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Bioceramics in Total Hip Arthoplasty: Hydroxyapatite Coating
Bioceramics in Total Hip Arthoplasty: Hydroxyapatite Coating William N. Capello, MD,* James A. D’Antonio, MD,† Michael T. Manley, PhD,‡ and Judy R. Feinberg, PhD* Our objective is to describe the use of hydroxyapatite coatings in total hip arthroplasty. Hydroxyapatite enhances bone ingrowth and provides for a durable and lasting construct. The femoral component has performed well at 15-year minimum follow-up, with aseptic revision rates routinely less than 1%. Initial results on the acetabular side were disappointing. However, after redesign, a second-generation arc-deposited hydroxyapatite-coated cup is performing well at 4- to 7-year follow-up. Hydroxyapatite-coated implants have also been used successfully in the revision setting. Future applications include the use of hydroxyapatite coating as a means of delivering bone-enhancing drugs such as the bisphosphonates. Semin Arthro 17:153-160 © 2006 Elsevier Inc. All rights reserved. KEYWORDS hydroxyapatite, total hip arthroplasty, bioceramics, implant design
I
nterest in the use of bioceramics in orthopedic surgery has been growing over the past four decades. Materials that are classified as bioceramics include alumina, zirconia, calcium phosphates (hydroxyapatite), silica-based glasses, and pyrolytic carbons. This article will focus on hydroxyapatite coating as one bioceramic material used in total hip arthroplasty to enhance implant fixation.
Biologic Characteristics of Hydroxyapatite As both a biocompatible and bioactive compound, hydroxyapatite has a number of key qualities that make it particularly attractive in enhancing implant fixation. The chemical structure and composition of synthetic hydroxyapatite is similar to that of naturally occurring hydroxyapatite in bone.1 Hydroxyapatite has been shown to be safe, nontoxic, and stable clinically in maxillofacial and dental surgery as well as in animal studies of orthopedic implant applications.2-5 In addition, although its mechanical properties of hydroxyapatite in granular or sintered-block forms have been found to be *Indiana University School of Medicine, Department of Orthopaedic Surgery, Indianapolis, IN. †Sewickley Valley Hospital, Moon Township, PA. ‡Stryker Orthopaedics, Mahwah, NJ. Address reprint requests to William N. Capello, MD, Indiana University School of Medicine, Department of Orthopaedic Surgery, 541 Clinical Drive, CL600, Indianapolis, IN 36202-5111. E-mail: [email protected]
1045-4527/06/$-see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1053/j.sart.2006.09.010
unsuitable for load-bearing applications,1 a 50-m-thick hydroxyapatite coating plasma-sprayed over a titanium femoral implant has demonstrated accelerated and enhanced apposition to bone without an intervening layer of fibrous tissue in both animal and clinical studies,5-7 and this bond has been shown to be durable and long lasting.6-8 No local or systemic toxicity to hydroxyapatite has been found in multiple in vivo studies,1,4,7,8 and the material is very slowly absorbed.2,9 Early concerns about possible delamination of the coating leading to increased polyethylene wear and subsequent implant loosening have been allayed in numerous laboratory10-14 and clinical studies14-19 performed in the past decade.
Clinical Experience with Hydroxyapatite-Coated Implants in Total Hip Arthroplasty Femoral Side Between January 1988 and November 1990, patients were enrolled in a multicenter, US Food and Drug Administration (FDA) study of one hydroxyapatite-coated femoral component with the aim of demonstrating the safety and efficacy of the implant. The design of this femoral component, the Omnifit-HA (Stryker Orthopaedics, Mahwah, NJ), is a gritblasted, collarless, straight titanium alloy with normalization steps to provide for a tight interference fit proximally. The stem has a 50-m-thick layer of hydroxyapatite, which was 153
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Figure 1 Photograph of the Omnifit-HA stem (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
plasma-sprayed onto the substrate over the proximal onethird of the stem (Fig. 1). During approximately the same time period (February 1987 through February 1989) a similar study was being performed in Europe using the same hydroxyapatite-coated femoral implant. FDA approval for the device was obtained in December 1990, thus ending the commitment of the contributing surgeons to the study. However, since that time, a portion of the patients, both in the
W.N. Capello et al. United States and in Europe, have continued to be followed under the auspices of the industry sponsor (Stryker). There have been many reports of the data from this multicenter study, including short-term20-22 and mid- to long-term15,23,24 clinical and radiographic results as well as reports of efficacy in special populations25-27 and critical issues.28,29 Only those hips with a minimum of 15-year follow-up from either the United States or European studies are included in this report of clinical and radiographic results. The study group consists of 146 patients (166 hips) with 15-year minimum follow-up (range,14.6 to 18.2 years). Average age at the time of surgery was 51 years (range 18 to 73 years), and males and females are equally represented (49 and 51%, respectively). The majority of hips (71%) had osteoarthritis, with the second largest subgroup (11%) having avascular necrosis of the hip. Clinical and radiographic outcome measures are obtained in a standardized manner preoperatively and at each scheduled postoperative visit. Clinical parameters include an assessment of hip pain, severity of limp, and a functional assessment using the calculation of a Harris Hip Score.30 Radiographs are blinded and then evaluated for the presence of radiolucent lines, periosteal cortical hypertrophy, and cancellous condensation by Gruen zone.31 In addition, implant stability is rated according to a modification of the Engh criteria for porous-coated implants32; any subsidence is measured; and the degree of heterotopic bone is recorded using the Brooker classification system.33 The hydroxyapatite-coated femoral component was paired with a variety of acetabular components, including some with hydroxyapatite-coating. The results on the acetabular side will be presented separately from the femoral component results. All hip arthroplasties in this study had a cobalt chrome femoral head coupled with a gamma-in-air sterilized ultra-high molecular weight polyethylene liner. At 15-year minimum follow-up, the clinical results show that 90% of the hips are pain-free or with slight pain only, and a similar percentage (91%) walk with no or a mild limp only. Two patients report slight thigh pain. Twenty-two (15%) patients use an assistive device to walk. Average Harris Hip Score is 91.5 (range 52 to 100). Of the 11 hips with a Harris Hip Score less than 70, two are related to cup loosening, and the others are unrelated to the hip. Of the unrevised stems, all but one are radiographically stable with one stem evaluated as stable fibrous, and none radiographically loose. Cancellous condensation and cortical hypertrophy are noted in at least one Gruen zone in 99 and 58% of hips, respectively. Twenty-three percent of hips show some degree of heterotopic bone formation, and there are no cases of stem subsidence. Osteolysis is present in about onehalf (49%) of hips and is limited to Gruen zones 1, 7, 8, or 14 only, with no cases of intramedullary osteolysis noted at 15year minimum follow-up. A typical radiographic series is seen in Figure 2. Note that the radiographs of this 61-yearold male with degenerative arthritis of his left hip show a stable stem with cancellous bone in the transition zones from the coated to uncoated portion of the stem. Small scalloping
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Figure 2 Case example of a 61-year-old male with osteoarthritis of the left hip. The anteroposterior radiographs are preoperative, postoperative, 5, 10, and 17 years after total hip arthroplasty using the Omnifit-HA stem and a porouscoated Dual Geometry cup.
is seen in Gruen zone 7, with no evidence of intramedullary osteolysis. Of 262 femoral components with reported study data, including those with less than 15-year follow-up, 13 have undergone revision for any reason. Of those 13 stems, only one revision was performed because of aseptic loosening. The one stem revision for aseptic loosening was done at 10 years postimplantation in a 29-year-old female with degenerative arthritis of the hip. Because there are no radiographically loose stems, both the femoral aseptic revision and the mechanical failure rates for this proximally hydroxyapatitecoated stem are 0.4%. Clearly, with the vast majority of patients pain-free and functioning well and with all but one stem radiographically bony stable and no cases of intramedullary osteolysis, these clinical and radiographic results are excellent. There has been only a single stem revision for aseptic loosening. Thus, the authors believe this proximally hydroxyapatite-coated femoral component is functioning very well after 15-year minimum follow-up. Comparing our results with those in the literature, it is evident that other surgeons have obtained similar results (Table 1). In fact, combining the five other reported series34-38 of midto long-term follow-up of a hydroxyapatite-coated stem with our results, there have been only two stem revisions for aseptic loosening out of a total of 715 hydroxyapatite-coated stems, for an overall aseptic revision rate of 0.3%. In addition to these now mid- to long-term results with this proximally hydroxyapatite-coated stem, we have used this same stem in another multicenter investigational device
exemption study that primarily focused on the safety and efficacy of an alumina-on-alumina bearing surface.39,40 That study, which began in October 1996, randomized patients into three groups, two of which had the alumina-on-alumina bearing system. The third group was a control group that had a gamma-irradiated in nitrogen and vacuum packaged polyethylene liner coupled with a cobalt chrome head. All three groups received the hydroxyapatite-coated stem. The shortterm results of the femoral component are presented here. There were 328 hips in 316 patients with an average follow-up of 5 years. The demographic characteristics of the total group were similar to those of the hydroxyapatitecoated stem study. Two-thirds were male; average age at time of surgery was 54 years; and the vast majority (80%) had a diagnosis of osteoarthritis. Four stems have been revised. Two stems were revised postfracture from falls, and one stem was revised at 1 month secondary to leg-length discrepancy. One stem was revised at 4.5 years due to subsidence and aseptic loosening in a patient who had suffered a fall, with no radiographic evidence of fracture in the first year after implantation. In addition, there was one radiographically loose and subsided stem in yet another patient who had fallen in the first year after implantation. Therefore, the femoral aseptic revision and mechanical failure rates within this study were 0.3 and 0.6%, respectively. Although these results are significantly shorter term than our previous study, this study involves more hips and yet the failure rates are less than 1%. Perhaps of greater interest is the variation in incidence of the scalloping around the femoral neck resection level between the two bearing surfaces. As in the longer-term
Table 1 Mid- to Long-Term Results with a Hydroxyapatite-Coated Femoral Component in Primary Total Hip Arthroplasty Author/Year
Hips (n)
Average Age (years)
Average Follow-up (years)
Stem Revisions for Aseptic Loosening
Oosterbos et al 200434 Parvizi et al 200435 Reikeras and Gunderson 200336 Robertson et al 200537 Singh et al 200438 Capello et al (in press)24
100 52 291 68 38 166
72 66.8 48 48 42 51
10 9.8 10 8.8 10 15.8
0 (0.0) 0 (0.0) 1 (0.3) 0 (0.0) 0 (0.0) 1 (0.6)
Values in parentheses are percentages.
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study, there are no cases of intramedullary osteolysis, indicating that the hydroxyapatite-coated stem consistently seals the intramedullary canal from the egress of wear particles. However, the incidence of scalloping is 10 times greater in the polyethylene-on-metal bearing group compared with the alumina-on-alumina group (14 versus 1.4%, respectively) at 5-year follow-up. Although the polyethylene used in this more recent study was improved over that used in the original study in that it was gamma-irratiated in nitrogen and vacuum packaged, it was not highly cross-linked, and polyethylene wear is still noted. Those hips with evidence of scalloping had significantly more linear wear per year than did those hips with the polyethylene-on-metal bearing surface (0.170 ⫾ 0.09 versus 0.116 ⫾ 0.08 mm/year, P ⫽ 0.02).
Acetabular Side A variety of cup designs were used in the initial multicenter study aimed at determining the safety and efficacy of the hydroxyapatite-coated femoral component. Three of those designs employed hydroxyapatite coating for fixation. Both the Dual Radius and Dual Geometry cups (Fig. 3A) were press-fit, and the third hydroxyapatite-coated cup was a threaded design (Fig. 3B), all from the same manufacturer (Stryker Orthopaedics). The peripheral surface of the pressfit cups had a machined knurling to a depth of 200 m to provide a somewhat roughened surface. During implantation, the acetabulum was reamed to a diameter 1 mm less than that of the cup to provide for a tight peripheral interference fit. The hydroxyapatite coating was applied to the cup in a similar manner as it was to the stem; that is, a 50-m-thick coating was plasma-sprayed onto the titanium substrate. While the hydroxyapatite-coated stems showed excellent early results, by 5-year minimum follow-up (range 5.3 to 9.1 years), 11% of the press-fit hydroxyapatite-coated cups had been revised secondary to aseptic loosening and another 14% were deemed radiographically loose.41 At the same time of follow-up, the aseptic revision rate for the hydroxyapatitecoated threaded cup was 1%. These widely disparate results between the hydroxyapatite-coated stem and threaded and press-fit cups led us to perform detailed histologic and biomechanical testing to determine the contributing factors to the failure of the press-fit cups. Clinically, the hydroxyapatite-coated press-fit cups appeared to have a stable interface with osseus ingrowth for the first 2 to 3 years postimplantation before a progressive radiolucent line, starting in DeLee and Charnley42 zone 3, appeared in the subchondral bone beneath the cup, leading to aseptic loosening. Finite element analyses of the interface stresses around acetabular implants may help explain the mode of failure of these cups in that the local tensile stresses are very high at the fixation interface beneath the cup as physiologic loads are applied to the construct.43,44 These tensile loads cause micromotion and distraction between the implant and underlying bony structure, eventually leading to implant failure. Furthermore, biomechanical studies showed that intraoperative fixation of the press-fit cups was provided by hoop and compressive stresses in the bone around the
Figure 3 (A) Photograph of the Dual Radius (left) and Dual Geometry (right) cups (Stryker Orthopaedics, Mahwah, NJ). (B) Photograph of the Omnifit- Threaded hydroxyapatite-coated cup (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
acetabular opening, whereas fixation of the threaded cups was achieved by interlocking of the acetabular bone and the threads of the implant. At approximately the same time as the US multicenter study of hydroxyapatite-coated implants was being conducted, Epinette and coworkers45 began a similar study in France in which he used a hydroxyapatite-coated threaded cup (HA-Arc2f, Osteonics Corporation, Allendale NJ, now Stryker Orthopaedics) (see Fig. 4). This threaded cup had a hemispherical design with a thin double-start cutting thread incorporated into the periphery of the shell. The hydroxapatite coating was plasma-sprayed onto the titanium shell substrate, and bone screws were used to provide secondary fixation. At 10-year minimum follow-up of 304 cups, survivorship with a defined endpoint of mechanical failure was 99.43 ⫾ 0.01014%. These excellent results at 10 years lend support to the need for a construct that can withstand the repetitive tensile and shear stress forces applied to the inferior cup interface (zone 3) as was seen with this threaded cup design. The analyses of the failure of the grit-blasted hydroxyapatite-coated cup led to the design of a second-generation arc-
Bioceramics in THA: hydroxyapatite coating
Figure 4 Photograph of the Arc2f threaded hydroxyapatite-coated cup (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
deposited hydroxyapatite-coated cup (Secur-fit HA, Stryker Orthopaedics) (see Fig. 5). This second-generation cup also has a 50-m-thick coating of hydroxyapatite over a titanium substrate. However it also has an increased surface roughness (RA ⫽ 20.7 m). At 4- to 7-year follow-up of 109 arc-deposited hydroxyapatite-coated cups, there are no radiolucent lines in any Gruen zone, no radiographically loose cups, and there have been no revisions of this cup for aseptic loosening.46 A case example of serial radiographs demonstrate a stable cup with no evidence of radiolucent lines at 9-year follow-up (Fig. 6). Using the same arc-deposited hydroxyapatite-coated cups, Hermida and coworkers47 have reported similar results to our multicenter study. They have had no revisions for aseptic loosening and no radiographically loose cups in 90 hips followed 2 to 5 years. Although these results are short term, clinical use is now approaching 10 years, and the overall results to date are vastly superior to the firstgeneration cup at the same time period. Thus we are hopeful that, in the absence of any clinical or radiographic signs of impending failure, that this arc-deposited hydroxyapatitecoated cup will continue to perform well in years to come.
157 gone multiple revisions. All but 2 patients had confirmed osseointegration at 2 years or later, and the majority of patients had better bone stock 2 years after revision compared with prerevision. Geesink48 noted that the pattern of ingrowth radiographically was more irregular compared with patients with primary total hip arthroplasty but that the speed and magnitude of bone regeneration around the implants were striking. Survival rate of the femoral component was 93% at 11 years with a femoral mechanical failure rate of 5%. In addition, Buoncristiani and coworkers49 compared results of a porous-coated versus a hydroxyapatite-coated revision stem (APR; Intermedics Orthopedics, Austin, TX) and found that those hips with hydroxyapatite coating added to the porous coating had significantly improved clinical outcome relative to pain and limp. On the acetabular side, Epinette and coworkers50 have used a hydroxyapatite-coated cup (Arc2f, Stryker Orthopaedics) (Fig. 4) in the revision setting. Their series includes 160 revisions in a patient group averaging 68 years of age. All grades of acetabular deficiency were included, and average follow-up was 8 years postrevision. Eight cups (5%) were re-revised: three for sepsis, two for recurrent dislocation, and three for mechanical failure. According to Epinette and coworkers,50 the data from this series suggest that the Arc2f cup may be used effectively in Paprosky51 type defects I through IIIa and that some type IIIb cases can be handled if primary stability can be obtained with the threads. In another series52 of 59 aseptic revisions using a hydroxyapatite-coated cup, the acetabular survival rate was 98.8% at a mean follow-up of 4.6 years with a prognostic survival rate of the cup at 10 years of 94.4%. The authors concluded that, even in cases in which the acetabular bone stock may be significantly diminished, bone growth onto the hydroxyapatite-coated cup can still occur, resulting in a reliable fixation.
Conclusion As clinical use of hydroxyapatite-coated total hip components approaches its third decade, a number of conclusions can be
Hydroxyapatite Coated Implants in Revision Total Hip Arthroplasty Hydroxyapatite-coated hip implants have been used selectively in the revision setting, primarily because of the variation and complexity of revision total hip arthroplasty. On the femoral side the hydroxyapatite coating must be in contact with viable bone, particularly in the area below the lesser trochanter to enhance early stability. Geesink48 recently reported on a series of 60 patients who underwent revision total hip arthroplasty using a hydroxyapatite-coated femoral component. Average age of the patients at the time of the revision procedure was 55 years, and 37 (61%) had under-
Figure 5 Photograph of the Secur-fit HA cup (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
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Figure 6 Case example of a 49-year-old male with osteoarthritis of the left hip. The anteroposterior radiographs are preoperative, postoperative, 1, 5, and 9 years after total hip arthroplasty using the Omnifit-HA stem and Secur-fit HA cup with two bone screws superiorly.
drawn. Radiographic data routinely show cancellous condensation, cortical hypertrophy, and bony stable implants that corroborate the findings of the early laboratory studies11,12,14 that a 50-m-thick hydroxyapatite coating over a titanium substrate provides for increased bone apposition. Early concerns about the potential for delamination of the hydroxyapatite coating itself contributing to increased polyethylene wear and/or increased osteolysis have been allayed. In our clinical experience, it appears that the hydroxyapatite-coated portion of the stem seals the intramedullary canal, thus protecting against the distal egress of polyethylene wear particles and preventing the development of subsequent osteolysis. In addition, with our more recent study utilizing an alternative bearing surface, aluminaon-alumina, the incidence of osteolysis in the form of proximal zone scalloping is minimal when paired with a hydroxyapatitecoated cup and stem. The early disparate results between the stem, threaded, and press-fit hydroxyapatite-coated implants led us to a greater understanding of the requirements for hydroxyapatite coating to be effective long term and to a redesign of the acetabular component. Although that redesigned cup, now with an arc-deposited and increased roughened surface, has significantly shorter follow-up than do the stem or threaded cups, it is important to note that the time of follow-up for this second-generation cup has surpassed the point where the first generation cups were starting to fail. Thus, we are hopeful that this second-generation hydroxyapatite-coated cup will have longevity similar to the stem and threaded cup. Although hydroxapatite-coated implants have been used less frequently in the revision setting, clinical and radiographic results suggest that hydroxyapatite-coated cups and stems can be successful, providing implant stability even in cases with moderate bone loss.
Future of Hydroxyapatite Coatings in Total Hip Arthroplasty With the safety and efficacy of hydroxyapatite coatings in total hip arthroplasty demonstrated, researchers are now initiating animal model studies aimed at determining the safety and efficacy of using hydroxyapatite-coated implants as a
vehicle for delivering drugs to restore lost bone stock. Bisphosphonates have a demonstrated inhibitory effect on bone resorption and adsorb effectively to hydroxyapatite. Oral bisphosphonate therapy has resulted in mitigation of the osteolytic effects of accumulated wear debris around joint implants53,54 and has been used to treat periprosthetic bone loss secondary to stress shielding.55-57 Currently researchers are working on the development of safe, simple, and effective means of locally delivering bisphosphonates through elution from a hydroxyapatite-coated implant. In a recent study by Tanzer and coworkers58 using a canine ulnar implant model, results showed a 2.3-fold increase in periimplant bone around bisphosponate-dosed hydroxyapatite-coated implants and an increase of 58% more bone within the pores of the implant compared with controls. Clinical implications for this drug-induced increase in bone include use in revision procedures in which bone stock has been lost or in postmenopausal and elderly patients with osteoporosis.
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droxyapatite-coated hip prostheses: 100 consecutive hips followed for 10 years. Acta Orthop Scand 75:127-133, 2004 Parvizi J, Sharkey PE, Hozack WJ, et al: Prospective matched-pair analysis of hydroxyapatite-coated and uncoated femoral stems in total hip arthroplasty. J Bone Joint Surg Am 86:783-786, 2004 Reikeras O, Gunderson RB: Excellent results of HA coating on a gritblasted stem: 245 patients followed for 8-12 years. Acta Orthop Scand 74:140-145, 2003 Robertson A, Lavalette D, Morgan S, et al: The hydroxyapatite-coated JRI-Furlong hip: outcome in patients under the age of 55 years. J Bone Joint Surg Br 87:12-15, 2005 Singh S, Trikha SP, Edge AJ: Hydroxyapatite ceramic-coated femoral stems in young patients: a prospective ten-year study. J Bone Joint Surg Br 86:1118-1123, 2004 Capello WN, D’Antonio JA, Feinberg JR, et al: Alternative bearing surfaces: alumina ceramic bearings for total hip arthroplasty. Instr Course Lect 54:171-176, 2005 D’Antonio JA, Capello WN, Manley MT, et al: A titanium-encased alumina ceramic bearing for total hip arthroplasty: 3- to 5-year results. Clin Orthop 441:151-158, 2005 Manley MT, Capello WN, D’Antonio JA, et al: Fixation of acetabular cups without cement in total hip arthroplasty: a comparison of three different implant surfaces at a minimum duration of follow-up of five years. J Bone Joint Surg Am 80:1175-1185, 1998 DeLee JG, Charnley J: Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop 121:20-32, 1976 Mann KE: The stress state at the socket/bone interface for well fixed press-fit and threaded HA coated cups. Internal report. Allendale, NJ, Osteonics Corporation, 1996 Rapperport DJ, Carter DR, Schurman DJ: Contact finite element stress analysis of porous ingrowth acetabular cup implantation, ingrowth, and loosening. J Orthop Res 5:548-561, 1987 Epinette JA, Manley MT, D’Antonio JA, et al: A 10-year minimum follow-up of hydroxyapatite-coated threaded cups: Clinical, radiographic and survivorship analyses with comparison to the literature. J Arthroplasty 18:140-148, 2003 Capello WN, D’Antonio JA, Manley MT, et al: Arc-deposited hydroxyapatite-coated cups. Clin Orthop 441:305-312, 2005 Hermida JC, D’Lima DD, Steklov N, et al: Outcome of an acetabular design with hydroxyapatite coating on a rough substrate. Clin Orthop 441:298-304, 2005 Geesink, RGT: Osteoconductive coatings for total joint arthroplasty. Clin Orthop 395:53-65, 2002 Buoncristiani AM, Dorr LD, Johnson C, et al: Cementless revision of total hip arthroplasty using the Anatomic Porous Replacement Revision Prosthesis. J Arthroplasty 12:403-415, 1997 Epinette J-A, Manley MT, Tillie B: The use of the hydroxyapatite-coated Arc2f cup in acetabular revision surgery, in Epinette J-A, Manley MT (eds): Fifteen Years of Clinical Experience with Hydroxyapatite Coatings in Joint Arthroplasty. Paris, France, Springer Verlag, 2004, pp 365-376 Paprosky WG, Perona PG, Lawrence JM: Acetabular defect classification and surgical reconstruction in revision arthroplasty: a 6-year follow-up evaluation. J Arthroplasty 9:33-44, 1994 Tonino AJ, van der Linde M, Meijers W, et al: Acetabular revision using cementless hydroxyapatite-coated components, in Epinette J-A, Manley MT (eds): Fifteen Years of Clinical Experience with Hydroxyapatite Coatings in Joint Arthroplasty. Paris, France, Springer Verlag, 2004, pp 377-383 Horowitz SM, Algan SA, Purdon MA: Pharmacologic inhibition of particulate-induced bone resorption. J Biomed Mater Res 31:91-96, 1996 Shanbag AS, Hasselman CT, Rubash HE: Inhibition of wear debris mediated osteolysis in a canine total hip arthroplasty model. Clin Orthop 344:33-43, 1997 Soininvaara TA, Jurvelin JS, Miettinen HJ, et al: Effect of alendronate on periprosthetic bone loss after total knee arthroplasty: a one-year, randomized, controlled trial of 19 patients. Calcif Tissue Int 71: 472-477, 2002
160 56. Venesmaa PK, Kroger HP, Miettinen HJ, et al: Alendronate reduces periprosthetic bone loss after uncemented primary total hip arthroplasty: a prospective randomized study. J Bone Miner Res 16:21262131, 2001 57. Wilkinson JM, Stockley I, Peel NF, et al: Effect of pamidronate in
W.N. Capello et al. preventing bone loss after total hip arthroplasty: a randomized, double-blind, controlled trial. J Bone Miner Res 16:556-563, 2001 58. Tanzer M, Karabasz D, Krygier CET, et al: The Otto Aufranc Award: bone augmentation around and within porous implants by local bisphosphonate elution. Clin Orthop 441:30-39, 2005
I
nterest in the use of bioceramics in orthopedic surgery has been growing over the past four decades. Materials that are classified as bioceramics include alumina, zirconia, calcium phosphates (hydroxyapatite), silica-based glasses, and pyrolytic carbons. This article will focus on hydroxyapatite coating as one bioceramic material used in total hip arthroplasty to enhance implant fixation.
Biologic Characteristics of Hydroxyapatite As both a biocompatible and bioactive compound, hydroxyapatite has a number of key qualities that make it particularly attractive in enhancing implant fixation. The chemical structure and composition of synthetic hydroxyapatite is similar to that of naturally occurring hydroxyapatite in bone.1 Hydroxyapatite has been shown to be safe, nontoxic, and stable clinically in maxillofacial and dental surgery as well as in animal studies of orthopedic implant applications.2-5 In addition, although its mechanical properties of hydroxyapatite in granular or sintered-block forms have been found to be *Indiana University School of Medicine, Department of Orthopaedic Surgery, Indianapolis, IN. †Sewickley Valley Hospital, Moon Township, PA. ‡Stryker Orthopaedics, Mahwah, NJ. Address reprint requests to William N. Capello, MD, Indiana University School of Medicine, Department of Orthopaedic Surgery, 541 Clinical Drive, CL600, Indianapolis, IN 36202-5111. E-mail: [email protected]
1045-4527/06/$-see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1053/j.sart.2006.09.010
unsuitable for load-bearing applications,1 a 50-m-thick hydroxyapatite coating plasma-sprayed over a titanium femoral implant has demonstrated accelerated and enhanced apposition to bone without an intervening layer of fibrous tissue in both animal and clinical studies,5-7 and this bond has been shown to be durable and long lasting.6-8 No local or systemic toxicity to hydroxyapatite has been found in multiple in vivo studies,1,4,7,8 and the material is very slowly absorbed.2,9 Early concerns about possible delamination of the coating leading to increased polyethylene wear and subsequent implant loosening have been allayed in numerous laboratory10-14 and clinical studies14-19 performed in the past decade.
Clinical Experience with Hydroxyapatite-Coated Implants in Total Hip Arthroplasty Femoral Side Between January 1988 and November 1990, patients were enrolled in a multicenter, US Food and Drug Administration (FDA) study of one hydroxyapatite-coated femoral component with the aim of demonstrating the safety and efficacy of the implant. The design of this femoral component, the Omnifit-HA (Stryker Orthopaedics, Mahwah, NJ), is a gritblasted, collarless, straight titanium alloy with normalization steps to provide for a tight interference fit proximally. The stem has a 50-m-thick layer of hydroxyapatite, which was 153
154
Figure 1 Photograph of the Omnifit-HA stem (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
plasma-sprayed onto the substrate over the proximal onethird of the stem (Fig. 1). During approximately the same time period (February 1987 through February 1989) a similar study was being performed in Europe using the same hydroxyapatite-coated femoral implant. FDA approval for the device was obtained in December 1990, thus ending the commitment of the contributing surgeons to the study. However, since that time, a portion of the patients, both in the
W.N. Capello et al. United States and in Europe, have continued to be followed under the auspices of the industry sponsor (Stryker). There have been many reports of the data from this multicenter study, including short-term20-22 and mid- to long-term15,23,24 clinical and radiographic results as well as reports of efficacy in special populations25-27 and critical issues.28,29 Only those hips with a minimum of 15-year follow-up from either the United States or European studies are included in this report of clinical and radiographic results. The study group consists of 146 patients (166 hips) with 15-year minimum follow-up (range,14.6 to 18.2 years). Average age at the time of surgery was 51 years (range 18 to 73 years), and males and females are equally represented (49 and 51%, respectively). The majority of hips (71%) had osteoarthritis, with the second largest subgroup (11%) having avascular necrosis of the hip. Clinical and radiographic outcome measures are obtained in a standardized manner preoperatively and at each scheduled postoperative visit. Clinical parameters include an assessment of hip pain, severity of limp, and a functional assessment using the calculation of a Harris Hip Score.30 Radiographs are blinded and then evaluated for the presence of radiolucent lines, periosteal cortical hypertrophy, and cancellous condensation by Gruen zone.31 In addition, implant stability is rated according to a modification of the Engh criteria for porous-coated implants32; any subsidence is measured; and the degree of heterotopic bone is recorded using the Brooker classification system.33 The hydroxyapatite-coated femoral component was paired with a variety of acetabular components, including some with hydroxyapatite-coating. The results on the acetabular side will be presented separately from the femoral component results. All hip arthroplasties in this study had a cobalt chrome femoral head coupled with a gamma-in-air sterilized ultra-high molecular weight polyethylene liner. At 15-year minimum follow-up, the clinical results show that 90% of the hips are pain-free or with slight pain only, and a similar percentage (91%) walk with no or a mild limp only. Two patients report slight thigh pain. Twenty-two (15%) patients use an assistive device to walk. Average Harris Hip Score is 91.5 (range 52 to 100). Of the 11 hips with a Harris Hip Score less than 70, two are related to cup loosening, and the others are unrelated to the hip. Of the unrevised stems, all but one are radiographically stable with one stem evaluated as stable fibrous, and none radiographically loose. Cancellous condensation and cortical hypertrophy are noted in at least one Gruen zone in 99 and 58% of hips, respectively. Twenty-three percent of hips show some degree of heterotopic bone formation, and there are no cases of stem subsidence. Osteolysis is present in about onehalf (49%) of hips and is limited to Gruen zones 1, 7, 8, or 14 only, with no cases of intramedullary osteolysis noted at 15year minimum follow-up. A typical radiographic series is seen in Figure 2. Note that the radiographs of this 61-yearold male with degenerative arthritis of his left hip show a stable stem with cancellous bone in the transition zones from the coated to uncoated portion of the stem. Small scalloping
Bioceramics in THA: hydroxyapatite coating
155
Figure 2 Case example of a 61-year-old male with osteoarthritis of the left hip. The anteroposterior radiographs are preoperative, postoperative, 5, 10, and 17 years after total hip arthroplasty using the Omnifit-HA stem and a porouscoated Dual Geometry cup.
is seen in Gruen zone 7, with no evidence of intramedullary osteolysis. Of 262 femoral components with reported study data, including those with less than 15-year follow-up, 13 have undergone revision for any reason. Of those 13 stems, only one revision was performed because of aseptic loosening. The one stem revision for aseptic loosening was done at 10 years postimplantation in a 29-year-old female with degenerative arthritis of the hip. Because there are no radiographically loose stems, both the femoral aseptic revision and the mechanical failure rates for this proximally hydroxyapatitecoated stem are 0.4%. Clearly, with the vast majority of patients pain-free and functioning well and with all but one stem radiographically bony stable and no cases of intramedullary osteolysis, these clinical and radiographic results are excellent. There has been only a single stem revision for aseptic loosening. Thus, the authors believe this proximally hydroxyapatite-coated femoral component is functioning very well after 15-year minimum follow-up. Comparing our results with those in the literature, it is evident that other surgeons have obtained similar results (Table 1). In fact, combining the five other reported series34-38 of midto long-term follow-up of a hydroxyapatite-coated stem with our results, there have been only two stem revisions for aseptic loosening out of a total of 715 hydroxyapatite-coated stems, for an overall aseptic revision rate of 0.3%. In addition to these now mid- to long-term results with this proximally hydroxyapatite-coated stem, we have used this same stem in another multicenter investigational device
exemption study that primarily focused on the safety and efficacy of an alumina-on-alumina bearing surface.39,40 That study, which began in October 1996, randomized patients into three groups, two of which had the alumina-on-alumina bearing system. The third group was a control group that had a gamma-irradiated in nitrogen and vacuum packaged polyethylene liner coupled with a cobalt chrome head. All three groups received the hydroxyapatite-coated stem. The shortterm results of the femoral component are presented here. There were 328 hips in 316 patients with an average follow-up of 5 years. The demographic characteristics of the total group were similar to those of the hydroxyapatitecoated stem study. Two-thirds were male; average age at time of surgery was 54 years; and the vast majority (80%) had a diagnosis of osteoarthritis. Four stems have been revised. Two stems were revised postfracture from falls, and one stem was revised at 1 month secondary to leg-length discrepancy. One stem was revised at 4.5 years due to subsidence and aseptic loosening in a patient who had suffered a fall, with no radiographic evidence of fracture in the first year after implantation. In addition, there was one radiographically loose and subsided stem in yet another patient who had fallen in the first year after implantation. Therefore, the femoral aseptic revision and mechanical failure rates within this study were 0.3 and 0.6%, respectively. Although these results are significantly shorter term than our previous study, this study involves more hips and yet the failure rates are less than 1%. Perhaps of greater interest is the variation in incidence of the scalloping around the femoral neck resection level between the two bearing surfaces. As in the longer-term
Table 1 Mid- to Long-Term Results with a Hydroxyapatite-Coated Femoral Component in Primary Total Hip Arthroplasty Author/Year
Hips (n)
Average Age (years)
Average Follow-up (years)
Stem Revisions for Aseptic Loosening
Oosterbos et al 200434 Parvizi et al 200435 Reikeras and Gunderson 200336 Robertson et al 200537 Singh et al 200438 Capello et al (in press)24
100 52 291 68 38 166
72 66.8 48 48 42 51
10 9.8 10 8.8 10 15.8
0 (0.0) 0 (0.0) 1 (0.3) 0 (0.0) 0 (0.0) 1 (0.6)
Values in parentheses are percentages.
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study, there are no cases of intramedullary osteolysis, indicating that the hydroxyapatite-coated stem consistently seals the intramedullary canal from the egress of wear particles. However, the incidence of scalloping is 10 times greater in the polyethylene-on-metal bearing group compared with the alumina-on-alumina group (14 versus 1.4%, respectively) at 5-year follow-up. Although the polyethylene used in this more recent study was improved over that used in the original study in that it was gamma-irratiated in nitrogen and vacuum packaged, it was not highly cross-linked, and polyethylene wear is still noted. Those hips with evidence of scalloping had significantly more linear wear per year than did those hips with the polyethylene-on-metal bearing surface (0.170 ⫾ 0.09 versus 0.116 ⫾ 0.08 mm/year, P ⫽ 0.02).
Acetabular Side A variety of cup designs were used in the initial multicenter study aimed at determining the safety and efficacy of the hydroxyapatite-coated femoral component. Three of those designs employed hydroxyapatite coating for fixation. Both the Dual Radius and Dual Geometry cups (Fig. 3A) were press-fit, and the third hydroxyapatite-coated cup was a threaded design (Fig. 3B), all from the same manufacturer (Stryker Orthopaedics). The peripheral surface of the pressfit cups had a machined knurling to a depth of 200 m to provide a somewhat roughened surface. During implantation, the acetabulum was reamed to a diameter 1 mm less than that of the cup to provide for a tight peripheral interference fit. The hydroxyapatite coating was applied to the cup in a similar manner as it was to the stem; that is, a 50-m-thick coating was plasma-sprayed onto the titanium substrate. While the hydroxyapatite-coated stems showed excellent early results, by 5-year minimum follow-up (range 5.3 to 9.1 years), 11% of the press-fit hydroxyapatite-coated cups had been revised secondary to aseptic loosening and another 14% were deemed radiographically loose.41 At the same time of follow-up, the aseptic revision rate for the hydroxyapatitecoated threaded cup was 1%. These widely disparate results between the hydroxyapatite-coated stem and threaded and press-fit cups led us to perform detailed histologic and biomechanical testing to determine the contributing factors to the failure of the press-fit cups. Clinically, the hydroxyapatite-coated press-fit cups appeared to have a stable interface with osseus ingrowth for the first 2 to 3 years postimplantation before a progressive radiolucent line, starting in DeLee and Charnley42 zone 3, appeared in the subchondral bone beneath the cup, leading to aseptic loosening. Finite element analyses of the interface stresses around acetabular implants may help explain the mode of failure of these cups in that the local tensile stresses are very high at the fixation interface beneath the cup as physiologic loads are applied to the construct.43,44 These tensile loads cause micromotion and distraction between the implant and underlying bony structure, eventually leading to implant failure. Furthermore, biomechanical studies showed that intraoperative fixation of the press-fit cups was provided by hoop and compressive stresses in the bone around the
Figure 3 (A) Photograph of the Dual Radius (left) and Dual Geometry (right) cups (Stryker Orthopaedics, Mahwah, NJ). (B) Photograph of the Omnifit- Threaded hydroxyapatite-coated cup (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
acetabular opening, whereas fixation of the threaded cups was achieved by interlocking of the acetabular bone and the threads of the implant. At approximately the same time as the US multicenter study of hydroxyapatite-coated implants was being conducted, Epinette and coworkers45 began a similar study in France in which he used a hydroxyapatite-coated threaded cup (HA-Arc2f, Osteonics Corporation, Allendale NJ, now Stryker Orthopaedics) (see Fig. 4). This threaded cup had a hemispherical design with a thin double-start cutting thread incorporated into the periphery of the shell. The hydroxapatite coating was plasma-sprayed onto the titanium shell substrate, and bone screws were used to provide secondary fixation. At 10-year minimum follow-up of 304 cups, survivorship with a defined endpoint of mechanical failure was 99.43 ⫾ 0.01014%. These excellent results at 10 years lend support to the need for a construct that can withstand the repetitive tensile and shear stress forces applied to the inferior cup interface (zone 3) as was seen with this threaded cup design. The analyses of the failure of the grit-blasted hydroxyapatite-coated cup led to the design of a second-generation arc-
Bioceramics in THA: hydroxyapatite coating
Figure 4 Photograph of the Arc2f threaded hydroxyapatite-coated cup (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
deposited hydroxyapatite-coated cup (Secur-fit HA, Stryker Orthopaedics) (see Fig. 5). This second-generation cup also has a 50-m-thick coating of hydroxyapatite over a titanium substrate. However it also has an increased surface roughness (RA ⫽ 20.7 m). At 4- to 7-year follow-up of 109 arc-deposited hydroxyapatite-coated cups, there are no radiolucent lines in any Gruen zone, no radiographically loose cups, and there have been no revisions of this cup for aseptic loosening.46 A case example of serial radiographs demonstrate a stable cup with no evidence of radiolucent lines at 9-year follow-up (Fig. 6). Using the same arc-deposited hydroxyapatite-coated cups, Hermida and coworkers47 have reported similar results to our multicenter study. They have had no revisions for aseptic loosening and no radiographically loose cups in 90 hips followed 2 to 5 years. Although these results are short term, clinical use is now approaching 10 years, and the overall results to date are vastly superior to the firstgeneration cup at the same time period. Thus we are hopeful that, in the absence of any clinical or radiographic signs of impending failure, that this arc-deposited hydroxyapatitecoated cup will continue to perform well in years to come.
157 gone multiple revisions. All but 2 patients had confirmed osseointegration at 2 years or later, and the majority of patients had better bone stock 2 years after revision compared with prerevision. Geesink48 noted that the pattern of ingrowth radiographically was more irregular compared with patients with primary total hip arthroplasty but that the speed and magnitude of bone regeneration around the implants were striking. Survival rate of the femoral component was 93% at 11 years with a femoral mechanical failure rate of 5%. In addition, Buoncristiani and coworkers49 compared results of a porous-coated versus a hydroxyapatite-coated revision stem (APR; Intermedics Orthopedics, Austin, TX) and found that those hips with hydroxyapatite coating added to the porous coating had significantly improved clinical outcome relative to pain and limp. On the acetabular side, Epinette and coworkers50 have used a hydroxyapatite-coated cup (Arc2f, Stryker Orthopaedics) (Fig. 4) in the revision setting. Their series includes 160 revisions in a patient group averaging 68 years of age. All grades of acetabular deficiency were included, and average follow-up was 8 years postrevision. Eight cups (5%) were re-revised: three for sepsis, two for recurrent dislocation, and three for mechanical failure. According to Epinette and coworkers,50 the data from this series suggest that the Arc2f cup may be used effectively in Paprosky51 type defects I through IIIa and that some type IIIb cases can be handled if primary stability can be obtained with the threads. In another series52 of 59 aseptic revisions using a hydroxyapatite-coated cup, the acetabular survival rate was 98.8% at a mean follow-up of 4.6 years with a prognostic survival rate of the cup at 10 years of 94.4%. The authors concluded that, even in cases in which the acetabular bone stock may be significantly diminished, bone growth onto the hydroxyapatite-coated cup can still occur, resulting in a reliable fixation.
Conclusion As clinical use of hydroxyapatite-coated total hip components approaches its third decade, a number of conclusions can be
Hydroxyapatite Coated Implants in Revision Total Hip Arthroplasty Hydroxyapatite-coated hip implants have been used selectively in the revision setting, primarily because of the variation and complexity of revision total hip arthroplasty. On the femoral side the hydroxyapatite coating must be in contact with viable bone, particularly in the area below the lesser trochanter to enhance early stability. Geesink48 recently reported on a series of 60 patients who underwent revision total hip arthroplasty using a hydroxyapatite-coated femoral component. Average age of the patients at the time of the revision procedure was 55 years, and 37 (61%) had under-
Figure 5 Photograph of the Secur-fit HA cup (Stryker Orthopaedics, Mahwah, NJ). (Color version of figure is available online.)
W.N. Capello et al.
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Figure 6 Case example of a 49-year-old male with osteoarthritis of the left hip. The anteroposterior radiographs are preoperative, postoperative, 1, 5, and 9 years after total hip arthroplasty using the Omnifit-HA stem and Secur-fit HA cup with two bone screws superiorly.
drawn. Radiographic data routinely show cancellous condensation, cortical hypertrophy, and bony stable implants that corroborate the findings of the early laboratory studies11,12,14 that a 50-m-thick hydroxyapatite coating over a titanium substrate provides for increased bone apposition. Early concerns about the potential for delamination of the hydroxyapatite coating itself contributing to increased polyethylene wear and/or increased osteolysis have been allayed. In our clinical experience, it appears that the hydroxyapatite-coated portion of the stem seals the intramedullary canal, thus protecting against the distal egress of polyethylene wear particles and preventing the development of subsequent osteolysis. In addition, with our more recent study utilizing an alternative bearing surface, aluminaon-alumina, the incidence of osteolysis in the form of proximal zone scalloping is minimal when paired with a hydroxyapatitecoated cup and stem. The early disparate results between the stem, threaded, and press-fit hydroxyapatite-coated implants led us to a greater understanding of the requirements for hydroxyapatite coating to be effective long term and to a redesign of the acetabular component. Although that redesigned cup, now with an arc-deposited and increased roughened surface, has significantly shorter follow-up than do the stem or threaded cups, it is important to note that the time of follow-up for this second-generation cup has surpassed the point where the first generation cups were starting to fail. Thus, we are hopeful that this second-generation hydroxyapatite-coated cup will have longevity similar to the stem and threaded cup. Although hydroxapatite-coated implants have been used less frequently in the revision setting, clinical and radiographic results suggest that hydroxyapatite-coated cups and stems can be successful, providing implant stability even in cases with moderate bone loss.
Future of Hydroxyapatite Coatings in Total Hip Arthroplasty With the safety and efficacy of hydroxyapatite coatings in total hip arthroplasty demonstrated, researchers are now initiating animal model studies aimed at determining the safety and efficacy of using hydroxyapatite-coated implants as a
vehicle for delivering drugs to restore lost bone stock. Bisphosphonates have a demonstrated inhibitory effect on bone resorption and adsorb effectively to hydroxyapatite. Oral bisphosphonate therapy has resulted in mitigation of the osteolytic effects of accumulated wear debris around joint implants53,54 and has been used to treat periprosthetic bone loss secondary to stress shielding.55-57 Currently researchers are working on the development of safe, simple, and effective means of locally delivering bisphosphonates through elution from a hydroxyapatite-coated implant. In a recent study by Tanzer and coworkers58 using a canine ulnar implant model, results showed a 2.3-fold increase in periimplant bone around bisphosponate-dosed hydroxyapatite-coated implants and an increase of 58% more bone within the pores of the implant compared with controls. Clinical implications for this drug-induced increase in bone include use in revision procedures in which bone stock has been lost or in postmenopausal and elderly patients with osteoporosis.
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