This study aims to research on the association between the inclination of the proximal tibiofibular joint surface and medial compartment knee osteoarthritis. The proximal tibiofibular joint radiographs of the participants will be taken. The angle of inclination of the proximal tibiofibular joint surface in two groups will be compared.

The nonuniform settlement of the tibial plateau has been introduced to explain the development of knee varus and medial compartment knee osteoarthritis. It is believed that the lateral support of the fibula to the tibial plateau leads to the nonuniform settlement and degeneration of the plateau bilaterally.

Bone mass decreases as part of the normal aging process. Varying degrees of settlement of bone mass exist in the load-bearing joints, such as the knees, hips, ankles, and spine. In the proximal tibia, the lateral support of the fibula to the lateral tibial plateau routinely leads to nonuniform settlement, which is more severe in the medial plateau than in the lateral plateau. The slope of the tibial plateau arising from nonuniform settlement results in a transverse shearing force, with the femoral condyle shifting medially during walking and sports. Furthermore, side-slip aggravates the nonuniform settlement of the tibial plateau, especially in the medial plateau. Accordingly, a cycle of increasing the load distribution in the medial compartment and nonuniform settlement occurs. This results in the load from the normal distribution shifting farther medially to the medial plateau and consequently leads to knee varus, aggravating the progression of medial compartment knee osteoarthritis.

However, the angle of inclination of the proximal tibiofibular joint surface in different individuals varied widely. The angle of inclination of the proximal tibiofibular joint may affect the nonuniform settlement of the bilateral tibial plateau.

This study aims to research on the association between the inclination of the proximal tibiofibular joint surface and medial compartment knee osteoarthritis. A sample size power analysis was conducted before the study, which revealed that a minimum of 39 subjects per group was needed to achieve statistical significance with two-sided α=0.05 and 1-ß=0.80 and an assumed mean differences between two groups = 65% of the standard deviation.

Participants will be recruited from the outpatient clinic of No.113 Hospital of Chinese People's Liberation Army, as well as advertising flyers within the clinic. Firstly, patients of the study group will be recruited. It is hoped that the number of males will be equal to the number of females. Then the controls will be recruited and matched for age (within 5 years) and sex to the cases.

Standing anteroposterior knee radiographs and special PTFJ radiographs will be taken for the participants in both groups. The standing anteroposterior knee radiographs will be obtained with the knee extended. The PTFJ radiographs will be acquired with the knee of the participants in approximately 45°-60° of internal rotation.

On weight bearing anteroposterior radiographs of knee, osteoarthritis will be graded by using the Ahlbäck grading scale, and femorotibial angle will be measured. The Ahlbäck classification Stage 0: No radiographic sign of arthritis. Stage I: Narrowing of the joint space (JSN) (with or without subchondral sclerosis). JSN is defined by a space inferior to 3 mm, or inferior to the half of the space in the other compartment (or in the homologous compartment of the other knee). Stage II: Obliteration of the joint space; Stage III: Bone defect/loss <5 mm. Stage IV: Bone defect/loss between 5 and 10 mm. Stage V: Bone defect/loss >10 mm, often with subluxation and arthritis of the other compartment.

For the PTFJ radiographs, the degenerative joint disease of the PTFJ will be staged with the Kellgren-Lawrence staging system, and the fibular inclination angle (FIA) and the tibial inclination angle (TIA) of the PTFJ surface to the longitudinal axis of both fibula and tibia will be measured separately. The Kellgren-Lawrence Grade 0: No X-ray changes of osteoarthritis. Grade I: Formation of osteophytes on the joint margins, in the knee joint, on the tibial spine. Grade II: Grade I + thinning of joint cartilage associated with sclerosis of subchondral bone. Grade III: Grade I + II + subchondral cysts. Grade IV: Grade I + II + III + deformation of the bone ends.

Demographics, laterality of included knees and FTA, the inclination angle of the proximal tibiofibular joint surface and the grade of the proximal tibiofibular joint degeneration in two groups will be evaluated and compared.

Inclusion Criteria:

• symptomatic knee osteoarthritis according to the American College of Rheumatology criteria for the classification and reporting of OA of the knee joint,
• predominance of self-reported pain over the medial aspect of the knee,
• radiographic evidence of medial compartment knee osteoarthritis with an Ahlbäck score of grade I or greater,
• and radiographic evidence of lateral compartment with Ahlbäck classification grade 0.

Exclusion Criteria:

• any previous surgery or severe trauma in the affected limb.
• inflammatory joint disease or tumors in the affected limb.

The outpatient clinic of No.113 Hospital of Chinese People's Liberation Army