Commentary & Perspective

Commentary & Perspective on
"Diagnostic Features of Pelvic Osteolysis on Computed Tomography: The Importance of Communication Pathways"
by Nobuto Kitamura, MD, et al.
and on
"Early Osteolysis Following Second-Generation Metal-on-Metal Hip Replacement"
by Youn-Soo Park, MD, et al.

Commentary & Perspective by
Seth S. Leopold, MD*,
Department of Orthopaedics and Sports Medicine, University of Washington Medical Center, Seattle, Washington

While the recent "minimally invasive hip replacement" phenomenon has diverted much of our specialty's attention, the fact remains that the principal cause of failure following total hip arthroplasty is not a function of the size, location, or number of incisions used to access the joint. Rather, most revision arthroplasties following cementless total hip arthroplasty, whether performed for late dislocation, component loosening, bone loss, or polyethylene exchange, were necessitated by failure of the materials used in the bearing surfaces of the prosthetic joints. For this reason, it is hard to imagine a topic more important or central to our decision-making than that of osteolysis, which is addressed from two different aspects in this month's issue of The Journal.

The study by Kitamura and colleagues compared the computed tomographic findings in autopsy-retrieved hemipelves containing osteolytic lesions with the radiographic appearance of those same hips immediately before total hip arthroplasty. Autopsy retrieval studies such as this one provide a unique glimpse into the "normal" host-arthroplasty biological response by focusing on implants that were functioning well in situ at the time of death from other causes. This report found fairly predictable differences between the study groups, the most important of which was the consistent finding of a "communication pathway" between the socket and the pelvic defect in patients with osteolytic lesions.

While the apparent mechanism of lysis observed in the study by Kitamura et al.—that of the effective joint space—has been well accepted since the early 1990s¹, Kitamura et al. did document pre-existing (e.g., non-osteolytic) cystic lesions in a surprising 34.8% of the hips studied (sixteen of forty-six), which could easily have been mistaken for osteolysis by a surgeon who did not have the preoperative films available. And while the study is limited by the absence of histological findings to confirm that the apparently osteolytic lesions are not the result of some other process, the authors recently published another study supporting their contentions with reasonable histological endpoints². In addition, the radiographic follow-up in the present study, which includes unassailable evidence that the putative lytic zones were not present preoperatively, is quite thorough. Since many revision hip surgeons now use computed tomography scans as part of the work-up of patients with osteolysis, and since many of these patients have been referred from other surgeons' practices with no accompanying pre-arthroplasty radiographs, this potential source of "false positive" computed tomographic results should be of great interest. Future investigators are likely to incorporate the work in this report into a set of diagnostic criteria for the computed tomographic diagnosis of osteolysis, which may help to avoid subsequent unnecessary imaging examinations and surgical procedures.

Given the volume and intensity of debate about alternative bearing surfaces, the disturbing results presented by Park et al. are likely to raise eyebrows. Proponents of hard-on-hard bearing surfaces such as metal-on-metal and ceramic-on-ceramic argue that the use of these bearings should become more widespread because they generate fewer and smaller particles than do metal-on-polyethylene bearing couples and may thus be associated with lower rates of clinically important osteolysis. Those who are reluctant to embrace metal-on-metal bearings note that there are systemic effects of metal-on-metal bearings that are well documented³, that no long-term comparative trials have been reported, and that our subspecialty has a four-decade-long track record of living with the law of unintended consequences when it comes to novel approaches to the bearing couple.

At this time, apart from anecdotes and case reports⁴, there is relatively little clinical evidence that contemporary metal-on-metal total hip implants can be the cause of real illness or premature failure. By contrast, Dr. Park's group found that the radiographs of 5.5% of patients who received a modern metal-on-metal bearing couple during total hip arthroplasty demonstrated visible osteolysis at the time of short-term follow-up (nine of 165 patients at minimum two-year follow-up). Two of these patients underwent revision procedures, one of which was to address a massive osteolytic lesion that threatened to result in pathological fracture. Perhaps even more troublesome than the proportion of patients with osteolysis—which is in fact higher than that found in most contemporary series of metal-on-polyethylene total hip replacements—is the apparently novel cause of bone loss in these patients: hypersensitivity reactions to elements of the metallic bearing. Although tissue specimens were obtained only from the two patients who had revision surgery, analysis of these specimens did not show the typical histology of osteolysis induced by particulate debris. Rather, microscopic analysis of these patients' lesions showed histology typical of an immune-mediated, possibly allergic, reaction to metals present in the bearing surfaces of the implants. Further substantiating this mechanism was the significant difference observed between patients with osteolysis and those without osteolysis when those patients underwent skin patch testing with cobalt chloride.

The experimental design in this paper is not sufficiently robust to demonstrate causality with respect to metal allergy or hypersensitivity; that is, we cannot determine whether the implants sensitized the patients and caused the condition or if metal-allergic patients may be prone to a worrisome frequency of osteolysis with this implant. This study is neither the first to suggest the existence of metal allergy nor the first to report on osteolysis found around metal-on-metal bearings, but it does provide both a denominator and a plausible mechanism for the findings observed. Since most of the papers that report good outcomes with modern metal-on-metal bearings are, at most, intermediate-term⁵ success stories from surgeons involved with the design process, we should be particularly attentive to cautionary reports, particularly when they come from high-volume early adopters. Dr. Park's study, which demonstrated an alarming frequency of osteolysis at short-term follow-up after implantation of a commercially-available metal-on-metal bearing, appears to be one such report.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. A commercial entity (Zimmer Inc., Warsaw, Indiana) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

1. Schmalzried TP, Jasty M, Harris WH. Periprosthetic bone loss in total hip arthroplasty. Polyethylene wear debris and the concept of the effective joint space. J Bone Joint Surg Am. 1992;74:849-63.
2. Leung S, Naudie D, Kitamura N, Walde T, Engh CA. Computed tomography in the assessment of periacetabular osteolysis. J Bone Joint Surg Am. 2005;87:592-7.
3. Jacobs JJ, Skipor AK, Doorn PF, Campbell P, Schmalzried TP, Black J, Amstutz HC. Cobalt and chromium concentrations in patients with metal on metal total hip replacements. Clin Orthop Relat Res. 1996;329 Suppl:S256-63.
4. Beaule PE, Campbell P, Mirra J, Hooper JC, Schmalzried TP. Osteolysis in a cementless, second generation metal-on-metal hip replacement. Clin Orthop Relat Res. 2001;386:159-65.
5. Dorr LD, Wan Z, Sirianni LE, Boutary M, Chandran S. Fixation and osteolysis with Metasul metal-on-metal articulation. J Arthroplasty. 2004;19:951-5.

Copyright © 2005 by the The Journal of Bone and Joint Surgery, Inc.