Purpose: This study was performed to determine the accuracy of correction in the frontal plane and to evaluate whether a frontal plane correction influences the sagittal slope of the proximal tibial surface in open-wedge high tibial osteotomy. Type of Study: Retrospective review. Methods: In this study, 30 patients (32 knees) with a mean age of 38 years (range, 20 to 66 years) were available for follow-up evaluation at a minimum interval of 24 months (mean, 42 months; range, 24 to 62 months). A normal axis was planned for a varus morphotype without evidence of osteoarthrosis. If one third of the thickness of the medial cartilage was lost, the new mechanical axis was planned to pass lateral to the center of the knee at the 10% position (where the 0% position is at the center of the knee joint and the 100% position is at the lateral border of the plateau). When two thirds were lost, the new mechanical axis was planned to pass through the 20% position. If total cartilage loss was present, the mechanical axis was planned to pass through the 30% position. The correction was estimated as good when the mechanical axis was found to be approximately +/- 5% of the width of the tibial plateau. The angle of the osteotomy and the size of the wedge-shaped bone graft were calculated using the "push" orthoradiogram. Results: Consolidation of the osteotomy was obtained after a mean period of 8.4 weeks (range, 6 to 13 weeks). The mean amount of valgus correction was 5.9degrees. Sixteen of 32 knees (50%) showed the desired position of the mechanical axis. Ten knees (31%) showed an undercorrection; 6 knees (19%) an overcorrection. The mean increase of the posterior tibial slope was 2.7degrees (range, -8degrees to 10degrees). Conclusions: The medial open-wedge osteotomy of the proximal tibia has a tendency to increase the posterior tibial slope. This effect is unfavorable in anterior knee instability because it increases the anterior translation of the tibia. Increasing the posterior slope can be favorable in posterior instability because it reduces the posterior sag. Level of Evidence: Level IV.
