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The Bone-Implant Interface of Femoral Stems with Non-Circumferential Porous Coating. A Study of Specimens Retrieved at Autopsy*

ROBERT M. URBAN, , JOSHUA J. JACOBS, M.D., DALE R. SUMNER, PH.D., CHRISTOPHER L. PETERS, M.D., FRANK R. VOSS, M.D. and JORGE O. GALANTE, M.D., D.SC., CHICAGO, ILLINOIS

Investigation performed at the Department of Orthopedic Surgery, Rush Arthritis and Orthopedics Institute, Rush-Presbyterian-St. Luke's Medical Center, Chicago.

	Abstract

Top Abstract Introduction Materials and Methods Discussion References

 
A histological study was performed of the bone-implant interface of fifteen titanium-alloy femoral stems with porous coating limited to three proximal areas that did not cover the full circumference of the device. The specimens were obtained at autopsy from ten cadavera at a mean of forty-six months (range, one to eighty-nine months) after the implant had been inserted without acrylic cement. The volume fraction of bone within the porous spaces (the percentage of the porous space that was filled with bone) and the extent of bone ingrowth (the percentage of the porous-coated surface covered with ingrown bone that was more than one-half fiber-diameter deep, as measured from the outer surface of the porous coating), were determined with histomorphometric methods. Eleven of the fifteen stems had bone within the porous coating that was in continuity with the surrounding medullary bone. The mean volume fraction of bone ingrowth in these specimens was 26.9 per cent (range, 12.2 to 61.0 per cent), and the mean extent of bone ingrowth was 64.3 per cent (range, 28.6 to 95.2 per cent). Both of these parameters increased with time. In the other four stems, the bone lacked continuity with the surrounding trabecular bed. Two of these stems had a limited amount of bone within the porous coating, and two stems (from one patient) had no bone ingrowth. Periprosthetic membranes surrounded by a shell of trabecular bone covered the uncoated surfaces of the stems. The membranes of implants that had been in situ for eight months or more demonstrated polyethylene wear debris, and other particles generated at the level of the joint, within histiocytes throughout the length of the femoral stem. CLINICAL RELEVANCE: The findings in this study are relevant to the utilization and mechanisms of failure of femoral stems inserted without cement. Bone ingrowth and the resulting stability of the implant can be achieved with porous-coated stems. However, the extent of the surface that is porous-coated must be sufficient to prevent trabecular fracture as a secondary mechanism of loosening. Interruptions in the circumferential extent of the porous surface are associated with the formation of periprosthetic membranes, which provide a pathway for migration of particulate wear and corrosion products to the distal part of the stem. A circumferential coating may retard the access of particles and thus decrease the possibility of diaphyseal osteolysis.

	Introduction

Top Abstract Introduction Materials and Methods Discussion References

 
Porous-coated femoral stems implanted without acrylic cement have been used widely in arthroplasty of the hip in younger patients, with the hope that a stable interface between the implant and bone would be obtained. However, the short-term clinical results of the first generation of these devices have included pain in the thigh, subsidence of the component, aseptic loosening, proximal loss of bone due to adaptive remodeling, and osteolysis associated with particulate debris^{2,4-7,11,14,19,26,27,30,33,35,37,41}. The biological and mechanical factors responsible for these findings are poorly understood.

It was thought initially that the elimination of acrylic cement from total hip arthroplasty would reduce the prevalence of periprosthetic osteolysis. However, it is now clear that osteolysis associated with particulate debris is a major cause of failure of femoral stems inserted with or without cement^{1,13,18,20,21,24,25,28,29,39,47,53,56}. Indeed, several authors have indicated that the prevalence of osteolysis actually has been higher in association with the first generation of porous-coated femoral stems than in association with stems that have been inserted with cement^15,17,21,32.

Examination of porous-coated femoral stems that have failed clinically and have been recovered in revision operations has shown that the prevalence and amount of bone ingrowth are far less than those reported in animal studies^8-10,43,49. The authors of one clinical retrieval study hypothesized that loss of fixation by ingrown bone is due to fracture of the supporting osseous trabeculae²³. At present, neither the basic design parameters of the location and extent of the porous coating nor the amount of bone ingrowth necessary to maintain a biologically stable interface have been determined.

The purpose of the present study was to determine the histological characteristics of the bone-prosthesis interface of fifteen femoral components recovered at autopsy from the cadavera of patients in whom the prosthesis had functioned well. The femoral components were all of the same design: the porous surface was limited to three proximal areas that did not cover the full circumference of the device.

	Materials and Methods

Top Abstract Introduction Materials and Methods Discussion References

 
Fifteen femoral components and the corresponding femora were recovered at autopsy from ten cadavera of nine patients (thirteen hips) who had had a primary total hip arthroplasty and one patient (Case 4) who had had a primary bilateral bipolar hemiarthroplasty. Six patients (nine hips) had been operated on at our institution, and four patients (six hips) had been managed at other hospitals.

Clinical information was obtained from office notes, medical records, and radiographs (Table I). The mean age of the six men and four women at the time of the operation had been fifty-three years (range, thirty-three to seventy-four years). The preoperative diagnosis had been osteonecrosis in five patients (ten hips), osteoarthrosis in four patients (four hips), and rheumatoid arthritis in one patient (one hip). The cause of death had been metastatic carcinoma in four patients, septicemia in two, myocardial infarction in two, pneumonia in one, and renal failure in one. The fifteen femoral components had been in situ for a mean of forty-six months (range, one to eighty-nine months). The mean body weight of the seven patients (Cases 4 through 10) for whom such information was available had been sixty-nine kilograms (range, fifty-seven to eighty-six kilograms).

For the six hips (four patients [Cases 1 through 4]) that had been treated at other hospitals, the result of the arthroplasty had been satisfactory according to the surgeon. Four of these hip replacements had been in place for only one to eight months. In the remaining two hips, both in the patient (Case 4) who had had bilateral bipolar hemiarthroplasty, the components had been in place for twenty-five and twenty-eight months. This patient had had a seven-year history of end-stage renal disease and had received ongoing hemodialysis. He had had normal function of the hips when he was last examined, five days before he died.

A Harris-Galante femoral component (Zimmer, Warsaw, Indiana), fabricated from Ti-6Al-4V alloy (nominal composition, 5.5 to 6.5 per cent aluminum and 3.5 to 4.5 per cent vanadium, with the balance consisting of titanium), had been used in all of the hips. There was a commercially pure titanium fiber-metal porous coating on the anterior, posterior, and medial surfaces of the proximal part of the stem (Fig. 1). One stem (Implant 12) also had a porous coating on the proximal-lateral surface. The femoral components had a cast cobalt-chromium-alloy modular head. In the thirteen hips in which the acetabulum had been reconstructed, a Harris-Galante cup (Zimmer) had been inserted without cement but with use of three, four, or five titanium-alloy screws for initial fixation.

View larger version (53K): [in this window] [in a new window]   Fig. 1 Photograph of the Harris-Galante titanium-alloy femoral component (Zimmer, Warsaw, Indiana) with a commercially pure titanium fiber-metal porous coating on the anterior, posterior, and medial surfaces of the proximal part of the stem. Note the interruptions in the circumferential extent of the porous surface. The modular head is made of cobalt-chromium alloy.

Methods of Analysis
After removal of the soft tissues, the femur with the prosthesis in place was fixed in neutral buffered formalin. Transverse cuts through the specimen were made at one-centimeter intervals along the length of the femur, beginning at the level of the collar and extending distal to the tip of the stem, with use of a diamond-blade cutoff saw. These pieces were embedded in methylmethacrylate without decalcification. The hardened specimen blocks were cut into sections, which were carbon-coated for quantitation of bone ingrowth with scanning electron microscopy; other sections were ground and stained for evaluation with light microscopy.

Bone ingrowth into the porous coating was measured on backscattered scanning electron microscopic images (model 840A; JEOL, Peabody, Massachusetts); these images were digitally stored and were analyzed with the aid of a computer (model 3000; Image Technology, Deer Park, New York)⁴². The measurements encompassed the entire interface between the porous coating and the bone at each of the four or five one-centimeter levels that had such coating. The measurements were expressed as both the volume fraction and the extent of bone ingrowth.

The volume fraction of bone ingrowth is defined as the percentage of the porous space that is occupied by mineralized bone^{38,42,43,48,49}. The volume fraction, which is a three-dimensional quantity, has been shown to be equal to the relative area of a structure as determined from a two-dimensional image³⁴. For the measurements of volume fraction, a mean of 196 square millimeters of the porous coating was analyzed.

The extent of bone ingrowth is defined as the percentage of the porous surface covered with ingrown bone that is more than one-half fiber-diameter deep, as measured from the outer surface of the porous layer. This measurement provides an indication of the topographic distribution of bone ingrowth over the surface of the porous coating^{12,36,44-46,50}. The extent of bone ingrowth was measured by classifying one-millimeter-wide fields as positive or negative for mineralized bone, with a mean of 126 fields assessed for each implant. The data are presented according to the location of the porous coating.

The plastic-embedded sections for histological analysis were ground to approximately 100 micrometers and were surface-stained with basic fuchsin and toluidine blue, as described previously³⁶. Regular and polarized light microscopy were performed to characterize the interface tissues within and adjacent to the porous-coated and uncoated surfaces of the femoral components.

In order to provide greater cellular detail and to identify particulate debris in the interface tissues, four-micrometer-thick sections were prepared from selected areas of the interface. The areas of interest were excised from the plastic sections and were deplasticized, re-embedded in paraffin, and cut into serial sections that were alternately stained with hematoxylin and eosin and left unstained. Particles of polyethylene were identified in the stained thin sections with polarized light microscopy. Other particles were identified in the unstained thin sections with electron microprobe analysis (model 8600; JEOL) with use of energy-dispersive and wavelength-dispersive x-ray analysis, as described previously^22,51. Selected specimens were examined with transmission electron microscopy (model JEM-4000FX; JEOL) and energy-dispersive x-ray analysis and with Fourier transform infrared microprobe spectroscopy (model I micro RS; Spectra-Tech, Stamford, Connecticut).

Statistical analyses (analyses of variance for repeated measures, paired t tests, and Spearman rank-order correlations) were performed to determine if the location of the porous coating, the duration of the implant in situ, or the Harris hip score at the last follow-up examination had influenced the amount of bone within the porous coating. It was assumed that the two implants from a bilateral arthroplasty could be analyzed independently. Actual significance levels are given, and a result was considered significant when p was less than 0.05.

Results
Two distinct conditions at the interface of the bone and the porous coating were recognized on the histological sections. The first condition, observed in eleven components (Implants 1 through 5 and 10 through 15), was characterized by bone within the porous coating that was in continuity with the surrounding medullary bone. The second condition, noted in two stems that had bone ingrowth (Implants 6 and 9) and in two stems that did not (Implants 7 and 8), was characterized by a lack of osseous connection between the porous coating and the surrounding medullary bone bed.

Stems with Bone Ingrowth in Continuity with the Surrounding Medullary Bone
The eleven femoral components that had bone ingrowth in continuity with the surrounding medullary bone had an over-all mean volume fraction (and standard deviation) of bone ingrowth of 26.9 ± 17.6 per cent (range, 12.2 to 61.0 per cent) and a mean extent of bone ingrowth of 64.3 ± 23.6 per cent (range, 28.6 to 95.2 per cent). These two measurements were correlated with each other (r² = 0.42, p = 0.03). Both the volume fraction and the extent of bone ingrowth increased with the duration of implantation (r² = 0.48, p = 0.02, and r² = 0.64, p = 0.003, respectively) (Fig. 2). There was no proximal-to-distal difference in the volume fraction of bone ingrowth as determined with analysis of variance for repeated measures. Neither the volume fraction nor the extent of bone ingrowth was correlated with the last recorded Harris hip score.

View larger version (25K): [in this window] [in a new window]   Fig. 2 Graph showing the increase with time in the extent of bone ingrowth in the eleven components that demonstrated continuity between the ingrown bone and the surrounding trabecular bed (r2 = 0.64, p = 0.003). The extent of bone ingrowth is the percentage of the porous surface covered with ingrown bone that is more than one-half fiber-diameter deep, as measured from the outer surface of the porous layer. This was determined by classifying one-millimeter-wide fields as positive or negative for bone ingrowth.

The nature of the ingrown bone varied with the duration that the implant had been in situ. The specimen that was retrieved at one month (Implant 1) had slender trabeculae of woven bone that had penetrated two-thirds of the depth of the fiber-metal porous coating (Fig. 3-A). Few lamellae were seen on these young trabeculae. In the medullary bone adjacent to the porous coating, new bone had been laid down on the original trabeculae. Between these trabeculae were fine spicules of new bone, similar in development to those seen within the porous layer. The volume fraction of bone ingrowth was 13.1 per cent, and the extent was 48.2 per cent.

View larger version (112K): [in this window] [in a new window]   Figs. 3-A and 3-B: Backscattered scanning electron microscopic images demonstrating the nature of the ingrown bone in the components that had continuity of bone between the porous layer and the adjacent medullary bone. Fig. 3-A: Implant 1. At one month, slender trabeculae of woven bone were present within and adjacent to the porous coating, and new bone had been laid down on the original trabeculae (x 45).

Implants that had been in place for more than two years (Implants 5 and 10 through 15) had lamellar trabeculae and dense plates of lamellar and, occasionally, woven bone (Fig. 3-B). The proximal-medial part of the medullary cavity and the medial porous coating had particularly dense bone that often completely filled the pores of the fiber metal. The mean volume fraction of bone ingrowth in these specimens was 34.5 per cent (range, 12.2 to 61.0 per cent), and the mean extent of bone ingrowth was 76.7 per cent (range 40.3 to 95.2 per cent).

View larger version (126K): [in this window] [in a new window]   Fig. 3-B Implant 14. Thickened trabeculae and dense plates of lamellar bone nearly fill the void spaces of the anterior porous coating of this specimen, obtained after seventy-eight months (x 40).

Uncoated Surfaces of the Stem
All eleven stems (Implants 1 through 5 and 10 through 15) that had bone ingrowth in continuity with the surrounding trabeculae had at least one proximal uncoated surface that was covered with a membrane surrounded by a shell of trabecular bone (Fig. 4). The membranes and the trabecular shells spanned the uncoated surfaces of the stem from the edge of one fiber-metal pad to that of a neighboring one (Fig. 5-A).

View larger version (132K): [in this window] [in a new window]   Fig. 4 Implant 14. Transverse histological section through the proximal portion of a stem retrieved after seventy-eight months. There is extensive bone ingrowth in the anterior (top), posterior (bottom), and medial (right) porous surfaces. Membranes surrounded by a shell of trabecular bone are present on the lateral (left), anteromedial, and posteromedial surfaces where the porous coating was absent. The membranes covering all of the implants that had been in place for eight months or more contained particulate debris (plastic-embedded, undecalcified, stained section; x 5).

View larger version (91K): [in this window] [in a new window]   Figs. 5-A and 5-B: Photomicrographs of proximal transverse sections. Fig. 5-A: Implant 15. The lateral half of the stem and the adjacent tissue are shown eighty-nine months after implantation. A membrane covered by a shell of bone spans the uncoated lateral (left) surface of the stem from the edge of the anterior fiber-metal pad (top) to that of the posterior pad (bottom). This membrane contained abundant polyethylene debris (plastic-embedded, undecalcified, stained section; x 7).

View larger version (115K): [in this window] [in a new window]   Fig. 5-B: Implant 10. The medial half of the stem and the adjacent tissue are shown sixty-two months after implantation. There is extensive bone ingrowth in the medial (right), anterior (top), and posterior (bottom) porous-coated surfaces. Particulate-laden membranes are present at the proximal-anteromedial and posteromedial surfaces where the porous coating is absent (plastic-embedded), undecalcified, stained section: x 9).

View larger version (168K): [in this window] [in a new window]   Fig. 6 Implant 14. Macroscopic image of an entire transverse section of the distal portion of the stem and femur after seventy-eight months. Membranes (arrows) are interposed between the stem and a shell of trabecular bone at the anterior (top), posterior (bottom), and lateral (left) aspects of the stem (plastic-embedded, undecalcified, stained section; x 3).

Particulate Debris at the Bone-Implant Interface
The periprosthetic membranes were generally composed of moderately dense connective tissue consisting of collagen fibers oriented parallel to the uncoated surface of the implant, few fibroblasts (except on the specimens obtained at one and four months), and widely variable amounts of histiocytic infiltration and particulate debris. The surface of the membrane facing the implant often was covered by a layer of necrotic substance and fibrin-like material, while the side of the membrane facing the shell of trabecular bone was composed of less dense connective tissue that was rich in small blood vessels (Fig. 7). On the specimens obtained at six months or less (Implants 1, 2, and 3), the membranes were free of metal or polyethylene debris and aggregates of histiocytes. However, the membranes on all of the stems that had been in situ for eight months or more demonstrated focal areas of abundant polyethylene particles in histiocytes, spindle cells, and, occasionally, foreign-body giant cells. On four of the stems that had been in situ for a longer duration (Implants 10, 13, 14, and 15), the infiltration of polyethylene-laden histiocytes was so great that it dominated the histological character of some parts of the membranes, even at the most distal level of the stem (Figs. 8-A and 8-B). Portions of the membranes had been almost entirely replaced by histiocytes, which were arranged between remaining thin bands of collagen with scarce fibroblasts. In most of the specimens, the histiocytes were confined to the periprosthetic membranes. Adjacent to three of the stems that had been in situ for a longer duration (Implants 11, 14, and 15), particulate-laden histiocytes had infiltrated the vascular spaces of the surrounding bone and the marrow at the distal portion of the stem.

View larger version (123K): [in this window] [in a new window]   Fig. 7 Implant 4. Photomicrograph, made under polarized light, showing a periprosthetic membrane from the proximal-lateral portion of the stem eight months after implantation. The membrane is composed of a layer of fibrin-like material at the surface facing the implant (top), a central layer consisting of parallel collagen fibers with relatively few fibroblasts (middle), and a layer rich in small blood vessels immediately adjacent to the shell of trabecular bone (bottom). In this early specimen, birefringent polyethylene particles can be seen primarily within the fibrin-like layer near the top of the image and to a much lesser degree in the fibrous layer (paraffin-embedded, hematoxylin-and-eosin-stained section; x 300).

View larger version (155K): [in this window] [in a new window]   Figs. 8-A and 8-B: Implant 14. Photomicrographs of the membrane between the distal portion of the stem and the shell of trabecular bone of the specimen shown in Fig. 6, seventy-eight months after implantation (paraffin-embedded, hematoxylin-and-eosin-stained section; x 375). Fig. 8-A: Photomicrograph demonstrating characteristics typical of the implants that had been in situ for a longer duration. Some areas of the periprosthetic membrane are completely dominated by rows of large histiocytes, which were arranged between thin collagenous bands with scarce fibroblasts. Scattered lymphocytes are also present in this specimen. Few metal particles were apparent under the light microscope. The aspect of the membrane facing the stem is at the top of the image.

View larger version (155K): [in this window] [in a new window]   Fig. 8-B Polarized light micrograph of the same field, demonstrating the abundant birefringent polyethylene debris within the histiocytes. The particles in this field are as large as thirty-five micrometers, but most are less than one micrometer.

View larger version (178K): [in this window] [in a new window]   Fig. 9 Implant 14. Photomicrograph of a membrane adjacent to the proximal-lateral portion of the stem of a specimen obtained after seventy-eight months. Two approximately 100-micrometer pale-green particles of corrosion products of a chromium orthophosphate hydrate-rich material are surrounded by giant cells. The particles were generated at the corroded modular head-neck junction of the component (paraffin-embedded, hematoxylin-and-eosin-stained section; x 750).

Stems without Osseous Connection to the Surrounding Trabeculae
Two stems (Implants 6 and 9) had bone within the porous coating without osseous connection to the surrounding trabeculae (Fig. 10). Examination of several additional slides, produced from between each of the one-centimeter levels of sectioning, confirmed the lack of continuity at all levels of the porous coating. The volume fraction (8.8 and 3.0 per cent) and the extent (33.8 and 13.8 per cent) of bone ingrowth in these two stems were less than those observed in the eleven stems in which ingrown bone was in continuity with the surrounding trabeculae. In these two stems, the outer surface of the porous coating was separated from the medullary bone bed by a membrane of collagen, fibrin, and necrotic bone fragments of various sizes. The surrounding medullary bone had multiple areas of resorption, evidenced by scalloped margins and osteoclasts.

View larger version (119K): [in this window] [in a new window]   Fig. 10 Implant 6. Backscattered scanning electron microscopic image of the bone-porous coating interface of a stem retrieved after twenty-eight months. The ingrown bone was not in continuity with the surrounding medullary bone (x 35).

Unlike most of the periprosthetic membranes of the specimens demonstrating continuity of bone, the membranes surrounding Implants 6 and 9 had accumulations of fine metal debris that were obvious on light microscopy. These particles were identified on electron microprobe analysis as titanium alloy and commercially pure titanium, the material from which the porous coating had been fabricated.

The two specimens without ingrowth of bone (Implants 7 and 8) had been obtained from one cadaver (Case 5). The right femoral stem (Implant 8) was almost entirely encompassed by loose, granular necrotic debris and fragments of dead trabeculae, beginning at the most proximal portion of the stem and extending distal to the tip of the prosthesis. Immediately beneath the collar of the prosthesis, the proximal-medial cortex demonstrated microfracture and fragmentation of the bone. Neither metal nor polyethylene debris was detected at the bone-prosthesis interface except in the region inferior to the collar, where aggregates of titanium-alloy particles were identified within the vascular spaces of the proximal-medial cortex. The left femoral stem (Implant 7) was completely surrounded by a thick, collagenous membrane that penetrated and filled the porous coating. There were fragments of necrotic bone and few histiocytes within the membrane. At the anteromedial and medial aspects of the proximal portion of the stem, islands of fibrocartilage had formed within the membrane.

	Discussion

Top Abstract Introduction Materials and Methods Discussion References

 
In this study of femoral stems retrieved at autopsy, the ingrowth of bone into the porous surface was of greater magnitude than has been reported previously in association with failed prostheses^8-10. Moreover, in the present study, bone ingrowth increased progressively with time. The early biological response to implantation, as demonstrated in the specimen obtained at one month, was characterized by the formation of slender trabeculae of intramembranous bone at the bone-implant interface. The continuity between the bone within the porous coating and the adjacent bone suggests that there was a certain degree of mechanical stability of the stem as early as one month after implantation. The implants that had been in situ from four to eight months also demonstrated time-dependent patterns of bone ingrowth in which trabecular thickness increased with time. One would expect a corresponding time dependence of the strength of fixation, which would provide a rationale for the clinical practice of protected weight-bearing in the early postoperative period. In several implants that had been in situ for more than five years, ingrown bone covered nearly 100 per cent of the porous surface. This finding indicates that substantial appositional growth of bone continues long after the earlier response to implantation has waned.

In the current study, bone ingrowth was evaluated with use of two indices: volume fraction and extent. Both measurements have been used widely to characterize the tissues within microporous metal coatings of hip^12,36,46 and knee^44,45,50 prostheses retrieved in experimental and clinical studies. The volume fraction of bone ingrowth represents the over-all percentage of the available pore space within the porous coating that is occupied by mineralized bone, while the extent is the topographic distribution of bone ingrowth over the porous surface of the prosthesis, expressed as the percentage of fields that are positive for bone ingrowth. Although the volume fraction and the extent of bone ingrowth are often directly related, it is possible to have a high value for extent coupled with a low volume fraction. This was observed in the femoral stems from one patient (Case 7), who had had steroid-induced osteopenia; much of the porous surface of the components had ingrowth of thin lamellar spicules of bone.

The amount of bone ingrowth that is required to provide stable, long-term fixation is directly related to the surface area of the porous coating on a device. In the current study, approximately 15 per cent of the surface area of the stem was occupied by a porous coating. As the extent of bone ingrowth (the percentage of the porous surface covered with ingrown bone) was a mean of approximately 64 per cent, fixation by the ingrowth of bone in these patients depended on a net of only 10 per cent of the over-all surface of the implant. This may explain, in part, the relatively high prevalence of loosening of the femoral component (eleven [9 per cent] of 121) reported after primary total hip replacement with use of this device³¹.

In two specimens in the current study (Implants 6 and 9), bone was present in the porous spaces but was not continuous with the surrounding trabecular bone bed. While it is not known for certain whether the bone was ever in continuity, a likely explanation for this observation is that these two specimens represent late failure of the fixation of the trabecular bone in association with limited amounts of bone ingrowth. A history of trauma with a subsequent well defined change in the clinical presentation was documented for one of these patients (Case 6). The other patient (Case 4) had osteomalacic renal osteodystrophy with woven bone, which may have failed because of mechanical inadequacy.

No bone was present in the porous coatings of two prostheses (Implants 7 and 8), from a patient (Case 5) who had had bilateral hip replacement for idiopathic osteonecrosis and excellent clinical scores at fifty-one and sixty-nine months postoperatively. The findings in the periprosthetic tissues, which included extensive necrosis, dense fibrous tissue with foci of cartilage metaplasia, and the accumulation of metal debris beneath the collar of the prosthesis, suggest relative motion between the implant and bone and are probably histological characteristics of unstable implants. There was no obvious clinical explanation for the lack of bone ingrowth in the prostheses or for the apparent absence of polyethylene particles at the bone-implant interface.

An important finding in this study was that membranes bordered by a trabecular shell of bone were present adjacent to the uncoated proximal surfaces of all of the specimens that demonstrated continuity of bone between the porous coating and the surrounding trabeculae. The fact that these membranes were detected as early as one month after insertion of the stem indicates that the formation of this tissue is related to factors other than particulate debris, which appeared later. A membrane bordered by a trabecular shell was present at the distal portion of all but one stem, and in several specimens these membranes contacted the endosteal surface of the cortex. Although no expansile, lytic lesions were found near the tip of the prosthesis, the membranes on several implants that had been in situ for a longer duration contained large numbers of histiocytes laden with particulate debris, including polyethylene from the bearing surface. In addition, several specimens demonstrated metal and polyethylene particles within the vascular spaces of the bone surrounding the tip of the stem. These findings graphically demonstrate transport pathways from the joint cavity to the most distal aspect of well fixed femoral stems and are relevant to the high prevalence of femoral diaphyseal osteolysis that has been reported with this design of stem^{13,14,21,26,31}. A circumferential coating might prevent the formation of these periprosthetic membranes and thus retard the distal migration of particulate debris. This has been demonstrated in both weight-bearing^43,48,49,52 and non-weight-bearing³ experimental models. For that reason, we believe that the porous coating of femoral stems should be circumferential.

Most of the particles within the periprosthetic membranes were polyethylene, but in several specimens there were also substantial amounts of corrosion products from the cobalt-chromium-alloy modular head and fewer particles of titanium alloy. In one specimen, stainless-steel-alloy particles from trochanteric wires were found. Although polyethylene has been implicated as the main factor responsible for periprosthetic osteolysis, the participation of other particles in this process should not be discounted^{17,21,22,40,51,56}.

The findings of this autopsy retrieval study suggest that most patients who have a stable porous-coated implant without complications have considerable bone ingrowth, in contrast to findings reported after previous large-scale retrieval studies of clinically unsuccessful devices^8-10. Although there are certain advantages to limiting the porous coating to the proximal portion of the stem, its extent must be great enough to allow an area of bone ingrowth that is adequate to support the loads likely to be encountered in active patients who have a well functioning hip replacement. It is also clear that interruptions in the circumferential extent of the porous coating promote the formation of periprosthetic membranes that serve as pathways for the distal migration of particles generated at the level of the joint. This process of formation of periprosthetic membranes and transport of particulate debris probably plays an important role in the development of femoral diaphyseal osteolysis, which has been identified as a major cause of failure of implants of this design.

NOTE: The authors thank research assistants Deborah Hall and Susan Infanger and research nurse Leslie Patterson for their technical contributions. The authors are also grateful to Richard Blakey, M.D.; John Hunter, M.D.; David Kyzer, M.D.; Aaron Rosenberg, M.D.; Mitchell Sheinkop, M.D.; and Erwin Stauffer, M.D., for their valuable cooperation. Manmohan Singh, M.D., provided consultation regarding the histological findings in the patients who had metabolic bone disease.

	Footnotes

 
*One or more of the authors have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. In addition, benefits have been or will be directed to a research fund or foundation, educational institution, or other non-profit organization with which one or more of the authors are associated. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were National Institutes of Health Grant AR39310 and Zimmer-USA.

Department of Orthopedic Surgery, Suite 103, 2242 West Harrison Street, Chicago, Illinois 60612.

Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, 1653 West Congress Parkway, Chicago, Illinois 60612.

	References

Top Abstract Introduction Materials and Methods Discussion References

 

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