The primary objective of this prospective multicenter study is to assess whether the functional outcome measured with the Knee Injury and Osteoarthritis Outcome Score (KOOS) for patients with medial unicompartmental osteoarthritis and osteonecrosis of the knee treated with open wedge high tibial osteotomy (HTO) using the TomoFix™ Small is better than the functional outcome after conservative treatment.

Osteoarthritis (OA) of the knee (gonarthrosis) is a common medical condition and causes pain and disability. By 65 years of age, the majority of the US population has radiographic evidence of OA and 11% have symptomatic OA of the knee. In Japan, OA of the knee is more common due to the common sitting style with maximally flexed knees and parallel position of both legs.

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.

Inclusion Criteria:

• 40 years of age and older
• Patients with OA Grade 1-4 (Kellgren-Lawrence) or osteonecrosis (ON) Stage 2-4 (according to the Koshino classification)
• Indication for open wedge HTO with the TomoFix™ Small
• No ligamental laxity
• Signed written informed consent (by the subjects or legal guardian) and agreement to attend all planned follow-ups
• Willing and able to comply with the postoperative management program
• Able to understand and read country national language at an elementary level

Exclusion Criteria:

• Femoro-tibial angle (FTA) > 185o (standing view x-ray)
• Flexion contracture > 15o
• Total knee replacement or unicompartmental knee on the contralateral side
• Infections located between the middle of the femur and the ankle
• Systemic bacterial infections
• Severe osteoarthritis or surgery of the hip joint
• Smoking of more than 20 cigarettes per day
• Immunodeficiency or compromised host
• Patients who have participated in any other device or drug related clinical trial within the previous month
• ACL/PCL reconstruction of the same knee
• Severe osteoarthritis of the ipsilateral ankle joint

For conservative group only:

• HTO on the contralateral side
• Major surgery on both knees (minor surgery as arthroscopic meniscectomy and synovectomy is allowed)

Patients with Indication for open wedge HTO with the TomoFix™ Small presenting in the orthopaedic department of the participating hospitals