Sonography after total hip replacement: Reproducibility and normal values in 47 clinically uncomplicated cases

Background and purpose Interpretation of sonographic data is difficult when hematoma after total hip replacement is suspected, as there are no normative data. We describe the normal sonographic image, focusing on the amount and location of postoperative fluid collections after a clinically uncomplicated, primary total hip replacement by the posterior approach. Inter- and intraobserver reproducibility is also considered.

Patients and methods We performed sonography of the hip in 47 patients between the second and the fifth postoperative day. Bone-to-capsule distance and deep and superficial extraarticular fluid collections were measured. Intraclass correlation coefficients were calculated.

Results The normal values of bone-to-capsule distance and amount of extraarticular fluid after total hip replacement were established from the upper bound of the 95% confidence interval. The upper bound for bonecapsule distance was 6 mm, for deep fluid collections 21 mm, and for superficial fluid collections 28 mm. In this clinically normal patient group, 4 patients had an extreme value (< 3 SD) for bone-to-capsule distance. For the deep and superficial fluid collections, no extremes were found. No correlation was found between bone-to-capsule distance and whether or not there was extraarticular fluid. Intraclass correlation coefficients were 0.98 for bone-to-capsule distance and 0.99 for fluid collection measurements.

Interpretation Sonography is a reproducible method for the evaluation of fluid collections after total hip replacement. The values measured can be helpful in decision making when there is clinical suspicion of postoperative hematoma after hip replacement by the posterior approach.