Digital protractor as an intraoperative guide to cup anteversion in total hip arthroplasty

Introduction To explore if digital protractor could guide the anteversion of acetabular cup during primary THA and make it consistent with preoperative. Methods We retrospectively reviewed 172 cases of primary THA with direct anterior approach (DAA) over 2 years. The anteversion of acetabular cup were measured from computed tomography (CT) scan preoperative and de-identified plain radiographs postoperative by two blinded investigators who were not involved in the surgery. The effect of the digital protractor on the anteversion was determined using regression analysis. Results The mean anteversion for the THAs in digital protractor group was 15.5 degrees and 21.4 degrees in control group (P < 0.01). The mean anteversion bias for the THAs in digital protractor group was 1.59 degrees and 6.63 degrees in control group (P < 0.01). Regression analysis identified a 10.7% difference in anteversion due to the use of digital protractor (P < 0.01), and THAs performed without digital protractor were six times more likely to result in anteversion of > 25 degrees. The correlation coefficient for the interobserver reliability of the measurement of the two investigators was 0.94. Conclusion The digital protractor is a practical tool in the DAA for THA to determine the anteversion of the acetabular prosthesis.