The diagnosis of FAI is based on a clinical examination in symptomatic patients combined with radiographic findings supporting a structural deformity that leads to hip damage. Understanding radiographic features consistent with impingement morphologies will help the impingement surgeon determine the best surgical approach to treat the impinging hip. We therefore (1) delineated the femoral and acetabular radiographic features associated with differing hip morphologies; (2) determined the prevalence of radiographic coxa profunda in each of these groups; (3) identified differences in radiographic features between hips with and without radiographic coxa profunda; and (4) determined the sensitivity and specificity of coxa profunda as a measure associated with global acetabular overcoverage or pincer FAI.
There are limitations to the present study. First, there may be some selection bias present as a result of not including patients who did not undergo SHD for FAI. However, this bias is somewhat reduced including all patients undergoing SHD within the given timeframe. Second, radiographic measurements may vary with patient position and could be subject to interpretation error [5
]. The radiographic technique is standardized to accommodate for these errors. All measurements were performed digitally on an in-house templating program that adjusts for technique differences. Third, these measurements are clearly subject to error, and we did not assess intraobserver or interobserver reliability. Fourth, we defined the morphologies according to established radiographic criteria, but these criteria have not been universally accepted. Finally, we classified hips according to intraarticular chondrolabral damage as noted on operative reports and intraoperative pictures. This was especially challenging for hips with combined morphology, because cam lesions typically dominate the damage picture. This might contribute to the differences between these groups.
Our analysis shows there are clear differences in pelvic and femoral parameters among impingement morphologies. We found differences, for example, in Angle of Sharp and femoral neck angles in between morphologic subgroups. Because the Angle of Sharp is the degree of lateral opening, a larger angle was seen in retroverted sockets when compared with all other morphologic subtypes. Retroverted sockets do not appear to be characterized by lateral overcoverage. Hips with global overcoverage or pincer morphology had considerably more valgus femoral neck angles compared with all other morphologies. One could theorize that the valgus of the femoral neck may be adaptive in nature, helping with clearance of the trochanter with ROM because the structure is further away from the pelvis.
We introduced a new measure, the total roof length, and its lateral and medial components and found differences in total roof length, Roof 2, and two calculated ratios (total roof/Roof 2 and Roof 1/Roof 2). Shorter total roof lengths were seen in hips with overcoverage morphology. Interestingly, the actual length of the weightbearing sourcil was no different among the groups (Roof 1), but the total roof length was actually shorter. The differences in total roof length between the groups seems to lie mainly in the differences in the distance between the ilioischial line and the medial aspect of the roof or sourcil that is substantially shorter in hips with global overcoverage. The differences in roof ratio were primarily the result of differences in Roof 2. Both total roof/Roof 2 and Roof 1/Roof 2 ratios were smaller in hips with global overcoverage. The anatomic difference in hip center of rotation without changes to true roof or sourcil length could indeed be responsible for the problems seen in hips with global overcoverage. Unfortunately, we were unable to determine a cutoff point that could help determine radiographically pincer hips despite these statistical differences. This could be related to the overlap of intraoperative criteria consistent with combined hips, erroneous classification of impinging morphology, or simply to lack of number of hips to determine these radiographic differences.
Coxa profunda, defined as the medial wall of the acetabulum at or medial to Kohler’s line, is commonly seen in patients with and without FAI and was seen in 55% of hips undergoing FAI surgery in this series [4
]. This common finding was also noted by Armbuster et al. [1
]. In 1978, these investigators evaluated over 300 AP radiographs of the normal adult hip and determined that coxa profunda as it is currently defined was present in over 70% of female radiographs. In their study, the average distance between the medial wall of the acetabulum and Kohler’s line was in fact 2 mm lateral in males and 1 mm medial in females [1
]. Despite its common occurrence, there are clear differences in radiographic parameters between hips that have this radiographic feature compared with those that do not. These differences in part relate to pincer-type morphologic features such as greater LCE angle, smaller Tönnis angle, or shorter total roof, but because of its prevalence in all impingement morphologies, it should not be used alone but combined with other measurements to describe global overcoverage morphology. In the present series, the specificity of the radiographic feature of coxa profunda in determining global overcoverage was 62%.
In 1971, Hooper and Jones [9
] defined protrusio acetabuli at a center-edge angle > 40º with no mention of the medial acetabular wall. Kutty et al. [10
] recently reported on the predictability of the center-edge angle in the assessment of pincer-type FAI. They found LCE angle ≥ 40º to be 84.2% sensitive and 100% specific for pincer-type FAI [10
]. There are however many hips that have pincer-type morphology and intraarticular damage consistent with pincer FAI that have an LCE angle less than 40°. It would seem that using LCE greater than 40° as a sole measurement to determine global overcoverage is too simplistic, because the pathomechanics of FAI are more complex than one simple, two-dimensional measurement. Accordingly, we describe the major differences between the various impingement morphologies.
As our understanding of FAI improves, it is important to look for objective measurements that can conveniently and reliably inform clinical decision-making. Although the measurements are imperfect and can vary somewhat with the rotation of the radiograph, developing objective criteria for global overcoverage and other impinging morphologies and then correlating the data with intraoperative findings and patient outcome will lay the foundation for more objective and reproducible approaches to preoperative decision-making. Our study demonstrates limitations of the currently accepted parameters used to determine pincer-type morphologies and presents novel measurements that may prove to be useful with further analysis.