Risk of patella baja after opening-wedge high tibial osteotomy

Purpose: Medial opening-wedge high tibial osteotomy (OWHTO) induces a lower patellar position, and the subsequent degree of patellar movement may not be predicted preoperatively. The purpose of this study was to clarify the relationship between preoperative and postoperative patellar height based on the correction angle of OWHTO and to create a formula to predict the appearance of patella baja following OWHTO. Materials and Methods: Seventy-five knees with varus knee osteoarthritis treated with OWHTO were included in this study. The Caton–Deschamps index was used to evaluate patellar height preoperatively and postoperatively, and the cut-off value for preoperative parameters was determined by a receiver operating characteristic curve to determine the risk ratio for postoperative patella baja. Results: The Caton–Deschamps index significantly decreased from 0.93 to 0.77 after OWHTO (p < 0.01). The OWHTO correction angle negatively correlated with the delta Caton–Deschamps index (r = −0.44, p < 0.01), and a 1.7% decrease in the Caton–Deschamps index was shown with a 1° correction angle. Receiver operating characteristic curve analysis revealed that a Caton–Deschamps index of 0.8 was the cutoff for OWHTO; knees with a preoperative Caton–Deschamps index of < 0.8 tended to develop patella baja after OWHTO, with a risk ratio of 9.5 (95% confidence interval [4.3–20.7]). Conclusions: OWHTO can induce patella baja, and a 1.7% decrease in the Caton–Deschamps index was shown with a 1°-correction angle. A preoperative Caton–Deschamps index < 0.8 should be considered a risk factor for postoperative patella baja. Level of Evidence: Retrospective study, Level IV