Long term clinical data and patient satisfaction is reported on 152 patients implanted with the UniSpacer interpositional spacer during the first four years of clinical use with a minimum 5-year, maximum 9-year follow-up. 156 UniSpacer™ Knee System implants were implanted in 152 patients (4 bilateral), for treatment of isolated medial compartment osteoarthritis over a 4-year period. The minimum follow-up for this group of patients is 5 years with a range of 60 to 108 months. Revisions to a TKR within one year of the implantation date during the first and second year of UniSpacer implantations were 6% and 5% respectively. By years three and four, the TKR revision rate within the first year had dropped to 0% (zero). The data reflects the improvement in surgical technique and the development of proper patient selection criteria. The long term data provides validation that the UniSpacerÔ can provide a successful, long term, bone preserving, treatment alternative to the current HTO, UKR or TKR procedures.