The external obturator footprint as a landmark in total hip arthroplasty through a direct anterior approach: a CT-based analysis

Background: Anatomical landmarks for templating of total hip arthroplasty (THA) that are visible both during surgery and on radiographs are rare. If surgery is performed through a direct anterior approach the external obturator tendon (EO) is consistently visible. To use this point as a reference the exact position and dimensions of the footprint need to be known. Aim: To determine the location and dimension of the EO footprint on pelvic radiographs by correlating the EO anatomy in CT scans with conventional radiographs. Methods: CT scans and radiographs of 200 patients were analysed. The EO tendon was identified on CT scans; the height of its footprint, and its distance to the tip of the greater trochanter and to the anatomical axis of the femur was measured. The accuracy and inter-rater reliability in the identification of the EO footprint was determined. Results: The EO tendon was visible on all CT scans and it's footprint was identifiable on all corresponding radiographs. It's cranio-caudal dimension was 6.4 +/- 1.4 mm. It's distance to the tip of the greater trochanter was 16.0 +/- 3.1 mm. The EO footprint was located 5.2 +/- 3.7 mm lateral to the femoral anatomical axis. There was no significant difference regarding the accuracy of EO footprint localisation on radiographs among the 2 readers. Conclusion: The EO footprint on the greater trochanter is consistently visible on CT scans and radiographs. As the variability of the footprint dimension is small, this structure may serve as a useful landmark in THA, particularly when performed through a direct anterior approach.