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Abstract

Background:

The anterior center edge angle (ACEA) is a radiographic parameter commonly utilized for the assessment of anterior acetabular coverage in the setting of various hip abnormalities. However, this measurement is greatly dependent on patient positioning and imaging technique.

Purpose:

To evaluate the utility of a measurement technique utilizing a horizontal reference line and femoral head perfect circles for better control of pelvic obliquity and the assessment of its potential as a more reproducible method for determining the ACEA.

Study Design:

Cohort study (diagnosis); Level of evidence, 3.

Methods:

There were 3 authors who retrospectively reviewed the false-profile radiographs of 21 patients. Traditional ACEA measurements and novel ACEA measurements utilizing a horizontal reference line and best-fit circles on each femoral head were recorded by each author on 2 separate occasions. A paired t test was used to establish the difference between traditional and novel measurements for each reviewer. The intraclass correlation coefficient (ICC) was used to determine interrater and intrarater reliability across both methods. Significance was set at P < .05.

Results:

The mean ACEA value obtained using the traditional method was 39.72°, and the mean ACEA value obtained using the novel method was 37.68° (P = .36). Intrarater reliability for the novel method was found to be strong (ICC = 0.805-0.966) in all instances. Interrater reliability was found to be strong in all instances utilizing the horizontal reference line (ICC = 0.831-0.902) but was weak for 2 of 3 reviewers using the traditional method (ICC = 0.420, 0.585, and 0.769).

Conclusion:

The novel method of measurement was found to have strong agreement within each reviewer across both measurement attempts as well as between all 3 physician reviewers. The traditional method was found to have weak intrarater reliability for 1 of 3 physician reviewers and weak interrater reliability for 2 of 3 reviewers. These results demonstrate that the novel method was more reproducible across individual reviewers and between different reviewers. As such, consideration should be given to utilizing the novel method for measuring the ACEA as a standardized technique in future research efforts on hip dysplasia and femoroacetabular impingement.

Keywords

femoroacetabular impingement hip arthroscopic surgery hip dysplasia false-profile view anterior center edge angle

The anterior center edge angle (ACEA) was first described by Lequesne and de Seze⁹ in the radiographic false-profile view to quantify anterior acetabular coverage in the evaluation of patients with dysplastic hips and femoroacetabular impingement (FAI).⁵ Since its introduction, the ACEA has been a common component in the evaluation of patients with various hip abnormalities that affect the degree of acetabular coverage.⁷ Patients with acetabular undercoverage experience increased contact stress about the hip joint, resulting in damage to the labrum and articular surface and thus making accurate measurements/classifications of this parameter an important component of the clinical evaluation.¹ Moreover, radiographic analysis of the pelvis is a critical segment of the preoperative assessment for patients with FAI and patients with hip dysplasia, as suboptimal outcomes from hip arthroscopic surgery are often seen in those with observable abnormal coverage if it is not addressed at the time of surgery.

Despite multiple studies reporting the high reliability of ACEA measurements,^{12,13,18,22,23} several studies have demonstrated the variability in the ACEA with patient positioning.^15,21 Positioning for adequate false-profile pelvic imaging is well described and reportedly reliable, but resultant radiographs often have some degree of error secondary to equipment difficulties, operator technique, or patient cooperation.^12,23 Furthermore, without a standardized method for establishing a horizontal reference, ACEA measurements are subject to variability with pelvic obliquity. Because traditional methods of measuring the ACEA do not consistently account for variable pelvic obliquity, templating the false-profile projection with a true horizontal reference, while also utilizing perfect circles to center the reference and account for positional femoral head magnifications, may lead to more accurate and reliable measurements.

A technique has previously been described that may allow for more precise radiographic assessments. This technique utilizes standard radiographic viewing tools to generate a true horizontal reference line and femoral head perfect circles to control for image obliquity, femoral head pathoanatomy, and image projection.² However, this technique has only been investigated with regard to the lateral center edge angle, as measured on standard anteroposterior (AP) pelvic radiographs. It is unknown if the application of this technique to false-profile views of the pelvis and the ACEA results in more reliable or accurate measurements. The aim of this study was to evaluate this measurement technique and its potential as a more reproducible method for determining the ACEA. We hypothesized that this novel method for measuring the ACEA would have better interrater and intrarater reliability compared with the traditional method for measuring the ACEA.

Methods

After obtaining exemption from institutional review board approval, a review of medical records was performed at a single academic institution. The institution's database was queried from 2014 to 2023 to establish a cohort of patients with a hip pain diagnosis using Current Procedural Terminology code M25.55. Inclusion criteria were age 14 to 60 years and the availability of full hip series of radiographs. Exclusion criteria were patients without both femoral heads visible on false-profile imaging, the presence of Tonnis grade ≥3 arthritis on AP pelvic imaging, or orthopaedic hardware in either hip.

False-profile radiographs were obtained in accordance with their initial description by Lequesne and de Seze.⁹ The image detector was placed in a vertical position, and the patient was asked to stand with the hip in question in contact with the image detector, such that the ipsilateral foot was parallel to the detector. The pelvis was rotated posteriorly 25°, such that the patient's body was then at a 65° angle to the image detector. The X-ray beam was centered between the femoral heads, and the focal distance was 110 cm. All radiographs were taken by a certified radiological technologist. Although there is minimal available information on the standardized appearance of a false-profile radiograph, we assessed the fitness of radiographs for inclusion by ensuring that the distance between the femoral heads was approximately one head diameter between each of the reference femoral heads, which has previously been described.¹⁴ Measurements were performed on the false-profile radiographs utilizing the following technique: a best-fit circle was drawn for each femoral head, a line connecting the center of both best-fit circles was drawn to serve as the horizontal reference, and a vertical ray was drawn starting at the center of the pathological hip at 90° to the horizontal reference line. The ACEA was defined as the angle subtended by the previously drawn vertical ray and a second ray projecting from the center of the femoral head to the lateral margin of the acetabular sourcil (Figure 1). Control measurements were performed utilizing the traditional method for calculating the ACEA, as described by Lequesne and de Seze.⁹ Specifically, we defined and utilized the sourcil as it has been described by Hanson et al⁷ rather than measuring the most anterior-lateral edge of bone visualized on radiographs.

Figure 1.

Anterior center edge angle measurement utilizing a horizontal reference line.

A priori power analysis was conducted, demonstrating a need for 21 patients to reach 80% power at an alpha of 0.05. There were 3 physician reviewers who independently measured the ACEA using both the traditional method (control) and the novel method (experiment). Reviewers consisted of 2 senior-level residents and 1 sports medicine fellow. After a 2-week interval, repeat measurements were taken to establish intrarater reliability. A paired t test was used to calculate the difference between traditional and novel measurements for each reviewer. The intraclass correlation coefficient (ICC) was used to determine interrater and intrarater reliability across both methods. Significance was set at P < .05.

Results

A total of 73 patients aged 14 to 60 years were identified during the index search with a diagnosis of hip pain and complete imaging. The following patients were excluded: 23 patients did not have false-profile views, 18 patients’ imaging did not capture the complete contralateral femoral head in the false-profile view, 6 patients had orthopaedic hardware in either hip, 3 patients had poor-quality imaging that prevented measurements, and 2 patients had Tonnis grade ≥3 arthritis. Overall, 21 patients met the inclusion criteria of appropriate age and complete imaging. The mean age was 33.63 ± 11.99 years, and 71.43% were female. The mean Tonnis grade was 0.33 ± 0.47. The mean ACEA value obtained using the traditional method was 39.72°± 6.73°. The mean ACEA value obtained using the novel method was 37.68°± 7.65°. A difference in measurements between the traditional and novel methods was found to reach significance once among the measurement attempts (reviewer 2: 40.5° vs 37.0°, respectively; P = .019). Mean ACEA values using both techniques are listed in Table 1.

Table 1

Measurements Using Traditional and Novel ACEA Techniques ^a

	Traditional Method	Novel Method	P
Reviewer 1
Round 1	40.1 ± 6.87	36.5 ± 9.06	.094
Round 2	40.0 ± 9.18	39.6 ± 8.46	.781
Reviewer 2
Round 1	40.5 ± 7.83	37.0 ± 7.58	.019
Round 2	40.5 ± 6.33	37.9 ± 7.81	.112
Reviewer 3
Round 1	38.1 ± 6.79	37.3 ± 7.18	.565
Round 2	39.1 ± 6.19	37.8 ± 7.34	.336

Data are shown as mean ± SD (in degrees). ACEA, anterior center edge angle. Bold indicates statistical significance.

The ICC was used to determine intrarater reliability, which was found to be strong in all instances of measurements, aside from the traditional method in reviewer 1. The ICC was likewise used to determine interrater reliability, which was found to be strong in all instances utilizing the novel method but was weak for 2 of 3 reviewers using the traditional method. Full ICC data are provided in Tables 2 and 3.

Table 2

Intrarater Reliability

	Intraclass Correlation Coefficient	Agreement
Reviewer 1
Traditional method	0.394	Weak
Novel method	0.805	Strong
Reviewer 2
Traditional method	0.705	Strong
Novel method	0.966	Strong
Reviewer 3
Traditional method	0.863	Strong
Novel method	0.951	Strong

Table 3

Interrater Reliability

	Intraclass Correlation Coefficient	Agreement
Traditional method
Reviewer 1	0.585	Weak
Reviewer 2	0.769	Strong
Reviewer 3	0.420	Weak
Novel method
Reviewer 1	0.870	Strong
Reviewer 2	0.902	Strong
Reviewer 3	0.831	Strong

Discussion

This study demonstrated that utilization of a horizontal reference line in the false-profile view when measuring the ACEA yielded more reproducible calculations, both between separate physicians and within a physician's individual measurements, compared with the traditional measurement method. Given the importance of the reproducibility of radiographic measurements in clinical decision making, conceptual understanding of hip abnormalities, and generalizability of research results, this novel method for measuring the ACEA may provide a valuable modification to measurements of this commonly used parameter.

Reproducible and accurate measurements of acetabular coverage are critical when addressing complex acetabular morphology seen with hip dysplasia and FAI. Herfkens et al⁸ demonstrated that the addition of the false-profile radiographic view in the assessment of young adult hips more than doubled the rate of accurate diagnoses of developmental dysplasia of the hip compared with the use of a single AP pelvic radiograph alone. However, studies have posited that ACEA measurements are subject to technique-related inconsistencies.²⁰ The traditionally described ACEA measurement technique involves drawing a vertical ray from the center of the femoral head parallel to the lateral margin of the radiograph, followed by an intersecting ray from the center of the femoral head to the lateral margin of the acetabular sourcil.⁹ Unfortunately, in clinical practice, perfect patient positioning is difficult to achieve, especially in the setting of abnormal lumbosacral anatomy and variable pelvic obliquity. Without establishing true vertical/horizontal lines, this method is subject to error secondary to patient anatomy/positioning and human error, as physicians are forced to estimate the vertical axis relative to the borders of the radiograph. Moreover, abnormal proximal femur anatomy can make estimations of the center of the femoral head challenging, especially in patients with cam deformities.² By establishing true horizontal and vertical references relative to the patient, we are able to obtain a more consistent 2-dimensional measurement of this complex 3-dimensional anatomy. Furthermore, as first noted by Anderson et al,² using a perfect circle technique about the femoral head helps to eliminate error when identifying the center of the femoral head, further increasing the reliability of hip measurements. It is for these reasons that we believe that there was greater interrater and intrarater reliability when using this novel method compared with the traditional method.

Accurate and reproducible measurements are a vital part of the diagnosis and treatment of FAI and hip dysplasia. Patient outcomes after the arthroscopic treatment of FAI hinge on the accuracy of preoperative measurements, as hip arthroscopic surgery in patients with hip dysplasia has been shown to have worse outcomes than in patients without hip coverage abnormalities.^13,18,22 To that end, no significant difference was found between mean ACEA measurements using the traditional method, which has been validated in vitro and in vivo, and the proposed alternative, which incorporates a horizontal reference line. The mean ACEA values of 39.72° and 37.68° for the traditional and novel methods, respectively, are in line with average values in previous studies on nondysplastic hips.^6,16,20,23 Similarly, Putnam et al¹⁵ highlighted that while changes in pelvic rotation and tilt do affect the measured ACEA, the variation in radiographic technique would have to be at an extreme (±20° of rotation in the false-profile view) to impart a clinically meaningful difference. Therefore, the findings in the present study are concordant with that described in previous literature. This ultimately suggests that establishing horizontal and vertical references does not affect the overall accuracy of ACEA measurements. However, the novel method may improve precision and reproducibility.

Advanced imaging also plays a role in the preoperative evaluation of periarticular bony morphology. Computed tomography (CT) is the most accurate modality to assess acetabular and femoral morphology.^5,11,20 However, despite the advent of low-dose CT hip protocols,¹⁹ it seems prudent to be able to effectively assess bony coverage via radiographs to allay issues with radiation exposure and health care cost, especially considering that all patients with FAI will have to undergo magnetic resonance imaging (MRI) to evaluate chondral and labral abnormalities. At a minimum, the effective interpretation of radiographs may help to determine when CT may be indicated or that more scrutiny should be given to MRI osseous findings rather than a catchall algorithm in which all patients have CT performed as part of their work-up. As time has progressed, MRI in the assessment of bony morphology of the acetabulum has been better studied.^10,17 There is now ample evidence that current MRI technology can provide sufficient detail to characterize osseous morphology and aid in preoperative decision making. Should its use become more commonplace, the requirement of additional radiation in a relatively younger patient population can be avoided while still providing a thorough diagnostic work-up.

Our study is not without limitations. First, this new method for measuring the ACEA is only able to control for patient mispositioning in the coronal plane. Thus, there may still be variability present secondary to inconsistent pelvic tilt and/or rotation that advanced imaging would demonstrate. Consistent with that variability, our interrater and intrarater reliability for the traditional method was lower than measurements provided in some previous literature while being similar or better than the measurements in other studies.^3,4,12,23 This is likely because of our rigorous methodology to prevent biased results based on previous measurements. Further, the radiographs were not taken as part of a study or work-up for FAI or hip dysplasia but rather represent those typically obtained during clinical visits for general hip pain identified with the corresponding International Classification of Diseases–10th Revision code. We attempted to control for any significant pathological atypia by only including radiographs with low Tonnis grades, but it is possible that radiographs from a population solely representative of patients with FAI may have different absolute values for the ACEA. Additionally, we utilized 3 reviewers, which likely increased the variability seen in our measurements. Likewise, we chose to measure the ACEA from the sourcil rather than the most lateral visible acetabular margin, which has historically shown worse interrater and intrarater reliability.³ In considering a thorough review of hip morphology, the assessment of acetabular coverage should include a combination of numerous radiographic parameters, whereas the focus of this study was solely the ACEA. Finally, the measurements obtained were not referenced against CT, which would strengthen the study; unfortunately, the majority of patients in the cohort did not have CT scans in our picture archiving and communication system, and thus, we were unable to perform this analysis.

Conclusion

The novel method of measurement was found to have strong agreement within each reviewer across both measurement attempts as well as between all 3 physician reviewers. The traditional method was found to have weak intrarater reliability for 1 of 3 physician reviewers and weak interrater reliability for 2 of 3 reviewers. These results demonstrate that the novel method was more reproducible across individual reviewers and between different reviewers. As such, consideration should be given to utilizing the novel method of ACEA measurements as a standardized technique in future research efforts on hip dysplasia and FAI.

Footnotes

Final revision submitted February 16, 2025; accepted April 21, 2025.

One or more of the authors has declared the following potential conflict of interest or source of funding: N.P. has received hospitality payments from Arthrex and Biotronik. S.B. has received nonconsulting fees from Arthrex, food and beverage from Wright Medical Technology, travel and lodging from Smith & Nephew, and hospitality payments from Stryker and Wright Medical Technology. J.P.S. has received travel from Arthrosurface and support for education from Liberty Surgical. 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.

Ethical approval for this study was waived, with institutional review board exemption from Jefferson Alumni Hall (iRISID-2022-0956).

ORCID iDs

Benjamin Miltenberg

William L. Johns

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The Use of a Horizontal Reference Line While Measuring the Anterior Center Edge Angle Is More Reliable Than the Traditional Method