What Is the Critical Tibial Resection Depth During Unicompartmental Knee Arthroplasty? A Biomechanical Study of Fracture Risk.

Background: Fracture after medial unicompartmental knee arthroplasty (UKA) is a rare complication. Biomechanical studies evaluating association between depth of resection and maximum load to failure are lacking. The purpose of this study is to establish the relationship between depth of resection of the medial tibial plateau and mean maximum load to failure.

Methods: Medial tibial resections were performed from 2 to 10 mm in 25 standardized fourth-generation Sawbones composite tibias (Sawbones, Vashon Island, Washington). A metal-backed tibial component with a 9-mm polyethylene bearing was used (Stryker PKR). Tibias were mounted on a biomechanical testing apparatus (MTESTQuattro) and axially loaded cyclically 10 times per cycle and incrementally increased until failure occurred.

Results: Load to failure was recorded in 25 proximal tibia model samples after medial UKA using sequential resections from 2 to 10 mm. Analysis of variance testing identified significant differences in mean maximum load to failure between groups (P = .0003). Analysis of regression models revealed a statistically significant fit of a quadratic model (R = 0.59, P = .0001). The inflection point of this quadratic curve was identified at 5.82 mm, indicating that the maximum load to failure across experimental models in this study began to decline beyond a resection depth of 5.82 mm.

Conclusion: In this biomechanical model, medial tibial resections beyond 5.82 mm produced a significantly lower mean load to failure using a quadratic curve model. Resections from 2 to 6 mm showed no significant differences in mean load to failure. Identification of the tibial resection depth at which the mean load to failure significantly decreases is clinically relevant as this depth may increase the risk of periprosthetic fracture after a medial UKA.