Objective:To explore the clinical efficacy of hip arthroscopy (HA) and periacetabular osteotomy (PAO) in the treatment of borderline developmental dysplasia of the hip (BDDH).Methods:The clinical data of 57 patients with BDDH treated with HA or PAO from July 2016 to June 2020 were retrospectively studied. According to surgical methods, they were divided into HA group and PAO group. There were 32 patients (6 males and 26 females) in HA group, with an average age of 34.5±6.4 years, ranging from 19 to 47 years. There were 25 patients (5 males and 20 females) in the PAO group, with an average age of 37.2±6.9 years, ranging from 21 to 50 years. Patients were followed up till May 2023. Imaging data of patients before the surgery and during follow-up were collected including lateral center edge angle (LCEA), anterior center edge angle (ACEA), extrusion index, T?nnis angle, femoral neck anteversion angle, and T?nnis grade. The modified Harris hip score (mHSS), non-arthritic hip score (NAHS) and hip outcome score activity of daily living scale (HOS-ADL) were evaluated at 6 months, 12 months and the final follow-up.Results:All patients were followed up for an average of 54.2±8.2 months in HA group and 55.5±9.4 months in PAO group separately. Before surgery, there were no significant differences in LCEA, ACEA, T?nnis angle, extrusion index, femoral neck anteversion angle and T?nnis grade between the two groups ( P>0.05). α angle of HA was 60.8°±5.8°, which was significant different from α angle of PAO 53.0°±5.9° ( t=5.07, P<0.001). At the final follow-up, in HA group, LCEA, ACEA, T?nnis angle, extrusion index were 20.0°±2.1°, 20.0°±2.6°, 30.6%±2.3%, 10.8°±1.6°, 39.8°±5.4, which were all significantly different from pre-operation data ( P<0.05). While there was no significant difference between pre- and post-operation in T?nnis grade (χ 2=5.72, P=0.057). In PAO group, LCEA, ACEA, T?nnis angle, extrusion index were 33.0°±3.0°, 33.3°±4.0°, 15.0%±4.0%, 2.8°±1.7° which were all significantly different from pre-operation data ( P<0.05). While there were no significant differences between pre- and post-operation in α angle and Tonnis grade ( P>0.05). Between HA group and PAO group, there were significant differences in LCEA, ACEA, T?nnis angle, extrusion index and α angle ( P<0.05), while no significant difference was found in T?nnis grade (χ 2=3.024, P=0.996). There were no significant differences between the two groups in preoperative mHHS, NAHS and HOS-ADL ( P>0.05). At 6 months,12 months and the final follow-up after operation, the mHHS, NAHS and HOS-ADL were significantly improved compared to pre-operation data ( P<0.05). At 6 months and 12 months after operation, the three hip function scores in HA group were significantly better than those in PAO group ( P<0.05). At the final follow-up, there was no significant difference in three hip scores between the HA group and the PAO group ( P>0.05). Conclusion:Both HA and PAO can achieve good short- and mid-term clinical efficacy in the treatment of BDDH. Specifically, HA is superior to PAO in the early clinical efficacy, and the mid-term clinical efficacy is similar. PAO is able to improve the acetabular radiographic parameters of BDDH (except α angle) while HA can only improve α angle. However, they all can effectively delay the imaging progression of hip osteoarthritis.