UniSpacer: insufficient data to support its widespread use.

The concept of a UniSpacer is not new. McKeever and MacIntosh metallic hemiarthroplasties have been available for more than 50 years. Two decades ago, published reports for patients with unicompartmental OA revealed good initial results in 85% of patients. This procedure is conservative and easily revised, if necessary, to any type of arthroplasty in the future. The UniSpacer can be thought of as a mobile McKeever or MacIntosh metallic hemiarthroplasty. Rather than attempting fixation to the tibial plateau via a keel or a roughened undersurface, it is designed to translate freely on the tibial plateau as determined by the conforming articulation of its top side surface with the femoral condyle. This mobility makes it inappropriate for use in the lateral compartment where the femoral roll-back could cause prosthetic dislocation or soft tissue impingement or both. The eventual role of the UniSpacer in arthroplasty currently is uncertain. There are no published reports of its effectiveness. Its indication should be similar to those for McKeever arthroplasty. A patient with unicompartmental OA in whom an osteotomy is contraindicated but is considered too young, heavy, or active for a metal-to-plastic arthroplasty is ideal. Less than 1% of patients with OA should be appropriate candidates. The procedure is technically demanding and sensitive, making its widespread success unlikely.