CORR Insights®: Does the Rule of Thirds Adequately Detect Deficient and Excessive Acetabular Coverage?

Where Are We Now?

Acetabular morphology influences hip function and can predispose patients to pathological conditions of the hip joint. Deficient acetabular coverage may cause instability and abnormal load bearing, resulting in the possibility of osteoarthritis in patients with developmental dysplasia of the hip (DDH). Acetabular overcoverage may cause impingement between the acetabular rim and femoral head-neck junction, causing degeneration of the labrum-cartilage junction in patients with femoroacetabular impingement (FAI). Orthopaedic surgeons generally use the lateral center-edge angle (LCEA) to define parameters of deficient and excessive coverage and to assess the superolateral acetabular coverage on plain anteroposterior radiographs.

Even within categories of acetabular over- or undercoverage as diagnosed on two-dimensional (2-D) radiographs, there may be large variations in three-dimensional (3-D) acetabular morphology [5, 8, 9]. For example, within the category of DDH, the degree of anterior acetabular deficiency and retroversion may vary widely from patient to patient; the same is true for patients with overcoverage or FAI.

One study [10] proposed acetabular wall indices to quantify anterior and posterior wall coverage on plain anteroposterior radiographs. The anterior wall index (AWI) and the posterior wall index (PWI) were strongly correlated with femoral head coverage by the anterior and posterior acetabulum, respectively, which were calculated using image analysis software [10]. These indices are helpful to the surgeons because 3-D acetabular morphology can be partly quantified without obtaining CT or MRI. If reliable classifications for acetabular coverage using the acetabular wall indices are established, diagnostic ability for DDH and FAI and treatment outcomes in these pathologies may be improved based on 3-D morphological assessment.

In the present study, Stetzelberger and colleagues [11] proposed deficient, normal, or excessive classifications for anterior and posterior coverage, using the rule of thirds for the AWI and PWI relative to the radius or diameter of the femoral head. This classification showed high specificity but relatively low sensitivity using the definition based on area-based acetabular coverage. These findings indicated that surgeons may easily diagnose abnormal acetabular coverage in cases classified into the abnormal third zones but require further 3-D examinations in cases classified into the normal zone due to high possibility of undetected 3-D morphological abnormality.

Where Do We Need To Go?

The current study’s classification for the anterior and posterior coverage appears easy to implement and practical for routine clinical use. However, for it to see wider use—like better-known indices, such as the LCEA—several gaps in our knowledge need to be filled.

First, the AWI and PWI were measured at the intersection point at the anterior and posterior acetabular rim along the femoral neck axis [10], resulting in measurements at variable points around the circumferential acetabular rim among individuals. This could cause the AWI to be measured at a superoanterior point around the acetabular rim in some hips and at an anterior point in others, depending on the neck-shaft angle of the femur. In a previous CT study, acetabular retroversion was mostly observed at the superior acetabular level relative to the center of the femoral head in cadaveric specimens [4], and acetabular wall coverage tended to increase at the superoanterior and inferoposterior points around the circumferential acetabular rim in normal volunteers and DDH patients [8]. Therefore, we need to determine the most appropriate point around the acetabulum to measure the AWI and PWI in patients with DDH and FAI.

Second, Stetzelberger and colleagues [11] obtained radiographs in the supine position. However, acetabular anteversion is likely to increase with standing, in association with posterior tilting of the pelvis, leading to a decrease in superoanterior acetabular coverage and an increase in superoposterior acetabular coverage in patients with DDH [12]. For this reason, we need to determine the most suitable position in which to obtain diagnostic radiographs for measuring acetabular coverage (supine, standing, or sitting).

Finally, Stetzelberger and colleagues [11] used reference threshold values for the acetabular deficient and excessive coverage, which were determined from a previous analysis in symptomatic patients receiving hip preservation surgery [13]. Although the degree and patterns of 3-D acetabular coverage in patients with DDH and FAI [7-9] and patients without those pathologies [5] have been investigated, reliable reference values to distinguish among deficient, normal, and excessive acetabular coverage have not been established. Additionally, while previous studies have examined the reference values of the AWI and PWI for DDH and FAI [1, 10], such examinations were conducted using 2-D plain radiographs and intraoperative findings [1, 10]. There may be structural abnormalities in the anterior and posterior regions that 2-D radiographs may not detect, even if the LCEA is considered normal. Therefore, we need further investigations to establish reference threshold values of the AWI and PWI as they relate to symptom, function, and risk of subsequent development of osteoarthritis.

How Do We Get There?

Computer-simulation studies that assess kinematic movement using 3-D models of the hip from CT or MRI can identify the sites where impingement between the acetabulum and femur is likely to occur [2, 7, 8]. One study examining patients with and without DDH observed the highest frequency of anterior impingement at the superoanterior site of the acetabular rim with the hip flexion and internal rotation movement but a relatively low frequency of posterior impingement at the posterior acetabular rim with extension and external rotation movement [8]. Advanced MR imaging such as open-bore MR scanning in various postures of the hip and real-time imaging during femoroacetabular motion may be able to detect impingement sites around the acetabular rim in vivo [2].

Clinical investigations to explore measurement points around the acetabular rim; selection of positions for radiographs among supine, standing, and sitting positions; and reference threshold values for deficient and excessive acetabular coverage by the AWI and PWI require cross-sectional and longitudinal large-cohort studies. Several previously published cross-sectional studies reported averages and ranges of AWI and PWI values for a small number of symptomatic patients [10] or asymptomatic volunteers [1] and categorized them into deficient, normal, and excessive acetabular coverage based on plain radiographic indices such as the LCEA and the crossover sign. Previously, associations of representative radiographic indices such as the LCEA and acetabular socket depth with incidence of symptoms and radiographic osteoarthritis were investigated in a population-based cross-sectional study of over 3000 subjects [3]. Prognostic abilities of the LCEA in DDH and the alpha angle in FAI for the risk of osteoarthritis progression were demonstrated in a longitudinal study over 20 years [14]. Such large-scale studies including patients with a range of hip morphologies will be necessary to establish reliability of the AWI and PWI along with defining their ideal threshold values for the acetabular coverage classification. Since the AWI and PWI can be measured only on AP pelvic radiographs, patient follow-up databases that accumulate previous data in population-based or multicenter studies should be used for this, and it should be possible to do at centers that perform a high volume of this type of surgery.

Even if future studies establish reliable AWI and PWI threshold values for differentiating deficient, normal, and excessive acetabular coverage, those values may not be useful as target values in surgical corrections, such as during acetabular rim trimming procedures for patients with FAI nor in acetabular reorientation osteotomies for patients with DDH. My reason for saying this is that FAI and DDH are influenced by factors on both sides of the joint, and AWI and PWI measure only the acetabular side; that is, they do not consider proximal femoral morphology. For example, femoral anteversion may influence both ROM and impingement locations in patients with FAI [6]. For this reason, I believe the next round of studies should combine 3-D radiographic indices of the acetabulum and proximal femur. These might be done by exploring new indices on anteroposterior radiographs, which are well correlated with femoral anteversion.