Good accuracy of the Phase III Oxford Mobile Bearing Unicompartmental Knee Instrumentation

Background and purpose - Unicompartmental knee arthroplasty (UKA) needs careful balancing of flexion/extension (F/E) gaps to prevent dislocation of the mobile meniscal bearing. Assessment of gaps is based on the surgeon's subjective insertion force of a feeler gauge with different thicknesses and/or the lift-off of a trial meniscal bearing. However, the accuracy of this method remains unclear. We assessed the accuracy of the technique. Patients and methods - A consecutive series of 33 UKAs in 32 patients (mean age 64 years, 24 women) were balanced using the Oxford Phase III (OP III) Instrumentation. The recommended technique for F/E gap assessment was performed using different feeler gauges with 1-mm increments and the meniscal bearing lift-off tests according to surgical technique. A tensiometer was inserted and both gaps were maximally distracted by hand. Measurements in mm were recorded and analyzed with a reading of 90 N for both gaps in 20 and 90 degrees of flexion. Results - The gaps measured were 12 (11-18) mm in extension and 13 (11-18) mm in 90 degrees of flexion. The difference between the gaps was 0.4 (-0.5 to 1.0) mm (p < 0.001). There were no statistically significant gender differences regarding composite implant thickness, laxity, flexion gap, extension gap, or gap difference. Interpretation - OP III instrumentation using feeler gauges and the lift-off test provides accurate balancing of F/E gaps with an accuracy of less than 1 mm.