The Journal of Bone and Joint Surgery

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Commentary & Perspective

Commentary & Perspective on
"Metal-on-Metal Hybrid Surface Arthroplasty: Two to Six-Year Follow-up Study"
by Harlan C. Amstutz, MD, et al.

Commentary & Perspective by
Daniel J. Berry, MD*,
Mayo Clinic, Rochester, Minnesota

Surface replacement arthroplasty of the hip, in various iterations, has been part of the orthopaedic armamentarium for almost three decades. Despite the promise of providing a bone-conserving hip arthroplasty with a more normal range of motion than that provided by conventional total hip replacement, surface arthroplasty, in its many iterations, has been unsuccessful for a variety of reasons. Early cemented surface arthroplasties with metal-on-polyethylene bearing surfaces failed due to a combination of acetabular component loosening and polyethylene wear. Subsequent versions of surface arthroplasty with uncemented acetabular component fixation reduced acetabular loosening but failed due to a combination of thin polyethylene, high volumetric polyethylene wear, and osteolysis. In the last five years, surface replacement arthroplasty has been the subject of renewed interest as metal-on-metal bearing surfaces have been combined with uncemented acetabular component fixation. The first author of "Metal-on-Metal Hybrid Surface Arthroplasty," Dr. Amstutz, should be credited with persevering over the last several decades in the quest to make surface arthroplasty a viable alternative to conventional hip arthroplasty.

The authors provide results of 400 surface arthroplasties with uncemented acetabular fixation, cemented femoral fixation, and a large-diameter metal-on-metal bearing surface. During the time period covered by the study, the senior author, who performed all of the operations, performed sixty-nine conventional hip arthroplasties, a fact which demonstrates that the author used surface replacement arthroplasty in most of his patients. The demographics of the patient group are notable for a young mean age (48.2 years), which needs to be borne in mind as the results are reviewed.

During the follow-up period, fourteen hips required revision or removal: seven for loosening of the cemented femoral component, three for fracture of the femoral neck, one for early acetabular component intrapelvic protrusion (cup exchange only), one for component mismatch (cup exchange only), one for recurrent subluxation, and one for infection. Of the 384 hips in which radiographic follow-up was available, sixteen unrevised hips had high-grade femoral radiolucencies, which the authors believe are probably indicative of early femoral loosening (most of these patients, however, were asymptomatic at the time of review). None of the sockets appeared loose. Thus, for the cohort of 400 hips, aseptic mechanical failure or impending failure was present in twenty-seven hips (6.8%) at two to six years.

The authors have developed a risk index for surface replacement arthroplasty that helps stratify patients according to the risk of implant failure. Patients with large femoral-head cysts, low weight, previous surgery, and high activity level are considered at risk for implant failure. Survivorship free of conversion to total hip replacement at four years was 97% for patients at low risk compared with 88.8% for patients at high risk. When conversion to total hip replacement or probable impending mechanical failure (as evidenced by high-grade radiolucencies around the femoral component) was used as an end point, the survivorship at four years was 73.8% in the high-risk group and 94.5% in the low-risk group.

The clinical results in patients with surviving implants were generally good, and the patients demonstrated a relatively high activity level that was consistent with their young age. The few functional problems were mostly caused by heterotopic ossification, some of which occurred early in the series in association with bilateral arthroplasty; subsequently, the authors modified their protocol to reduce heterotopic ossification. The high mean hip range of motion obtained in this series demonstrates that surface arthroplasty can provide excellent range of motion with a stable hip for most patients.

One of the more common complications of surface replacement arthroplasty in other series has been fracture of the femoral neck, which occurred infrequently in this series. The relatively low rate of fracture was probably related to the senior author's extensive experience with the technique.

An important ingredient of whether or not metal-on-metal surface replacement arthroplasty ultimately will be successful is the level of metal ion release associated with large-diameter metal-on-metal bearings. The authors do not provide detailed information on metal levels in this group of patients, but they do state that the levels were consistent with those encountered in metal-on-metal bearings in conventional arthroplasty. It is anticipated that the authors will provide further information on this important subject as it becomes available.

The results in this paper demonstrate that metal-on-metal surface replacement arthroplasty of this design provided reasonable clinical results at two to six years. On the basis of this report, it can also be concluded that with the technique, indications, and prosthesis used in the present report, the rate of failure of surface replacement arthroplasty is higher in the short term than that which would be expected from present-day conventional uncemented hip arthroplasty with alternative bearing surfaces. The main difference between the results of surface arthroplasty and conventional hip arthroplasty is the relatively high early femoral failure rate of hybrid surface arthroplasty. Perhaps with modifications to the surgical technique (such as full cementation of the metaphyseal stem, which the authors employed later in the series), the frequency of this mode of failure can be reduced. The authors' valuable experience with the use of a risk index for surface arthroplasty demonstrates that certain populations are at higher risk for failure, and this may allow refinement of indications for the procedure and a further improvement in the success rate.

The authors have provided a detailed and valuable report on the early results of surface replacement arthroplasty of the hip in a large group of patients. The senior author should be commended for decades of effort to refine, optimize, and make successful a procedure that all recognize has many theoretically appealing features.

*The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. One or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (DePuy). 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 authors are affiliated or associated.