Ultrasound-guided Ischial Bursa Injection: Technique and Positioning Considerations

Objectives: To 1) describe and validate an ultrasound-guided ischial bursa injection technique in an unembalmed cadaveric model and 2) to compare the distance between the ischial tuberosity and the sciatic nerve in a hip neutral versus 90° flexed hip position in asymptomatic volunteers. Design: The first part was a single-blind prospective study. The second part was a prospective cohort study. Setting: An academic institution procedural skills laboratory and outpatient clinic. Participants: The first part of the study involved 1 cadaveric specimen. The second part of the study involved 20 asymptomatic subjects. The mean age of the subjects was 28 years, and the mean (standard deviation) body mass index was 23.2 ± 2.8 kg/m² (minimum, 18.3 kg/m²; maximum, 29.5 kg/m²). Methods: In the first part of the study, a single operator completed bilateral ultrasound-guided ischial bursa injections in an unembalmed cadaveric specimen by using diluted colored latex. In the second part of the study, ultrasound was used in 20 asymptomatic volunteer subjects (10 men and 10 women) to measure the distance from the lateral edge of the ischial tuberosity to the sciatic nerve. Main Outcome Measurements: The injections were graded for accuracy as follows: accurate (all injectate contained within the ischial bursa), accurate with overflow (injectate within the ischial bursa but also located in adjacent structures other than the needle track), or inaccurate (injectate not within the ischial bursa). The second part of the study measured the distance from the ischial tuberosity to the sciatic nerve with subjects in 2 different positions (prone and side lying with the tested hip flexed to 90°). Results: Postinjection cadaveric dissections revealed that both ultrasound-guided injections accurately placed liquid latex within the ischial bursae. There was no evidence of injury to surrounding neurovascular structures. Among asymptomatic volunteers, the average distance between the ischial tuberosity and the sciatic nerve increased from 28.4 mm (range, 20.5-38.9 mm) in the neutral position to 41.9 mm (range, 30.9-66.0 mm) with the hip flexed to 90° (average change, 13.5 mm away from the ischial tuberosity; P = .0001). Conclusions: Ultrasound-guided ischial bursa injections are technically feasible. Flexing the hip to 90° increases the distance between the ischial tuberosity and the sciatic nerve in asymptomatic volunteers, thus potentially resulting in a safer needle trajectory when ischial bursa injections are clinically indicated. Further investigation in clinical settings is warranted to validate these findings.