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SELECTED PROCEEDINGS FROM THE 2020 BERNESE HIP SYMPOSIUM GUEST EDITOR: KLAUS-ARNO SIEBENROCK MD

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

Smith, Justin T. MD; Jee, Young MD; Daley, Erika MD; Koueiter, Denise M. MS; Beck, Martin PD, MD; Zaltz, Ira MD

Author Information
Clinical Orthopaedics and Related Research: May 2021 - Volume 479 - Issue 5 - p 962-971
doi: 10.1097/CORR.0000000000001610

Abstract

Introduction

Proper classification of symptomatic hips with mild structural deformities ranging from dysplasia to femoroacetabular impingement (FAI) has become increasingly complex and is often controversial [4, 8, 13, 21, 25, 39]. Wiberg [36] first described a mild structural deformity as borderline hip dysplasia based on a lateral center-edge angle (LCEA) of 20° to 25°. Given this limited definition, which accounts only for lateral acetabular coverage, hips in this borderline group can demonstrate clinical features ranging from instability to impingement. This diagnostic dilemma carries substantial gravity because an improper diagnosis may lead to delayed treatment, resulting in progressive chondral damage, or worse, morbidity from mechanically inappropriate surgical treatment. For example, recent studies have reported that isolated hip arthroscopy in hips with mild structural deformities is associated with high rates of reoperation and conversion to THA [9, 11, 18, 20, 23, 28, 40]. Furthermore, a growing number of periacetabular osteotomies are being performed after failed hip arthroscopy procedures [13]. Accordingly, there is an increasing emphasis on accurate diagnosis to guide operative management.

Numerous radiographic measures have been used to assist clinicians in accurately categorizing hips with mild structural deformities. Measures of instability are the acetabular index (AI) and Tönnis angle, LCEA [29], anterior center-edge angle (ACEA) [1, 7, 37], lateral femoral head migration, a break in the Shenton line, distance from the ilioischial line [6], coxa valga, increased femoral anteversion, labral hypertrophy, and volume of the iliocapsularis muscle [2]. However, no single measure can definitively classify a hip. Rather, diagnosing a hip as stable or unstable is based on the compilation of clinical findings and imaging measures [39], with no exact criteria for differentiating hip stability.

An increasingly recognized radiographic measure with high intrarater and interrater reliability, the Femoro-Epiphyseal Acetabular Roof (FEAR) index, originally described by Wyatt et al. [38], offers improved diagnostic guidance in hips with mild structural deformities [3, 22, 33].

The FEAR index was originally described in a group of young adults who were skeletally mature. It measures the relationship between the central portion of the proximal femoral physeal scar and the acetabular sourcil. It is based upon the observation that during hip development, the proximal femoral physis will orient itself perpendicular to the joint reactive forces [24]. The relationship between the sourcil and the central one-third of the physeal scar may serve as a developmental marker of a hip’s stability. In addition, the FEAR index has neither been characterized nor assessed in the skeletally immature.

We therefore asked: (1) What are the characteristics of the FEAR index in children and how does the index change with skeletal maturation? (2) How does the FEAR index correlate with clinical diagnosis and surgical treatment in a large cohort of symptomatic hips and asymptomatic controls? (3) How does the FEAR index correlate with clinical diagnosis in the borderline (LCEA 20°-25°) group?

Patients and Methods

Study Design and Setting

The study was a retrospective case-control series of patients selected between January 2008 and January 2018 from a single-surgeon series of patients treated for FAI or dysplasia. Data on an age-approximate control cohort were then collected from a database of patients with trauma at the authors’ institution from January 2017 to December 2017.

Participants

We retrospectively reviewed 220 patients with a clinical diagnosis of dysplasia or FAI. The senior author (IZ) established the diagnosis using clinical, radiographic, and advanced imaging techniques including MRI-arthrography and three-dimensional (3-D) CT. Patients with hip dysplasia presented with complaints of hip instability, deep pain in the joint (C sign)/groin pain, and/or apprehension/giving way with activities (such as with hip hyperextension or external rotation). On physical exam, these hips exhibited one or multiple positive signs of hip instability: anterior apprehension test, prone external rotation test, abduction-extension-external rotation test, increased ROM [17, 19], and/or posterior apprehension test. Radiographically, patients with hip dysplasia had an LCEA < 25°, ACEA < 25°, AI > 10°, and/or a broken Shenton line [6, 10, 29, 30, 37]. Advanced imaging (CT, MRI) was used to further characterize the acetabular dysplasia, articular cartilage status, and any additional intraarticular pathologies. Alternatively, patients with FAI presented with complaints of hip stiffness, deep pain in the joint (C sign)/groin pain, and/or pain with prolonged hip flexion or activities with hip flexion/internal rotation. On physical exam, an FAI hip exhibited one or multiple signs of hip impingement: slouched sitting posture (avoidance of hip flexion), decreased hip flexion [17, 19], decreased internal rotation at 90° of hip flexion, a positive flexion-abduction-external rotation test, and/or a positive posterior impingement test [26]. Radiographically, FAI hips demonstrated multiple radiographic findings consistent with FAI: a pistol grip deformity, alpha angle > 60°, LCEA > 20°, AI < 0°, presence of an impingement cyst, and/or a head-neck offset ratio < 0.17 [6, 27]. As with hip dysplasia, advanced imaging (CT, MRI) was used to characterize the acetabular and femoral impingement morphology, articular cartilage status, and any additional intraarticular pathologies.

Exclusion criteria included any femoral head abnormalities preventing LCEA measurement (for example, Perthes disease), Tönnis osteoarthritis grade greater than 1, prior hip surgery, or prior femoral osteotomy. In the 220 participants, 395 hips met inclusion criteria and were reviewed. Once exclusion criteria were applied, 15 hips were excluded due to prior hip surgery and 12 hips were excluded due to femoral head deformity. One hip was then randomly selected from each patient, resulting in 144 dysplastic hips and 62 FAI hips (Fig. 1). Hip treatments included a reorienting acetabular osteotomy for dysplasia, open or arthroscopic treatment for FAI, or nonoperative management. A decision for a reorienting acetabular osteotomy was made when a dysplastic hip had failed conservative management, was congruent, and contained < Grade 1 Tönnis osteoarthritis. A decision for open/arthroscopic FAI treatment was made when the patient had failed conservative management and the hip contained < Grade 1 Tönnis osteoarthritis.

F1
Fig. 1.:
Flow chart of all analyzed participants and those who were excluded from the study.

Data on asymptomatic age-approximated controls (n = 73 participants) in a 3:1 ratio were collected from the senior author’s (IZ) institutional trauma database for comparison. Inclusion criteria included a standardized AP pelvis radiograph, defined as a coccyx centered over the pubic symphysis, and within 1 to 3 cm of the superior aspect of the symphysis. Patients were excluded if they had any fracture to the pelvis or femur or had prior hip/pelvis surgery. After exclusion criteria were applied, 16 hips were excluded due to pelvis/femur fracture. One hip was then randomly selected from each participant, resulting in 65 control hips (Fig 1).

Description of Experiment, Treatment, or Surgery

Standardized AP pelvis radiographs, in both the investigational and control cohorts, were used to measure the FEAR index, LCEA, and acetabular index/Tönnis angle (Fig. 2A-C). The standardized investigational group’s radiographs were obtained to minimize variation between radiographs with the patient standing in stocking feet with both patella pointing forward, feet at neutral standing width distance, and the beam centered between the anterior superior iliac spine and pubic symphysis. Similarly, in the investigational cohort, standardized false-profile radiographs (performed in the same standing posture to the investigational group’s AP pelvis radiographs) were used to measure the ACEA (Fig. 2D). Control group AP pelvis radiographs were standardized by the inclusion of AP pelvis radiographs that had the coccyx centered over the pubic symphysis and within 1 to 3 cm of the superior aspect of the symphysis. Investigational hip measurements were made using the Joints® PACS imaging software (Medstrat, Downers Grove, IL, USA) from the senior author’s (IZ) practice, and control hip measurements were made using the IntelliSpace PACS imaging software (Philips, Amsterdam, Netherlands) from the senior author’s affiliated hospital. Interrater reliability was calculated from two blinded researchers’ (JTS, YJ) independent FEAR index measurements. Intraclass correlation coefficients for the FEAR index were excellent (intraclass correlation coefficient 0.92 [95% CI 0.84 to 0.96]).

F2
Fig. 2.:
A-D These AP pelvis radiographs show measurements performed in the study: (A) FEAR index; (B) LCEA; (C) Tönnis angle. Additionally, the false profile pelvis radiograph (D) demonstrates the ACEA measurement.

To answer the question of FEAR variation with maturation, we compared radiographic measures among age subgroups (childhood: younger than 10 years; adolescence: 10 to 14 years old; maturity: older than 14 years) in dysplastic, FAI, and control hips to provide evidence for the developmental basis of the FEAR index. These subgroups were selected given developmental morphologic changes that occur during prepubertal growth [35]. To further validate the FEAR index in a large cohort of hips, we compared the FEAR index among clinical diagnoses and treatments in the entire cohort. To investigate the association of FEAR index in borderline hips, we compared the FEAR index among clinical diagnoses and treatments in borderline hips (defined by an LCEA of 20°-25°). A receiver operating characteristic (ROC) curve was used to determine the diagnostic ability and optimal cutoff of the FEAR index for the diagnosis of dysplasia and FAI in the entire cohort and in borderline hips.

Description of Study Population

In the investigational group (hips diagnosed with dysplasia or FAI), there were 206 hips in 206 patients. The mean age at the time of radiography was 13 ± 3 years in dysplastic hips (144) and 14 ± 1 years in FAI hips (62). The mean BMI at presentation was 23 ± 5 kg/m2. There were 22% (46 of 206) male patients in the investigational group. Of these hips, 70% (144 of 206) had a diagnosis of dysplasia and 30% (62 of 206) had FAI. Of patients with a clinical diagnosis of dysplasia, 15% (22 of 144) were male. Of patients with a clinical diagnosis of FAI, 39% (24 of 62) were male. A reorienting acetabular osteotomy for hip dysplasia was performed in 49% (70 of 144), and operative open or arthroscopic treatment of FAI was performed in 63% (39 of 62). In the control group of asymptomatic hips, there were 65 hips in 65 patients. The mean age in this group was 16 ± 8 years. There were 55% (36 of 65) male patients and 45% (29 of 65) female patients (Table 1).

Table 1. - Demographics of investigational versus control patients
Parameter Investigational (n = 206) Control (n = 65) p value
Age in years, mean ± SD 13 ± 3 16 ± 8 0.02
BMI in kg/m2, mean ± SD 23 ± 5 22 ± 7 0.80
Sex, males, % (n) 22 (46) 55 (36) < 0.001

Primary and Secondary Study Endpoints

The primary study goal was to evaluate how the FEAR index varies with skeletal maturation in dysplasia and FAI. This was done by analyzing the FEAR index by diagnosis in three age groups (childhood: younger than 10 years; adolescence: 10 to 14 years old; maturity: older than 14 years). The secondary study goal was to further validate the FEAR index association with dysplasia/FAI and to define an optimal threshold. This was done by analyzing the FEAR index in a large cohort of symptomatic dysplasia and FAI hips to age-matched controls. The third study goal was to further validate the FEAR index’s association with dysplasia/FAI and define an optimal threshold in borderline hips. This was done by analyzing the FEAR index in a borderline hip cohort of symptomatic dysplasia and FAI hips with borderline age-matched controls.

Ethical Approval

Ethical approval for this study was obtained from our institutional review board (IRB# 2018-180).

Statistical Analysis

Changes in the FEAR index based on skeletal maturity was assessed using a two-way ANOVA with the factors of age subgroup (younger than 10 years, 10 to 14 years, and older than 14 years) and diagnosis (dysplasia, FAI, and control). Subsequently, we compared the FEAR index across clinical diagnoses in the entire cohort using a one-way ANOVA and post-hoc Bonferroni correction. For each diagnosis (dysplasia and FAI), an independent t-test was used to determine any difference in the FEAR index between hips that had surgical treatment and those that did not. This analysis was repeated in borderline hips (defined by an LCEA of 20°-25°). We used an ROC curve to determine the diagnostic ability and optimal cutoff of the FEAR index for the diagnosis of dysplasia versus FAI and control in the entire cohort. Using the coordinate points of the curve, we identified the FEAR index cutoff at which most hips were accurately diagnosed (we defined this as the optimal cutoff). This analysis was repeated for only borderline hips. p < 0.05 was the threshold for statistical significance. All analyses were performed using SPSS (version 26, IBM Corp, Armonk NY, USA).

Results

Characteristics of the FEAR Index During Maturation

The FEAR index was found to be lower in hips of advanced age for both dysplastic and control hips (p < 0.001), while also remaining diagnosis-specific in childhood, adolescence, and maturity (p < 0.001). Similarly, the FEAR index’s relationship with diagnosis (higher in dysplasia and lower in FAI and control hips) remained consistent within each age category (p = 0.11) (Fig. 3).

F3
Fig. 3.:
This figure and table demonstrate the comparison of measured FEAR indices in asymptomatic control hips (Control), hips clinically diagnosed as dysplastic (Dysplasia), and hips clinically diagnosed as FAI (FAI) across ages younger than 10 years, 10 to 14 years, and older than 14 years. Bars represent the mean ± SD.

Correlation of FEAR Index with Clinical Diagnosis and Surgical Treatment

The FEAR index was greater in all 144 dysplastic hips (mean 5° ± 10°) than in 65 asymptomatic controls (mean -13° ± 7°; p < 0.001) and all 62 FAI hips (mean -10° ± 11°; p < 0.001). The FEAR index was lower in all 62 FAI hips (mean -10° ± 11°; p < 0.001) compared with all 144 dysplastic hips (mean 5° ± 10°; p < 0.001), but not in the 65 asymptomatic controls (mean -13° ± 7°; p > 0.05) (Table 2).

Table 2. - Demographics and radiographic measures by diagnosis
Parameter Dysplasia (n = 144) FAI (n = 62) Control (n = 65) p value
Age in years 13 ± 3 14 ± 1 16 ± 8
Sex, males 15 (22) 39 (24) 55 (36)
LCEA in ° 15 ± 11 27 ± 6 28 ± 7 < 0.001
 LCEA < 20° 58 (83) 11 (7) 11 (7)
 LCEA 20°-25° 36 (52) 34 (21) 25 (16)
 LCEA > 25° 6 (9) 55 (34) 64 (42)
FEAR index
 Entire cohort in ° 5 ± 10 -10 ± 11 -13 ± 7 < 0.001
 Borderline hips (LCEA 20°-25°) -1 ± 5 -10 ± 8 -10 ± 7 < 0.001
Acetabular index in ° 15 ± 9 5 ± 4 4 ± 5 < 0.001
ACEA in ° 15 ± 14 29 ± 8 < 0.001
Break in Shenton line 19 (27) 0 (0) < 0.001
Data are presented as mean ± SD or % (n); LCEA = lateral center-edge angle, ACEA = anterior center-edge angle.

An optimal FEAR index cutoff associated with dysplasia was determined to be greater than -1.3° (Fig. 4). The ROC curve generated an area under the curve of 0.91 (95% CI 0.88 to 0.94) for the entire cohort.

F4
Fig. 4.:
This scatter plot compares measured FEAR indices in the entire cohort with respect to clinical diagnosis of dysplasia, FAI, or control. Individual data points are represented as gray dots and bars for mean ± SD. Optimal ROC calculated FEAR cutoff represented as black line at -1.3°.

Furthermore, operative management was associated with a more increased FEAR index within hips diagnosed with dysplasia (8° ± 11° versus 1° ± 7°; p < 0.001). However, operative management was not associated with FEAR index within hips diagnosed with FAI (-12° ± 10° versus -7° ± 11°; p = 0.10) (Fig. 5).

F5
Fig. 5.:
This figure illustrates the FEAR indices in dysplastic hips and FAI that were treated operatively with a reorienting acetabular osteotomy or were treated nonoperatively with conservative measures as well as FAI patients treated operatively with an arthroscopic or open FAI procedure or were treated nonoperatively. Bars represent the mean ± SD.

Correlation of FEAR Index with Diagnosis in Borderline Hips (LCEA 20°-25°)

Within the group of borderline hips, those with a clinical diagnosis of dysplasia had a higher FEAR index than those that did not. The borderline dysplastic hips had a higher FEAR index (n = 52; mean -1° ± 5°) than both asymptomatic borderline controls (n = 16; mean -10° ± 7°; p < 0.001) and borderline FAI hips (n = 21; mean -10° ± 8°; p < 0.001). Borderline FAI hips (n = 21; mean -10° ± 8°) had a lower FEAR index than the borderline dysplastic hips (n = 52; mean -1° ± 5°; p < 0.001), but not the asymptomatic borderline controls (n = 16; mean -10° ± 7°; p > 0.05) (Fig. 6). The optimal FEAR index threshold for diagnosing a dysplastic hip among borderline hips remained -1.3° (area under the curve 0.86 [95% CI 0.81 to 0.92]).

F6
Fig. 6.:
This scatter plot compares measured FEAR indices in the subgroup of borderline hips (LCEA 20°-25°) with respect to clinical diagnosis of dysplasia, FAI, or control. Individual data points are represented as colored points and bars for mean ± SD. Optimal ROC calculated FEAR cutoff represented as black line at -1.3°.

Discussion

Making the diagnosis of hip dysplasia or impingement requires complex decision-making, particularly in patients with borderline hips (LCEA 20°-25°), where the radiographic picture is often complicated by overlapping morphology. Surgeons must rely on clinical history, physical examination, and radiographic markers to make the correct diagnosis and minimize the risk of inferior clinical outcomes from delayed or inappropriate treatment. There are numerous examples of failed hip preservation procedures in hips with mild deformities that are caused by an inaccurate diagnosis [12, 15, 18, 34]. The urgency in finding superior diagnostic tools is further reinforced by a nearly twofold increase in periacetabular osteotomies for hip dysplasia after hip arthroscopy from 2008 to 2015 [13]. The FEAR index has emerged as a promising radiographic finding to distinguish dysplasia from impingement, especially in the setting of mild deformity [38]. The purpose of this study was to broaden our understanding of the FEAR index by evaluating a broader age group, comparing borderline and nonborderline hips and including a large control group of asymptomatic hips. This study demonstrated that the FEAR index has a developmental basis correlating with the diagnosis of dysplasia and FAI in a young patient population. In the symptomatic and asymptomatic control groups, a positive (greater than -1.3°) FEAR index correlated with a dysplastic hip in 89% of the entire study population and 90% of borderline hips.

Limitations

There are several limitations to this study. First, this was a single-surgeon study with a diagnosis established by the senior author (IZ) based on a combination of clinical history, physical examination findings, plain radiographs, and advanced 3-D imaging, as detailed in the methods. Selection bias could have been limited by utilizing multiple surgeons, objective diagnostic criteria, and treatment-related outcome scores; however, using multiple surgeons could introduce similar biases. Second, variability in patient positioning during pelvic radiography may have altered radiographic measures as demonstrated in prior studies [16, 31]. This variability is minimized by use of standardized investigational group standing pelvic radiographs (detailed in methods), in a highly protocolized hip preservation clinic, and use of control group AP pelvis radiographs in which the coccyx is centered over the pubic symphysis and within 1 to 3 cm of the superior aspect of the symphysis. Furthermore, as the AI has been shown to not be influenced by pelvic tilt or rotation, the FEAR index is likely to not be influenced as well [31]. Yet limb rotation and hip abduction angles may impose variability, which could not be standardized in the control cohort, given the study design. Despite this, the FEAR index measures were as expected in the control group, compared with the FAI and dysplasia hips, suggesting limb positioning had a minimal contribution. Third, the approximate age of patients in each cohort resulted in a statistically different mean age between investigational and control groups. This difference should not alter the FEAR index measures because they were shown in this study to remain diagnosis-specific with age. Fourth, there are expected errors in measurement of the FEAR index as there are with measurements of AI [5]. This may lead to an incorrect association given the narrow FEAR index cutoff of -1.3°. However, this study demonstrated an excellent intraclass correlation coefficient (ICC 0.92 [95% CI 0.84 to 0.96]) as have prior FEAR index studies, suggesting high reproducibility and consistency [3, 33, 38]. Fifth, there was a noted limitation of the FEAR index and its lack of variance between asymptomatic control hips and FAI hips, though this is expected as both hip types are considered stable and the FEAR index is a marker of mechanical stability [38].

Characteristics of the FEAR Index During Maturation

Younger patients demonstrated higher FEAR index values than older ones in both the dysplastic and nondysplastic groups, suggesting a developmental basis of the FEAR index. As expected, the FEAR index correlated highly with dysplasia and impingement. In addition, the FEAR index was shown to decrease in dysplasia and become more negative in FAI hips with age. This coincides with the recent analysis by Hingsammer et al. [14] of morphological changes in a developing hip, which exhibited an increase in acetabular coverage with skeletal maturation. The applicability and understanding of the FEAR index in this younger population had been previously limited because prior research included older populations, consisting of participants in their late 20s to early 30s [3, 21, 33, 38].

Correlation of FEAR Index with Clinical Diagnosis and Surgical Treatment

In each age group, the FEAR index was higher in dysplastic hips than in asymptomatic controls and in hips with an FAI diagnosis. There was little or no difference in the FEAR index between the hips of asymptomatic control patients and the hips of patients with FAI. As such, this study reinforces the findings from prior FEAR index research of a significantly greater FEAR index in dysplastic hips compared with controls or symptomatic impinging hips [3, 33, 38]. However, this study included a significantly larger population and a control group not selected by an LCEA greater than 20°, thereby increasing the generalizability and reducing Type II error. With this larger study population, we found an optimal FEAR index threshold of -1.3°. This FEAR index threshold is most similar to that of Batailler et al. [3] (+2°); this is compared with the original threshold of Wyatt et al. [38], which was +5° and that of Truntzer et al. [33], which was -5°. Additionally, the inclusion of nonoperative dysplastic and impingement hips in this study allowed for an analysis of the association of the FEAR index with treatment within each diagnostic category. It was demonstrated that an increasingly high FEAR index trend and the LCEA are associated with operative intervention in patients with dysplasia. Though the LCEA is commonly used, an LCEA greater than 25° has been shown to not correlate with normal, increased, or decreased acetabular surface area [32].

Correlation of FEAR Index with Diagnosis in Borderline Hips (LCEA 20°-25°)

In the group of borderline hips, those with dysplasia had a distinctly different FEAR index than those who did not. Original data from Wyatt et al. [38] demonstrated a 79% association with nondysplastic hips in borderline hips below a FEAR index of +5°. More recent data from Batailler et al. [3] showed a 90% association using a FEAR index of +2°. However, these study populations were limited to borderline LCEA investigational hips and a control group of hips with an LCEA greater than 25°. In this study, our inclusion of a control group of asymptomatic borderline hips provides evidence that the FEAR index may be used to differentiate symptomatic dysplastic borderline patients from both symptomatic FAI borderline hips and asymptomatic borderline hips. The ROC calculated FEAR index threshold remained -1.3° in borderline hips.

Conclusion

This study supports the premise that the FEAR index is developmentally specific and subject to the unique hip forces generated by acetabular morphology. The FEAR index is a useful screening measure for hip stability in patients with a wide range of ages and various degrees of acetabular deformity. Specifically, a FEAR index greater than -1.3° is associated with a dysplastic hip and a FEAR index less than -1.3° is associated with an FAI/control hip. The FEAR index may help hip preservation surgeons establish the correct diagnosis and select appropriate surgical procedures on the basis of a hip’s pathomechanics. Future prospective studies using the FEAR index and defined outcome instruments should further validate the clinical utility of this measurement and enable surgeons to improve diagnostic specificity through prospective analysis in larger patient cohorts.

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