Design and Accuracy Evaluation of a Depth Gauge-Referencing Guide System for Measuring Articular Cartilage Thickness on the Distal Femoral Condyles During Unrestricted Kinematically Aligned Total Knee Arthroplasty

Abstract Regardless of pre-operative deformity, unrestricted kinematically aligned (unKA) total knee arthroplasty (TKA) seeks to restore the native femoral joint lines which requires that the thickness of articular cartilage be determined in vivo at the location on a resection where thickness is greatest. Available methods for making this measurement are complex, costly, and offer challenges in pinpointing the location of maximum resection thickness. The purposes were to describe a new mechanical depth gauge-referencing guide system for making this measurement on the distal femoral resections and quantify accuracy of the measurement during unKA TKA. A new mechanical depth gauge was designed which offered a blunt tipped probe to avoid penetrating the subcondylar bone and a calibrated plunger. To ensure that cartilage thickness was measured where the resection is thickest, a custom transparent distal femoral referencing guide was designed which correctly located the depth gauge. To determine accuracy, the system was used to measure articular cartilage thickness on 100 resections taken from 93 patients undergoing unKA TKA. To serve as a gold standard, cartilage thickness was measured following resection using a calibrated photograph. Both systematic (bias) and random (precision) errors were computed. The depth gauge-referencing guide system provided thickness measurements which were unbiased and with precision less than the 0.5 mm resolution of the calibrated plunger, which is the resolution needed for restoration of native joint lines within an acceptable tolerance. Accordingly, the new mechanical depth gauge-referencing guide system provides surgeons with a proven tool for intra-operative measurement of cartilage thickness.