Diagnostic Utility of CT-Based Acetabular Sector Angles in Pincer-Type Femoroacetabular Impingement: Limitations and Clinical Integration

Abstract

Background:

Pincer-type femoroacetabular impingement (FAI) results from acetabular overcoverage. While computed tomography (CT) with multiplanar reformation (MPR) enables precise measurement of acetabular sector angles (ASAs), the diagnostic utility and limitations of these measurements remain unclear.

Purpose:

To evaluate the diagnostic accuracy of ASA measurements in pincer-type FAI and determine whether these measurements can serve as stand-alone diagnostic criteria or should be integrated with other clinical parameters.

Study Design:

Cross-sectional study.

Methods:

The authors evaluated 104 hips in 52 patients with surgically confirmed pincer-type FAI and 31 asymptomatic controls. ASAs were measured at 4 levels using CT with MPR correction for pelvic tilt. Receiver operating characteristic (ROC) analysis determined diagnostic cutoff values.

Results:

Intermediate anterior ASA (IAASA) showed the most significant difference between FAI and controls (88.7°± 21.4° vs 67.8°± 13.6°; P < .001; d = 1.11). However, ROC analysis revealed only moderate diagnostic accuracy (area under the curve, 0.79). An IAASA cutoff of 97° provided 88% specificity but only 40% sensitivity.

Conclusion:

IAASA is significantly elevated in pincer-type FAI but has only moderate diagnostic accuracy due to substantial population variation. ASA measurements should be integrated with clinical findings, traditional radiographic parameters, and surgical findings rather than used as stand-alone diagnostic criteria.

Keywords

femoroacetabular impingement acetabular sector angle pincer impingement 3-dimensional computed tomography hip arthroscopy acetabular overcoverage

Hip disorders represent a spectrum from acetabular undercoverage (dysplasia) to overcoverage (femoroacetabular impingement [FAI]), both leading to pain and premature osteoarthritis. FAI syndrome (FAIS) has emerged as a significant cause of hip pain in young active adults, with incidence rates of 54 per 100,000 person-years and peak diagnosis ages of 18 to 35 years.^10,12 The condition comprises cam-type (femoral asphericity), pincer-type (acetabular overcoverage), or combined morphology, with 67% presenting as mixed type.¹²

Historically, acetabular coverage assessment relied on 2-dimensional (2D) radiographic measurements including the lateral center-edge angle (LCEA) of Wiberg and anterior center-edge angle (ACEA).^13,23 However, these conventional measurements have significant limitations: positioning variability substantially affects measurements, 2D projections cannot capture regional variations in acetabular anatomy, and pelvic tilt creates systematic errors in coverage assessment.^7,21 The Warwick Agreement on FAIS emphasizes that diagnosis requires concordance between clinical symptoms, physical examination findings, and imaging features, acknowledging the limitations of imaging alone.¹¹

Computed tomography (CT) imaging has advanced acetabular morphology assessment through multiplanar reformation (MPR) and 3-dimensional (3D) reconstruction capabilities.^5,24 CT-based measurements offer potential advantages including standardization of pelvic orientation and assessment of regional coverage variations at different femoral head levels. Anda and colleagues^1,2 pioneered acetabular sector angle (ASA) measurements on axial CT slices, demonstrating that acetabular coverage varies substantially at different femoral head levels. This methodology has been refined and validated, with recent studies establishing normative values and cutoff thresholds primarily for hip dysplasia.^16,22

Pincer-type FAI occurs when excessive acetabular coverage causes abnormal contact between the acetabular rim and femoral neck during hip motion, leading to progressive labral and cartilage damage.^4,18 While global overcoverage can occur, focal anterior or anterosuperior overcoverage predominates, with lesions characteristically localized between the 1- and 3-o'clock positions.^6,14 Despite the recognized importance of coverage assessment in FAI, most ASA studies have focused on dysplastic hips with limited data specifically addressing pincer-type impingement.^16,22

The purpose of this study was to evaluate the diagnostic utility of ASAs measured at multiple femoral head levels in patients with pincer-type FAI compared with asymptomatic controls using standardized CT protocols with MPR correction. We hypothesized that patients with pincer-type FAI would demonstrate increased anterior ASA (AASA), and we sought to determine whether specific cutoff values could provide clinically useful diagnostic criteria or whether ASA measurements are better suited as adjunctive tools for surgical planning.

Methods

Patient Selection and Study Design

This retrospective cross-sectional study was conducted at a single tertiary referral center in Istanbul, Turkey (Metin Sabanci Bone Diseases Training and Research Hospital), between January 2020 and December 2023. Institutional review board approval was obtained, and all patients provided informed consent for use of imaging data.

For the FAI patient group (group 2), we identified 52 consecutive patients (104 hips) who underwent hip arthroscopy for pincer-type FAI. Inclusion criteria were (1) unilateral hip pain with positive impingement testing (flexion-adduction-internal rotation test), (2) radiographic evidence of acetabular overcoverage (LCEA >40° or positive crossover sign), (3) preoperative pelvic CT scan with 1-mm slices for surgical planning, (4) intraoperative confirmation of labral pathology consistent with pincer impingement localized between the 1- and 3-o'clock positions, and (5) minimum 6-month clinical follow-up. Exclusion criteria included (1) cam-type deformity requiring femoral osteochondroplasty (alpha angle >55° requiring resection), (2) previous hip surgery, (3) hip dysplasia (LCEA <20°), (4) Tönnis grade ≥2 osteoarthritis, (5) inflammatory arthritis, and (6) inadequate CT image quality.

The FAI group comprised 35 male patients and 17 female patients with a mean age of 36.5 years (range, 18-58 years). All patients underwent hip arthroscopy with acetabular rim trimming. Intraoperative records were reviewed to confirm pure pincer-type morphology without cam resection, excluding mixed-type cases.

The control group (group 1) consisted of 31 asymptomatic individuals (31 hips) who underwent CT imaging for non–hip related indications (trauma evaluation of remote body regions, preoperative planning for nonorthopaedic procedures). Inclusion criteria included (1) no current or historical hip pain or dysfunction verified through medical record review, (2) age 18 to 60 years, (3) and CT imaging including pelvis with adequate quality (1-mm slices). Exclusion criteria mirrored the FAI group. The control group included 21 male patients and 10 female patients with a mean age of 32.5 years (range, 19-56 years).

Finally, the 52 patients (52 hips) with unilateral symptomatic FAI had their asymptomatic contralateral hips analyzed separately (group 3) to investigate bilateral morphological patterns. These hips had no clinical symptoms but were in the same patients as symptomatic FAI hips.

Sample Size Considerations

On the basis of previous studies reporting mean intermediate anterior ASA (IAASA) values of approximately 70° in normal populations with standard deviation of 15°, we estimated that detecting a 15° difference (effect size, d = 1.0) between FAI patients and controls would require 17 patients per group with 80% power at α = .05. Our sample sizes of 52 FAI patients and 31 controls exceeded this requirement. However, as this was a retrospective study, our sample size was determined by available consecutive patients meeting inclusion criteria rather than prospective power calculation.

CT Imaging Protocol and Measurement Methodology

All patients underwent pelvic CT using standardized protocols (120 kVp, auto-mA modulation, 1-mm slice thickness, 0.5-mm increment) on multidetector CT scanners. Images were obtained with patients in the supine position, feet internally rotated 15°.

It is important to clarify our measurement methodology. While our study utilized CT data sets capable of MPR and 3D reconstruction, the actual ASA measurements were performed on MPR-corrected axial images after the established 2D methodology of Anda et al.^1,2 MPR was used specifically to standardize pelvic orientation by adjusting the axial plane to pass through both femoral head centers, thereby eliminating measurement errors from pelvic tilt. This approach differs from true volumetric 3D surface reconstruction and impingement simulation studies, which analyze acetabular coverage using rendered 3D models. Our methodology represents an intermediate approach: more standardized than standard axial CT due to pelvic tilt correction, but not equivalent to full 3D morphometric analysis.

ASA Measurements

All measurements were performed using a dedicated picture archiving and communication system workstation with MPR capabilities by 2 experienced musculoskeletal radiologists. The axial plane was adjusted to pass through both femoral head centers, ensuring consistent measurement planes across all patients (Figure 1).

Figure 1.

(A) Measurement technique for acetabular sector angles using multiplanar reformation (MPR). Axial computed tomography slices demonstrating pelvic tilt before correction. (B) MPR adjustment to ensure both femoral head centers lie in the same plane.

ASAs were measured according to Anda et al’s^1,2 methodology. Four standardized levels were used: (1) proximal: most superior axial slice through the femoral head center; (2) intermediate: midway between the proximal and equatorial levels; (3) equatorial: axial slice through the true femoral head center (largest diameter); and (4) superior: measured on coronal reformatted images.

For each axial level, AASA and posterior ASA (PASA) were measured. ASA was defined as the angle between (1) a line connecting both femoral head centers and (2) a line from the ipsilateral femoral head center to the anterior or posterior acetabular rim. Superior ASA (SASA) was measured on coronal images as the angle between the line connecting the femoral head centers and the line to the superolateral acetabular rim.

The following angles were recorded: proximal AASA (PAASA) and proximal PASA (PPASA) (Figure 2); IAASA and intermediate PASA (IPASA) (Figure 3); equatorial AASA (EAASA) and equatorial PASA (EPASA) (Figure 4); and SASA (Figures 5 and 6).

Figure 2.

(A) Proximal anterior acetabular sector angle (ASA) and (B) proximal posterior ASA measurement at superior femoral head level.

Figure 3.

(A) Intermediate anterior acetabular sector angle (ASA) and (B) intermediate posterior ASA measurement at mid–femoral head level.

Figure 4.

(A) Equatorial anterior acetabular sector angle (ASA) and (B) equatorial posterior ASA measurement at the level of maximal femoral head diameter.

Figure 5.

Superior acetabular sector angle measured on coronal reformatted images.

Figure 6.

Three-dimensional computed tomography visualization of acetabular sector angle measurement points. Red points indicate the acetabular rim landmarks used for sector angle measurements, whereas the horizontal yellow lines represent the reference line connecting both femoral head centers and the measurement lines used to calculate the respective acetabular sector angles.

Additionally, conventional radiographic parameters—the LCEA and the ACEA—were measured on standardized anteroposterior pelvic radiographs and false-profile radiographs.

Reliability Assessment

To assess measurement reliability, the primary observer (10 years of musculoskeletal radiology experience) repeated all measurements after a 10-day interval, blinded to initial values. An independent observer (experienced hip arthroscopy surgeon) performed measurements on identical images for interobserver reliability. Intraclass correlation coefficients (ICCs) were calculated for all measurements.

Statistical Analysis

Statistical analyses were performed using SPSS Version 27.0 (IBM Corp). Normality was assessed using Shapiro-Wilk test. For normally distributed data, independent-samples t tests compared groups; for nonnormal distribution, Mann-Whitney U tests were applied. Paired analysis compared symptomatic versus asymptomatic hips within the same patients. Cohen d assessed clinical meaningfulness of significant differences. Receiver operating characteristic (ROC) curves were constructed for each ASA parameter to determine diagnostic accuracy. For parameters with area under the curve (AUC) >0.60, optimal cutoffs were determined using Youden index. ICC values with 95% CIs were calculated for reliability. Statistical significance was set at P < .05. Pearson or Spearman correlation coefficients were calculated to evaluate the relationship between CT-based ASA measurements and conventional 2D radiographic parameters (LCEA, ACEA). Correlation strength was interpreted as negligible (r < 0.30), low (0.30-0.49), moderate (0.50-0.69), high (0.70-0.90), or very high (>0.90).

Results

Patient Demographics

No significant age differences existed between controls (32.5 ± 8.2 years) and FAI patients (36.5 ± 9.4 years; P = .08). Sex distribution showed similar male predominance (controls, 67.7%; FAI patients, 67.3%; P = .96).

Sex Differences in Control Population

Within the normal population (group 1), significant sex differences in acetabular coverage appeared at the equatorial level (Table 1). Male patients demonstrated greater anterior coverage (EAASA, 62.2°± 9.3° vs 53.7°± 7.5°; P = .01) while female patients showed greater posterior coverage (EPASA, 102.3°± 7.0° vs 95.4°± 9.7°; P = .04). At the intermediate level, male patients exhibited greater anterior coverage (IAASA, 71.3°± 13.5° vs 60.4°± 11.0°; P = .02). No sex differences appeared at proximal or superior levels. Conventional radiographic parameters showed no significant sex differences in controls (LCEA: male, 31.2°± 5.5°; female, 29.1°± 4.4°; P = .54; ACEA: male, 36.1°± 3.4°; female, 34.3°± 3.4°; P = .51).

Table 1

Sex Differences in Acetabular Sector Angles Within Control Population (Group 1) ^a

ASA Parameter	Male(n = 21)	Female(n = 10)	P	Test Used	Cohen d
Proximal PASA, deg	133.2 ± 10.4	134.3 ± 11.9	.80	t test	–0.10
Proximal AASA, deg	124.1 ± 11.0	116.2 ± 9.7	.056	t test	0.74
Intermediate PASA, deg	107.9 ± 10.9	111.7 ± 13.6	.45	t test	–0.32
Intermediate AASA, deg	71.3 ± 13.5	60.4 ± 11.0	.02	Mann-Whitney	0.85
Equatorial PASA, deg	95.4 ± 9.7	102.3 ± 7.0	.04	t test	–0.76
Equatorial AASA, deg	62.2 ± 9.3	53.7 ± 7.5	.01	t test	0.96
Superior ASA, deg	126.2 ± 4.4	125.1 ± 5.5	.58	t test	0.24
LCEA, deg	31.2 ± 5.5	29.1 ± 4.4	.54	t test	0.29
ACEA, deg	36.1 ± 3.4	34.3 ± 3.4	.51	t test	0.28

Data are presented as mean ± SD. Bold values indicate statistical significance (P < .05). AASA, anterior ASA; ACEA, anterior center-edge angle; ASA, acetabular sector angle; LCEA, lateral center-edge angle; PASA, posterior ASA.

Comparison of ASA Values Between Groups

Table 2 presents mean ASA values for all 3 groups with statistical comparisons.

Table 2

Comparison of Acetabular Sector Angles and LCEA, ACEA Between Groups ^a

Angle	Normal Population Group 1(n = 31)	FAI Symptomatic Group 2(n = 52)	P (G1 vs G2)	FAI Asymptomatic Group 3(n = 52)	P (G3 vs G2)
Proximal PASA, deg	133.5 ± 10.7	138.8 ± 18.8	.35	134.4 ± 15.6	.23
Proximal AASA, deg	121.5 ± 11.1	128.2 ± 19.1	.09	120.1 ± 23.1	.08
Intermediate PASA, deg	109.1 ± 11.8	112.5 ± 17.7	.83	109.4 ± 13.4	.76
Intermediate AASA, deg	67.8 ± 13.6	88.7 ± 21.4	<.001	84.7 ± 21.9	.31
Equatorial PASA, deg	97.6 ± 9.4	97.0 ± 11.9	.47	95.7 ± 10.5	.82
Equatorial AASA, deg	59.5 ± 9.6	63.4 ± 12.2	.14	65.8 ± 12.2	.35
Superior ASA, deg	125.9 ± 4.7	126.3 ± 11.0	.61	125.1 ± 7.5	.37
LCEA, deg	30.6 ± 6.1	47.1 ± 6.3	<.001	43.1 ± 5.3	<.17
ACEA, deg	35.9 ± 3.6	56.3 ± 9.4	<.001	54.0 ± 7.5	<.24

Bold values indicate statistical significance (P < .05). Data are presented as mean ± SD. AASA, anterior ASA; ACEA, anterior center-edge angle; ASA, acetabular sector angle; FAI, femoroacetabular impingement; LCEA, lateral center-edge angle; PASA, posterior ASA.

For the IAASA, the most striking finding was significant IAASA elevation in FAI patients versus controls. Symptomatic FAI hips (group 2) demonstrated a mean IAASA of 88.7°± 21.4° versus 67.8°± 13.6° in controls (P < .001; Cohen d = 1.11; large effect). Asymptomatic contralateral hips (group 3) also showed elevated IAASA (84.7°± 21.9°) compared with controls (P < .001). No significant difference existed between symptomatic and asymptomatic hips within FAI patients (P = .31).

Mean PAASA values were 121.5°± 11.1° in controls, 128.2°± 19.1° in symptomatic FAI hips (P = .09), and 120.1°± 23.1° in asymptomatic FAI hips (P > .05 vs controls). While a trend toward increased PAASA appeared in symptomatic hips, this did not reach statistical significance.

Regarding other ASA parameters, no significant differences appeared for PPASA, IPASA, EPASA, EAASA, or SASA between any groups (all P > .05).

ROC Curve Analysis and Diagnostic Cutoff Values

Table 3 and Figure 7 present ROC analysis results for distinguishing FAI patients from controls.

Table 3

ROC Analysis Results for Diagnostic Cutoff Values (Group 1 vs Group 2) ^a

Angle	AUC	Cutoff Value, deg	Sensitivity, %	Specificity, %
Proximal PASA	0.566	136	55.8	60.2
Proximal AASA	0.605	118	75.0	44.6
Intermediate PASA	0.517	149	9.6	100.0
Intermediate AASA	0.782	97	40.4	88.0
		77	60.0	81.0
Equatorial PASA	0.490	116	9.6	96.4
Equatorial AASA	0.503	68	38.5	74.7
Superior ASA	0.545	130	46.2	74.7

Two cutoff values shown for intermediate AASA: 97° for high specificity, 77° for optimal balance. AASA, anterior ASA; ASA, acetabular sector angle; AUC, area under the curve; PASA, posterior ASA; ROC, receiver operating characteristic.

Figure 7.

Receiver operating characteristic (ROC) curves for acetabular sector angles (ASAs) in differentiating femoroacetabular impingement patients (group 2) from controls (group 1). (A) Intermediate anterior ASA (IAASA) showing highest area under the curve (AUC) of 0.79. (B) Proximal anterior ASA with AUC of 0.61. Note that IAASA demonstrates superior diagnostic performance compared with all other parameters.

IAASA demonstrated the highest diagnostic accuracy (Table 3):

Group 1 vs group 2: AUC, 0.79 (95% CI, 0.70-0.88)

Group 1 vs group 3: AUC, 0.74 (95% CI, 0.64-0.84)

Optimal cutoff, 77°: sensitivity, 60%; specificity, 81%

High-specificity cutoff, 97°: sensitivity, 40%; specificity, 88%

PAASA showed modest discriminatory ability:

Group 1 vs group 2: AUC, 0.61

Cutoff, 118°: sensitivity, 75%; specificity, 45%

Other parameters including the remaining ASA measurements (PPASA, PAASA, IPASA, EPASA, EAASA, SASA) showed poor to fair discriminatory ability with AUC values 0.45 to 0.60, indicating insufficient diagnostic accuracy for clinical application.

Conventional Radiographic Parameters and Correlation With ASA

Mean LCEA was significantly higher in FAI patients compared with controls (47.1°± 6.3° vs 30.6°± 6.1°; P < .001), confirming acetabular overcoverage. Similarly, mean ACEA was elevated in FAI patients (56.3°± 9.4° vs 35.9°± 3.6°; P < .001). Asymptomatic contralateral hips (group 3) demonstrated intermediate values for both LCEA (43.1°± 5.3°) and ACEA (54.0°± 7.5°), which were significantly higher than controls (both P < .001) but not significantly different from symptomatic hips (Table 2). Correlation analysis between ASA measurements and conventional radiographic parameters revealed moderate positive correlations for anterior coverage angles (Table 4). IAASA demonstrated the strongest correlation with both LCEA (r = 0.58; P < .001) and ACEA (r = 0.54; P < .001). EAASA showed moderate correlation with LCEA (r = 0.47; P < .001) and ACEA (r = 0.52; P < .001).

Table 4

Conventional Radiographic Parameters and Correlation With ASA ^a

ASA Parameter	LCEA, r	P	ACEA, r	P	Correlation Strength
Proximal AASA	0.36	<.01	0.44	<.001	Moderate
Intermediate AASA	0.58	<.001	0.54	<.001	Moderate-strong
Equatorial AASA	0.47	<.001	0.52	<.001	Moderate
Superior ASA	0.39	<.01	0.18	.12	Low

AASA, anterior ASA; ACEA, anterior center-edge angle; ASA, acetabular sector angle; LCEA, lateral center-edge angle.

Reliability Analysis

Both intra- and interobserver reliability were excellent for all ASA measurements (Table 5). Intraobserver ICC values ranged from 0.89 to 0.96 (all P < .001). Interobserver ICC values ranged from 0.85 to 0.94 (all P < .001). Highest reliability was SASA (intraobserver ICC, 0.96; interobserver ICC, 0.94). Lowest reliability was EAASA (intraobserver ICC, 0.89; interobserver ICC, 0.85).

Table 5

Intraobserver and Interobserver Reliability ^a

Parameter	Intraobserver ICC (95% CI)	Interobserver ICC (95% CI)
PPASA	0.92 (0.88-0.95)	0.89 (0.84-0.93)
PAASA	0.91 (0.87-0.94)	0.88 (0.83-0.92)
IPASA	0.93 (0.90-0.96)	0.90 (0.86-0.94)
IAASA	0.90 (0.86-0.94)	0.87 (0.81-0.91)
EPASA	0.94 (0.91-0.96)	0.92 (0.88-0.95)
EAASA	0.89 (0.84-0.93)	0.85 (0.79-0.90)
SASA	0.96 (0.94-0.98)	0.94 (0.91-0.96)
LCEA	0.93 (0.89-0.95)	0.91 (0.87-0.94)
ACEA	0.90 (0.88-0.92)	0.87 (0.84-0.92)

All ICC values were statistically significant (P < .001). ACEA, anterior center-edge angle; ASA, acetabular sector angle; EAASA, equatorial anterior ASA; EPASA, equatorial posterior ASA; IAASA, intermediate anterior ASA; ICC, intraclass correlation coefficient; IPASA, intermediate posterior ASA; LCEA, lateral center-edge angle; PAASA, proximal anterior ASA; PPASA, proximal posterior ASA; SASA, superior ASA.

Comparison With Published Normative Data

Comparing our control group values to previously published studies revealed substantial differences from Verhaegen et al's²² European cohort but close agreement with Nahal et al's¹⁶ North American data (Table 6). Our mean PPASA (133.5°± 10.7°) differed significantly from Verhaegen et al's 162°± 17° (P < .001) but resembled Nahal et al's 149.1°± 23.1° (P = .06). These discrepancies likely reflect population differences, age variations, and methodological factors including pelvic tilt correction protocols.

Table 6

Comparison of Control Group Values With Published Literature ^a

Angle	Current Study Group 1	Verhaegen et al²²	P vs Verhaegen et al²²	Nahal et al¹⁶	P vs Nahal et al¹⁶
Proximal PASA, deg	133.5 ± 10.7	162 ± 17	<.001	149.1 ± 23.1	.06
Proximal AASA, deg	121.5 ± 11.1	151 ± 26	<.001	128.8 ± 23.0	.28
Intermediate PASA, deg	109.1 ± 11.8	117 ± 11	.02	106.5 ± 6.8	.70
Intermediate AASA, deg	67.8 ± 13.6	74 ± 12	.10	69.0 ± 10.6	.80
Equatorial PASA, deg	97.6 ± 9.4	100 ± 8	.20	93.4 ± 7.1	.40
Equatorial AASA, deg	59.5 ± 9.6	60 ± 7	.70	54.9 ± 8.5	.30
Superior ASA, deg	125.9 ± 4.7	123 ± 5	.30	—	—

Data are presented as mean ± SD. Bold values indicate statistical significance (P < .05). Dashes indicate not available. AASA, anterior ASA; ASA, acetabular sector angle; PASA, posterior ASA.

Discussion

The principal finding was that IAASA was significantly elevated in patients with pincer-type FAI compared with asymptomatic controls, with a large effect size (Cohen d = 1.11). However, and importantly, the diagnostic accuracy of this measurement is only moderate (AUC, 0.79), with sensitivity of 40% to 60% and specificity of 81% to 88% at optimal cutoffs. This finding has critical clinical implications: ASA measurements should not serve as stand-alone diagnostic criteria but rather should be integrated with other clinical and radiographic parameters in comprehensive FAI evaluation.

Clarification of CT Measurement Methodology

It is essential to distinguish between different CT-based approaches to acetabular morphology assessment. Our study utilized MPR to standardize pelvic orientation before performing traditional 2D axial measurements according to Anda et al's^1,2 methodology. This approach corrects for pelvic tilt variability—a significant source of measurement error—but does not constitute true 3D surface reconstruction or volumetric analysis. Advanced 3D CT–based impingement simulation studies, which render the acetabulum and femur as 3D surfaces and model their interaction during hip motion, represent a distinct and more sophisticated methodology.^14,15 While such approaches offer theoretical advantages in predicting impingement location and extent, our MPR-corrected axial measurement approach offers practical advantages including widespread availability, established normative data, and direct comparability with the existing ASA literature.

Integration With Traditional Radiographic Parameters

Traditional radiographic parameters including LCEA and ACEA have been the mainstay of acetabular coverage assessment.^13,23 Our findings suggest that ASA measurements provide complementary rather than replacement information. While LCEA measures global lateral coverage, ASA measurements at different levels provide spatial information about regional coverage variations—particularly important in pincer-type FAI where focal anterior overcoverage predominates. IAASA exhibited the strongest correlation with both LCEA (r = 0.58; P < .001) and ACEA (r = 0.54; P < .001), suggesting that intermediate anterior coverage most closely reflects global and anterior acetabular overcoverage. The finding that IAASA was the most discriminating parameter aligns with the known location of pincer lesions between the 1- and 3-o'clock positions, corresponding to the midfemoral head level captured by intermediate ASA measurements.¹⁵

We recommend that clinicians integrate ASA measurements with traditional parameters as follows: LCEA >40° suggests global overcoverage; positive crossover sign indicates acetabular retroversion. IAASA >77° to 97° provides quantitative confirmation of anterior overcoverage at the critical midfemoral head level. This multiparametric approach may improve diagnostic accuracy compared with any single measurement, though validation in larger cohorts is needed.

Clinical Implications: Why Moderate Diagnostic Accuracy Is Meaningful

Our finding of only moderate diagnostic accuracy (sensitivity, 40%-60%; specificity, 81%-88%) should not be viewed as a failure of the methodology but rather as an important insight into the nature of pincer-type FAI. The substantial overlap in ASA values between FAI patients and controls reflects genuine biological reality: acetabular morphology exists on a continuum, and the boundary between normal variation and pathological overcoverage is inherently imprecise. Many individuals with elevated IAASA values remain asymptomatic, while others with borderline values develop symptoms—a phenomenon explained by factors beyond bony morphology including activity level, labral tissue properties, cartilage quality, and neuromuscular hip control.

This population variability underscores why the Warwick Agreement emphasizes the need for concordance between symptoms, clinical signs, and imaging findings for FAIS diagnosis.¹¹ Our data support this multiparametric approach and argue against using any single radiographic measurement—including ASA—as a stand-alone diagnostic criterion.

Surgical Planning Applications: Rim Trimming Quantification

While ASA may have limited value as a primary diagnostic tool, our findings suggest potential utility for surgical planning. The significant IAASA elevation in FAI patients quantifies the degree of anterior overcoverage at the midfemoral head level—the precise location requiring rim trimming in most pincer cases. An IAASA substantially above the normal range (>90°; approximately 1.5 SD above control mean) combined with normal posterior coverage (PASA values) indicates focal anterior overcoverage amenable to targeted rim trimming.

Recent evidence demonstrates that residual pincer lesions of ≥20% are associated with poorer postoperative outcomes, emphasizing the importance of adequate rim resection.⁸ CT analysis with ASA measurements may be particularly valuable in revision cases where previous rim trimming was inadequate or excessive, helping identify residual or iatrogenic coverage abnormalities.^19,25 However, we caution against rigid application of any numerical target, as the optimal degree of coverage is influenced by individual factors including patient activity demands and associated intra-articular pathology.

Population-Specific Variations and Normative Values

Our data demonstrate substantial variability in ASA values even within normal populations, with significant sex differences (men showing greater anterior coverage, women greater posterior coverage). This variability, combined with documented racial/ethnic variations in hip morphology,^17,20 underscores the need for population-specific normative databases rather than universal cutoff values. The discrepancy between our normative values and previously published European cohorts likely reflects population genetics, age variations, and methodological factors.^16,22

Bilateral Morphological Abnormalities

A striking finding was elevated IAASA in asymptomatic contralateral hips of FAI patients (84.7°± 21.9°) compared with controls (67.8°± 13.6°) (P < .001). This suggests morphological FAI predisposition is often bilateral, even when symptoms are unilateral. This observation aligns with previous studies showing that asymptomatic contralateral hips in FAI patients frequently develop symptoms over time, with reported rates of 60% to 81% becoming symptomatic within 2 to 5 years.^3,9 The lack of significant IAASA difference between symptomatic and asymptomatic hips within FAI patients further supports the concept that symptom development depends on factors beyond morphology alone.⁸

Limitations

This study has several limitations. The retrospective design introduces potential selection bias, and the relatively small sample size may have limited statistical power for subgroup analyses. Furthermore, the study population was specific to a Turkish cohort, which may limit generalizability. Femoral version was not assessed, although it may influence FAI presentation. Control patients were not evaluated for asymptomatic hip pathology. We also did not perform direct correlation analyses between ASA values and conventional radiographic parameters. In addition, static CT measurements may not fully capture dynamic impingement patterns. Finally, we did not evaluate the relationship between preoperative ASA values and the extent of rim trimming or postoperative outcomes.

Conclusion

IAASA is significantly elevated in pincer-type FAI but has only moderate diagnostic accuracy because of substantial population variation. These findings indicate that ASA measurements should be integrated with clinical findings, traditional radiographic parameters, and surgical findings rather than used as stand-alone diagnostic criteria. The primary clinical value of ASA may lie in surgical planning—quantifying anterior overcoverage to guide rim trimming—rather than in primary diagnosis. Future studies should correlate preoperative ASA measurements with surgical technique and postoperative outcomes to determine optimal utilization of these measurements in comprehensive FAI management.

Footnotes

Final revision submitted January 13, 2026; accepted March 3, 2026.

The authors declared that they have no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

This study was approved by the ethics committee of Baltalimani Metin Sabanci Bone Diseases Training and Research Hospital (approval No. 2024/189).

ORCID iDs

Murat Önder

Abdulhamit Misir

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