The primary objective of this registry is to assess 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 conservatively. The aim of the Focused Registry is to complete the data collection and enable a sound analysis for the Tomofix Small size study.

The aim of the Focused Registry is to complete the data collection and enable a sound analysis for the Tomofix Small size study.

The incidence of knee osteoarthritis (OA) is higher in Japan than in other populations. Respective prevalences of 47.0 and 70.2% in men and women for Kellgren-Lawrence Grades 2-4 knee OA are reported in Japanese patients aged 60 years and over.

First treatment options are pharmacological (i.e. NSAIDs) and non-pharmacological therapies (i.e. use of assistive devices, exercises, osteopathy, physical therapy), followed by injections of hyaluronic acid and other substances. If no such therapies are successful, surgical treatment options as unicompartmental knee replacement, total knee arthroplasty and high tibial osteotomy (HTO) are recognized. The discussion of which is the best therapy is still controversial. HTO may decrease the number of complications compared to total knee replacement or unicompartmental knee arthroplasty and leave the option for later replacement surgery.

In this project, patients are included who would be surgically treated with HTO with Tomofix small based on the investigator's assessment but refuse to have surgical treatment. They will be treated conservatively and are investigated to serve as reference to an ongoing clinical study investigating invasive treatment modalities.

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
• Able to understand and read country national language at an elementary level
• Ability to understand the content of the patient information / informed consent form
• Willingness and ability to participate in the registry according to the registry plan (RP)
• Signed and dated IRB/EC-approved written informed consent

Exclusion Criteria:

• Femoro-tibial angle (FTA) > 190° (standing view x-ray)
• Flexion contracture > 15°
• 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
• 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