Anatomical landmarks for guiding a surgeon during total hip arthroplasty (THA) that are visible both during surgery and on radiographs are rare. If a surgeon is planning to perform THA using the direct anterior approach, the external obturator tendon (EO) is one of those rare, but valuable landmarks. It is consistently visible during surgery and radiographically. But how accurate is it as a reference as to the exact position and dimensions of the footprint?
A team of researchers from Belgium decided to measure the accuracy of the EO tendon’s footprint as an anatomical THA landmark. Their work, “The External Obturator Footprint Is a Usable, Accurate, and Reliable Landmark for Stem Depth in Direct Anterior THA,” was published in the August 2021 edition of Clinical Orthopaedics and Related Research.
According to the authors of this study, use of the external obturator footprint for stem depth hasn’t been tested in daily practice. As a result, they decided to precisely measure (intraoperatively) the distance between the superior border of the external obturator tendon and the shoulder of the stem in all patients (n = 135) who were treated with primary THA via a direct anterior approach as performed by the senior author (Stijn Ghijselings, M.D.) between November 2019 and October 2020.
Co-author Georges Vles, M.D., Ph.D. told OTW, “There are basically two previous studies on this topic. The first paper was by Rudiger et al., who in a CT study showed that the height of this landmark is very consistent, i.e., 6.4 ± 1.4 mm. The second study, performed by our own group, used 2D and 3D anatomical mapping of the external obturator footprint in cadavers to come up with instructions on exactly how to template this anatomical landmark on a conventional planning radiograph of the pelvis prior to total hip arthroplasty, so that it can be useful intra-operatively.”
For this study, wrote the authors, “The landmark was considered useful when two of three evaluators agreed that the intersection of the vertical line comprised of the lateral wall of the trochanteric fossa and the oblique line formed by the intertrochanteric crest was clearly visible on the preoperative planning radiograph, and when the landmark was furthermore identified with certainty during surgery.”
“Although strictly speaking it was found useful in 77% of patients in this study, we found that this percentage of usability can easily be improved to around 90% by providing the radiology lab technician with instructions to correct external rotation of the foot during the taking of the planning radiograph. Future studies could compare the established (in)equality in leg length in patients using the external obturator landmark with computer-assisted surgery.”
Dr. Vles said to OTW, “When performing a direct anterior THA, the landmark is useful in around 90% of patients and has shown good-to-excellent accuracy (ICC0.75-087), and excellent intrarater (ICC 0.99) and interrater reliability (ICC 0.99).”
“This easy, accessible landmark allows surgeons to position the stem within a range of the templated depth that is beneath the threshold for the development of unphysiological gait parameters. Therefore, it is probably the best landmark for stem depth when performing a non-fluoroscopy-assisted total hip arthroplasty via a direct anterior approach.”
