The John Charnley Award

Abstract

The reintroduction of surface arthroplasty of the hip with metal-on-metal bearings has the potential to eliminate or substantially reduce long-term wear-induced osteolysis as the major cause of failure. To determine important modes of failure, implant retrieval analysis was done on 98 failed surface arthroplasty components from different manufacturers. Analysis involved sectioning the components, measuring cement mantle thickness and the depth of penetration, histopathology, and measurement of the bearing wear. A finite element model was constructed to examine cement thermal necrosis. Femoral neck fracture and femoral loosening were the main causes of failure. The finite element model showed thermal necrosis could occur when cysts were filled with cement. Histologic observations verified necrosis of interfacial bone, although adaptive remodeling was commonly seen. The amount of cement varied considerably with implant type, and failure mode and was greater in loosened components. Although implant failure is multifactorial, these observations should be a cause for concern in current cementing techniques in which controlling mantle thickness and extent of penetration may be difficult. Optimizing cement technique to avoid leaving the component proud, and to avoid extensive cementation of the femoral head, may be important in reducing some modes of failure. With the application of metal-on-metal bearings, surface arthroplasty is again being performed in a growing number of centers worldwide. Although relatively few procedures have been performed in the United States, thousands of surface arthroplasty components have been implanted in Europe and Australia. We anticipate the problems faced by the first generation of metal-polyethylene surface arthroplasties, primarily related to debris-induced osteolysis caused by polyethylene wear,3,5,6,23 can be overcome by the current generation of low wearing metal-on-metal surface arthroplasties. Short-term clinical followup reports of metal-on-metal surface arthroplasties have been encouraging,4,15-17 although femoral neck fractures4,6,33,34 and femoral loosening2 have been identified as causes of failure. Risk factors in surface arthroplasty highlight the importance of patient selection criteria and good bone quality for implant survival.11 Currently, the role of femoral head vascularity in implant durability is controversial; some surgeons are concerned the posterior surgical approach sacrifices the important extraosseous blood supply to the femoral head,8,10,32 whereas others believe adequate blood supply will be provided intraosseously.21 Although the reduced wear of metal-on-metal bearings is well recognized, there have been concerns that heat-treating the components after casting can lead to higher wear, possibly sufficient to cause osteolysis.15 The unknown long-term consequences of metal wear debris are also a concern.26,28 Despite these concerns, the conservative nature of surface arthroplasty and the restoration of a high degree of function, including the ability to return to sports, make this surgery appealing to young, active patients. In our experience, patients are willing to travel long distances to specialty, high-volume centers, often at their own expense, for this surgical option. With the introduction of any new device there will often be a learning curve as surgeons gain experience and understand the limitations and the factors involved in clinical success and failure. This occurred with the introduction of cementless fixation and it will likely be no different for surface arthroplasty of the hip, which is recognized as a more technically demanding operation than standard total hip replacement. Before the orthopaedic community moves forward with the widespread use of surface arthroplasty, it is paramount we identify mechanisms of failure not fully understood in the previous metal-polyethylene surface arthroplasty experience. Despite many procedures performed over the past several years, failure analysis reports of revised metal-on-metal surface arthroplasty specimens are limited. We examined the failure mechanisms in metal-on-metal surface arthroplasty components submitted to our laboratories over the past 8 years. We compared the findings with failure modes identified in the metal-polyethylene surface arthroplasty era to examine the influence of removing the problem of wear-debris induced osteolysis as a primary mode of failure. The primary goal of these analyses was to understand which failures may be preventable through optimized patient selection and surgical techniques before the widespread reintroduction of surface arthroplasty. Second, we looked for new complications that were unique to this generation of metal-on-metal surface arthroplasty. MATERIALS AND METHODS One hundred forty specimens from four metal-on-metal surface arthroplasty designs were submitted to our implant retrieval laboratory from 1997 to 2005. Forty-two of the revised components had insufficient clinical information or lacked a femoral component that was suitable for inclusion in the analysis of failure mechanisms, leaving 98 for analysis. There were 50 male and 48 female patients with average ages of 49 years (range 33-66 years) and 51 years (range 24-78 years), respectively. The implants have been described in detail elsewhere.2,9,15,16,29 To assess failure modes of the various designs, the retrieved implants were studied by type and we summarized the design features of these components (Table 1). We examined 58 Conserve® Plus (Wright Medical Technology, Arlington, TN), 23 McMinn (Corin, Cirencester, United Kingdom), 13 Birmingham Hip Replacement (Smith & Nephew, Memphis, TN), and four Cormet 2000 (Corin) components. All femoral components were cemented, and acetabular components were noncemented, except for most of the McMinn components. The implants were inserted using a posterior approach. Most of the 40 patients from our institution had intraoperative photographs of the femoral head taken before and after surgical preparation for implantation.TABLE 1: Surface Arthroplasty FailuresAt revision, the femoral components were resected with a portion of the femoral neck where possible, and were immediately fixed in buffered formalin. In 49 revisions, the acetabular components also were removed. The components were inspected, photographed, and measured for wear depth and clearance using a coordinate measuring machine. Initially, femoral components were sectioned into a variable number of sections to allow inspection of the cement-bone interfaces and access to samples of the bone from various locations for decalcified histologic analysis. Later, a more systematic sectioning protocol was followed to facilitate cement interface analysis on a subgroup of cases (see below). For each of these 98 cases, a mode of failure was determined based on our previously applied criteria.5 Cement fixation and bone histology were quantitatively analyzed in a group of 45 failed metal-on-metal surface arthroplasty implants, including 24 that had failed because of femoral neck fracture or loosening and 21 that failed because of other causes. All 45 implants were sectioned by cutting a 2-mm thick coronal section from the middle of the metaphyseal stem and from the middle of each resulting portion to yield three 2-mm thick slices from the anterior, middle, and posterior portions of the femoral head. Because only specimens with an intact cement-bone interface were suitable for this analysis, implants that had loosened to the point of disassociation of the bone from the cement could not be included. The sections were radiographed and photographed. The following analyses were then performed. The thickness of the cement mantle (defined as the cement layer between the metal and the outer edge of the prepared surface of the bone) and the depth of cement penetration into cancellous bone (ie, the interdigitation of cement into cancellous bone starting from the outer edge of the prepared bone) were measured in a blinded manner in 11 BHR implants, 22 Conserve® Plus implants, 11 McMinns implants, and one Cormet implant. The cut section photographs and microradiographs were scanned into a computer, calibrated to within 0.1 mm, and an image analysis software program (MetaMorph® Version 4.6, Universal Imaging Corp, Downingtown, PA) was used to measure the thickness and area of the cement mantle and the depth and area of penetration of cement into the cancellous bone in 10 sites across the entire section. The data were averaged to give cement mantle thickness, penetration depth, total mantle and penetration areas, and the percentage of the femoral head cross-section within the component occupied by cement (combining the areas of the cement mantle, penetration and cement-filled fixation pegs or cysts) in each of the anterior, middle, and posterior sections. Specimens from all 98 femoral components were examined histologically and the general appearance of the bone and marrow was used to determine if the failure was related to osteonecrosis, fracture, cement interface loosening or infection using standard histopathologic criteria.30 The bone sections were carefully removed from the sectioned metal shell and divided into two or three smaller pieces that included the cement. The pieces were photographed to preserve orientation, then decalcified and embedded in paraffin for routine sectioning and staining with hematoxylin and eosin. Because this involved paraffin embedding, the bone cement was removed, but the bone within the cement and the interfacial soft tissues were preserved. The presence of interfacial membranes and bone necrosis was recorded for each of the 45 cases used for cement analysis. Semiquantitative histologic analysis of features at the proximal cement interface, the middle of the head and the component-femoral neck edges was carried out in a blinded fashion on 25 of these sections, using a modification of the scheme used by Howie et al.24 Bone viability, bone formation, marrow viability, and interface membrane formation were rated as none, low if the feature occupied less than 10% of the 4× field of view, moderate if it occupied 10% to 50%, and high if more than 50% of the field of view showed the feature. Bone viability was judged by the presence of osteocyte nuclei in most lacunae of the bone trabeculae. Image analysis software (Image One, West Chester, PA) was used to position a 100-point grid over the field of view. The proportion of each feature was calculated by recording which feature lay at the intersecting points of the grid. The thickness of interfacial membranes was measured in 10 places using a calibrated caliper function. The clinical and radiographic histories of each case were reviewed together with the results of the above analyses to determine the factors associated with their failures. Statistical analysis was done using SPSS version 11.5 analytic software (SPSS, Inc, Chicago, IL). Means and standard deviations of all were calculated for each of failure implant type, and then were compared using there were three or more analysis of was For as or implant type, analysis was used to the of failures in each analysis then was used to assess the of each variable all were in the on the of failure or compared with One hundred components out of the in our were measured for wear to the sectioning which was not performed in the only components used in the failure analyses above were specimens were of one and were for the analysis of clearance and wear. were performed from less than to depth was measured with a coordinate measuring at to points over the surface of the implant. The clearance between the and was calculated from this analyses were performed to examine between clinical and implant factors with wear The cement thickness analysis to the being to have a or less for a cement mantle within the femoral head, the thickness of this layer often was Cement penetration was often well into the cancellous bone, and femoral head cysts were filled with cement. A finite element model was constructed using software Corp, and analyzed using software Inc, to the within the bone to the cement. The finite element model had a metal shell with a of and a thickness of at the A bone cement layer thick was To the the of the cancellous and bone the extent of the metal shell were as This model was with The femoral shell was by the cement layer by the cancellous bone by and the shell by The and interfaces were as The thermal analyses were performed on of the cement no cement penetration into the cancellous bone (ie, the penetration a cement-filled head of and penetration a cement-filled head of The depth of cement penetration and the presence of a cement-filled were by the of the of the model from of cancellous bone to of cement. the and were the for the of the cement The thermal of bone cancellous bone, and bone were based on previous The from the of the metal shell and bone was by with an of The analysis analysis of the finite element model and the the finite element model for the generation cement The amount of in the cement layer was calculated using the for cement by et where is the degree of is the of generation and is the total amount of which is The thermal analysis was performed in a of The of the was taken as as by et which to a of for all of the cement and a of for the first using a were into the finite element model to the first of the analysis. The results of the first then were used to the of the of the and of the for again using the the were into the finite element model to the of analysis. This was the cement was The main for failure were femoral neck fracture and femoral loosening loosening for failures of the first generation McMinn components and for failure in 10 (Table component problems as edge or in the failure of four Birmingham Hip Replacement components, one Cormet 2000 and one Conserve® Plus caused failures. failed arthroplasties to included one one case of and and performed for analysis femoral loosening and and an extensive of the tissues of a metal with some of these cases more than one of these of the 98 were the main cause of failure 1). was in only a few cases, but most occurred occurred less than after surgery femoral neck occurred in cases to failure, and were caused by extensive of the femoral head, by the of viability of the bone and There was no to the cut the occurred at the of a with of the A bone A the implants in good after a Conserve® Plus the patient had a fracture of the femoral This a cut section through the retrieved femoral head. The component stem the fracture A of an area of new bone at the component edge with of a previous fracture or through which the fracture This feature was in most of the hematoxylin and loosening occurred from to after The degree of loosening and of bone femoral loosening was associated with a the stem that often or more years before failure loosening failures were associated with an area of bone in the middle of the femoral head Histologic features were to a of the 21 failures caused by femoral loosening were associated with of fixation and femoral head because the proximal bone had been by thick that was often In the cases, loosening was associated with membrane formation between the cement and bone, from 50 to The membranes were and variable of and bone cement. Bone to the membrane often was active A had of the The femoral component loosened after the using a Conserve® Plus implant. taken before surgery a the metaphyseal stem of the femoral The cut section of the retrieved femoral component is A the component was proud, and also the presence of a within the femoral head. There was bone at this A a of bone by the membrane that had between the cement and the bone in a Conserve® Plus component after of the bone is on the There has been new bone formation a of bone hematoxylin and cases from a first generation metal-on-metal surface arthroplasty were revised in with failure, including from a bearing in a acetabular component of an were failures years to and in most cases, the femoral components were We an of a femoral head revised after 10 years This had McMinn metal-on-metal for of the The failed on the acetabular but the femoral component was well fixed at the of to total hip replacement. A cut section of the retrieved femoral component and the the presence of cement cysts that have the The staining of the is a cutting A of the cement-bone interface the of the cement to the bone and The previous of the bone nuclei but the bone is in this long-term hematoxylin and were performed for acetabular position that to edge and The acetabular from to The total wear depth was measured in four and from after 10 to at A wear on the femoral component in three was with the and edge of the components A a Birmingham Hip Replacement component for of the the patient had an and general and the acetabular component had been implanted at surgery 13 after the was and there was metal staining of the This wear wear depth of the femoral which measured wear depth of the acetabular component was the clearance was penetration not failure Cement mantle thickness and area and the depth and area of cement penetration were different in components that failed because of problems compared with failures the average cement mantle thickness in cases that failed from femoral loosening averaged mm, whereas failures to neck fracture averaged and with failures averaged (Table A the thickness of cement by the mode of failure with A cement mantle thickness in cement penetration depth in and amount of cement in the section mantle, penetration, and cement-filled fixation pegs and of Cement cement mantle area and thickness were different the implant with Conserve® Plus implants a average cement mantle and Birmingham Hip the average depth of penetration There was a for cement penetration to be when the cement mantle thickness was A cement mantle thickness in and cement penetration depth in total percentage of the femoral head sections occupied by cement cement-filled fixation pegs or and penetration from to and was more in failures compared with all other modes of failure. cysts were more in cases that failed by femoral loosening compared with failures. Bone was in areas by cement. There was a higher of bone the cement in the cases in which necrosis of bone and marrow to several from the the edges of the where cement to be had the osteonecrosis, where new bone had a was the cement and was associated with blood but was in most of the other membranes were this interface was more likely to be by marrow and bone Bone was associated with areas of bone formation or with membranes bone cement. bone was commonly in femoral but also was in the femoral from failures because the cement the bone through the and cement were within the femoral of four of these acetabular A histologic features of the cement-bone interface in the middle of the femoral head in a Birmingham Hip Replacement implant with cement after The marrow has been by and several this area is The bone is but is by a of bone hematoxylin and summarized wear depth and clearance (Table The wear depth varied from to varied from to that failed because of femoral fracture or loosening had wear to that failed because of acetabular infection or other but the may have been to this of Birmingham Hip which were revised for acetabular had higher average wear and clearance than the other The of in on wear was for the total which from less than one to more than 10 years, but when examined for cases retrieved at less than years in wear in the Conserve® and Cormet was higher than components in than years. the bone cement mantle was thick and there was no cement penetration into the femoral head only of the the bone and cement In with penetration or a cement-filled of or the within the bone were and respectively. The in the bone above for and with the above cement The in the bone was above to a depth the cement-bone interface with of penetration, with penetration a cement-filled of and with penetration a cement-filled of of the finite element model of cement within a femoral surface replacement. were from to penetration, and penetration and a cement-filled of The portions a high to The introduction of metal-on-metal bearings in surface arthroplasty has loosening caused by bearing wear fixation and failures are the primary for a and surface arthroplasty. Although cement is used for femoral fixation in most of the designs in the and of cement application and the amount of the cement mantle and bone penetration are This showed in cement in failed and femoral using thickness and area The of cysts with the problem of components, and the of low cement into bone of variable quality can in of cement in the femoral head. In some cases in this retrieval of the bone had We the amount of cement was greater in loosened femoral components. This should be a cause for concern in of the current cementing techniques where the of the cement mantle thickness and extent of penetration may be to as well as the of cementing bone is optimized to the applied to the femoral head, but the depth of cement penetration that could this as by bone from remodeling to new when an implant is is are to determine cement-filled bone or One of the concerns of cement use is thermal necrosis of the The results of the when cement penetration was with a relatively cement-filled the within the femoral head interfacial bone were high and the were long to cause thermal bone The model the amount of cement in many of the Because it not for as extensive of the prepared bone and the only of the cement bone interface will if the occur in Bone and marrow necrosis were to several to cement-filled cancellous bone or cysts in but not to cause failure. bone formation within to the cement also was in some The interfaces in several of the specimens with cement showed this membrane This may the ability of the bone to some degree of but this ability may be when several can only occur with the or restoration of an adequate femoral head blood In this there were failures. bone interfacial membrane formation and were in few cases with cement In membranes between bone and cement were where there was or no cement penetration, and there often was active bone from the We thick interfacial bone and of bone into in several femoral metal-on-metal surface arthroplasty components. features were in loosening failures of metal-polyethylene surface arthroplasty. Howie et loosening of metal-polyethylene surface arthroplasties with the of polyethylene at the cement-bone interface, to bone membrane formation, and of the femoral head bone as loosening In the of polyethylene the most likely factors interfacial membrane formation are and thermal necrosis. of components measured with from because of the layer that bone the bone cement. Femoral neck fracture after hip is a for the and the mechanisms this failure mode and The associated with the surface arthroplasty also may be a in neck in this retrieval occurred with surgeons new to the sections and cement mantle thickness showed the implants were not fully This the femoral neck at in several The may or to the to if there is bone or component bone may and could as a and the thick mantle may reduce bone cement penetration for as by our analysis. Although of metal-on-metal surface arthroplasty components has been bone after fracture is a of and our histologic analysis showed that occurred in these One of the major in this era of metal-on-metal bearings is the in wear provided by The wear of these failed components was with the of implants, when acetabular in of This may be a unique of metal-on-metal surface arthroplasty, but it should be preventable with and to surgical technique and implant of the components submitted to our laboratories were revised for and implant retrieval analysis was in in most of was the cause of in three This has been a concern implants were reports have some have an to wear of this Although this is a should be to a of In in a at a on surface arthroplasty, main to be overcome in the potential success of the arthroplasty are failures to femoral acetabular and The clinical the selection of a suitable patient with adequate bone and the associated with the the neck and the components by interdigitation of years with arthroplasty being used more than we some of the and but has been The of and clinical laboratory including wear implant design and retrieval will information in the of the success and failure of the new generation of surface arthroplasties. The and the at and all of the surgeons specimens for analysis.