Radiographic measurement of leg-length change in the nonoperative leg during total hip arthroplasty: a potential indicator of imaging error?

Abstract

Background: Anteroposterior (AP) pelvic radiographs are subject to errors that may cause measurement inaccuracy in total hip arthroplasty (THA). Such errors may be detected by measuring pre- to postoperative leg-length changes in the nonoperative leg, which experiences no physical changes during THA. Methods: From AP pelvic radiographs, we measured pre- to postoperative leg-length changes (LLC) in the nonoperative legs of 67 patients who underwent primary THA using the trans-ischial line method. Results: An LLC of 0 mm was observed in the nonoperative leg in only 14 cases (21%). A LLC ⩾ 2 mm was observed in 27% (18/67) of cases, including 13% (9/67) with LLC ⩾ 3 mm and 6% (4/67) with LLC ⩾ 4 mm. A post-hoc analysis used a validated method to measure change in pelvic tilt between pre- and postoperative images and found that changes in pelvic tilt ⩾ 4° in the anterior and posterior directions created apparent lengthening (2.0 ± 1.4 mm, p  < 0.001 vs. 0–3° of tilt) and shortening (−2.1 ± 1.6 mm, p  < 0.001 vs. 0–3° of tilt) of the nonoperative leg, respectively. Conclusions: The current study provides evidence of measurement errors in leg length using AP pelvic radiographs following THA. Changes in pelvic tilt may be in part responsible for these errors, with the direction of change in pelvic tilt influencing the apparent lengthening or shortening of the lower limb. Ultimately, these findings may influence the radiographic measurement and interpretation of leg-length changes following THA.