Commentary & Perspective

Commentary & Perspective on
"Prospective Matched-Pair Analysis of Hydroxyapatite-Coated and Uncoated Femoral Stems in Total Hip Arthroplasty: A Concise Follow-up of a Previous Report"
by Javad Parvizi, MD, FRCS, et al.

Commentary & Perspective by
Lawrence D. Dorr, MD*,
The Arthritis Institute at Centinela Hospital Medical Center, Inglewood, California

For this study, the authors made use of a matched-pair analysis of ten characteristics, including patient and operative factors as well as follow-up time. Matched pair analysis with this level of sophistication is a good method for comparing the long-term results of implants and is probably as good as randomization, a method that can lose efficacy over time if death and loss of follow-up of patients affects one group of patients more than another¹. In this study, one member in nine pairs died and forty-three of the original fifty-two matched pairs were surviving at a mean 9.8 years of follow-up. Ordinarily, the occurrence of nine deaths in one arm of a randomized study would have affected the statistical significance of the randomization; however, in this study, the sample size of forty-three pairs was adequate and was associated with 97% power to detect differences in pain, motion, and function.

The authors compared the results of a porous-coated, tapered stem with those of an identical stem that had hydroxyapatite added to the porous coating. The results were first reported at a mean 2.2 years of follow-up, at which time the scores for pain, motion, and function were nearly identical and there had been no revisions. The authors now report on the same patients at a mean follow-up time of 9.2 years for the group with hydroxyapatite-coated stems and 10.1 years for the group with non-hydroxyapatite-coated stems. At the latest follow-up, no stems have been revised. Scores were essentially identical with no statistical differences.

The study has two weaknesses. The first is that not all of the radiographs were current; for some patients, the latest radiographs were the ones made at the five-year follow-up. Although the authors did not identify the number of matched pairs that had current radiographic follow-up, the fact remains that fixation was exactly the same in both groups: 100% had radiographic bone ingrowth of the stems, and no patient had osteolysis or calcar atrophy. The second weakness of the study is that ten of the acetabular components required revision. The activity level of a patient who has undergone a revision is distinctly different from that of a patient who has not had a revision, and this difference could affect the outcome of the stem. However, because revisions were necessary for four hips in one group and six in the other, the effect should not have been any greater on one group than on the other. Furthermore, eight of the ten cups that required revision had accelerated polyethylene wear, but there was no difference in results between the groups. Therefore, there was no evidence from this study to suggest that either porous coating or hydroxyapatite particles are associated with accelerated wear.

The authors accurately conclude that hydroxyapatite does not "confer detectable advantages" when added to a porous-coated cementless tapered stem. However, they incorrectly compare their results to those of D'Antonio et al.², Capello et al.³, and others who reported the results of hydroxyapatite-coated stems on a rough surface (but not a porous-coated surface). This comparison, along with the implication that the use of hydroxyapatite was not necessary on the stems used in the studies of D'Antonio and Capello, is not justified. D'Antonio and Capello "advocated" hydroxyapatite applied to a roughened surface as the primary fixation surface on a stem.

It now seems clear that hydroxyapatite is not beneficial when used as an adjunct fixation with porous-coated stems. It also seems clear that hydroxyapatite is beneficial when used as a primary fixation surface^2,3. We observed in a study of porous-coated anatomic porous replacement stems (Zimmer, Warsaw, Indiana), that there was no difference in fixation results between stems that had porous coating alone and stems that had hydroxyapatite added to the porous coating⁴. The study of Parvizi et al. has soundly confirmed that observation. We have also observed that porous coating is not necessary if hydroxyapatite is used as the primary fixation surface⁵. In a study of bilateral total hip replacements, there was no difference in fixation between stems with hydroxyapatite as the primary fixation and those with porous coating as the primary fixation and hydroxyapatite added to the porous coating⁵.

What is not clear at this time is which surfacing method is best—hydroxyapatite, porous coated, or grit-blasted—for use as the primary surface for fixation. Only when we have the results from the ten to thirty-year outcomes will we know if one surface emerges as better in terms of progressive loosening, greater wear of the articulation surface (perhaps from increasing particulate debris), and cost of the implant.

*In support of his research or preparation of this manuscript, the author received grants or outside funding from Zimmer. In addition, the author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Zimmer). 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. Laupacis A, Bourne R, Rorabeck C, Feeny D, Tugwell P, Wong C. Comparison of total hip arthroplasty performed with and without cement: a randomized trial. J Bone Joint Surg Am. 2002;84:1823-8.
2. D'Antonio JA, Capello WN, Crothers OD, Jaffe WL, Manley MT. Early clinical experience with hydroxyapatite-coated femoral implants. J Bone Joint Surg Am. 1992;74:995-1008.
3. Capello WN, D'Antonio JA, Feinberg JR, Manley MT. Ten-year results with hydroxyapatite-coated total hip femoral components in patients less than fifty years old. A concise follow-up of a previous report. J Bone Joint Surg Am. 2003;85:885-9.
4. Kang JS, Dorr LD, Wan Z. The effect of diaphyseal biologic fixation on clinical results and fixation of the APR-II stem. J Arthroplasty. 2000;15:730-5.
5. Won YY, Dorr LD, Wan Z. Comparison of proximal porous-coated and grit-blasted surfaces of hydroxyapatite-coated stems. J Bone Joint Surg Am. 2004;86:124-8.

Copyright © 2004 by the The Journal of Bone and Joint Surgery, Inc.