Orthopedics

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Feature Article 

Center Edge Angle Measurement for Hip Preservation Surgery: Technique and Caveats

Lucas A. Anderson, MD; Jeremy Gililland, MD; Christopher Pelt, MD; Samuel Linford, MD; Gregory J. Stoddard, MPH; Christopher L. Peters, MD

Abstract

Anterior and lateral center edge angles have traditionally been used to determine acetabular coverage, and thereby strongly influence the decision to perform acetabular reorientation versus osteochondroplasty in patients with dysplasia and/or femoroacetabular impingement. We propose templating the center of the contained articular femoral head in aspherical hips to provide reliable assessment of acetabular coverage. Digital radiographs of 30 patients with various combinations of femoral and acetabular morphologies were evaluated using 2 methods to identify the anterior center edge angle and lateral center edge angle. The control method used an estimated femoral head center for angle apex. The study technique determined the center of the femoral head by templating the congruent aspect of the femoral head contained by the acetabulum while ignoring the increasing lateral and anterior radius associated with cam deformities. Four readers measured lateral center edge angles on anteroposterior radiographs and anterior center edge angles on false-profile radiographs. Two reads were performed by each reader using both the estimated and the templated methods for a total of 4 reads. Interobserver reliability using the proposed method compared to the standard was much improved for anterior center edge angles (intraclass correlation coefficient of 0.76 vs 0.55) as well as with lateral center edge angles (ICC of 0.80 vs 0.42). Decreased correlation was most commonly associated with abnormal sourcil morphology, posterior wall deficiency combined with calcified labra, and os acetabuli. Including the anterolateral cam deformity in identifying the center of the femoral head for measuring center edge angles leads to an underestimation of acetabular coverage, which may negatively affect hip preservation surgical decision making.

Abnormal hip morphology often affects both the femur and acetabulum and leads to a spectrum of symptoms from instability to femoroacetabular impingement.1,2 Both the development of acetabular coverage as well as the measurement thereof is influenced by femoral head morphology.3 The 3-dimensional nature of the acetabulum makes it possible for a hip’s coverage to be deficient in 1 region and over-covered in another. An example of this is the retroverted acetabulum that may have anterior overcoverage and yet have lateral and/or posterior deficiency.4-9

When assessing complex hip pathomorphology, it is paramount to determine the sufficiency of acetabular coverage. Acetabular deficiency may be found in the coronal or sagittal planes and combinations thereof. Traditional measurements of acetabular coverage include anterior and lateral center edge angles that use the estimated center of the femoral head for the apex of the measurement.8,10,11 These measurements strongly influence the decision to perform acetabular reorientation versus open or arthroscopic osteochondroplasty in patients with femoroacetabular impingement and/or acetabular deficiency. However, it is difficult to identify the center of the femoral head accurately or reproducibly in hips with aspherical heads and therefore, measured coverage may be neither valid nor consistent between readers.

We propose a variation of the Mose templates method by templating the center of the articular head contained by the acetabulum with digital computer templates and ignoring the anterolateral asphericity commonly found in complex hip dysplasia and femoroacetabular impingement.12 The purpose of this study is to: (1) demonstrate that our templated method (templated center of contained femoral head with Picture Archiving and Communication System) is superior to the traditional method (estimated center of the femoral head) in both intra- and interreader reliability, (2) demonstrate that our templated method has consistent reliability in hips with varying degrees of femoral asphericity, and (3) analyze potential errors in measuring center edge angles.

Preoperative anteroposterior (AP) and false-profile radiographs of 30 patients were collected to represent a cross-section of our young adult hip dysplasia population who either underwent surgical dislocation and osteochondroplasty or reorienting periacetabular osteotomy. Major categories…

Abstract

Anterior and lateral center edge angles have traditionally been used to determine acetabular coverage, and thereby strongly influence the decision to perform acetabular reorientation versus osteochondroplasty in patients with dysplasia and/or femoroacetabular impingement. We propose templating the center of the contained articular femoral head in aspherical hips to provide reliable assessment of acetabular coverage. Digital radiographs of 30 patients with various combinations of femoral and acetabular morphologies were evaluated using 2 methods to identify the anterior center edge angle and lateral center edge angle. The control method used an estimated femoral head center for angle apex. The study technique determined the center of the femoral head by templating the congruent aspect of the femoral head contained by the acetabulum while ignoring the increasing lateral and anterior radius associated with cam deformities. Four readers measured lateral center edge angles on anteroposterior radiographs and anterior center edge angles on false-profile radiographs. Two reads were performed by each reader using both the estimated and the templated methods for a total of 4 reads. Interobserver reliability using the proposed method compared to the standard was much improved for anterior center edge angles (intraclass correlation coefficient of 0.76 vs 0.55) as well as with lateral center edge angles (ICC of 0.80 vs 0.42). Decreased correlation was most commonly associated with abnormal sourcil morphology, posterior wall deficiency combined with calcified labra, and os acetabuli. Including the anterolateral cam deformity in identifying the center of the femoral head for measuring center edge angles leads to an underestimation of acetabular coverage, which may negatively affect hip preservation surgical decision making.

Abnormal hip morphology often affects both the femur and acetabulum and leads to a spectrum of symptoms from instability to femoroacetabular impingement.1,2 Both the development of acetabular coverage as well as the measurement thereof is influenced by femoral head morphology.3 The 3-dimensional nature of the acetabulum makes it possible for a hip’s coverage to be deficient in 1 region and over-covered in another. An example of this is the retroverted acetabulum that may have anterior overcoverage and yet have lateral and/or posterior deficiency.4-9

When assessing complex hip pathomorphology, it is paramount to determine the sufficiency of acetabular coverage. Acetabular deficiency may be found in the coronal or sagittal planes and combinations thereof. Traditional measurements of acetabular coverage include anterior and lateral center edge angles that use the estimated center of the femoral head for the apex of the measurement.8,10,11 These measurements strongly influence the decision to perform acetabular reorientation versus open or arthroscopic osteochondroplasty in patients with femoroacetabular impingement and/or acetabular deficiency. However, it is difficult to identify the center of the femoral head accurately or reproducibly in hips with aspherical heads and therefore, measured coverage may be neither valid nor consistent between readers.

We propose a variation of the Mose templates method by templating the center of the articular head contained by the acetabulum with digital computer templates and ignoring the anterolateral asphericity commonly found in complex hip dysplasia and femoroacetabular impingement.12 The purpose of this study is to: (1) demonstrate that our templated method (templated center of contained femoral head with Picture Archiving and Communication System) is superior to the traditional method (estimated center of the femoral head) in both intra- and interreader reliability, (2) demonstrate that our templated method has consistent reliability in hips with varying degrees of femoral asphericity, and (3) analyze potential errors in measuring center edge angles.

Materials and Methods

Preoperative anteroposterior (AP) and false-profile radiographs of 30 patients were collected to represent a cross-section of our young adult hip dysplasia population who either underwent surgical dislocation and osteochondroplasty or reorienting periacetabular osteotomy. Major categories of hip morphology were assessed including isolated cam femoracetabular impingement, pincer femoacetabular impingement, traditional hip dysplasia, acetabular retroversion, Perthes-like deformity, and combinations of the above.

All 30 patients had plain upright AP pelvis and false-profile hip radiographs. Anteroposterior pelvis views were obtained upright with neutral rotation of the femurs. False-profile views were obtained with the patient standing, and the patient rotated 30° posterior oblique with the affected hip against the radiograph cassette.13

Alpha angles were measured to assess the degree of femoral head asphericity on both AP and false-profile radiographs.14-16 We hypothesized that accuracy of center edge angle measurements would be reduced using the estimated technique but maintained using the study or templated technique in hips with greater asphericity (alpha angles >50°) compared to hips with relatively normal sphericity (alpha angles <50°).10,16,17 With the proposed templated method, the center of the femoral head was determined with a spherical template on digital radiographs (Picture Archiving and Communication System) by placing the radius of the template congruent with the aspect of the head contained by the acetabulum while ignoring the increasing lateral and anterior radius associated with cam-type impingement deformities (Figure 1). Anteroposterior radiographs were corrected for leg-length inequality or obliquity by determining the vertical bases on a plane perpendicular to a line through the ischial tuberosities, tear drops, or inferior border of the obturator foramina depending on which was more symmetric and assessable (Figure 1). The center edge angle was formed by the intersection of the vertical line (corrected for obliquity) through the center of the femoral head with the line extending to the lateral edge of the sourcil (radiographic eye brow of the weight-bearing region or roof of the acetabulum).12,18,19

Figure 1: The method for adjusting for pelvic obliquity/leg length
Figure 1: AP pelvis radiograph demonstrating the method for adjusting for pelvic obliquity/leg length equality when measuring lateral center edge angles. Lateral center edge angle (right hip) is the angle formed by a line perpendicular to the tilt of the pelvis and through the center of the spherical femoral head (bold right-angled line through the inferior aspect of the obturator foramina and the center of the femoral head) and a line from the center of the femoral head to the lateral aspect of the congruent sourcil (medial to the calcified labra and the up-sloping sourcil and even with the posterior wall).

Readers 1 and 4 performed 2 reads 1 week apart on 30 sets of radiographs with the traditional estimated method first, followed by 2 additional reads 1 week apart using the proposed templated method. Each read was blinded, randomized, and performed 1 week prior to minimize reader familiarity. Readers 2 and 3 performed 2 reads each with the templated method first and estimated method second. The differing order for reads using estimated and templated methods between readers 1 and 4 versus readers 2 and 3 were designed to control the potential for falsely rejecting the null hypothesis based on improving skill or familiarity of readers as well as falsely rejecting the hypothesis based on reader fatigue.

Additionally, one reader measured anterior center edge angle and lateral center edge angle on false-profile and AP radiographs respectively with computer templates contoured to the anterior and lateral asphericity as well as the superior head and once as previously described, ignoring the asphericity and including the contained articular head.

The 5 radiographic sets with the greatest standard deviation for anterior center edge angles and lateral center edge angles (worst reliability) for the 16 reads (4 readers×2 reads×2 methods) using both estimated and templated techniques were analyzed further for common errors and inconsistencies of technique between readers.

Intra- and interrater reliabilities were computed as the intraclass correlation coefficient derived from mixed effects linear regression models. To compare the intraclass correlation coefficient for the new technique with the intraclass correlation coefficient for the old technique, a bootstrapped normal approximation 1-sample test for the intraclass correlation coefficient difference was used. A bias-corrected bootstrapped 95% confidence interval was provided for the intraclass correlation coefficient difference. A 2-sided Student’s t test was used for comparing measured angles between techniques.

Results

Interobserver reliability was significantly improved with the templated technique when compared to the estimated technique with an intraclass correlation coefficient of 0.76 versus 0.55 (P<.001) for measuring anterior center edge angles and 0.80 versus 0.42 (P=.001) when measuring lateral center edge angles (Table 1). Our study did not demonstrate a statistically significant difference in intraobserver reliability between the estimated and templated techniques with either anterior center edge angles and lateral center edge angles (Table 2). This indicates that individual reader precision was not improved with the templated technique. However, this lack of statistical difference in intraobserver reliability between techniques does not indicate that both yield equally valid measurements of coverage.

Table 1: Interreader Reliability

Table 2: Intrareader Reliability

Alpha angles were used to categorize the alpha angle for intraobserver reliability. The mean of the 4 readers for their first AP read and their first false-profile read was computed. The intra and interobserver reliability of center edge angle measurements was examined with alpha angles <50°, between 50° and 70°, and those >70°. The interobserver reliability degraded with increasing alpha angles (more aspherical hips) with both techniques. Better reliability was associated with the templated technique (Table 3).

Table 3: Interreader Reliability of Templated Technique by Alpha Angle Category

Measuring with a template that includes the femoral asphericity yielded lateral center edge and anterior center edge angles that averaged <2.7° and 1.9° respectively when the template did not include the anterolateral cam lesion (SD 2.4° and 2.0°; P<.00001 and P<.0001). When there was minimal to moderate asphericity (alpha angles of <70°), the technique that included the asphericity averaged 0.55° less for lateral center edge angles and 1.77° less for anterior center edge angles compared to measurements based on the spherical contained head while ignoring the lateral asphericity.

When measuring radiographs with severely aspherical heads (alpha angles >70°), the templated technique that included the asphericity significantly underestimated the lateral center edge angle (3.9° less than the templated method that ignored the asphericity, P=.00002). While there was a trend towards underestimating anterior center edge angles with the templating technique that included the asphericity, this was not significant (3° less than the templated method that ignored the asphericity, P>.05). This may be due to the more superior (superolateral) asphericity of cam deformities on the AP and more inferior (anteroinferior) asphericity on false-profile projections. Translation of the head center in the lateral plane reduces measured center edge angles more than inferior translation. When the deformity is low, the mistemplated center of the head is translated inferior more than lateral (anterior in the case of the false-profile) whereas when deformity is high on the head, the mistemplated center of the head is translated mostly in the lateral plane.

The 5 radiographic sets with the greatest standard deviation (worst reliability) for measured anterior center edge angles and lateral center edge angles were analyzed further for common errors and inconsistencies of technique between readers. Standard deviation ranged from 3.9° to 12.5°. Of the 5 radiographic sets associated with the worst reliability between reads, 4 were common to both techniques. This indicates that there were characteristics of the radiographs that made consistency in measurement of center edge angle (center edge angle) difficult despite templates. Four of the 5 radiographic sets had retroversion with a cross-over sign4 (3 of which also had a positive posterior wall sign20 indicating posterior wall deficiency). One had an os acetabuli, 1 had a cyst in the lateral acetabular rim, and 1 had a calcified labrum that was continuous with the sourcil and thus, difficult to determine the end of the sourcil and the beginning of the labrum. Average alpha angle for these 5 sets was greater but not statistically significant, measuring 78.8° on the AP radiograph and 67° on the false-profile radiograph compared to 65° and 58° respectively for all sets (P=.32 and .49).

Discussion

Wiberg12 first described the center edge angle as a measurement of acetabular coverage using transparent spherical templates, Mose circles, with angles in 5° increments from 0° to 45°. He stated, “In a normal hip joint and in the joints with a maldeveloped acetabular roof, the femoral head is always spherical enough to enable practically exact determination of its center.”12 However, we find that in aspherical femoral heads, it is easy to lateralize the hips’ centers of rotation by including the anterolateral cam deformity in the spherical template, thus underestimating the center edge angle.

Additionally, Wiberg12 noted that the “lateral acetabular border” at the lateral margin of the “dense zone of acetabular roof” (sourcil) should be considered the “point where the curving of the acetabular border laterosuperiorly begins, ie, where the bony support may be considered to end.” Ogata et al21 demonstrated that in pediatric hips, acetabular coverage measured on computed tomography (CT) more reliably correlated with lateral center edge angles measured to the lateral border of the sourcil rather than the lateral rim. Acetabular rim extending beyond the sourcil does not contribute to load sharing and therefore, they felt it should not be used to determine the lateral margin of center edge angles. Sakai et al22 compared measuring to the rim versus to the anterior sourcil on radiographs to coverage determined by 3-dimensional CT. They found that normal hips measuring to the rim correlated best with coverage, whereas with dysplastic hips, measuring to the sourcil correlated best and the rim overestimated coverage. Of note, Omeroglu et al23,24 found larger variation when measuring lateral center edge angles in pediatric hips due to the sourcil’s less defined margin compared to the lateral rim.

The clinical usage of Wiberg’s center edge angle has evolved to differentiate not only between normal and deficient acetabuli but also overcovered acetabuli. The left center edge angle measured on the AP radiograph may distinguish between acetabular insufficiency (<20°; 21°-24°=borderline)10,25-27 versus lateral acetabular over-coverage (>40°)28,29 on the other extreme. Additionally, measuring Wiberg’s angle on the false-profile radiograph13,30 has been described as the anterior center edge angle19 and can diagnose anterior undercoverage (<20°)31 or overcoverage (>40°), resulting from acetabular retroversion, or either global or focal anterior acetabular overcoverage.8,32

Valid and reproducible measurements of acetabular coverage in both coronal and sagittal planes are necessary to direct surgical approaches tailored to correct the complex acetabular pathomorphologies found in this varied population. While a symptomatic hip with cam impingement and a measured lateral center edge angle of 18° would typically be addressed by an anterior osteochondroplasty and periacetabular osteotomy, an anteverted acetabulum with a lateral center edge angle of >20° might undergo isolated osteochondroplasty via arthroscopy or surgical dislocation. However, it is difficult to consistently and accurately measure the center edge angles of hips with cam femoroacetabular impingement as it is difficult to determine the center of the aspherical femoral heads. This is supported by our finding of an incremental decrease in intra- and interreader reliability when measuring center edge angle in hips with increasing alpha angles. The center of an aspherical hip can be estimated more lateral (AP radiographs) or anterior (false-profile radiographs) due to the typical anterior and lateral asphericity associated with cam impingement, which tends to underestimate the measured center edge angles (Figure 2). Based on these observations, we propose a method using a computer template of the contour of the articular head contained by the acetabulum to determine the center of the femoral head for measuring the center edge angles. This method measures the acetabular coverage that will result with anticipated complete correction of the femoral cam deformity, ignoring the aspherical peripheral head. We have found that shifting the estimated head’s center lateral even 3 mm can change the measured center edge angle by 6°. In a hip with combined mild acetabular insufficiency and femoral asphericity, this underestimation of coverage could influence the decision to perform acetabular reorientation as opposed to osteochondroplasty alone.

Figure 2: Estimated versus templated femoral head center
Figure 2: AP hip radiographs demonstrating estimated versus templated femoral head center used in determining lateral center edge angle, a measurement of lateral acetabular coverage. When measuring center edge angle with the aspherical heads associated with cam femoroacetabular impingement, it is common to lateralize the estimated head center and thus underestimate coverage.

Several studies have looked at the reliability of measuring center edge angles. Nelitz et al33 found a standard deviation of 7.8° for lateral center edge angles in children and adolescents with excellent inter- and intraobserver reliability, 0.85 to 0.88 and 0.88 to 0.92 respectively. Omeroglu et al34 found the average intra- and interobserver differences for center edge angle to be 3.1° and 4.0° for radiographs of 33 children. Broughton et al35 found the average intra- and interobserver differences for center edge angle to be 9.3° and 9.1° for radiographs of 170 children over 5 years. We found interobserver reliability to be improved using the templated method for measuring both anterior center edge angles (ICC of 0.76 versus 0.55) and lateral center edge angles (ICC of 0.80 versus 0.42) (Table 1).

When we analyzed the 5 hips with the worst intra- and interreader reliability, 1 or more of the following were associated: abnormal sourcil morphology, os acetabuli, calcified labra, and retroverted acetabuli. This is consistent with our clinical experience and has led us to consider the endpoint of weight-bearing anatomy in these cases. In the case of the upsloping terminal rim, we measured the coverage at the point that the subchondral bone of the acetabulum deviated from the contour of the sourcil (Figure 3A).

Figure 3A: Hips with acetabular retroversion Figure 3B: Hips with acetabular retroversion
Figure 3: AP (A) and false-profile (B) radiographs showing hips with acetabular retroversion demonstrated by bilateral caudal crossover signs. Note the sclerotic sourcil extending to the lateral rim and confluous with the calcified labrum on both the AP and false-profile radiographs. Lateral center edge angle on the AP radiograph should be measured to the lateral sourcil where it meets the posterior wall but not to the calcified labrum, focal retroversion, or anterolateral pincer extension. The anterior center edge angle measured on the false-profile radiograph should be to the end of the congruent sourcil while disregarding the calcified labrum and noncongruent extension of the anterior roof that drifts away from the femoral head and offers no weight-bearing coverage.

During surgical planning, we measure center edge angles to the medial aspect of both calcified labra and os acetabuli if we are planning on trimming to determine if excision will lead to acetabular undercoverage postoperatively (Figures 3B, 4). In anteverted or neutral hips, the posterior wall terminates at the lateral rim while the anterior wall terminates medial to or at the lateral rim (Figures 3A, 5A). The opposite is true in a retroverted acetabulum with a cross-over sign4,8,9,36 (anterior wall crossing lateral to the posterior wall before terminating at the superolateral rim), in that the posterior wall terminates more medial to the lateral rim. When we perform a focal cranial rim trimming of a retroverted acetabulum with a positive cross-over sign, the lateral ray of the lateral center edge angle is measured to where the posterior wall meets the lateral border of the sourcil and not to the lateral rim (Figure 5). In essence, we attempt to measure the lateral acetabular coverage (lateral center edge angle) that will exist after a rim resection to determine whether a rim resection or acetabular repositioning osteotomy is indicated.

Figure 4A: Templated femoral head center for measuring anterior center edge angle Figure 4B: The incorrect inclusion of the calcified anterior labrum
Figure 4: False-profile radiographs demonstrating templated femoral head center for measuring anterior center edge angle, a measurement of anterior acetabular coverage. The correct method of anterior center edge angle measurement including only the sourcil (sclerotic eyebrow) (A). The incorrect inclusion of the calcified anterior labrum (B).

Figure 5A: A cropped AP pelvis radiograph of a retroverted acetabulum Figure 5B: A cropped AP pelvis radiograph Figure 5C: Technique for measuring lateral center edge angle
Figure 5: A cropped AP pelvis radiograph of a retroverted acetabulum (A) demonstrating a sourcil ending medial to the lateral acetabular rim extension. A cropped AP pelvis radiograph (B) demonstrating the sourcil ending medial to the lateral acetabular rim extension on both the right and left retroverted acetabuli. A drawing of Figure 5B (C) demonstrating the technique for measuring lateral center edge angle in retroverted acetabuli with the sourcil confluent with sclerosis of the lateral rim; the lateral ray should meet the sourcil at the junction of the posterior wall and the lateral sourcil. Measured lateral center edge angles were 27° and 37° degrees respectively for coverage measured to sourcil and lateral rim, respectively.

One of the limitations of our method is measuring center edge angle in Perthes and Perthes-like hips. Other methods for determining femoral head coverage have attempted to adjust for femoral head deformity such as the extrusion index,37 the acetabular head quotient (acetabular coverage divided by the head diameter on the AP radiograph),38 and the anterior acetabular head index30 (same ratio on false-profile radiograph). However, these techniques use the horizontal width of the femoral head and therefore, are similarly vulnerable to underestimation of acetabular coverage.

A second potential limitation of our study was the limited sample size. However, we believe our radiographic sets included a satisfactory range of acetabular morphologies to adequately evaluate our hypotheses. Additionally, our statistician (G.J.S.) warned in planning our methods that 4 reads of >30 radiographic sets would more likely lead to fatigue error.

We believe templating the center of the spherical articular femoral head is essential in radiographic assessment of acetabular coverage in hips with complex combinations of femoroacetabular impingement and acetabular deficiency. Without this technique, it is easy to lateralize the center of the head based on the asphericity associated with cam-type impingement, and therefore underestimate acetabular coverage. Continued study to better understand how 2-dimensional radiographic imaging correlates with 3-dimensional advanced imaging and how both correspond to actual anatomic acetabular coverage and morphology is necessary.

References

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Authors

Drs Anderson, Gililland, Pelt, Linford, and Peters and Mr Stoddard are from the Department of Orthopedic Surgery, University of Utah School of Medicine, Salt Lake City, Utah.

Drs Anderson, Gililland, Pelt, Linford, and Peters and Mr Stoddard have no relevant financial relationships to disclose.

Correspondence should be addressed to: Christopher L. Peters, MD, Department of Orthopaedic Surgery, University of Utah School of Medicine, 590 Wakara Way, Salt Lake City, UT 84108 (chris.peters@hsc.utah.edu).

doi: 10.3928/01477447-20101221-17

10.3928/01477447-20101221-17

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