Wang T, et al. - Investigators analyzed 65 subjects to analyze the surgical injury of hip external rotators after posterior total hip arthroplasty (THA) and also to investigate the influence of the muscle repair on the muscle recovery using the MRI three-dimensional reconstruction technique combined with the clinical assessment. Preoperatively, they observed homogeneous bilateral piriformis and internal obturator muscle. They recorded an increase in volume atrophy and a fat-muscle ratio of the piriformis on operated side by 1.64%, 0.26% at 6 weeks and 1.33%, 0.20% at 12 weeks and 7.28%, 2.09% and 15.71%, 5.14% for the internal obturator muscle, respectively as compared with the contralateral side. A significant increment in muscle atrophy and fatty infiltration at 52 weeks, was recorded, indicating harms to hip external rotators and the early hip external rotation function with THA via a posterior approach.