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© 1997 The Journal of Bone and Joint Surgery, Inc. Hydroxyapatite-Coated Total Hip Femoral Components in Patients Less Than Fifty Years Old. Clinical and Radiographic Results after Five to Eight Years of Follow-up* , INDIANAPOLIS , MOON TOWNSHIP, PENNSYLVANIA, INDIANAPOLIS, INDIANA , FRANKLIN LAKES, NEW JERSEY*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. No funds were received in support of this study.
One hundred and thirty-three patients (152 hips) who were an average of thirty-nine years old (range, sixteen to forty-nine years old) received a proximally hydroxyapatite-coated femoral prosthesis as part of a total hip arthroplasty and were followed for a minimum of five years (average, 6.4 years; range, five to 8.3 years) or until revision. The average Harris hip score was 47 points (range, 22 to 77 points) preoperatively and 93 points (range, 49 to 100 points) at the time of the latest clinical evaluation. Two patients who had a well fixed femoral implant had activity-limiting pain in the thigh at the time of the most recent examination. Radiographic changes consistent with bone-remodeling (cortical hypertrophy and bone condensation) typically were seen around the mid-part of the shaft of the prosthesis. Forty-eight (32 per cent) of the 148 hips that were included in the radiographic analysis demonstrated a small amount of erosive scalloping in either zone 1 or zone 7 of Gruen et al., and intramedullary osteolysis was suspected in only one hip. All stems were radiographically osseointegrated according to a modification of the criteria described by Engh et al. Four stems were revised, but none of the revisions were performed because of mechanical failure (two stems were revised in conjunction with a revision of the cup because of pain; one, because of an infection; and one, after a traumatic femoral fracture that occurred six years postoperatively). Thus, the rates of aseptic and mechanical failure were both 0 per cent. The combined rate of failure, which included the two stems that were revised because of pain and the two stems that were associated with pain that limited activity, was 2.6 per cent (four of 152 stems). The over-all clinical results associated with hydroxyapatite-coated femoral components were excellent in this group of young patients after intermediate-term follow-up. A review of serial radiographs showed mechanically stable implants with osseous ingrowth, evidence of stress transmission at the middle part of the stem, and minimum endosteal osteolysis.
The treatment of hip disease in young patients remains a major challenge to the orthopaedic surgeon. Numerous operative techniques and prostheses have been evaluated over the years. Surface-replacement components, which were designed to preserve as much bone stock as possible, have not been associated with good long-term results4,25. Early studies of cemented total hip prostheses in young patients also have demonstrated high rates of failure8,10,13. Advances in cementing technique have produced moderately to greatly improved results after a minimum duration of follow-up of ten years1,2. In addition, it is hoped that techniques that do not involve the use of cement will provide long-term stability of femoral implants in young patients. However, early evidence suggests that this may not be the case. Although the rates of aseptic loosening of stems inserted without cement have been lower than those of cemented stems, pain in the thigh, subsidence, and osteolysis have been noted after five to seven years of follow-up5,19,22. The increased level of activity and the potential longevity of young patients managed with total hip arthroplasty continue to provide the impetus for the development of methods of more durable and longer-lasting fixation. The preservation of bone stock in such patients also is important because of the potential need for a revision operation in the future. Synthetic hydroxyapatite is a safe, non-toxic, and highly biocompatible material18. Although studies of hip implants with a thin coating of hydroxyapatite have demonstrated rapid and early bonding with bone18, evidence of new-bone formation, and excellent clinical results after five years of follow-up11,12,17, the durability and strength of this coating over time have not yet been demonstrated, to our knowledge. The purpose of the present report is to describe the clinical and radiographic results of implantation of hydroxyapatite-coated femoral components as part of a total hip arthroplasty in patients who were less than fifty years old and who were followed for at least five years or until revision.
As part of a prospective, multicenter study, thirteen surgeons at eleven institutions implanted 370 hydroxyapatite-coated femoral components (Omnifit-HA; Osteonics, Allendale, New Jersey) during total hip arthroplasties that were performed between January 1988 and November 1990. Although there were no specific age limits for inclusion in that study, the investigators were provided with guidelines that indicated that this component had been developed to fulfill the needs of younger, more active patients. The average age of the patients at the time of the arthroplasty was fifty years (range, sixteen to eighty-one years), and 226 procedures (61 per cent) were performed in male patients. Only arthroplasties that had been done as primary procedures in patients who were less than fifty years old and who had been followed for at least five years (or until revision) were included in the present analysis. Of the 169 implants in patients who were less than fifty years old, five had been inserted during revision operations and twelve had been in place for at least five years but the patient had not returned for assessment or the surgeon had not forwarded data for analysis. The present study therefore included 152 hips in 133 patients who were an average of thirty-nine years old (range, sixteen to forty-nine years old); ninety-six hips (63 per cent) were in male patients and fifty-six, in female patients. The most common diagnoses were osteoarthrosis (seventy hips; 46 per cent) and avascular necrosis (forty-two hips; 28 per cent). Four hips had a revision of the femoral component; the other 148 hips were followed for an average of 6.4 years (range, five to 8.3 years). The femoral component was a double-wedged, grit-blasted, collarless, straight, titanium-alloy implant with normalization steps on the anterior and posterior surfaces. A fifty-micrometer-thick circumferential coating of hydroxyapatite was applied to the proximal one-third of the stem with use of a plasma-spray process. Ninety-eight hydroxyapatite-coated acetabular cups, fifty-one porous-coated cups, and three bipolar components were implanted in conjunction with the hydroxyapatite-coated stems. Clinical and radiographic data were collected preoperatively, in the early postoperative period, at six months, at one year, and annually thereafter. Because of the multicenter nature of this study, standardized forms were used for the collection of data. An individual at each center was designated to complete all clinical forms and to return them to a central location for analysis. All radiographs also were sent to the central location and were reviewed in a standardized manner by one of the senior two of us (W. N. C. or J. A. D.), both of whom were blinded to the clinical status of the patients. The clinical assessment included an evaluation of pain as well as of functional parameters such as walking, stair-climbing, use of external support, limp, and ability to perform daily activities involving the hip. A composite Harris hip score23 was calculated on the basis of these data. Radiographs were evaluated for the presence of radiodense lines in the modified zones of Gruen et al.21,26, resorption of the calcar, subsidence, periosteal cortical hypertrophy, cancellous condensation, and osteolysis. Stability of the femoral implant was determined according to a modification of the criteria described by Engh et al.15. A stem was considered to be stable with osseous ingrowth if there was an absence of radiodense lines as well as accretion of endosteal bone (that is, cancellous condensation or spot welds) in all hydroxyapatite-coated zones. A stem was considered to be stable with fibrous ingrowth if there were parallel radiodense lines involving the hydroxyapatite-coated zones but there was no subsidence. A stem was considered to be unstable if it was surrounded by non-parallel radiodense lines or if it had subsided. The performance of the implant was evaluated according to three different indices. The first was simply the rate of aseptic failure, which was based on the number of stems that had been revised because of aseptic loosening. The second was the rate of mechanical failure, which was based on the number of stems that had been revised because of aseptic loosening or that were radiographically loose. The third and most critical was the combined rate of failure, which was based on the sum of clinical and mechanical failures. A stem was considered to have failed clinically if it was associated with pain that limited activity or that necessitated a revision.
One hundred and forty-eight hips were included in the analysis of clinical and radiographic results; the other four hips had had a revision of the femoral component and therefore were excluded from this portion of the analysis. The average Harris hip score23 was 47 points (range, 22 to 77 points) preoperatively and 93 points (range, 49 to 100 points) after five to eight years of follow-up. Of the eleven hips that had a Harris hip score of less than 70 points, nine were in patients who had a loose acetabular component for which a revision was pending at the time of follow-up, one was in a patient in whom the functional deficits were attributed to back pain, and one was in a patient who had chronic pain in the thigh but no evidence of loosening of either the femoral or the acetabular component. One hundred and fifteen patients (130 hips; 88 per cent) had no or slight pain in the hip. Similarly, 115 patients (130 hips; 88 per cent) walked without support. One hundred and three patients (115 hips; 77 per cent) were able to walk for an unlimited distance, and 122 patients (137 hips; 93 per cent) had no limp or a mild limp. Nine patients reported slight pain in the thigh, and two patients had moderate pain in the thigh that limited activity. The type of bone was determined on the basis of preoperative radiographs with use of the method described by Dorr et al.14. Fifty hips (34 per cent) were found to have type-A bone; eighty-five hips (57 per cent), type-B bone; and thirteen hips (9 per cent), type-C bone. At the time of the latest radiographic assessment, forty-eight hips (32 per cent) demonstrated a small amount of erosive scalloping of the femoral neck at the level of resection (in either zone 1 or zone 7 of Gruen et al.21). Intramedullary osteolysis was suspected in one patient (one hip), in whom a small lesion (less than one centimeter in its largest dimension) in zone 926 initially was noted on a lateral radiograph that was made five years postoperatively. This patient also was noted to have excessive polyethylene wear as well as osteolytic lesions in zones 1 and 7 of Gruen et al.21. Six years postoperatively, she had an exchange of the polyethylene liner and bone-grafting of the lesions in zones 1 and 7. The acetabular and femoral components were both stable at the time of the reoperation, and they were left in situ. At the time of the most recent radiographic evaluation (eight years postoperatively), the bone graft was noted to have been incorporated and the lesion in zone 926, while still present, was no larger than it had been when first noted (Figs. 1-A, 1-B, 1-C, 1-D).
Remodeling of the femoral neck with loss of height or thinning was noted in eighty-six hips (58 per cent). Changes consistent with bone-remodeling (cortical hypertrophy and condensation of endosteal bone) typically were seen around the mid-part of the shaft of the prosthesis. Ninety-three hips (63 per cent) demonstrated cortical hypertrophy and 148 hips (100 per cent) demonstrated condensation of bone in at least one zone21 on the most recent anteroposterior radiograph (Figs. 2-A, 2-B, 2-C, 2-D). Parallel radiodense lines, with no evidence of formation of a so-called pedestal, were seen around the distal, uncoated portion of the stem in eighty-seven hips (59 per cent). Radiodense lines were uncommon around the hydroxyapatite-coated part of the stem; only four hips (2.7 per cent) had such lines in a single coated zone. No hip had radiolucent lines in all zones, and all of the stems were osseointegrated.
There were twelve intraoperative and four postoperative femoral fractures. The twelve intraoperative fractures were small and minimally displaced; all were located in the proximalmost area of the femur, and none was distal to the level of the lesser trochanter. These fractures occurred either during broaching or when the stem was inserted. Six intraoperative fractures necessitated fixation with cerclage wires, and the other six did not necessitate intervention. No patient had any sequelae as a result of an intraoperative fracture. The fact that these twelve fractures were reported by seven surgeons reflects the multicenter nature of this study. The four postoperative fractures occurred as the result of trauma fifteen to sixty-eight months postoperatively. One of these fractures was treated with traction, two were treated with open reduction and internal fixation, and one necessitated revision to a long-stem prosthesis. All four postoperative fractures subsequently healed. In addition to the fractures, there were seven dislocations (reported by five surgeons), two nerve palsies (one resolved within two months and the other resolved within one year postoperatively), and one deep joint infection (which necessitated removal of the component twenty-three months postoperatively). The component that was removed from the infected hip was reimplanted successfully two years later. Thirty-two components were revised in twenty-nine hips: only the cup was revised in twenty-five hips; both the cup and the stem, in three; and only the stem, in one. Of the twenty-five revisions that involved the cup only, seventeen were done because of aseptic loosening and eight were done because of excessive polyethylene wear or acetabular osteolysis. Two of the three revisions that involved both the cup and the stem were performed because of pain in the hip or the thigh; these procedures were performed twenty-four and fifty months postoperatively by surgeons who were not involved in the study. In both instances, the operative report indicated that the stem was well fixed and was difficult to remove. The third revision that involved both the cup and the stem was performed twenty-three months postoperatively because of an infection, as mentioned previously. The one revision that involved only the stem was necessary because of a traumatic femoral fracture that occurred distal to the stem approximately six years postoperatively; the stem was radiographically stable at the time of the revision. No femoral component was revised because of aseptic loosening or was radiographically loose in this group of young patients. The rates of aseptic and mechanical failure of the hydroxyapatite-coated stems in the present study therefore were 0 per cent after a minimum duration of follow-up of five years. The two stems that were revised because of pain and the two stems that were associated with pain that limited activity were considered to have failed clinically; the combined rate of failure therefore was 2.6 per cent (four of 152 stems).
Most of the young patients in the present study had a good or excellent clinical result after a minimum duration of follow-up of five years. All of the stems were well fixed proximally, and the routine finding of bone condensation at the junction between the coated and uncoated portions of the stem suggests that stress transfer to the femur occurred in this region. The radiographic results in this subgroup of patients who were less than fifty years old were similar to those in the over-all group of patients who were managed with hydroxyapatite-coated stems12. In addition, no femoral component in the present study was revised because of aseptic loosening or was radiographically loose at the time of the most recent examination. Barrack et al.2, in a study of fifty total hip arthroplasties that were performed with cement in patients who were less than fifty years old, reported that no femoral component had been revised and only one was radiographically loose after an average duration of follow-up of twelve years. However, other authors have reported much higher rates of failure in studies of young patients who were managed with total hip arthroplasty with cement8,10,13. In several studies of total hip arthroplasty without cement in which the average age and duration of follow-up were comparable with those in the present series, the rate of aseptic loosening was 3.7 per cent (nine of 242 hips)19, 3 per cent (two of seventy-three hips)22, and 0 per cent (zero of 100 hips)24. The prevalence of moderate pain in the thigh in the present study (two hips; 1.3 per cent) was lower than that in other studies of implants inserted without cement7,24,30. Therefore, our results appear to be comparable with the best results in published studies of implants inserted either with or without cement in young patients who were followed for similar periods of time. Proponents of hydroxyapatite theorize that, because of the biocompatibility of the material and the potential for circumferential osseous apposition, hydroxyapatite-coated implants may prevent metal and polyethylene wear debris from migrating distally and thereby minimize the loss of endosteal bone. Taylor29, in a study of dogs, demonstrated that hydroxyapatite-coated femoral implants formed a protective barrier that prevented the intrusion of articular wear debris into the bone-implant interface. In the present series, there was only one possible case of intramedullary osteolysis; this finding suggests that a hydroxyapatite coating on a femoral component may indeed provide a barrier against distal migration of articular wear debris into the endosteal interface. Critics of hydroxyapatite argue that particulate debris of that substance may contribute to third-body wear, thereby accelerating the loss of bone and the loosening of components6,9,28. Although there have been isolated reports of hydroxyapatite contributing to third-body wear in analyses of retrieved implants6,9, the clinical evidence to date3 (including the results of the present study) suggests that this is not a major problem. In an attempt to examine objectively the issue of whether hydroxyapatite contributes to third-body wear, Bauer et al.3 used laser interference microscopy to measure and to compare the surface roughness of the heads from forty-five retrieved femoral components (fifteen each from hydroxyapatite-coated stems, porous-coated stems, and cemented stems). Those authors found that the heads from the hydroxyapatite-coated stems had significantly less surface roughness and fewer deep scratches than the heads from either of the other two types of stems (p < 0.05). The results described by Bauer et al.3 suggest that the problem of third-body wear is not greater in association with hydroxyapatite-coated implants compared with porous-coated or cemented implants. One driving force behind the development of implants to be inserted without cement was concern about the loss of endosteal bone that generally is attributed to the presence of acrylic cement16. However, loss of endosteal bone also has been seen in association with implants inserted without cement. Heekin et al.24, for example, observed femoral osteolysis around seven (8 per cent) of ninety-one porous-coated prostheses that had been followed for five to seven years. Similarly, Glassman et al.19 observed femoral osteolysis around forty-three (18 per cent) of 238 unrevised porous-coated stems in patients who were less than fifty years old and had been followed for an average of eight years. In that series, most lesions were confined to the proximal part of the femur and progressive femoral osteolysis was noted in only one patient. Because access of wear debris to the periprosthetic interface appears to be a critical factor in the production of femoral osteolysis, some investigators have suggested that the use of so-called third-generation cementing techniques for fixation of the femoral stem may protect against this complication20. However, in a recent report on the results of total hip arthroplasty performed with use of third-generation cementing techniques in 100 patients who were an average of seventy-one years old (range, forty-one to ninety-two years old), Oishi et al.27 observed endosteal cavitation in five (6 per cent) of eighty-one surviving patients who had been followed for an average of seven years (range, six to eight years). Thus, osteolysis remains a major long-term concern after total hip arthroplasty, especially in young patients who may need a revision procedure in the future. A limitation of the present study is the non-random, non-consecutive manner in which the patients were selected. The surgeons who participated in the study, which was approved by the Food and Drug Administration, followed a standardized protocol for the enrollment of patients according to specific criteria for inclusion and exclusion and were required to obtain informed consent from each patient. These factors limit the generalizability of these results to the broader population of patients managed with total hip arthroplasty. However, although this was a limitation of the research design, the patient-selection process that was used reflects the usual and customary practice of surgeons who perform total hip arthroplasty; that is, the surgeon selects a specific implant for a given patient on the basis of a variety of factors related both to the patient and to the implant. The prospective and multicenter nature of the present study provides a strength that is not commonly seen in the orthopaedic literature. Most reported clinical studies are retrospective and are based on data collected at a single institution, frequently from the practice of an individual surgeon. Such studies raise concerns about the possible uniqueness of the setting or the surgeon and have inherent limitations regarding the collection of data. The present study, however, does not have these limitations. The multicenter approach facilitated the enrollment of a large number of patients in a relatively short period of time (thirty-four months) and allowed for the collection of specific, standardized, detailed information on nearly all hips at designated time-periods. Our findings suggest that this implant can be used, with excellent results, by a variety of surgeons in multiple settings. Several questions remain. Additional studies and longer durations of follow-up are needed to determine if hydroxyapatite-coated implants provide long-term stability, decrease the likelihood of osteolysis due to wear debris, and prevent bone loss in this and other populations of patients. Until these questions are answered, the issue of cost-effectiveness cannot be addressed. In summary, preclinical and early clinical reports of hydroxyapatite-coated stems have been encouraging. In this longer-term report of a series of young patients, hydroxyapatite-coated femoral components remained stable and the clinical results were consistently excellent. The fact that intramedullary osteolysis was suspected in only one hip suggests that a coating of hydroxyapatite may delay the distal migration of wear debris.
NOTE: The authors acknowledge the following investigators who participated in the study: Benjamin Bierbaum, M.D., Boston, Massachusetts; Michael Christie, M.D., Nashville, Tennessee; Omar Crothers, M.D., Portland, Maine; Joseph Dimon, III, M.D., Atlanta, Georgia; Vincent Eilers, M.D., St. Paul, Minnesota; William L. Jaffe, M.D., New York, N.Y.; Randall Lewis, M.D., Washington, D.C.; David Mattingly, M.D., Boston, Massachusetts; William Stillwell, M.D., Smithtown, New York; Anthony Unger, M.D., Washington, D.C.; and Richard Zimmerman, M.D., Portland, Oregon.
Department of Orthopaedic Surgery, Indiana University School of Medicine, 541 Clinical Drive, Suite CL 600, Indianapolis, Indiana 46202.
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