When performing a medial opening wedge upper tibial osteotomy, a fracture into the lateral cortex can lead to loss of stability of the construct. The aim of this study was to assess different intra-operative techniques to overcome this problem, and test the stability under axial compression. Twenty eight calf tibias had a medial opening wedge osteotomy and lateral cortical fracture created and then were tested in four groups depending on how this fracture was fixed; none, a plate and screws, two staples, and a circular external fixator. Loss of medial height of the osteotomy line and lateral cortex micromotion was evaluated under maximal axial loading of 2,500 N. No fixation had significantly inferior results compared with the other three types. The plate and screws and staples showed better results than circular external fixation in terms of preserving the medial height, whereas the plate and screws and circular external fixator showed better results compared with staples in terms of lateral cortex stability. This study suggests that plate and screw fixation for lateral cortex disruption during the medial opening wedge upper tibial osteotomy has better stability under axial compression than staples or a circular external fixator.