Commentary & Perspective

Commentary & Perspective on
"UniSpacer Arthroplasty of the Knee"
by Domenick J. Sisto, MD, and Isaac L. Mitchell, MD, et al.

Commentary & Perspective by
Bryan D. Springer, MD, and Richard D. Scott, MD*,
OrthoCarolina, Charlotte Hip and Knee Specialists, Charlotte, North Carolina (B.D.S.), and Brigham and Women's Hospital and New England Baptist Hospital, Boston, Massachusetts (R.D.S.)

The treatment of unicompartmental knee arthritis in the young patient is a challenging problem. Despite nonoperative treatment, many young patients will ultimately require surgery to relieve pain and restore function. The surgical options include osteotomy, unicompartmental knee arthroplasty, and total knee arthroplasty. A major concern is that because these procedures may have a finite life span in young active patients, the initial procedure may often compromise future surgical results when revision is required. Additionally, there is a subgroup of patients who are not ideal candidates for any of these procedures. For example, an osteotomy may be unacceptable to patients who are unwilling to accept a cosmetic deformity, and an arthroplasty is a relative contraindication in patients who are too young and/or active for a total joint prosthesis.

Metallic interposition hemiarthroplasty is not a new concept. In the 1950s, both McKeever and MacIntosh described a fixed metallic tibial hemiarthroplasty for the treatment of isolated unicompartmental arthritis^1,2. In 1985, Scott et al. reported 70% good/excellent results in forty-four patients at an average follow-up of eight years³. Likewise, in the same year, Emerson and Potter reported on sixty-one McKeever hemiarthroplasties with 72% good/excellent results at an average follow-up of five years⁴. Despite these early reports and because of the increasing success of total knee arthroplasty, the McKeever metallic hemiarthroplasty did not gain widespread acceptance.

Currently, because of the early results of the McKeever and MacIntosh prostheses and an increasing interest in minimally invasive procedures that may produce shorter hospitalization and quicker recovery, metallic interposition hemiarthroplasty is being revisited. The UniSpacer implant (Centerpulse Orthopaedics, Austin Texas) was introduced as a conservative, self-centering, non-fixed metallic component that requires no osseous resection. The purpose of the UniSpacer is to restore mechanical alignment, maintain stability with retention of the cruciate and collateral ligaments, adapt to the kinematics of the knee, and delay the need for joint arthroplasty.

In the first published study on the UniSpacer, Hallock and Fell reported on seventy-one knees with a minimum follow-up of one year⁵. Although, overall, the patients had improvement in knee Scores and functional Scores, 21% of the patients required reoperation at one year. Five patients (7%) required conversion to total knee arthroplasty, and an additional 10 patients (14%) underwent exchange to another UniSpacer because of dislocation (6%) or pain (4%). These results were inferior to those previously reported by Scott and Emerson for the McKeever metallic hemiarthroplasty^3,4, and the eventual role of the UniSpacer remained uncertain⁶. Subsequently, in unpublished data, Hallock et al., reported that their reoperation rate decreased to 5% in their second series of seventy-eight patients.

In the current study, Sisto and Mitchell evaluated thirty-seven UniSpacers placed in the medial compartment of thirty-four knees. The age of the patients ranged from forty-two to seventy-five years (mean, fifty-five years). They were evaluated an average of twenty-six months postoperatively. Despite strict inclusion criteria, there were no excellent results, ten good results, fifteen fair results, and twelve poor results. There were six anterior dislocations, and all twelve patients with a poor result required conversion to a total knee prosthesis. Functional scores did not improve significantly from the preoperative values, and thirty-two of the thirty-seven patients continued to have pain. Only 27% of patients had an improved activity level, and 27% had persistent postoperative swelling.

The authors described three mechanisms of failure in the twelve knees that required reoperation: (1) anterior dislocation of the prosthesis occurred in six knees (all of which required conversion to a total knee prosthesis) and could not be attributed to a learning curve or poor surgical technique; (2) lack of internal or external rotation of the prosthesis in flexion, attributed to the development of adhesions postoperatively; and (3) unexplained persistent pain in the knee. On the basis of their experience, the authors no longer recommend the UniSpacer arthroplasty to their patients.

The ideal procedure for the treatment of unicompartmental knee arthritis in young patients would be bone sparing, allow for a quick recovery, relieve pain, restore activity, be durable, and not compromise future arthroplasty when failure occurs. Clearly, no one treatment option exists at this time that meets all of these criteria. These most recent results on the UniSpacer, along with previously published data, make future successful treatment of unicompartmental knee arthritis with this specific device uncertain and unlikely.

*The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They 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 authors are affiliated or associated.

References

1. MacIntosh DL. Hemiarthroplasty of the knee using a space occupying prosthesis for painful varus and valgus deformities. In: Proceedings of the Joint Meeting of the Orthopaedic Associations of the English-Speaking World. J Bone Joint Surg Am. 1958;40:1431.
2. McKeever DC. Tibia plateau prosthesis. Clin Orthop Relat Res. 1960;18:86-95.
3. Scott RD, Joyce MJ, Ewald FC, Thomas WH. McKeever metallic hemiarthroplasty of the knee in unicompartmental degenerative arthritis. Long-term clinical follow-up and current indications. J Bone Joint Surg Am. 1985;67:203-7.
4. Emerson RH Jr, Potter T. The use of the McKeever metallic hemiarthroplasty for unicompartmental arthritis. J Bone Joint Surg Am. 1985;67:208-12.
5. Hallock RH, Fell BM. Unicompartmental tibial hemiarthroplasty: early results of the UniSpacer knee. Clin Orthop Relat Res. 2003;416:154-63.
6. Scott RD. UniSpacer: insufficient data to support its widespread use. Clin Orthop Relat Res. 2003;416:164-6.

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