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Commentary & Perspective | ||||||||
Commentary on In this issue of The Journal, Tonino et al. describe the results of the histological analysis of six clinically satisfactory, hydroxyapatite-coated acetabular components (ABG; Howmedica, Staines, UK) retrieved at autopsy. Much of the hydroxyapatite on these relatively smooth implants had disappeared at retrieval, but there was adequate bone apposition to retain satisfactory implant stability. One implant showed evidence of osteolysis associated with polyethylene particles, and several other implants showed bone resorption adjacent to empty screw-holes. This article raises several questions with respect to acetabular component fixation: 1) What is the natural history of a hydroxyapatite coating in situ, and what are the mechanisms of the loss of hydroxyapatite coating? 2) What are the consequences of this loss with respect to implant stability and third-body wear? 3) What substrate design and surface texture are optimum? Tonino and colleagues noted that much of the hydroxyapatite coating had disappeared from the surface of these clinically satisfactory implants, and that the amount of bone apposition at post mortem was independent of the amount of residual hydroxyapatite. One implant had no histologically recognizable hydroxyapatite after 6.5 years in vivo, while hydroxyapatite covered only 26% or less of the surface of the other implants. Previous authors also have noted the loss of visually recognizable hydroxyapatite in retrieved hydroxyapatite-coated implants1-3. Tonino et al. described at least one focus of osteoclasts in a location that suggests cell-mediated hydroxyapatite-resorption, an observation also noted in previous studies1. Because osteoclasts dissolve endogenous bone mineral at a pH of approximately 4.8, and because even hydroxyapatite coatings with high crystallinity are soluble at low pH, it is reasonable to suspect that osteoclasts would actively dissolve hydroxyapatite coatings in the bone-remodeling process. Partly on the basis of a study of clinically successful femoral components from the same group of patients4, Tonino et al. further suggested that areas of increased mechanical load were likely to show greater hydroxyapatite resorption due to a higher rate of bone remodeling at that site when compared with areas of the implant that transmit a lower mechanical load. While I agree that osteoclastic resorption is one mechanism of the degradation of hydroxyapatite coating, I have a different interpretation of the patterns of hydroxyapatite coating loss. Our own observations of clinically satisfactory hydroxyapatite-coated femoral and acetabular implants retrieved at autopsy reveal hypertrophy of bone and preservation of the hydroxyapatite in areas of expected load transfer, with preferential resorption of bone and hydroxyapatite in areas of the implant not active in load transmission. For example, our study of hydroxyapatite-coated threaded acetabular cups showed relative bone hypertrophy and preservation of the hydroxyapatite over the peak of some of the threads, a site expected to transmit mechanical load, with absent bone apposition and absent hydroxyapatite in the valleys between threads, an area not expected to transmit mechanical load2 (Fig. 1-A). This pattern is the opposite of what would be expected according to the hypothesis of Tonino et al. Tonino and colleagues also reported less bone apposition with these acetabular cups when compared with hydroxyapatite-coated stems in the same patients4,5, results identical to our own observations of hydroxyapatite-coated implants of a different design1,2. Tonino et al. suggested that this reflects a greater rate of bone remodeling in the acetabulum when compared with that in the femur. However, we speculate that the extent of bone apposition and hydroxyapatite preservation is highest where the implant transmits load to the cortex. We tend to see bone hypertrophy and hydroxyapatite preservation along the medial and anterior aspects of hydroxyapatite-coated femoral stems where the implant is relatively close to the cortex, while the posterior and lateral stem surfaces show less hydroxyapatite preservation and less bone apposition. Tonino et al. described this same distribution of bone apposition in a previous study of retrieved hydroxyapatite-coated femoral stems4. Dissolution of hydroxyapatite coatings in the acid environment created by osteoclasts represents one mechanism of coating loss, but some retrieved implants seem to demonstrate more hydroxyapatite loss than would be expected if osteoclast resorption were the only mechanism of coating degradation. Additional possible mechanisms of coating loss include dissolution at neutral pH, delamination, and abrasion6. Plasma-sprayed hydroxyapatite coatings do not exhibit uniform internal crystallinity. While the areas of high crystallinity are relatively stable at neutral pH, areas of lower crystallinity have higher solubility. Indeed it has been suggested that dissolution of a portion of the coating is necessary for bioactivity7. Thus, it is possible that at neutral pH dissolution of the areas of hydroxyapatite with low crystallinity, followed by the release of hydroxyapatite granules of higher crystallinity, is another mechanism of hydroxyapatite degradation that would be very difficult to pinpoint by histological evaluation alone. Another potential mechanism of hydroxyapatite degradation is delamination from the substrate. As described by Tonino et al. in the current article, the results of mechanical tests and retrieval studies suggest that delamination might be expected for either very thick coatings or for those prepared by simply pressing granules of hydroxyapatite onto a metal substrate8,9, but no delamination was identified in either these acetabular components, or in the femoral components4 studied previously by Tonino et al. We also have been unable to identify delamination in implants with hydroxyapatite coatings having a composition and thickness similar to those used in the ABG implants1,2. On the other hand, we have identified delamination of the hydroxyapatite in a canine total hip prosthesis that was retrieved by Dr. R. G. T. Geesink ten years after total hip arthroplasty, at post mortem. In this clinically satisfactory specimen, the delaminated hydroxyapatite became surrounded by bone, resembling a bone-graft substitute material rather than an abrasive particle (Fig. 2). Certainly delamination of an implant coating is undesirable, but in this canine hip, delamination did not lead to implant loosening or to accelerated polyethylene wear. Whether by osteoclastic resorption, delamination, dissolution, or abrasion, it seems apparent that hydroxyapatite coatings are subject to degradation. The results obtained by Tonino et al. demonstrate that a gradual loss of the coating is not necessarily associated with loss of fixation. Indeed, Tonino et al. showed approximately 20-56% bone apposition to femoral and acetabular components, independent of the amount of residual hydroxyapatite. They speculated that the implant design and the substrate texture are important factors that influence long-term stability of the implant after the hydroxyapatite coating disappears, a concept with which I and others10 agree.  Figure 1-A: Low-magnification photomicrograph of a threaded hydroxyapatite-coated acetabular component, retrieved at autopsy 25 months after implantation. Bone shows hypertrophy in areas of the maximum load transfer (thread tips) expected with this implant design (Hematoxylin and eosin).  Figure 1-B: Higher-magnification photomicrograph of the threads from the same implant. There is evidence of bone apposition and residual hydroxyapatite (HA) at the tips of the threads, while both bone and hydroxyapatite are absent from the valleys between the threads, areas expected to transmit less mechanical load (hematoxylin and eosin).  Figure 2: High-magnification photomicrograph showing delaminated hydroxyapatite coating (HA) adjacent to a canine titanium femoral stem (Ti), evaluated at autopsy after ten years in vivo. Lamellar bone (B) is present, surrounding the hydroxyapatite and immediately adjacent to the implant. There is no debris, and the implant shows histological features consistent with mechanical stability (hematoxylin and eosin). (Specimen contributed by Dr. R.G.T. Geesink.) *In support of his research or preparation of this manuscript, the author received grants or outside funding from Howmedica-Osteonics and Smith and Nephew Richards. The author did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated. References 1. Bauer TW, Geesink RG, Zimmerman R, McMahon JT. Hydroxyapatite-coated femoral stems. Histological analysis of components retrieved at autopsy. J Bone Joint Surg Am. 1991;73:1439-52. | ||||||||
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