The results of the first step then were used tocalculate the duration of the second step, magnitude of the heatflux, and accumulated rate of the polymerization for individualelements, again using the user-complied program. Lastly, thevalues were put into the finite element model to perform thesecond step of analysis. This cycle was repeated until the cementpolymerization was complete.

RESULTS

The main reasons for failure were femoral neck fracture(28) and femoral loosening (23). Acetabular loosening ac-counted for 18 cemented socket failures of the first gen-eration McMinn components and for failure in 10 porous-coated sockets (Table 2). Acetabular component malalign-ment causing mechanical problems such as impingement,edge loading, or subluxation resulted in the failure of fourBirmingham Hip Replacement components, one Cormet2000 component, and one Conserve� Plus component.Sepsis caused five failures. Eight failed arthroplastiesoriginally attributed to “other causes” included one recur-rent late dislocation, one case of effusion and joint swell-ing, and six revisions performed for unexplained pain.Retrieval analysis found femoral loosening and collapse(one case), osteonecrosis (two cases) and an extensivelymphocytic infiltration of the tissues suggestive of ametal sensitivity immune response (three cases), withsome of these cases having more than one of these find-ings.

Fractures were the main cause of short-term failure (Fig1). Trauma was reported in only a few cases, but mostfractures occurred suddenly. Twenty-three occurred lessthan 6 months after surgery (median, 2 months). Longer-

TABLE 2. Clinical Details of the 98 Retrievals

Reason for RevisionConserve�

Plus McMinn BHRCormet

2000

AverageStandardDeviation

Time Months

AverageAge at

Revision(years)

TotalNumber

Femoral neck fracture 25 1 2 0 3.7 ± 4.7 54.5 ± 10.6 28Femoral loosening 19 0 3 1 50.9 ± 20.3 50 ± 17.9 23Cemented socket failure n/a 18 n/a n/a 61.7 ± 40.4 48 ± 13 18Other 3 1 3 1 35.1 ± 29.6 52.4 ± 11.5 8Acetabular loosening 6 2 1 1 16.1 ± 14.7 46.2 ± 6.1 10Socket malposition 1 0 4 1 12.5 ± 7.9 48.7 ± 9.2 6Sepsis 4 1 0 0 17.8 ± 13.3 53.6 ± 17.6 5Total 58 23 13 4 30.6 ± 31.8 50.2 ± 12.7 98



term femoral neck fractures occurred in five cases (mediantime to failure, 12.4 months) and were caused by extensiveosteonecrosis of the femoral head, shown by the completelack of viability of the bone and marrow. There was norepair to the original cut surfaces, suggesting the ischemiaoccurred at the time of surgery.

Femoral loosening occurred from 17 to 100 monthsafter surgery. The degree of loosening and loss of under-lying bone varied. Radiographically, femoral looseningwas associated with a sclerotic and/or radiolucent linearound the short stem that often appeared 2 or more years

before failure (Fig 2A). Three loosening failures were as-sociated with an area of ongoing bone repair in the middleof the femoral head (Fig 2B). Histologic features weresimilar to a nonunion. Five of the 21 failures caused byfemoral loosening were associated with complete loss offixation and femoral head shape because the proximalbone had been replaced by thick fibrous tissue that wasoften partly necrotic. In the nondissociated cases, loosen-ing was associated with fibrous membrane formation be-tween the cement and bone, ranging from 50 �m to 5 mm.The membranes were vascularized and contained variable

Fig 1A–E. Illustrations show a 56-year-old woman with end-stage osteoarthritis of the right hip. (A) A preoperative anteroposteriorradiograph shows excellent bone quality. (B) A postoperative anteroposterior radiograph shows the implants in good position. (C)Two months after receiving a Conserve� Plus implant, the patient had a fracture of the femoral neck. (D) This illustration showsa cut section through the retrieved femoral head. The component stem bent during the fracture event. (E) A light micrograph ofan area of new bone at the component edge consistent with repair of a previous stress fracture or similar damage, through whichthe fracture occurred. This feature was seen in most of the short-term fractures. (Stain, hematoxylin and eosin; original magni-fication ×40).



numbers of macrophages, giant cells, and particulate bonecement. Bone adjacent to the membrane often was under-going active osteoclastic erosion (Fig 3).

Eighteen cases from a first generation metal-on-metalsurface arthroplasty (McMinn design) were revised in as-sociation with socket-cement failure, including socket-cement dissociation from a bearing cemented in a large-diameter acetabular component of an older design. Thesegenerally were later term failures (5 years average, range2 months to 131 months) and in most cases, the femoralcomponents were radiographically stable. We present anexample of a nonfailed femoral head revised after 10 years(Fig 4).

Six revisions were performed for poor acetabular posi-tion that led to impingement, edge loading, and pain. Theacetabular angles ranged from 56° to 85°. The total maxi-mum wear depth was measured in four revisions andranged from 14 �m after 10 months to 261 �m at 17months. A focal stripe wear pattern found on the femoralcomponent in three revisions was consistent with the sub-luxation and edge loading of the components (Fig 5).

Cement penetration did not differ among failure modes.Cement mantle thickness and area and the depth and areaof cement penetration were different in those componentsthat failed because of femoral-related problems comparedwith nonfemoral failures (Fig 6). Specifically, the averagecement mantle thickness in cases that failed from femoralloosening averaged 2.90 ± 1.8 mm, whereas failures at-tributable to neck fracture averaged 2.1 mm ± 0.9 (p �

0.03) and those with nonfemoral failures averaged 2.3 mm± 1.1 (p < 0.04) (Table 3).

Average cement mantle area and thickness were differ-ent among the implant types, with Conserve� Plus im-plants having a larger average cement mantle (2.7 mm ±

Fig 3. A light micrograph shows a piece of bone undergoingosteoclastic resorption by the membrane that had formed be-tween the cement and the bone in a Conserve� Plus compo-nent after 70 months. Scalloping of the bone is obvious on thetop surface. There has been new woven bone formationaround a central core of older, dead bone (healing osteone-crosis). (Stain, hematoxylin and eosin; original magnification×100).

Fig 2A–C. A 43-year-old woman had developmental dysplasia of the right hip. The femoral component loosened 39 months afterthe resurfacing procedure using a Conserve� Plus implant. (A) An anteroposterior radiograph taken shortly before revision surgeryshows a wide radiolucency around the short metaphyseal stem of the femoral component. (B) The cut section of the retrievedfemoral component is shown. (C) A corresponding microradiograph shows the component was left proud, and also shows thepresence of a nonunion within the viable femoral head. There was ongoing bone repair at this site.



1.1) and Birmingham Hip Replacements having the high-est average depth of penetration (6.7 mm ± 3.9; p < 0. 001)(Fig 7). There was a tendency for cement penetration to belower (R � 0.48, p < 0.003) when the cement mantlethickness was greater.

The total percentage of the femoral head sections oc-cupied by cement (mantle, cement–filled fixation pegs orcysts, and penetration combined) ranged from 11% to 89%and was more (p � 0.001) in loose failures compared withall other modes of failure. Cement-filled cysts were more

prevalent (p < 0.025) in cases that failed by femoral loos-ening compared with nonfemoral failures.

Bone was necrotic in areas penetrated by cement. Therewas a higher incidence of necrotic bone below the cementzone, especially in the short-term cases in which necrosisof bone and marrow extended up to several millimetersfrom the cement; however, the edges of the component,where cement tended to be minimal, had the least (p <0.01) necrotic bone. Healing osteonecrosis, where newbone had formed around a necrotic trabecular core, wascommon near the cement and was associated with abun-

Fig 4A–C. This 53-year-old woman had all-cemented McMinnmetal-on-metal resurfacing for osteoarthritis of the right hip.The prosthesis failed on the acetabular side but the femoralcomponent was well fixed at the time of conversion to total hipreplacement. (A) A cut section of the retrieved femoral com-ponent and the corresponding microradiograph (B) shows thepresence of large, cement debris-filled cysts that have erodedthe bone. The dark staining of the tissue is a cutting artifact.(C) A light micrograph of the cement-bone interface illustratingthe close proximity of the cement to the bone and marrow. Thescalloped surfaces indicate previous osteoclastic remodeling.Parts of the bone lack nuclei but the bone is mostly viable inthis long-term retrieval. (Stain, hematoxylin and eosin; originalmagnification ×40).

Fig 5A–C. A 54-year-old woman received a 46-mm Birming-ham Hip Replacement component for osteoarthritis of the lefthip. Postoperatively, the patient had an abnormal gait and gen-eral discomfort. Anterior–posterior (A) and lateral radiograph(B) show the acetabular component had been implanted at85°. At revision surgery 13 months after implantation, the jointfluid was gray, and there was metal staining of the surroundingtissues. (C) This wear plot shows wear depth of the femoralcomponent, which measured 100 µm (the maximum weardepth of the acetabular component was 37 µm; the diametralclearance was 202 µm).



dant blood vessels, but was sparse in most of the otherregions examined. Fibrous membranes were uncommonaround deeply penetrated cement; however, this interfacewas more likely to be characterized by fibrotic marrow andbone trabeculae having healing osteonecrosis (Fig 8).Bone resorption usually was associated with areas of wo-ven bone formation or with nearby membranes containingparticulate bone cement. Active bone resorption was com-monly seen in femoral loosening, but also was common inthe unfailed femoral heads from cemented socket failuresbecause the cement debris-filled granulomas invaded thebone through the cement-implant interface. Osteolysis andlarge defects containing cement debris-filled macrophageswere seen within the femoral heads of four of these ace-tabular failures.

We summarized wear depth and clearance measure-ments (Table 4). The wear depth varied from undetectable(< 2 �m) to 164 �m. Diametral clearances varied from123 to 400 �m. Components that failed because of femoralfracture or loosening had similar wear to those that failedbecause of acetabular loosening, malposition, infection orother reasons, but the study may have been underpoweredto address this point.

The Birmingham Hip Replacements, which were re-vised mostly for acetabular malposition, had higher aver-age wear and clearance than the other types (p < 0.001).The effect of time in vivo on wear was small for the totalgroup, which ranged from less than one month to morethan 10 years, but when examined for cases retrieved atless than 2 years in vivo, wear in the Conserve� Plus,McMinn, and Cormet types was higher (p < 0.05) thanthose components in vivo longer than 2 years.

When the modeled bone cement mantle was 1.5 mmthick and there was no cement penetration into the femoralhead (or only 1.5 mm of penetration), the temperaturesthroughout the bone and cement remained below bodytemperature (Fig 9). In contrast, with 6 mm penetration ora cement-filled cyst of 1 cm3 or 2 cm3, the peak tempera-tures within the bone were 55°C, 61°C, and 66°C, respec-tively. The temperature in the bone remained above 50oCfor 37, 64, and 117 seconds respectively with the abovecement configurations. The temperature in the bone wasabove 50°C to a depth 0.5 mm below the cement-boneinterface with 6 mm of penetration, 1.25 mm with 6 mmpenetration plus a cement-filled cyst of 1 cm3, and 2 mmwith 6 mm penetration plus a cement-filled cyst of 2 cm3.

DISCUSSION

The introduction of metal-on-metal bearings in surfacearthroplasty has nearly eliminated aseptic looseningcaused by particulate bearing wear debris. Consequently,achieving lasting fixation and avoiding mechanical fail-

Fig 6A–C. A box plot demonstrates the thickness of cementplotted by the mode of failure with p values. (A) A box plotshows cement mantle thickness in millimeters, (B) cementpenetration depth in millimeters, and (C) amount of cement inthe section (percentage), ie, mantle, penetration, and cement-filled fixation pegs and cysts.



ures are the primary requirements for a successful anddurable surface arthroplasty. Although cement is used forfemoral fixation in most of the resurfacing designs cur-rently in use, the method and timing of cement application

and the amount of the cement mantle and bone penetrationare variable. This study showed wide variation in cementin failed and nonfailed femoral heads using thickness andarea measurements. The practice of filling cysts with ce-ment, the problem of unseated components, and the pres-surization of low viscosity cement into bone of variablequality can result in large amounts of cement in the resur-faced femoral head. In some cases in this retrieval group,almost complete filling of the bone had occurred.

We found the amount of cement was greater in loos-ened femoral components. This should be a cause for con-

TABLE 3. Results of Cement Analyses

Variable

Thickness ofCement Mantle

(mm)

Area of CementMantle(mm2)

Depth of CementPenetration

(mm)

Area of CementPenetration

(mm2)

Percent of HeadSlice Filled

with Cement

Failure modeFemoral loosening 2.9 ± 1.8 176.6 ± 113.8 3.9 ± 3.2 161.4 ± 167.8 50.8 ± 10.4Femoral fracture 2.1 ± 0.9 166.2 ± 60 3.3 ± 2.7 161.8 ± 160.6 36.1 ± 12.9Other 2.3 ± 1.1 144.6 ± 8.3 3.0 ± 3.3 119.7 ± 111.3 39.5 ± 16.6Implant typeBHR 1.5 ± 1 101 ± 74.6 6.7 ± 3.9 262.9 ± 191.7 58.2 ± 13.1Conserve Plus 2.7 ± 1.1 171.2 ± 73.5 2.1 ± 1.5 102.8 ± 81.8 34.9 ± 10.9McMinn 2.7 ± 1.4 184.1 ± 89.7 2 ± 2.1 82.8 ± 92.2 35.5 ± 12.4Cormet 2 ± 0.9 157 ± 98.6 3.2 ± 2.3 160.3 ± 100.5 42 ± 16.6

Fig 7A–B. A box plot shows (A) cement mantle thickness inmillimeters and (B) cement penetration depth in millimeters.

Fig 8. A light micrograph shows histologic features of the ce-ment-bone interface in the middle of the femoral head in aBirmingham Hip Replacement implant with deeply penetratedcement after 1 month. The marrow has been replaced bygranulation tissue and several blood-filled vessels nearby in-dicate this area is undergoing repair. The paler stained bone isdead, but is surrounded by a seam of newly formed bone(healing osteonecrosis). (Stain, hematoxylin and eosin; originalmagnification ×40).



cern in light of the current cementing techniques where thecontrol of the cement mantle thickness and extent of pen-etration may be difficult to achieve, as well as the commonpractice of cementing cystic lesions. Trabecular bonestructure is optimized to withstand the dynamic stressesapplied to the natural femoral head, but the depth of ce-ment penetration that could affect this subtle balance, suchas by preventing bone from remodeling according to newstresses when an implant is present, is unknown. Addi-tional studies are required to determine how cement-filledbone behaves under changing stresses or conditions.

One of the concerns of excessive cement use is thermalnecrosis of the surrounding bone.7,18,19,31 The results ofthe present FEA indicated when deep cement penetrationwas combined with a relatively small cement-filled cyst,the peak temperatures within the femoral head interfacialbone were high enough, and the durations were longenough to cause thermal bone damage. The model under-estimated the amount of cement seen in many of the re-trievals. Because it did not account for practices such asextensive cold saline irrigation of the prepared bone sur-faces and the seated component, only real-time tempera-ture probe measurements of the cement bone interface willconfirm if the modeled temperature changes occur in vivo.Bone and marrow necrosis were observed up to severalmillimeters deep to cement-filled cancellous bone or cystsin short-term failures, but did not seem to cause failure.New bone formation within close proximity to the cementalso was noted in some cases. The bone-cement interfacesin several of the longer-term specimens with deep cement

showed evidence this damage healed without membraneformation. This may reflect the ability of the remainingbone to withstand some degree of insult, but this abilitymay be overwhelmed when several accumulative insultsoccur. Healing can only occur with the preservation orrestoration of an adequate femoral head blood supply. Inthis series, there were seven osteonecrosis-induced fail-ures.

Ongoing bone resorption, interfacial membrane forma-tion and cement-interface instability were noted in fewcases with deep cement penetration. In contrast, mem-branes between bone and cement were seen where therewas little or no cement penetration, and there often wasactive bone resorption, presumably from the bone-cement–related histiocytic tissue. We observed thick in-terfacial membranes, bone resorption, and transformationof bone into fibrous tissue in several loose femoral metal-on-metal surface arthroplasty components. Similar fea-tures were reported in loosening failures of metal-polyethylene surface arthroplasty. Howie et al23 proposedaseptic loosening of cemented metal-polyethylene surfacearthroplasties started with the accumulation of polyethyl-ene particles at the cement–bone interface, leading to boneresorption, membrane formation, and eventual fibroustransformation of the femoral head bone as loosening pro-gressed. In the absence of polyethylene debris, the mostlikely factors initiating interfacial membrane formation aremechanical instability and thermal necrosis. Mjoberg31

postulated migration of resurfacing components measuredwith roentgen stereophotogrammetry resulted from insta-

Fig 9. Results of the finite elementmodel of temperatures reached dur-ing cement polymerization within acemented femoral surface replace-ment. Three conditions were mod-eled, from left to right: 1.5 mm pen-etration, 6 mm penetration; and 6mm penetration and a cement-filledcyst of 2 cm3. The red portions indi-cate a temperature high enough todamage surrounding bone.

TABLE 4. Results of Wear Measurements

MeasurementBirmingham Hip

Replacement Conserve� Plus McMinn Cormet

Average femoral maximum wear depth (µm) 54.7 ± 48.8 7.03 ± 13.5 33.4 ± 54.1 32.7 ± 31.6Average cup maximum wear depth (µm) 31.8 ± 49.8 2.89 ± 5.54 17.4 ± 29.1 5.5 ± 3.5Average diametral clearance (µm) 271.4 ± 66 173.5 ± 30 267 ± 48 264 ± 21.8



bility because of the fibrous tissue layer that replaced ther-mally damaged bone under the polymerizing bone cement.

Femoral neck fracture after hip resurfacing is a risk forthe patient, and the mechanisms precipitating this failuremode include patient-related, surgery-related, and biome-chanical factors.6,11,19,33 The technical difficulty associ-ated with the surface arthroplasty also may be a factor inneck fracture. Several fractures in this retrieval series oc-curred with surgeons new to the procedure. Cut sectionsand cement mantle thickness measurements showed theimplants were sometimes left not fully seated. This putsthe femoral neck at risk in several ways. The surgeon mayapply extra pressure or hammer blows to seat the proudcomponent, leading to stress fractures if there is weakbone or component misalignment. Reamed bone may re-main uncovered and could act as a stress riser, and thethick mantle may reduce bone cement penetration for fixa-tion, as suggested by our analysis. Although healing ofstress fractures occurring under metal-on-metal surface ar-throplasty components has been reported,14 bone under-going repair after stress fracture is a site of weakness, andour histologic analysis showed that fractures occurred inthese areas.

One of the major improvements in this era of metal-on-metal bearings is the reduction in wear provided by metal-on-metal. The wear of these failed components generallywas low, with the exception of poorly functioning im-plants, particularly when acetabular malpositioning re-sulted in steep cup angles (> 55° of abduction).12 This maybe a unique complication of metal-on-metal surface arthro-plasty, but it should be preventable with education andcareful attention to surgical technique and implant place-ment.

Ten of the components submitted to our laboratorieswere revised for unexplained pain and implant retrievalanalysis was helpful in finding explanations in most ofthose cases. Metal sensitivity was the suspected cause ofpain in three revisions. This has been a concern sincecobalt chromium implants were introduced.20 Recent re-ports have shown some individuals have an immunologi-cal reaction to wear products made of this alloy.1,35 Al-though this is a rare complication, efforts should be con-tinued to devise a means of preimplantation testing.22

In 1982, in a paper presented at a symposium on surfacearthroplasty, Clarke noted “the main flaws to be overcomein realizing the potential success of the double-cup arthro-plasty procedure are failures due to femoral cup loosening,acetabular cup loosening, and femoral-neck fractures. Theclinical uncertainties include the selection of a suitablepatient with adequate bone stock and the technical diffi-culties associated with (1) reaming the acetabulum ad-equately, (2) reaming down onto the neck without violat-

ing it, and (3) anchoring the components securely by in-terdigitation of acrylic cement.”13

Twenty-four years later, with resurfacing arthroplastybeing used more widely than ever, we face some of thesame questions and issues, but progress has been made.The combination of careful and thorough clinical fol-lowup, laboratory studies including wear simulation, im-plant design modeling, and retrieval studies will provideuseful information in the understanding of the success andfailure of the new generation of surface arthroplasties.

AcknowledgmentsThe authors thank Dr. Joseph Mirra, Mr. Bill McGarry, Dr. FredDorey, Mr. Billy Lundergan, Dr. Brook Wager, Dr. Roel DeHaan, Dr. Harry McKellop, Ms. Cathy Fischl and the techniciansat Impath, and all of the surgeons who contributed specimens foranalysis.

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