‘E-Spot’ (Obturator ‘E’xternus) as an Intraoperative Landmark for Femoral Preparation During Anterior Approach Total Hip Arthroplasty: A Prospective Cohort Study of 306 Cases

Anterior THA has gained popularity despite challenges with femoral exposure, particularly when performed “off-table”. Femoral-sided complications during anterior THA may be mitigated by utilizing fluoroscopy, navigation/robotics, and/or anatomical landmarks as references (historically the femoral greater/lesser tuberosity). The purpose of this study was to assess use of the obturator externus tendon femoral origin (termed ‘E-spot’) as an intraoperative landmark during anterior approach THA. Aims were to determine whether ‘E-spot’ could 1) be identified reliably and 2) aid in the intraoperative judgement of femoral component position. A prospective cohort study of consecutive patients undergoing primary THA by the senior author between October 2022 and October 2024 was conducted. Early clinical outcomes and complications were assessed up to 2 years of follow up. THA was performed similarly to standard descriptions of DAA or ABMS with the patient positioned supine on a regular OR table. Systematic capsular release facilitated femoral preparation in all cases. The pubofemoral ligament, superior capsule/obturator internus were released in all cases (while protecting gluteus medius and minimus) to enable femoral mobilization and elevation via a femoral elevator retractor behind the greater trochanter. We attempted to visualize and preserve the obturator externus tendon in all cases as a marker of completeness of femoral release. Anatomic referencing via relation to ‘E-spot’ (rather than fluoroscopy or navigation) was utilized to judge femoral component position and placement. There were 460 primary hip arthroplasty procedures completed by during the study period. 154 patients (33%) were excluded for analysis as they underwent a posterior approach, being performed for patients having severe morbid obesity (BMI>45) or hip resurfacing. 306 hips in 300 patients that were performed via DAA (51) or ABMS (255) approaches were analyzed. Indications in 306 cases were primary/post-traumatic osteoarthritis, dysplasia, protrusio and displaced femoral neck fracture. 12 THAs were performed bilateral simultaneously (2.6%) and the rest unilateral. In terms of outcomes, ‘E-spot’ could 1) be reliably identified as well as 2) the stem to E-spot distance measured in every case (306 out of 306). There was 1 periprosthetic fracture (Vancouver B2) requiring revision surgery at 2 months postoperatively and 1 dislocation at 3 months postoperatively (0.65% overall early complication rate). No other surgical complications were observed (including greater trochanteric fracture, femoral perforation, or femoral component subsidence). In anterior approach THA performed supine on a regular OR table, identifying ‘E-spot’ via systematic capsular release is a safe and reliable method of enabling femoral exposure and referencing femoral component position. This technique does not require the use of fluoroscopy or other adjunctive technology. Similar to the way the transverse acetabulum ligament (‘TAL’) can be used on the acetabular side, ‘E-spot’ may be useful as an intraoperative landmark or 'lighthouse' during femoral preparation (i.e. ‘TAL of the femur’). Based on these results, we now routinely use E-spot to determine the completion of femoral capsular release, assess femoral component position, assure hip stability, and protect the trochanter during off-table anterior THA.