The Femoro-Epiphyseal Acetabular Roof (FEAR) Index: A NewMeasurement Associated With Instability in Borderline Hip Dysplasia?

Presenter’s name (Last, First): Wyatt, Michael

Qualifications: BSc MBChB MD MFSTEd FRCSEd FRACS(Orth)

Affiliations: Massey University

Other authors:
Weidner, Jan MD / Clinic for Orthopaedic andTrauma Surgery, Luzerner Kantonsspital ; Pfluger, Dominic PHD Clinic for Orthopaedic and TraumaSurgery, Luzerner Kantonsspital; Beck, Martin PD MD Clinic for Orthopaedic and Trauma Surgery,Luzerner Kantonsspital


Background / Introduction:
The definition of osseous instability in radiographic borderline dysplastic hips is difficult. A reliable radiographic tool that aids decision-making—specifically, a tool that might be associated with instability—therefore would be very helpful for this group of patients.

Patients / Methods:

We defined and validated the FEAR index in asymptomatic controls using two blinded independent observers. We compared its reliability with LCEA and AI. We performed a case-control study using standardized radiographs of symptomatic borderline radiographically dysplastic hips and 20 age-matched controls with asymptomatic hips (a 2:1 ratio).
Patient demographics were assessed using univariate Wilcoxon two-sample tests. Treatment received was either a periacetabular osteotomy (if the hip was unstable) 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 first using Wilcoxon two-sample tests (two-sided)followed by stepwise multiple logistic regression. Sensitivity, specificity, and receiver operator curves were calculated. The primary end point was the association between
the FEAR index and instability, which we defined as migration of the femoral head either already visible on conventional radiographs or re centering of the head on AP abduction views, a break of Shenton’s line, or the appearance of a crescent-shaped accumulation of gadolinium in the posteroinferior joint space at MR arthrography.


The FEAR index showed excellent intra- and interobserver reliability, superior to the AI and LCEA. The FEAR index was lower in the stable borderline group (mean, −2.1 ±8.4; 95% CI, −6.3 to 2.0) compared with the unstable borderline group (mean, 13.3 ± 15.2; 95% CI, 6.2– 20.4) (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 and unstable, respectively (sensitivity 78%;specificity 80%).

A painful hip with a LCEA of 25° or less and FEAR index less than5° is
likely to be stable, and in such a situation, the diagnostic focus might more productively be directed toward femoroacetabular impingement as a potential cause of a patient’s pain, rather than instability.

Level of Evidence & Study type: 3 Retrospective Audit

Declarations of Conflict: No conflicts of interest