The goal of this clinical trial is to compare the efficacy of hip arthroscopy versus individualized conservative treatment in patients with borderline developmental dysplasia of the hip (BDDH), defined by a lateral center-edge angle (LCEA) of 18-25°. The main question it aims to answer is: which treatment approach, surgical or conservative, provides superior pain relief, functional recovery, and quality of life improvement at 12 and 24 months post randomization? Participants will be randomly assigned (1:1) to either standardized hip arthroscopy, comprising capsular management, acetabular rim limited trimming, labral repair, and Cam deformity correction when indicated; or a 3-month individualized conservative protocol including patient education, nonsteroidal anti-inflammatory drugs (NSAIDs) for 2-4 weeks, chondroprotective supplementation for 8 weeks, and supervised progressive exercise training. All participants will undergo follow-up assessments at baseline, 12, and 24 months. The primary outcome is the modified Harris Hip Score (mHHS). Secondary outcomes include patient-reported outcome measures (VAS pain, iHOT-12, SF-12, and patient satisfaction), clinical assessments, and safety monitoring.

This multicenter, prospective, randomized controlled trial aims to compare the efficacy of hip arthroscopy versus individualized conservative treatment in patients with borderline developmental dysplasia of the hip (BDDH). BDDH, defined by a lateral center-edge angle (LCEA) of 18-25°, represents a distinct condition between normal hips and developmental dysplasia of the hip, and its optimal management remains controversial. A total of 248 eligible patients aged 18-42 years will be recruited from 23 qualified clinical centers across China and randomly assigned (1:1) to either hip arthroscopy surgery or individualized conservative therapy using minimization dynamic randomization stratified by age and LCEA. The conservative treatment protocol includes patient education, NSAIDs for 2-4 weeks, chondroprotective supplementation for 8 weeks, and supervised progressive exercise training for 3 months. The surgical group will undergo standardized hip arthroscopy with capsular management, acetabular rim limited trimming, labral repair, and Cam deformity correction when indicated. The primary outcome is the modified Harris Hip Score (mHHS) , with a minimal clinically important difference (MCID) of 9 points and patient-acceptable symptom state (PASS) threshold of 78 points. Secondary outcomes include patient-reported outcome measures (pain VAS, iHOT-12, SF-12, and patient satisfaction), clinical assessments (hip range of motion and impingement tests), and safety monitoring (adverse events recorded up to 24 months).

Inclusion Criteria:

1. Age 18-42 years (studies have reported age ≥42 years as a risk factor for hip arthroscopy in BDDH: HR=11.6, 95% CI: 2.5-53.9), either sex;
2. Symptoms (anterior-lateral groin, lateral, or posterolateral hip pain or discomfort), physical examination findings (limp, clicking, locking, and limited range of motion), and imaging findings consistent with BDDH diagnosis (standing anteroposterior pelvic radiograph with Wiberg lateral center-edge angle [LCEA] 18-25°);
3. Surgeon's assessment that the patient with BDDH will benefit from hip arthroscopy or conservative treatment;
4. Full understanding of trial benefits and risks, willingness to participate in the intervention and complete follow-up, and signed informed consent.

Exclusion Criteria:

1. Standing anteroposterior pelvic radiograph showing Tönnis grade ≥2 and/or joint space <2 mm;
2. Standing anteroposterior pelvic radiograph showing FEAR index >5°;
3. Positive physical examination for joint instability: anterior apprehension test (HEER test), abduction-extension-external rotation test in lateral decubitus position (AB-HEER test), or prone external rotation test;
4. History of high-energy hip trauma, previous ipsilateral or contralateral hip surgery, avascular necrosis of the femoral head, slipped capital femoral epiphysis, Perthes disease, severe osteoporosis, tumor, acetabular stress fracture, or hypertrophic or inflammatory hip disease;
5. Concomitant consciousness disorder, psychiatric illness, or neuromuscular dysfunction affecting lower limb function;
6. Contraindications to hip arthroscopy (such as systemic or local infection).