The goal of this observational study is to describe the radiographic and kinematic features in patients with femoroacetabular impingement syndrome (FAI). The main questions it aims to answer are:

• Is structural knee abnormalities associated with knee pain symptoms in FAI patients?
• Does FAI patients with knee pain demonstrate unique kinematic characteristics？ Participants will receive routine preoperative radiographic examinations, which include ipsilateral knee MRI for the purpose of femoral version measurement. Participants will also receive markerless motion capture analysis.

Researchers will compare preoperative radiographic and kinematic features based on knee pain status, which is routinely collected as part of clinical investigation procedures.

This observational study aims to study the association between preoperative subjective knee pain and baseline knee radiographic and kinematic characteristics. MRI of the ipsilateral knee is screened for structural abnormalities, including cartilage defects, meniscal tears and ligament injuries. Markerless motion capture data are evaluated for transverse plane kinematic features, with a primary focus on foot progression angles during normal gait.

Inclusion Criteria:

• (1) diagnosed with FAI based on clinical and radiographical findings;
• (2) failed conservative treatment for at least 6 months and indicated for surgical treatment;
• (3) received routine preoperative hip and ipsilateral knee MRI.

Exclusion Criteria:

• (1) history of fracture or surgery of hip and knee;
• (2) avascular necrosis;
• (3) Legg-Calve-Perthes disease;
• (4) Ehlers-Danlos syndrome;
• (5) pigmented villonodular synovitis (PVNS);
• (6) osteoid osteoma (OO);
• (7) synovial chondromatosis;
• (8) severe hip osteoarthritis (OA) with Tönnis grade > 1;
• (9) developmental dysplasia of hip (DDH)

Consecutive patients diagnosed with FAI and admitted into the sports medicine department at Peking university third hospital for hip arthroscopic surgery by a single surgeon.