EO Footprint as a Landmark During DAA THA

One of the goals of Total Hip Arthroplasty (THA) is to reconstruct leg length as adequately as possible. In order to achieve this one needs landmarks that are visible both on the templating X ray as well as during surgery. The classical posterior and lateral approaches often rely on the distance from the greater trochanter (GT) to the shoulder of the femoral stem or the distance from the lesser trochanter (LT) to the side of the neck osteotomy. If the surgeon finds out on the digital template that the distance from the GT to the shoulder of the implant should be X mm to achieve equal leg length, than the surgeon can try to reconstruct this during surgery. During the Direct Anterior Approach (DAA) these landmarks usually are not visible or require additional dissection injuring important soft tissue structures along the way. One anatomical structure that appears to be always visible during the DAA is the External Obturator tendon (EO). It was recently confirmed that the level of insertion of the EO onto the proximal femur can also always be determined on pre-operative X rays. It therefore represents one of the few landmarks that is visible both on the templating X ray as well as during DAA THA. Indeed, many hip surgeons, including ourselves, believe that if the shoulder of the femoral stem is near the insertion of the EO leg length cannot be far off. However, there is no clinical data to support this. The investigators would therefore like to the correlate the distance observed intra-operatively to the actual established distance on the post-operative X ray.