CORR Insights®: Can the Femoro-Epiphyseal Acetabular Roof (FEAR) Index Be Used to Distinguish Dysplasia from Impingement?

Where Are We Now? This one is too big. This one is too small. This one is just right. What is the ideal treatment for patients who have painful nonarthritic hips and mild dysplasia? The dilemma began with Dr. Ganz’s original description of the periacetabular osteotomy (PAO) [3] and the recognition of femoral acetabular impingement (FAI) [7]. The original treatment called for open surgery that was sufficiently large and complicated that it could only be performed by a few surgeons who had extensive and very-specific experience with this type of pelvic surgery. With the evolving use of hip arthroscopy, FAI could be treated by a new group of surgeons; although they had the technical skills to maneuver a scope around the hip joint, at first this group was somewhat less well versed in the nuances of dysplasia. There was a steep learning curve, but as surgeons’ understanding of hip biomechanics continued to evolve, and as techniques improved, arthroscopic interventions opened new avenues for the treatment of nonarthritic hip pain [8]. Inevitably, surgeons pondered how to treat patients with mild dysplasia (center-edge angles [CEA] between 20° and 25°), and treatment was divided between reorientation osteotomies and hip arthroscopy. Patient preference often is for the least-invasive treatment, which has inspired surgeons to develop soft tissue procedures that add stability, such as capsular plication [2]. When reviewing hip arthroscopy patients with mild dysplasia, an increase in the Tönnis angle (acetabular index) is associated with decreased patient-reported outcome scores and an increased revision rate of arthroscopic hip surgery [6]. As the arthroscopic treatment for mild dysplasia has increased, so too has the frequency of conversion to a PAO from prior arthroscopy [4]. The original Femoro-Epiphyseal Acetabular Roof (FEAR) index article sought to identify anatomic parameters that could help determine stability [1]. That small pilot study used the epiphyseal scar and acetabular index to represent the forces acting on the femoral head. The authors concluded that the FEAR index is a viable measurement in determining hip stability. A simple way to use the measurement is to look at the angle between the acetabular roof and epiphyseal scar; if the angle opens medially, the hip is stable. If it opens laterally, it is unstable. In the current study, Smith and colleagues [10] present a retrospective study of a young group of patients with dysplasia, FAI, and a group of controls. With a much larger cohort (206 patients), they measured the FEAR index and showed excellent interclass reliability in measuring the index in the three groups. A clinical determination of instability was substantial for the FEAR index that was either positive or more positive than -1.3. The study also found that the FEAR index becomes more negative (and thus more stable) with age; likewise, others have found that acetabular coverage increases with increasing age [5]. Patients with mild dysplasia and symptoms consistent with instability had a more positive FEAR index, while patients with mild dysplasia and symptoms consistent with impingement had a negative FEAR index. The clinician who is seeing these patients now has a quick and easily obtainable measurement at his or her disposal. During the standard work up of a patient with nonarthritic hip pain, the surgeon should collect the CEA and Tönnis angle. With one more measurement of the epiphyseal scar in patients with CEA between 20° and 25°, patients can be directed toward the appropriate intervention. Where Do We Need To Go? The increase in stability with age may lead an orthopaedic surgeon to pause when treating a young patient on the borderline of both the CEA and the FEAR index. Would waiting longer lead to greater hip stability? Future research should follow patients with contralateral asymptomatic hips as well as those with mild hip dysplasia, open triradiate cartilage, and unilateral symptoms to determine whether the contralateral hip becomes more stable as the FEAR index becomes more negative (and thus more stable) with age. Building beyond the original article on the FEAR index, the Smith et al. [10] study shows that patients with dysplasia and a CEA < 20° have a high FEAR index. These findings will give the treating physician one more measurement to assess dysplasia. But for patients in pain after hip arthroscopy with mild hip dysplasia, should orthopaedic surgeons consider revision hip arthroscopy or move to a PAO? There is often some doubt as to the pain generator. Adding to the uncertainty is that patients often present with a combination of both dysplasia and impingement in the form of either a CAM-type bump or decreased offset of the head and neck junction [9]. In these instances, is a PAO enough if the FEAR index is in the less-stable range, or should a combination of a revision proximal femoral osteochondroplasty and PAO be performed? Another question to consider is whether the FEAR index needs to be used if there is a high Tönnis angle. The FEAR index is an angle formed between the acetabular roof and the epiphyseal scar. If the Tönnis angle is greater than 10, how often does the epiphyseal scar lead to a FEAR index more negative than -1.3 (and thus more stable)? Also, is there a cutoff where it would make a difference to use both in the FEAR index? Would a high Tönnis angle of 10° or above be enough to create a less-stable hip [6]? How Do We Get There? The next step is to prospectively examine hips with mild dysplasia in both a group of surgeons whose practice focuses on PAOs and another group of surgeons who primarily perform arthroscopy using the new FEAR index and the -1.3 cutoff. This could be done as a multicenter study between institutions or within institutions who have a high volume of hip patients and surgeons performing the two operations. All symptomatic patients above (and more positive than) -1.3 would have a PAO, and all patients -1.4 and below would be treated arthroscopically. Revision surgery and patient-reported outcomes would then be compared prospectively. Additionally, the epiphyseal scar can be hard to see on a radiograph, particularly in more skeletally mature patients. It could be helpful to examine the use of MRI or CT to determine the FEAR index. A retrospective review looking at a cohort of patients comparing the FEAR index measurements on radiographs to either or both CT and MRI could help to make it easier for the provider to measure the angle. Choosing the right operation for the patient can help to avoid unwanted reoperations and improve function for those with hip pain and moderate dysplasia.