Geometrical Analysis of the Opening Gap after Tibial Condylar Valgus Osteotomy for Proper Hinge Point Selection.

Background: Preoperative deformity and hinge position are associated with the magnitude of the gap opening during corrective osteotomy. A larger opening gap angle is associated with a higher risk of complications. This cross-sectional study sought to identify a suitable hinge position that results in the smallest opening angle during tibial condylar valgus osteotomy (TCVO).Methods: The data of 66 arthritic knees treated by TCVO were included, comprising 16 knees with the hinge points selected medial to the center (group M), 21 knees with the hinge points selected at the center (group C), and 29 knees with the hinge points selected lateral to the center of the intercondylar eminence (group L). The opening gap angles and the correction amounts of the medial proximal tibial angle (Delta MPTA) were compared among the 3 groups to identify the preliminary relationship between the hinge positions and the opening gap angle. A simplified geometric model with the hinge positions selected at the medial beak, the center, and the lateral beak of the intercondylar eminence was constructed to simulate the realignment process. Several anatomical points were allocated as Cartesian coordinates. The opening gap angle with different hinge positions was mathematically formulated with MATLAB (MathWorks, Natick, MA, USA).Results: The average Delta MPTAs were 9.4 +/- 2.9 degrees, 9.4 +/- 3.5 degrees, and 9.3 +/- 3.0 degrees in groups L, C, and M, respectively. The opening angle of the osteotomy gap was the largest in group M and the smallest in group L (29.7 +/- 11.1 degrees and 16.9 +/- 5.3 degrees; P<0.01). The comparison of the opening angle per the Delta MPTA revealed a similar pattern. The simulated realignment process indicated that the hinge point at the lateral beak of the intercondylar eminence led to the smallest opening angle. The opening angle during TCVO was mathematically derived in terms of the Delta MPTA, the position of the intersection of the pre-and postoperative joint lines, and the position of the hinge point.Conclusions: The hinge point at the lateral beak of the intercondylar eminence results in the smallest opening angle and may be suitable for TCVO.