Methods The investigators conducted a non-randomized comparative study to compare component positioning and clinical outcomes of the unilateral primary THA patients with DAA techniques and those with micro-PA techniques. The investigators received approval from institutional review board in National Taiwan University Hospital. The investigators recruited consecutive patients receiving unilateral primary THAs performed by a single surgeon (the senior author) between October 2013 and September 2014. The eligibility criteria were patients with unilateral hip arthritis indicated for THA. The excluding criteria were patients with previous THAs or patients indicated for revision THAs. A total number of 141 patients were recruited. Twenty-six of the patients underwent THAs with DAA and 115 of the patients underwent THAs with micro-PA. The patient demographics was shown in table 1. All the patients were regularly follow-up at least 2 year in out-patient clinics. The investigators used Trilogy acetabular cup system and ML-taper femoral components (Zimmer Inc., Warsaw, IN, United States) in all patients.