The UniSpacer (Zimmer, Inc., Warsaw, IN) is a metallic tibial hemiarthroplasty for treatment of isolated osteoarthritis of the medial compartment of the joint. It is a mobile-bearing, self-centering shim that is introduced into the knee through a limited medial arthrotomy without requiring bone cuts or fixation to the tibia or femur. Clinical data are now available that define the efficacy of the procedure in addition to defining the ideal clinical setting for its use in the treatment of osteoarthritis of the medial compartment of the knee. Osteoarthritis of the medial compartment of the knee leaves the medial compartment partially devoid of articular and meniscal cartilage. The collapse of the compartment shifts the weight-bearing of the knee into varus alignment. Two degrees of varus leads to 75% of the load shifted to the medial compartment. Two degrees of valgus leads to 50/50 load distribution between the compartments. Thus, just a 40 correction of alignment shifts 25% of the load off the medial compartment. Acting as an intra-articular shim, the results of radiograph review demonstrate that the UniSpacer corrects the axial alignment from an average of 2.2 degrees of varus to an average of 2.7 degrees of valgus, thus off-loading the medial compartment by more than 25%. The UniSpacer does not require any bone resection for implantation and has no bone fixation. Bone and ligament preservation allows for future procedures to be performed without compromise. There is no currently available hemiarthroplasty that allows for the same degree of anatomic preservation of the knee as the UniSpacer. Eliminating fixation also allows the device to be used in certain patients, especially the obese, without the fear that mechanical failure may result from stresses at the bone/cement/ implant interface. Because the device is not fixed to the bone, failure from loosening or stress riser fracture is not possible. These two new design premises combined with the prior history of the McKeever/ McIntosh [1-3] allow the UniSpacer to fill a niche as a "bridge" procedure for younger patients trying to preserve their anatomy for future, yet probably inevitable, total knee replacement (TKR)s. This rationale is similar to that used in the employment of a high tibial osteotomy, but without the potential morbidity or alteration of the tibial joint line.
