High Tibial Osteotomy (HTO) for knee OA is a standard procedure in orthopedic surgery. Six years ago, HTO using TomoFix™ was introduced. Because Asian people have different geometric dimensions of the tibiae, an adaptation of size and shape was necessary, and the new TomoFix™ Small was developed. Although there is widespread literature concerning osteotomy for treating knee OA, literature assessing patient outcome after open wedge HTO using TomoFix™ is rare and for TomoFix™ Small, this data is still nonexistent. Therefore, the primary aim of the present multicenter study is to evaluate patient functional outcome using the KOOS. Since a proportion of patients with the indication for HTO refuse surgery and opt to receive conservative treatment, the functional results of patients treated with HTO using TomoFix™ Small will be compared with these patients.