DOI:10.2214/AJR.07.3258
AJR 2008; 190:1260-1262
© American Roentgen Ray Society
Femoroacetabular Impingement: Can the Alpha Angle Be Estimated?
Mohamed R. Nouh1,2,
Mark E. Schweitzer1,
Leon Rybak1 and
Jodi Cohen1
1 Department of Radiology, Hospital for Joint Disease, New York University, New
York, NY.
2 Present address: Department of Radiodiagnosis and Diagnostic Imaging, Faculty
of Medicine, Alexandria University, 1 Kolya-El Teb St., Mahata El-Ramel,
Alexandria, Egypt.
Received October 4, 2007;
accepted after revision November 12, 2007.
Address correspondence to M. R. Nouh
(mragab73{at}yahoo.com).
Abstract
OBJECTIVE. Femoroacetabular impingement is an important entity with
well-described radiographic findings. One of the criteria of the cam type of
femoroacetabular impingement is femoral head–neck dysplasia, denoted
mathematically as the "alpha angle." Several observers have
reported that direct measurement of the angle may not be necessary because
subjective appraisal may yield similar results. We sought to scientifically
determine the accuracy of a subjective assessment, using the calculated angle
as the gold standard.
MATERIALS AND METHODS. At 1.5 T, 50 consecutive patients' hips were
evaluated on sets of oblique axial images. Two musculoskeletal radiologists
recorded their subjective opinion as to the alpha angle using a confidence
scale of 1–5. Direct mathematic measurement of the alpha angle was done
by a third independent observer and correlated with the subjective results.
Correlations between the subjective and measured angles and interobserver
variation were calculated.
RESULTS. Statistically, significant variability was seen in the
subjective assessment of the alpha angle. When the alpha angle was >
55°, the area under the receiver operating characteristic curve (AUC) was
0.606, indicating that visual assessment is a poor predictor of a wide alpha
angle. Even in patients with a measured normal alpha angle (< 55°),
slightly fewer than half were subjectively thought to possibly, likely, or
definitely have abnormal angles. Similarly, more than half of the abnormal
cases (alpha angles > 55°) were subjectively thought to possibly or
probably be normal.
CONCLUSION. Subjective assessment of alpha angles is suboptimal
unless one is quite confident of a bone abnormality.
Keywords: alpha angle • femoroacetabular impingement • hip joint
Introduction
Femoroacetabular impingement is a major cause of early osteoarthritis of
the hip
[1–3].
It is characterized by a repetitive abnormal contact between skeletal
prominences of the anterosuperior femoral head–neck junction and the
acetabular rim at the extreme of range of motion
[1–4].
Although many if not most cases are mixed, two classic types of
femoroacetabular impingement are described. Pincer impingement is direct
linear contact between the femoral neck and an overcovered acetabulum. Cam
impingement is the result of contact between a nonspherical femoral
head–neck junction and the acetabulum
[1–8].
This dysmorphic bump is thought to cause mechanical impingement of the
acetabular rim during hip flexion, which results initially in abrasion and
eventually in tearing of the anterosuperior acetabular labrum
[1–8].
Several imaging findings have been described for diagnosing the cam type of
femoroacetabular impingement
[1,
3,
4,
8]. One objective method is the
measurement of the head–neck relationship on CT and MR scans using the
"alpha angle" as described by Notzli et al. in 2002
[6]. Because several authors
[9] have suggested that direct
measurement of this angle may not be necessary, our aim was to compare the
subjective assessment of femoral neck–acetabular relationship with the
absolute measurement.
Materials and Methods
Patients
Fifty consecutive patients who were being evalu ated for hip pain of any
cause were examined at 1.5 T. Although multiple planes and sequences were
available, only the oblique axial images were used for this assessment of the
alpha angle. Institutional review board approval was obtained.
MR Image Interpretation
Initially, one independent observer performed the direct measurement of the
alpha angle using an oblique axial image delineating the narrowest portion of
the femoral neck. Along with the femoral head contour, the angle was obtained
by placing a circle around the oblique axial circumference of the femoral
head, placing a line in the center of the femoral neck along its longitudinal
axis, and placing a second line that extends from the intersection of the
first line and the center of the femoral head to the point where the osseous
anterior femoral head intersects the circle
[6]
(Fig. 1). The same observer
also provided a second assessment of the alpha angle in 20 patients; these
data were used only to assess intrareader variation in the measurement of the
alpha angle.
At separate sittings, two independent musculoskeletal radiologists with 3
and 6 years of subspecialty experience, respectively, used a subjective
approach to estimate the alpha angle on axial oblique images, using a 5-point
scoring system in which 1 indicates definitely normal; 2, probably normal; 3,
possibly abnormal; 4, probably abnormal; and 5, definitely abnormal. Because
our aim was to evaluate the accuracy of a subjective assessment of the alpha
angle in diagnosed cases of femoroacetabular impingement, we omitted
evaluating any other associated finding such as labral abnormalities and
femoral torsion.
Statistical Analysis
Receiver operating characteristic (ROC) curve analysis was used to assess
the diagnostic usefulness of the ordinal femoroacetabular impingement scores
for the detection of patients having an alpha angle 
55°
(Fig. 1). Cohen's kappa
statistic was used to assess interreader agreement in terms of the ordinal
scores, and intrareader variation in alpha angle was assessed using a
Spearman's rank correlation and 95% limits of agreement.
Results
Table 1 shows the
significant variability in the subjective assessment of the alpha angle. Even
in patients with a measured normal alpha angle (< 55°)
(Fig. 1), slightly fewer than
half were subjectively thought to possibly, probably, or definitely have
abnormal angles.
Similarly, in abnormal alpha angles 55°), more than half of the
cases were subjectively thought to possibly or probably be normal
(Fig. 2).
View larger version (144K):
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Fig. 2 —Axial oblique fat-saturated T2-weighted image in 35-year-old
woman with femoroacetabular impingement. Alpha angle is estimated to be
60.5° with a confidence score of 4, probably abnormal, by first reader and
a score of 2, probably normal, by second reader.
|
|
Table 2 shows the
specificity and sensitivity for the 5-point scores provided by both readers
for subjective evaluation of femoroacetabular impingement.
View this table:
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TABLE 2: Specificity and Sensitivity of 5-Point Scores Given by Both Readers for
Subjective Evaluation of Femoroacetabular Impingement
|
|
Figure 3 displays the ROC
curve for visual scoring of the neck shaft angle, which was averaged over both
readers using an alpha angle of 
55° as the normal standard. The area
under the ROC curve (AUC) was 0.606. Because the AUC was not significantly
higher than 0.5 (p = 0.273), the data indicate that visual assessment
is a poor predictor of a large alpha angle.
Figure 3 displays the AUC of
both readers. The subjective evaluation of both readers for the alpha angle
was not statistically significant (p = 0.518). Therefore, experience
does not seem to play a major role in the visual assessment of alpha
angles.
Figure 4 shows that repeated
assessment of alpha angles by the same observer plotted a wide range of
responses around the line of equality. This shows some potential inconsistency
of this measurement.
Discussion
Femoroacetabular impingement is a common condition of the hip joint in
which repetitive contact between skeletal prominences of the acetabulum and
the femoral head–neck junction leads to premature degenerative changes
[1–8].
Cam impingement results from an abnormal morphology of the anterior femoral
head–neck junction in which variations in the anatomy of the proximal
femur appear to lead to labral impingement. The identification of the abnormal
head–neck junction is critical in treating patients with cam-type
impingement. If only the labral and cartilage abnormalities are identified and
treated, the underlying cause of impingement may remain present, possibly
resulting in persistent pain and leading to further cartilage and labral
damage [2,
7,
8].
At imaging, the abnormal morphology of the anterior femoral head–neck
junction is best evaluated on the axial oblique plane. The alpha angle
measurement is a commonly used method for this assessment. The alpha angle was
first described by Notzli et al.
[6] in 2002 as an objective
tool to evaluate the femoral head–neck junction. Angles greater than
55° were thought to be associated with femoroacetabular impingement, and
these higher values are believed to be associated with cam-type impingement
[6].
Measurements are tedious. Consequently, in practice many radiologists
"eyeball" or estimate angles, potentially including the alpha
angle. We sought to evaluate on a limited basis whether direct measurement of
the alpha angle is necessary. We found that subjective evaluation of this
angle is not accurate and experience does not seem to play a role, although
both observers were only moderately experienced in skeletal imaging, mimicking
the experience level of practicing radiologists.
Similar work has been done to assess other angular measurements in
musculoskeletal imaging. Knight et al.
[10] assessed the possibility
of estimating the Boehler angle and the critical Gissane angle in assessing
the presence of calcaneal fractures and used CT as the gold standard. They
found excellent interobserver agreement for visual assessment of the Boehler
angle but suboptimal subjective evaluation of the critical Gissane angle.
We acknowledge that our study is limited by lack of correlation with
clinical and surgical data, because we did not confirm whether the patients
had complained of impingement symptoms or had arthroscopic evidence of it. We
only sought to determine whether the alpha angle could be estimated. The study
is also limited by the potential bias of the readers, who may have noted other
signs of femoroacetabular impingement as well. We conclude that the alpha
angle has a limited ability to be subjectively assessed.
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Abstract
Introduction
–b line). Alpha angle is drawn from transition of head into neck
where neck radius exceeds head radius (
