Unicompartmental Knee Replacement Introduction: Where Have We Been? Where Are We Now? Where Are We Going?

The early reported series of unicompartmental knee replacement done by experienced surgeons did not have encouraging results, with 70% survivorship reported at 5 to 7 years,⁵ and 65% and 53% survivorships reported at 11 and 12 years, respectively.^8,19,22 However, much was learned from that early experience. Patients with moderate to severe deformities were not optimal candidates for the procedure. Overweight patients also were poor candidates. Results were better if a patient’s anterior cruciate ligament was intact. Technical consideration was learned from the initial experience, including the need to undercorrect the coronal plane deformity and to avoid femoral component impingement on the patella. Also component-on-component malalignment resulted in edge loading, high contact stress, accelerated polyethylene wear, and implant loosening. Additionally, there was concern that revision to total knee replacement could be compromised by the unicompartmental replacement.¹² Contralateral compartment degenerative changes also were considered poor prognostic indicators. Increasing conformity of the femoral tibial articulation in fixed bearing designs and thin tibial polyethylene (many were thinner than 6 mm) correlated with failure. Cementless fixation was found to provide less durable results than cemented fixation with unicompartmental replacement.

However, more recently the reports of unicompartmental knee replacement have been more encouraging with 94% survivorship at 10 years^1,3,11,21 and 90% survivorship at 18 years.²⁰ Authors of comparison studies of unicompartmental knee replacement and high tibial osteotomy have shown higher patient satisfaction, quicker recovery, less blood loss, less chance for infection, and easier revision to total knee replacement with unicompartmental knee replacement.²³ In patients with unicompartmental knee replacement in one knee and total knee replacement in the other knee, motion and satisfaction were higher and recovery was earlier in the knee with unicondylar replacement.⁶

Authors more recently showing encouraging results with skillfully done unicondylar knee replacements,^1,3,11,20,21 newer techniques for improving surgical precision and allowing the use of smaller incisions, and the newer designs available^14,15,16,18 have brought renewed enthusiasm for the procedure. Additionally, there is little compromise to doing a conversion to total knee replacement with newer techniques and designs.^2,4,7,9 Currently, techniques vary: tibial inlay,^16,18 bone cut using bone landmarks technique with traditional but modified knee replacement instruments,^1,3 and implant-to-implant preparation.¹⁷ Polyethylene quality has improved.¹⁰ Fixed-bearing and mobile-bearing implants are available. In addition, with the encouraging results and the newer techniques and designs, indications for the procedure are broadening.¹³ Only time will tell whether one technique or design will provide more optimal results and only time will tell how far the indications can be expanded and still provide adequate durability of the implant in the patient with unicompartmental arthritis.