CORR Insights®: The Acetabular Wall Index Is Associated with Long-term Conversion to THA after PAO

Where Are We Now? Around 40 years ago, one of my first mentors told me that orthopaedic surgery has evolved over three periods: The first period was mostly based on amputation, the second on substitution, and the third on regeneration. At that time, I believed that we were already living in the period of regeneration, and thus I set off on the path to explore the reorientation of dysplastic hips. Since then, I have come to understand that we still very much are living in the substitution period, despite the fact that the first reorientations of the acetabulum were performed more than 80 years ago [5]. We have not moved forward very much but, nevertheless, we have understood that the most biological way to correct a dysplastic acetabulum is by reorientation [2, 8], with special respect to rotating the acetabulum around a craniocaudal axis [4]. Despite this knowledge, we rarely use all of the tools that have been developed to achieve a “perfect outcome.” Why is this precision so important, and why should we work so hard to reproduce normal three-dimensional (3-D) anatomy? We have learned that projecting the contours and surface of the “facies semilunaris” of the acetabulum along the craniocaudal axis onto the femoral head in normal asymptomatic hips results in a quotient of 70% ± 4% SD [4]. This implies that nature made the anatomy very precise and that coverage does not allow for an inexact correction. This is why precision is important, and why we need 3-D anatomical imaging of the anatomy. In my view however, many researchers, including the authors of the current study [7], remain focused on extracting data from plain radiographs to assess coverage of the femoral head within all of its angles. Before evaluating the current study, we must first look at a previously published paper that examined the acetabular wall index and found that there is a desire to evaluate the acetabular anteversion, and indirectly, the anterior and the posterior coverage of the femoral head on plain radiographs [6]. But we have yet to determine how relevant the anterior and posterior coverage of the femoral head are in evaluating the results after reorientation of the acetabulum. In the current study, Stetzelberger and colleagues [7] fill this gap by linking the acetabular wall index with the need for joint replacement. They found that a poor anterior wall index is associated with decreased survivorship of the native hip in the long-term after periacetabular osteotomy (PAO) [7]. Indeed, insufficient or imprecise reorientation leads to joint destruction. Where Do We Need To Go? Let us compare the action of correcting the coverage of the femoral head with another which has historically undergone a faster evolution: air travel. In the 1930s, pilots had trouble creating a flight plan for the postal route from Europe to South America. Weather conditions including high winds, the mechanical “fitness” of the aircraft, and refuelling posts were all variables to contend with, jeopardizing the safety and timely arrival of the aircraft. Today, at the moment of take-off from Europe, one can predict with more than 90% certainty the arrival time in Buenos Aires or Santiago de Chile. Mechanical reliability, tracking with satellite assistance, and flight plans have all spectacularly improved. Normalizing the coverage of the femoral head through reorienting the acetabulum must undergo a similar evolution. We must first be able to tell the patient whether the “normalization” of coverage is possible. If it is possible, we must then develop a “flight plan” that will help ensure a straightforward and uncomplicated surgical procedure. The course of surgery should include a minimum amount of indecision. We should first aim at liberating the acetabular fragment from the pelvic frame and testing its ability to be rotated within the three planes separately, as well as its ability to shift longitudinally (especially medially and posteriorly). We should then apply the “flight plan” integrally, thereby reducing considerably the intraoperative radiological needs. Training in software management as well as surgical teaching in cadaver labs must be increased or introduced where it is not yet being performed. Planning and simulation should be included in our preoperative actions, and it should be billable to insurers and other payers. How Do We Get There? We need to integrate the tools that currently exist to plan our surgical corrections. Training in reorienting the acetabulum should be done using 3-D imaging through reconstructed CT and simulation of a corrected hip on the screen [1]. We must then collect the angular measurements within three planes, as well as longitudinal data in three planes, to either maintain or to bring the center of rotation of the hip joint at its anatomical location in relation to the whole pelvis. Teaching surgeons how to plan and perform osteotomies is critically important. Performing an osteotomy that frees the acetabulum from the pelvis without destabilizing the pelvic girdle while preserving perfusion to the freed fragment calls for specific training and education. A training program’s learning curve for this technique is shallow and long, and the risks of getting it wrong are severe. We could reduce the surgical approach by applying an endoscopic technique like “ballooning” the spaces to isolate the bone to be cut. Surgeons should also consider using innovative tools for osteotomies. For example, we have been using oscillating saws for decades and have generally ignored the evolving technologies that apply ultrasound or piezoelectricity to cut cortical bone. These tools require much less space and do not harm soft tissues [3]. Finally, educating our patients about the proposed surgery is an important part of our task; doing this well helps us avoid misunderstandings and it can help patients prepare for complications, which sometimes do occur.