No Differences in Hip Joint Space Measurements Between Weightbearing or Supine Anteroposterior Pelvic Radiographs

Purpose: To to assess whether there are any significant differences in hip joint space width (JSW) between weight-bearing versus supine pelvic radiographs.

Methods: Standing and supine anteroposterior pelvic radiographs of 86 patients (146 hips) were included. Sample size was sufficiently powered to assess for equivalence between standing and supine films for JSW measurements made at the medial, lateral, and central aspects of the sourcil line. Measurements were made by 2 independent reviewers blinded to patient positioning. Each reviewer repeated a subset of the measurements to assess intra-rater reproducibility. Mean differences in joint space measurements between standing and supine radiographs were reported for each point of the sourcil. Intraclass correlation coefficients (ICCs) for inter and intra-rater reliability were also calculated.

Results: There were no significant differences between JSW measurements made on standing and supine pelvic radiographs (P = .468). Furthermore, equivalence testing demonstrated statistical equivalence between standing and supine JSW measurements made based on an equivalence threshold of ±0.5 mm. Inter-rater reliability demonstrated good agreement with an overall ICC of 0.775 (95% confidence interval [CI] 0.734-0.809). Intra-rater reliability also demonstrated good agreement with ICCs of 0.84 (95% CI 0.758-0.889) and 0.798 (95% CI 0.721-0.851) for the 2 reviewers, respectively.

Conclusions: JSW measurements on standing and supine pelvic radiographs were not significantly different, and their inter-rater agreement and intra-rater reproducibility demonstrated good reliability and repeatability. Therefore, either may be used to assess JSW, including measurements that may impact treatment decisions for hip arthroscopy.

Level of evidence: Level III; retrospective comparative study.