gms | German Medical Science

Objectives: Definig osseous instability in borderline dysplastic hips is difficult. A reliable radiographic tool that might be associated with instability would be helpful for decision-making in this patient group. Goals of this study are: (1) To compare a new radiographic measurement, which we call the Femoro- Epiphyseal Acetabular Roof (FEAR) index, with the lateral center-edge angle (LCEA) and acetabular index (AI), with respect to intra- and interobserver reliability; (2) to correlate AI, neck-shaft angle, LCEA, iliocapsularis volume, femoral antetorsion, and FEAR index with the surgical treatment received in stable and unstable borderline dysplastic hips; and (3) to assess whether the FEAR index is associated with clinical instability in borderline dysplastic hips.

Methods: The FEAR index was defined and validated in 10 standardized radiographs of asymptomatic controls using two blinded independent observers. Interrater and intrarater coefficients were calculated. Its reliability with LCEA and AI was compared. We performed a case- control study using standardized radiographs of 39 surgically treated symptomatic borderline dysplastic hips and 20 age-matched controls with asymptomatic hips. Treatment received was either a periacetabular osteotomy (for unstable hips) or, for patients with femoroacetabular impingement, either an open or arthroscopic femoroacetabular impingement procedure. The association of received treatment categories with the variables AI, neck-shaft angle, LCEA, iliocapsularis volume, femoral antetorsion, and FEAR index were evaluated. Sensitivity, specificity, and receiver operator curves were calculated. The primary endpoint was the association between the FEAR index and instability, which was evaluated on conventional radiographs and MR arthrography.

Results: The FEAR index showed excellent intra- and interobserver reliability, superior to the AI and LCEA. FEAR index was lower in the stable borderline group (mean, -2.1 ± 8.4) compared with the unstable borderline group (mean, 13.3 ± 15.2) (p < 0.001) and had the highest association with treatment received. A FEAR index less than 5° had a 79% probability of correctly assigning hips as stable or unstable (sensitivity 78%; specificity 80%).

Conclusion: A painful hip with a LCEA of 25° or less and FEAR index less than 5° is likely to be stable. In such a situation, the diagnostic focus might more productively be directed toward FAI as a potential cause of pain, rather than instability.