CORR Insights®: The External Obturator Footprint Is a Usable, Accurate, and Reliable Landmark for Stem Depth in Direct Anterior THA.

Abstract

Where Are We Now? During preoperative planning for THA, femoral templating helps surgeons determine the best size and position of the stem to be used and aims to restore the desired leg length and hip offset. Intraoperative landmarks and tests are used during surgery to guide and confirm the desired implant position [2]. The stem insertion depth within the femur during THA affects both leg length and offset of the hip. Both are important factors for improving joint function. Change of leg length and hip offset can have not only cosmetic consequences but also functional ones, including altered long or short leg gait or abductor dysfunction [5], hip instability [9], neurologic loss [10], and increased trochanteric [13] and back pain [3]. Not surprisingly, the inability to restore leg length and hip offset at THA influences patient-reported outcomes [13]. Surgeons use several different approaches for preoperative planning of stem size and position, all of which continue to evolve. In the past, implant dimensions were manually drawn onto printed radiographs or surgeons used implant templates on transparent sheets overlaying radiographs. With the introduction of digital radiographs, templating is increasingly performed digitally using similar principles. As new technologies associated with navigation and computer-assisted THA evolve and are adopted into clinical practice, they will continue to change the way preoperative hip templating is performed [7]. Templating the femoral component uses the greater and lesser trochanters as reference points on radiographs. Not surprisingly, a major limitation of templating is the variability of the radiographic projection of these landmarks due to the pathology of the hip and its amount of rotation along the three axes of freedom at the time the radiographs were taken. Few studies have discussed the accuracy and reliability of templating, and their findings have differed. For example, the accuracy of templating in the prediction of stem size (within one size)—a factor that can influence both leg length and offset—has varied widely, from 37% [6] to 69% [4] and 95% [8]. The accuracy of templating in predicting hip offset is approximately 90% [4, 6]. While digital templating is handy, some evidence suggests it is less accurate than analogue templating both for predicting stem size and offset [4]. Therefore, exploring additional landmarks less prone to imaging variability, which can be identified on both the preoperative and intraoperative images, is clinically relevant and may improve the current preoperative planning methodology. The paper in this month’s Clinical Orthopaedics and Related Research® by Vles et al [12] investigates the efficacy, accuracy, and reliability of a new landmark, the obturator externus footprint, for stem depth position within the femur in THA performed through an anterior approach. This landmark was identified on 87% (117 of 135) of patients on preoperative radiographs and in 87% (118 of 135) of patients intraoperatively. Considering only those where the landmark was identified preoperatively, it was also identified intraoperatively in 88% of patients. Anatomical landmarks that are easily identifiable during surgery are particularly appealing to guide implant positioning. One such example, now well established and routinely used, is the use of the transverse acetabular ligament to guide cup anteversion during THA. Based on the discoveries of Vles et al [12], surgeons should consider using the obturator externus footprint to guide stem depth during THA. Where Do We Need To Go? Digital templating based on sophisticated imaging techniques, such as three-dimensional reconstructions and robotic technology, has the potential to be much more accurate when compared to previous radiographic methods. However, this superiority is yet to be proven and usage is not yet clinically widespread because of its recent introduction and cost. Therefore, ongoing modification of our current methods remains extremely important. New insights into the surgical anatomy of the hip, stemming from the recent increase in wider use of the direct anterior approach for THA, led to the development of the new obturator externus footprint landmark in the paper in this month’s CORR® [12]. Future investigations to improve our knowledge and understanding of surgical and radiological anatomy may assist surgeons to not only make better use of known landmarks but also help identify and assess the efficacy of novel ones as well [12]. Vles et al [12] suggest the obturator externus footprint on the greater trochanter is a useful landmark that can be identified both preoperatively, on radiographs, and during surgery to guide femoral stem insertion when performing THA through a direct anterior approach. To better appreciate the value of this landmark, future studies from independent centers should investigate the feasibility of identifying this landmark on radiographs and during surgery. Regarding identifying this landmark at surgery, Vles et al. [12] could not identify the obturator externus tendon at surgery in 13% (17 of 135) of patients. This could be a result of either inadequate exposure of the tendon or the tendon being unintentionally released. The need for surgical release, and therefore exposure, of the obturator externus during a direct anterior approach for THA is thought to be rare but has been acknowledged [1, 11]. This raises the question of whether further dissection could have improved the tendon detection at surgery as the tendon should be identifiable in every case. Investigating the role of this landmark when performing THA through other approaches, like the posterior approach, in which not only the tendon is routinely exposed but other trochanteric landmarks are readily available for comparison, can help better define the role of this new method. Once the role of the obturator externus footprint for stem depth insertion is better defined, the effects of the potential extra dissection required to expose the tendon in every patient should be investigated. How Do We Get There? The role of the obturator externus footprint as a landmark for stem depth insertion at THA should be compared with other landmarks on the greater and lesser trochanter currently in use. For example, many surgeons measure stem depth insertion relative to the tip of the greater trochanter; comparing the accuracy and reliability of these two landmarks in the same cohort will help establish the role of the obturator externus footprint in clinical practice. Other research groups will be able to build on the results of the current study by investigating the efficacy of obturator externus insertion as a landmark for determining the stem depth insertion into the femur during THA surgery. Employing this landmark in clinical practice requires validation studies by several independent research groups. Despite being experts in the field, the authors of the current CORR paper [12] could not identify the outline of the trochanteric fossa on plain radiographs in 13% of patients (18 of 135). Of these, only two had the landmark affected by previous trauma. Excluding these two patients, the landmark could not be identified in 12% of patients (16 out of 133) not affected by previous local pathology. We should train surgeons to identify the outline of the trochanteric fossa on plain radiographs, and further investigations to improve imaging techniques are warranted. These could be done by making surgeons aware of this landmark at workshops. We can also investigate whether the position of the leg during radiographs affects the projection of the landmark on plain radiographs. Although Vles et al. [12] performed this landmark for total hip replacement through an anterior approach, it is important to investigate its utility in other surgical approaches. In fact, using this landmark while performing THA through any exposure that routinely dissects and exposes the obturator externus tendon will eliminate the instances where it would otherwise be difficult to identify the tendon intraoperatively. All the proposed future research will be useful to consolidate the clinical utility of the obturator externus footprint on the greater trochanter in THA surgery.