Design and accuracy evaluation of a new surgical instrument for determining cartilage thickness on unworn posterior femoral articular surfaces intraoperatively in kinematically aligned total knee arthroplasty.

Abstract

Kinematic alignment (KA) total knee arthroplasty (TKA) strives to restore limb and knee alignments to native (i.e. pre-arthritic state) which in turn requires that the posterior femoral joint line be restored to native. However, restoration is challenging when the posterior femoral articular surface is worn since the cartilage thickness is unknown. A method that holds promise is to measure thickness on the contralateral unworn posterior condyle and use this to predict thickness on the worn condyle. Hence, the purpose was to describe the design and accuracy evaluation of a new surgical instrument for measuring cartilage thickness at the apex on the unworn posterior articular surface. The surgical instrument was L-shaped featuring a handle connected to a probe with a blunt tip and a scale in 1 mm increments. The probe was used to measure cartilage thickness at the estimated apices of 100 unworn posterior femoral articular surfaces prior to resection during KA TKA surgery. To serve as a reference measurement of cartilage thickness, the resection was bisected, the cross-section photographed, and the photograph calibrated. The error in the probe measurement was the difference in thickness between the probe and the calibrated photograph. The new surgical instrument provided apex thickness measurements with a root mean square error less than the 0.5 mm resolution of the probe. The new surgical instrument provides surgeons with a proven method for intra-operative measurement of cartilage thickness in the unworn posterior femoral condyle.