Open wedge valgus tibial osteotomies: affecting the distinct ACL bundles

Valgus alignment affects the biomechanical behavior of the ACL, resulting in an increase of the in situ forces in the posterolateral (PL) bundle of the graft. The amount of valgus alignment creating such effects on the force remains unclear. Furthermore, the mechanistic rational for the increase in situ forces on the PL bundle when compared with the AM during the valgus osteotomy remained unclear. We hypothesized that increasing valgisation of the knee results in significantly increased obliquity and a resulting increasing elongation pattern and apparent strain of the PL bundle compared to the AM bundle. Six cadaver legs were used utilizing two commercial navigation systems including navigated high tibial osteotomy (HTO) and ACL measurements simultaneously. ACL footprints were registered for the central and identified AM and PL bundle. A stepwise oblique open wedge osteotomy was performed for 7.5° and 15° valgisation. Length changes and coronal plane obliquity were determined at 0° and 30° flexion. The apparent strain for each fiber was calculated. Valgisation significantly affects the length and obliquity of the PL portion of ACL. Valgisation of 7.5° appears to be a threshold for affecting PL length and obliquity without significant changes compared to 15°. The mean apparent strain for the PL bundle increases up to 14% with a 15° osteotomy in full extension, compared to 4% for the central and 2% for the AM bundle. HTO should be done prior to fixation the ACL graft in combined procedures as valgisation does affect graft length. Relative alignment influence on obliquity and length of the PL bundle should be considered in valgus knees.