Objective To investigate the effectiveness of MAKO robotic-arm assisted total hip arthroplasty (THA) via direct anterior approach (DAA) for bony fused hips in ankylosing spondylitis (AS). Methods Between June 2021 and January 2022, MAKO robotic-arm assisted THA via DAA was applied to treat 10 cases (12 hips) of AS with bony fused hips. There were 7 males and 3 females; the age ranged from 30 to 71 years, with an average age of 42.4 years. The duration of AS was 12-35 years, with an average of 21.4 years. The preoperative hip was ankylosed in flexion in 3 cases, with a flexion angle of 20°, 30°, 35°, respectively; 9 cases were ankylosed in extension. The operation time and complications were recorded; the visual analogue scale (VAS) score, Harris score, and Oxford hip scale (OHS) score before and after operation, and postoperative range of motion (flexion, extension, internal rotation, external rotation, adduction, and abduction) were used to evaluate the recovery of joint function; according to the postoperative anteroposterior X-ray film and CT scan of both hip joints, the abduction angle, anteversion angle, the difference between bilateral combined off-set and the lower limb length discrepancy were measured. Results The operation time ranged from 80 to 190 minutes (mean, 134.6 minutes). All 10 patients were followed up 5-11 months (mean, 7.4 months). There was no serious adverse events such as incision infection, deep vein thrombosis of lower extremities, hip redislocation, aseptic loosening of the prosthesis, or death. At 5 months after operation, the acetabular prosthesis angle of abduction was 37°-45° (mean, 40.3°), anteversion angle was 9°-20° (mean, 15.8°). The difference between bilateral combined off-set was 0-10 mm (mean, 4.3 mm); the lower limb length discrepancy was 0-12 mm (mean, 3.5 mm). At last follow-up, the average range of motion of the hip joint was 89.2° in flexion (range, 80°-100°), 1.7° in extension (range, –5°-10°), 7.1° in internal rotation (range, 0°-15°), 20.4° in external rotation (range, 10°-30°), 7.9° in adduction (range, 0°-20°), and 16.5° in abduction (range, 10°-25°). At last follow-up, the VAS score, Harris score, and OHS score significantly improved when compared with those before operation (P<0.05). ConclusionThe MAKO robotic-arm assisted THA via DAA can achieve satisfactory results in the treatment of AS with bony fused hip, which has the advantages of accurate prosthesis installation, soft tissue release, and less trauma during operation.