Increase in posterior tibial slope would result in correction loss in frontal plane after medial open‐wedge high tibial osteotomy

PURPOSE: The purpose of this study was to clarify the causes of the increase in the posterior tibial slope during open-wedge high tibial osteotomy (HTO) and to investigate whether its changes influenced the correction angle in frontal plane. METHODS: We retrospectively reviewed 20 patients (26 knees) treated with open-wedge HTO. They were divided into the following two groups. Group A consisted of the knees whose opening gaps were fixed using a spacer plate having the trapezoidal block with a 2° posterior slope. In Group B, anterior and posterior opening gaps were fixed separately. The posterior tibial slope and the hip-knee-ankle angle were measured based on CT data. The relationship between the correction rate in frontal plane and the changes of posterior tibial slope was investigated. RESULTS: Increase in the posterior tibial slope was 2.1 ± 2.5° in Group A and 0.2 ± 1.2° in Group B, which showed a statistical difference (P = 0.02). The difference between the hip-knee-ankle angles before and after operation was 5.2 ± 2.3° in Group A and 5.5 ± 2.5° in Group B. The correction rate was statistically correlated with the changes of posterior tibial slope (R = -0.55, P = 0.003). CONCLUSION: To avoid increase in the posterior tibial slope, the trapezoidal block with a only 2° posterior slope in a spacer plate was not sufficient, and it was necessary to fix anterior and posterior gaps separately. The correction angle in frontal plane had a trade-off relationship with the changes in posterior tibial slope. Thus, we thought that increase in the posterior tibial slope might result in correction loss. LEVEL OF EVIDENCE: Therapeutic study, Retrospective comparative study, Level III.