The quality of cartilage regeneration after marrow stimulation is well documented to be unpredictable, because they do not regenerate consistent amount of cartilage. To overcome the shortcomings of the microfracture technique, various augmentation techniques using synthetic collagen matrix, scaffolds or plug devices have been developed. However, their efficacy remains unclear. The purpose of this prospective randomized controlled study is to evaluate whether the microfracture in combination with collagen gel augmentation could improve the quality of cartilage regeneration in patients undergoing medial open wedge high tibial osteotomy (HTO) for the treatment of medial unicompartmental knee osteoarthritis (OA).