Lateral tibial plateau fractures or lateral cortex fractures may occur when performing medial opening wedge osteotomy. Some special tools have been described to prevent lateral tibial plateau or lateral cortex fracture during the medial separation of the osteotomy planes in medial opening wedge osteotomy. The author has developed a distractor with angle-scale to prevent such complications. Between December 2001 and April 2004, 40 opening wedge osteotomies stabilised with the plate with wedge were performed on 38 patients. The distraction of initial osteotomies in 25 knees was done with different techniques and there were three lateral tibial plateau fractures and nine lateral cortex fractures. The remaining, recently operated, 15 knees were treated with the aid of the above described angle-scale distractor to prevent lateral tibial plateau and lateral cortex fractures. There was no lateral tibial plateau fractures during the surgery when we used this instrument for the distraction of the osteotomy sites. A subsequent prospective study evaluated this new device.