ObjectiveTo explore the method of acetabular orientation determination in total hip arthroplasty (THA) for bony ankylosed hip and the accuracy of the postoperative evaluation. MethodsBetween January 2009 and March 2013, 33 consecutive patients (49 hips) underwent THA. There were 25 males and 8 females with a mean age of 35.8 years (range, 18-69 years). The left hip was involved in 10 cases, the right hip in 7 cases, and bilateral hips in 16 cases. The causes were ankylosing spondylitis in 18 patients, tuberculosis in 6 patients, traumatic arthritis in 6 patients, osteoarthritis in 2 patients, and suppurative infection in 1 patient. The disease duration was 7-15 years with an average of 10.8 years. The acetabular orientation was determined with periacetabular bone marks (the upper margin of the obturator foramen, acetabular notch etc.) and soft tissue signs (acetabulum transverse ligament etc.). The hip or pelvic radiograph was taken to measure the acetabular prosthesis anteversion and abduction angle, and upward or downward, inward or outward acetabular migration degree. The acetabular anteversion angle of 15 degrees, the abduction angle of 45 degrees, and upward or downward, inward or outward acetabular migration degree of 0 served as a reference value to evaluate the accuracy of acetabular position. ResultsThere was no complications of neurovascular injury, fracture, joint dislocation, and infection. All of patients were followed up 13-63 months (mean, 30.3 months). The anteversion angle and abduction angle were (13.904±4.034)° and (42.898±7.474)° at last follow-up, showing no significant difference when compared with reference value (t=1.386, P=0.178; t=1.969, P=0.055). The inward or outward and upward or downward acetabulum migration degree were (2.530±2.261) mm and (3.886±3.334) mm respectively, showing significant differences when compared with reference value (t=7.830, P=0.000; t=8.159, P=0.000); it was less than 5 mm in 29 hips, 5-10 mm in 18 hips, and more than 10 mm in 2 hips; the acetabulum center coincidence rate was 59.2%. ConclusionFor bony ankylosed hip having loss of normal anatomy structure, intraoperative residues and permanent anatomical structure should be used for acetabular positioning.