High tibial osteotomy is a well-established treatment option for the young patients (aged 40~55years) with knee osteoarthritis which is confined in medial compartment of the knee. Classical technique was lateral closing wedge osteotomy, but recently medial open wedge osteotomy has gained popularity with the advent of new fixation devices and refined surgical techniques. The surgeon can correct the deformity more precisely in both coronal and sagittal planes simultaneously with medial opening technique. And it can avoid complications associated with lateral closing technique like tibial shaft offset or peroneal nerve palsy. But medial opening technique inevitably creates large bone defect, which has to be addressed to avoid complications like loss of correction or delayed/non-union. Autologous bone is widely accepted as a standard for filling bone defects, but its supply is limited and harvesting autologous bone adds to surgical morbidity like bleeding, pain or fracture at the donor site. Therefore, there has been much effort to find materials to substitute autologous bone. Many studies reported the results of using allogenic bone for addressing bone defects and most of them showed favorable results. But some allogenic bone products are cumbersome to process to make it fit to the defect, and there are potential risk of disease transmission, if the products are not properly treated. Bone cements of several different composition has been developed and when used for filling bone defect, they also showed good results in general. Recently, a new synthetic bone substitute based on calcium phosphate and calcium sulfate (geneX®, Biocomposites Co.,Ltd.) has been introduced and is commercially available. While providing initial mechanical strength, its calcium sulfate component is rapidly absorbed to provide space for new bone ingrowth and its surface is made to negatively charged, which helps accelerate new bone formation. It is provided as an injectable paste, which is easier to handle than allogenic bone, so it may help reduce operation time. With these theoretical advantages, there are some anecdotal reports that patients treated with geneX® presented less postoperative pain and bleeding than patients treated with allogenic bone graft. Therefore, we conducted this study to determine whether the new synthetic bone substitute (geneX®) is better than allogenic bone for addressing bone defect created in medial open wedge high tibial osteotomy.