The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 92, Issue 14

Scientific Articles | October 20, 2010

Prevalence of Cam-Type Femoroacetabular Impingement Morphology in Asymptomatic Volunteers

Kalesha Hack, MD¹; Gina Di Primio, MD, FRCPC¹; Kawan Rakhra, MD, FRCPC¹; Paul E. Beaulé, MD, FRCSC¹

¹ Departments of Diagnostic Imaging (K.H., G.D.P., and K.R.) and Adult Reconstruction (P.E.B.), The Ottawa Hospital—General Campus, 501 Smyth Road, Ottawa, ON K1H 8L6, Canada. E-mail address for P.E. Beaulé: pbeaule@ottawahospital.on.ca

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Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from the Ottawa Hospital Orthopaedic Discovery Fund. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.

Investigation performed at the Division of Orthopedic Surgery, The Ottawa Hospital, Ottawa, Ontario, Canada

J Bone Joint Surg Am, 2010 Oct 20;92(14):2436-2444. doi: 10.2106/JBJS.J.01280

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Abstract

Background:

Femoroacetabular impingement is a cause of hip pain in adults and is a possible precursor of osteoarthritis, with the cam type of impingement being the most common. The purpose of this study was to determine the prevalence of cam-type morphology of the hip in asymptomatic patients.

Methods:

Two hundred asymptomatic volunteers with no prior hip surgery or childhood hip problems underwent magnetic resonance imaging targeted to both hips. The subjects were examined at the time of magnetic resonance imaging for internal rotation of the hips at 90° of hip flexion and for a positive impingement sign. The contour of the femoral head-neck junction was quantified with use of the alpha angle. A value of >50.5° was considered positive for cam morphology. Measurements were performed independently by two musculoskeletal radiologists.

Results:

The mean age of the individuals was 29.4 years (range, 21.4 to 50.6 years); 79% were white, and 55.5% were women. The mean alpha angle anteriorly at the three o'clock position was 40.9° ± 7.0° on the right and 40.6° ± 7.1° on the left, whereas the mean alpha angle anterosuperiorly at the 1:30 position was 50.2° ± 8.0° on the right and 50.1° ± 8.3° on the left. Fourteen percent of the volunteers had at least one hip with cam morphology: 10.5% had an elevated alpha angle on either the right or the left side, and 3.5% had the deformity in both hips. Seventy-nine percent (twenty-two) of twenty-eight individuals who had an elevated alpha angle were men, and 21% (six) were women. Individuals with an elevated alpha angle on at least one side tended to be male (p < 0.001), with 24.7% (twenty-two) of eighty-nine men having cam morphology compared with only 5.4% (six) of 111 women.

Conclusions:

The prevalence of cam-type femoroacetabular impingement deformity is higher in men as well as in individuals with decreased internal rotation. Defining what represents a normal head-neck contour is important for establishing treatment strategies in patients presenting with prearthritic hip pain.

Figures in this Article

Introduction

Osteoarthritis of the hip is a debilitating and painful condition affecting a substantial proportion of the population. Idiopathic osteoarthritis accounts for approximately 40% to 50% of all cases of degenerative arthritis^1-3, and a growing body of literature supports the hypothesis that idiopathic osteoarthritis is frequently caused by subtle, and often radiographically occult, abnormalities at the femoral head-neck junction or acetabulum that result in abnormal contact between the femur and the acetabulum^4-9. This condition, known as femoroacetabular impingement (Fig. 1), has been proposed as the leading cause of labral tears, as well as degenerative hip arthritis. The structural abnormalities associated with femoroacetabular impingement were noted several decades ago^1,3,10, when abnormalities of the femoral head-neck contour were referred to as tilt or pistol grip deformities, which Ganz et al.¹¹ classified as cam morphology. In addition, Ganz et al. identified another mechanism of femoroacetabular impingement known as pincer (Fig. 1), which is secondary to acetabular overcoverage of the femoral head and is present in a third of the patients with a cam deformity^11,12.

Through observations at the time of open hip surgery, Ganz et al. defined the mechanisms of cartilage damage associated with femoroacetabular impingement as well as its clinical and radiographic features^8,11. However, with any new diagnosis, it is important that the diagnostic criteria be accurate and reproducible in order to properly assess the efficacy of different treatments as well as the natural course of the disease. More specifically, in the diagnosis of cam-type femoroacetabular impingement, specialized lateral radiographic projections of the hip are required^13,14 in order to best visualize the deformity. Magnetic resonance imaging is particularly suited for this purpose because of its ability to image and reconstruct the image in multiple planes^6,12,15,16. Consequently, with the increasing use of magnetic resonance imaging in the diagnosis of femoroacetabular impingement, quantifying the normal head-neck contour becomes very important in defining thresholds and ranges of normality.

The primary aim of this study was to gather normative data for the contour of the femoral head-neck junction in a large cohort of asymptomatic individuals with use of noncontrast magnetic resonance imaging. Our secondary aim was to examine the prevalence of cam morphology in a population of patients without hip pain or preexisting hip disease.

Fig. 1

Diagrammatic illustration of femoroacetabular impingement³⁶. A: A normal joint. B: Cam morphology with an insufficient femoral neck offset. C: Pincer morphology with acetabular overcoverage. D: A combined deformity. (Reprinted, with permission, from: Lavigne M, Parvizi J, Beck M, Siebenrock KA, Ganz R, Leunig M. Anterior femoroacetabular impingement: part I. Techniques of joint preserving surgery. Clin Orthop Relat Res. 2004;418:61-6.)

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Materials and Methods

This study was approved by our institutional review board, and all participants provided informed consent. Participants were recruited by means of a standardized letter that was circulated electronically within the hospital and medical school at the institution where the research was carried out. The hospital where the recruitment was done employs over 13,000 individuals with a varied work force such as maintenance, housekeeping, and skilled trades as well as the allied health professions. The letter included background information about femoroacetabular impingement and clearly outlined the study objectives as well as the inclusion criteria: an age of less than fifty-one years, no history of ongoing hip or groin pain¹⁷, no prior hip surgery or posttraumatic deformity, no arthritis in any joint, and no contraindication to magnetic resonance imaging. Participation in the study was voluntary, and no compensation was offered.

We recruited 200 asymptomatic volunteers providing a total of 400 hips. With an expected prevalence of 15%¹⁸, this would allow us to estimate a 95% confidence interval with a margin of error of 5%. Since the flyer specifically mentioned the presence or history of hip pain as an exclusion criterion, only six patients who responded to the letter were excluded: five had ongoing hip and/or groin pain, and one had a history of a childhood hip disorder.

All patients underwent physical examination of the hips by one physician at the time of the magnetic resonance imaging. The examination was done with the patient in the supine position, and internal and external rotation with the hip flexed at 90° were assessed with use of a goniometer⁶. An impingement test was also performed with the patient in the supine position: the hip was rotated internally as it was flexed passively to approximately 90° and adducted. This maneuver results in approximation of the femoral neck and acetabulum and results in pain among patients with damage to the acetabular rim¹⁹. Although often positive in patients with femoroacetabular impingement²⁰, the specificity and sensitivity of the impingement test in the diagnosis of femoroacetabular impingement is not known.

The magnetic resonance imaging examinations were carried out on a 1.5-T scanner (Symphony Quantum; Siemens, Erlangen, Germany) with a phased-array surface coil placed anteriorly over the pelvis and with spine phased-array coils placed posteriorly. Subjects were supine with the feet held in the neutral position. The magnetic resonance imaging sequence was a three-dimensional isotropic, T1-weighted spoiled gradient echo (MP-RAGE/TurboFLASH [magnetization-prepared rapid acquired gradient-echo sequence/fast low-angle shot sequence with preceding inversion pulse]; 38.4-cm field of view; 1-mm slice thickness; 384 × 384 matrix; 1250-ms repetition time; 2.44-ms echo time; 15° flip angle; and one average). Multiplanar reformation for each hip was carried out to generate 2-mm-thick oblique axial and oblique sagittal plane images, parallel and perpendicular to, respectively, the long axis of the femoral neck. The latter plane was used to prescribe the radial multiplanar reformation, with use of the center of the femoral neck as the axis of rotation, with 2-mm-thick images generated at 15° intervals (Fig. 2). Acquisition and multiplanar reformation images were sent to PACS (picture archiving and communication system) software (Horizon Rad Station 3.3; McKesson, San Francisco, California) for review. Due to the large field of view used to assess both hips in one single, axially acquired sequence, the resolution did not allow for optimal visualization of the labrum or cartilage lesions.

Fig. 2

T1-weighted axial oblique spoiled gradient echo magnetic resonance image demonstrating measurement of the alpha angle anteriorly at the three o'clock position (arrow).

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The alpha angle was selected as a means of evaluating for cam morphology as it is the current standard for quantifying femoral head-neck concavity with high intraobserver and interobserver agreement^6,21. Alpha angles were measured with use of the previously published method by Nötzli et al.⁶. A best-fit circle was drawn around the perimeter of the femoral head. The first arm of the angle was the long axis of the femoral neck, defined as a line drawn between the center of the femoral neck at its narrowest point and the center of the circle. The second arm of the angle was drawn from the center of the circle, anteriorly to the point where the femoral head extended beyond the margin of the circle (Figs. 3-A, 3-B, and 3-C). An alpha angle of >50.5° was considered elevated and thus positive for cam-type morphology^6,14,22,23.

Fig. 3-A Fig. 3-B

Figs. 3-A, 3-B, and 3-C T1-weighted spoiled gradient echo magnetic resonance images. Fig. 3-A Radial image demonstrates that the axis of rotation is the center of the femoral neck (asterisk) as demonstrated on an oblique sagittal image showing the femoral neck in cross section along its short axis (dashed line in inset). Fig. 3-B The localizer image demonstrates superimposed radial reference lines at 15° intervals. A clock-face nomenclature was implemented with the superior femoral head-neck junction denoted as twelve o'clock. Subsequent rotation of the axis (large arrow) generated images around the circumference of the femoral neck going from superior to anterior, from twelve o'clock to three o'clock.

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Fig. 3-C

Radial image demonstrating the femoral head-neck contour (asterisk) at the 1:30 vector²⁴.

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On the basis of the previous work of Nötzli et al.⁶, Pfirrmann et al.¹⁵, and Rakhra et al.²⁴, specific images were selected for measuring the alpha angle. A clock-face nomenclature was adopted for localization around the femoral head-neck junction with the anterior and superior locations designated three o'clock and twelve o'clock, respectively. From the oblique axial series, the image through the middle of the neck was chosen to optimally demonstrate the anteriormost region of the femoral head-neck junction (three o'clock). From the radial multiplanar reformation series, the image corresponding to the 1:30 position was used to evaluate the anterosuperior region of the femoral head-neck junction. This location was chosen since previous studies involving radial magnetic resonance imaging of the hip have found that the greatest degree of contour deformity, and thus the largest alpha angle value, occurs anterosuperiorly rather than anteriorly^15,24,25. In addition, as the oblique axial plane extends superiorly, the surface of the head-neck junction is no longer orthogonal to the imaging plane because of the curvature of the femoral neck. As a result, the oblique axial plane underestimates and may not even demonstrate the deformity. In contrast, radial images are based on a rotating plane such that each image is orthogonal to the femoral surface and depicts the head-neck junction in profile²⁴.

Two fellowship-trained musculoskeletal radiologists each independently measured alpha angles for 100 patients (200 hips). To determine interrater reliability, each radiologist also measured the alpha angle for another sixty patients (120 hips). The intraclass coefficient was 0.810 on the axial view and 0.796 on the radial view.

Source of Funding

There was no external source of funding for this study.

Results

The mean age of the 200 subjects in the study group was 29.4 years (range, 21.4 to 50.6 years), and 79% (158) were white. Fifty-six percent (111) were women, and 8% (sixteen individuals) reported a family history of a hip replacement in a parent, sibling, or grandparent. The mean body-mass index (and standard deviation) for men and women was 25.49 ± 3.18 and 23.04 ± 3.38, respectively. On physical examination, internal rotation of <20° was seen on at least one side in twenty-seven (13.5%) of the 200 individuals and bilaterally in thirteen (6.5%). Three (1.5%) had a positive impingement test (one was bilateral). The average internal rotation in 90° of flexion was 44° (range, 0° to 85°).

The mean alpha angles in the anterior (three o'clock) and anterosuperior (1:30) images are presented in Table I. A paired t test showed that measurements at the 1:30 position were significantly higher than measurements at the 3:00 position (p < 0.001). Figure 4 shows the percentage distribution of alpha angles at the 3:00 and 1:30 positions for all 400 hips. Using the predefined cutoff of 50.5°, at the 3:00 position, 14% of the volunteers had at least one hip with cam morphology: 10.5% had an elevated alpha angle on either the right or the left side, and 3.5% had bilateral deformity. At the 1:30 position, 53% (106) of the volunteers had an alpha angle of >50.5° (26% were unilateral and 27%, bilateral), and 33.5% (sixty-seven) had an alpha angle of >55° (20.5% were unilateral and 13%, bilateral).

Fig. 4

The percentage distribution of alpha angles (in degrees) for the 3:00 (anterior) compared with the 1:30 (anterosuperior) positions. The error bars indicate the 95% confidence intervals.

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TABLE I Mean Alpha Angles at Anterior (3:00) and Anterosuperior (1:30) Locations by Laterality

	Mean Alpha Angle (deg)	Standard Deviation	Range	95% Confidence Interval
Anterior
Right (n = 200)	40.94	7.00	29.30-66.90	39.97 to 41.92
Left (n = 200)	40.62	7.11	27.00-69.90	39.63 to 41.61
Overall (n = 400)	40.78	7.05	27.00-69.90	40.09 to 41.48
Anterosuperior
Right (n = 200)	50.23	8.01	32.60-72.00	49.12 to 51.35
Left (n = 200)	50.07	8.27	35.60-76.30	48.92 to 51.23
Overall (n = 400)	50.15	8.13	32.60-76.30	49.35 to 50.95

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TABLE I Mean Alpha Angles at Anterior (3:00) and Anterosuperior (1:30) Locations by Laterality

	Mean Alpha Angle (deg)	Standard Deviation	Range	95% Confidence Interval
Anterior
Right (n = 200)	40.94	7.00	29.30-66.90	39.97 to 41.92
Left (n = 200)	40.62	7.11	27.00-69.90	39.63 to 41.61
Overall (n = 400)	40.78	7.05	27.00-69.90	40.09 to 41.48
Anterosuperior
Right (n = 200)	50.23	8.01	32.60-72.00	49.12 to 51.35
Left (n = 200)	50.07	8.27	35.60-76.30	48.92 to 51.23
Overall (n = 400)	50.15	8.13	32.60-76.30	49.35 to 50.95

A chi-square analysis showed that individuals with an elevated alpha angle on at least one side measured at the 3:00 position tended to be male (p < 0.001), with 24.7% (twenty-two) of the eighty-nine men in our study population having cam morphology compared with only 5.4% (six) of the 111 women. Seventy-nine percent (twenty-two) of the twenty-eight who had an elevated alpha angle were men, and 21% (six) were women. Figure 5 shows the percentage distribution of alpha angles by sex at the 3:00 position for all 400 hips. With use of the 1:30 position, this same trend was apparent: 75.3% (sixty-seven) of the eighty-nine men compared with 35.1% (thirty-nine) of the 111 women had an alpha angle of >50.5° (p < 0.001), and 51.7% (forty-six) of eighty-nine men compared with 18.9% (twenty-one) of 111 women had an alpha angle of >55° (p < 0.001). Table II summarizes the mean alpha angles and standard deviations for men compared with women at both the anterior 3:00 and the anterosuperior 1:30 positions. Independent t tests showed that men had significantly higher alpha angles than women at the 3:00 position (44.02° and 38.19°, respectively; p < 0.001) and at the 1:30 position (54.09° and 46.99°, respectively; p < 0.001).

Fig. 5

The percentage distribution of alpha angles (in degrees) measured anteriorly (3:00) by sex in 400 hips. The error bars indicate the 95% confidence intervals.

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TABLE II Mean Alpha Angles at Anterior (3:00) and Anterosuperior (1:30) Locations by Sex

	Mean Alpha Angle (deg)	Standard Deviation	Range	95% Confidence Interval
Males
Anterior (n = 178)	44.02	7.82	30.00-69.90	42.86 to 45.17
Anterosuperior (n = 178)	54.09	8.54	37.10-76.30	52.83 to 55.36
Females
Anterior (n = 222)	38.19	5.06	27.00-57.00	37.52 to 38.86
Anterosuperior (n = 222)	46.99	6.19	32.60-70.00	46.17 to 47.81

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TABLE II Mean Alpha Angles at Anterior (3:00) and Anterosuperior (1:30) Locations by Sex

	Mean Alpha Angle (deg)	Standard Deviation	Range	95% Confidence Interval
Males
Anterior (n = 178)	44.02	7.82	30.00-69.90	42.86 to 45.17
Anterosuperior (n = 178)	54.09	8.54	37.10-76.30	52.83 to 55.36
Females
Anterior (n = 222)	38.19	5.06	27.00-57.00	37.52 to 38.86
Anterosuperior (n = 222)	46.99	6.19	32.60-70.00	46.17 to 47.81

The 200 study subjects (400 hips) were divided into three age categories: twenty to twenty-nine years (260 hips), thirty to thirty-nine years (120 hips), and forty to fifty years (twenty hips). A chi-square analysis showed no significant difference in the prevalence of an alpha angle of >50.5° (either unilateral or bilateral) measured at the 3:00 position among the three age groups: 10.76%, 20%, and 20%, respectively (p = 0.20). In terms of ethnicity, a chi-square analysis showed that there was no significant difference in the prevalence between whites (13.3%; twenty-one of 158) and nonwhites (16.7%; seven of forty-two) (p = 0.58). Internal rotation was negatively correlated with an elevated alpha angle (r = —0.33, p < 0.001). A chi-square analysis revealed that a significantly greater proportion of patients with limited internal rotation (=20°) also had an alpha angle of >50.5° (25%) compared with patients with internal rotation of >20° (5.7%) (p < 0.001).

Of the three individuals who had a positive impingement sign on at least one side, two had an alpha angle of >50.5° on at least one side (chi square = 7.02, p = 0.008). Independent t tests showed no significant difference in body-mass index between individuals who had positive morphology on at least one side compared with those who did not (p = 0.647 for men, and p = 0.640 for women). There was no correlation between a family history of hip replacement and an elevated alpha angle (chi square = 0.868, p = 0.352).

Discussion

Femoroacetabular impingement has been implicated as an etiologic factor for both acetabular labral pathology and idiopathic arthritis of the hip^{5,9,11,17,26,27}. The proposed mechanism of labral cartilage damage is excessive shear stresses caused by abnormal osseous contact within the hip joint as a result of the abnormal contour of the femoral head-neck junction (insufficient femoral head-neck offset or cam impingement) or acetabular overcoverage (pincer impingement)⁸. In our study, the prevalence of cam morphology was found to be 14% in a population of 200 asymptomatic volunteers. This is in keeping with the estimated 15% prevalence of femoroacetabular impingement reported in the literature²⁸. More importantly, the relatively higher prevalence of cam morphology in men (24.7% compared with 5.4% for women) correlates well with the fact that cam impingement is primarily a disease of young males^8,20. The strong sex difference in our study may also support the current belief that subclinical slipped capital femoral epiphysis could be an etiologic factor in the development of cam morphology as it is known that the prevalence of slipped capital femoral epiphysis is much higher in adolescent males than in females^27,29. This finding is also consistent with the Copenhagen Osteoarthritis Study¹⁷ of 4151 individuals, in which 17% of men and 4% of women were found to have cam morphology on anteroposterior radiographs. Nevertheless, the relatively high prevalence of cam morphology in asymptomatic individuals suggests that other environmental or geographic factors may be important in the development of hip osteoarthritis, as the prevalence of primary osteoarthritis of the hip on the basis of radiographic findings is close to 8%³⁰. Other factors may include the severity of the deformity³¹ or genetics²⁵ as well as associated pincer deformity^8,31.

A limitation of our study sample is that it may not truly represent the normal population as it was based in a hospital setting. However, the hospital where this study was conducted employs over 13,000 individuals with a varied work force. Although we did not specifically record occupational exposure in this patient group, Gosvig et al.¹⁷, as part of the Copenhagen Osteoarthritis Study, did not find body-mass index or occupational exposure to heavy workloads as risk factors for cam deformity. In addition, although no general health or disease-specific questionnaires were administered to better characterize the study sample, the large sample size taken from a diverse working environment does provide some reassurance regarding the generalizability of our findings. Also, for practical reasons, a short magnetic resonance imaging sequence that facilitated the necessary radial reformats for measuring the alpha angle accurately was needed. Consequently, a fluid-sensitive, smaller field of view, high resolution sequence to better assess for labral pathology could not be included. Thus, although patients did not complain of hip pain, we do not know whether patients with cam morphology had associated findings of impingement such as labral tears.

We believe that our study findings provide important information relevant to patients presenting with impingement-like symptoms. The high prevalence of cam morphology detected in asymptomatic volunteers also supports the concept that an elevated alpha angle, in isolation, is not diagnostic of femoroacetabular impingement. Rather, this radiographic finding should continue to be carefully considered along with clinical symptoms, physical examination, and other radiographic findings including cartilage damage and labral tears³². In a recent study of 113 patients with symptomatic cam-type femoroacetabular impingement, Allen et al.³¹ noted that although 78% (eighty-eight) of 113 patients had bilateral cam deformities and 42% (forty-eight) had an associated pincer deformity, only 26% (twenty-three of eighty-eight) had pain in both hips, with an alpha angle of >60° being a significant predictor of having hip pain. The alpha angle as a predictor of hip pain is consistent with the surgical finding that the presence of acetabular cartilage damage correlates with the severity of the alpha angle³³. Similarly, in this study, an elevated alpha angle correlated with restricted internal rotation, which supports the theory of impingement, as internal rotation is one of the movements that results in approximation of the femoral head-neck junction and the acetabulum³⁴. It follows that patients with increased osseous convexity at the femoral head-neck junction, as manifested by an elevated alpha angle, would therefore also have restricted internal rotation, as bone-on-bone contact prevents a complete range of motion. There was no association between an elevated alpha angle and a positive impingement test, which is in keeping with the current thought that a positive impingement sign is an indicator of labral abnormality³⁵. In our study, we measured the alpha angle anteriorly (3:00) and anterosuperiorly (1:30) in asymptomatic individuals and found it to average 40.9° and 40.6° for the right and left hips, and 50.2° and 50.1° for the right and left hips, respectively. Further studies examining the alpha angle in asymptomatic individuals will help to firmly establish the normal alpha angle and to refine the cutoffs for an abnormal angle, so as to maximize the sensitivity and specificity of this measurement in conjunction with other radiographic and clinical parameters to more accurately diagnose femoroacetabular impingement syndrome.

This study suggests that the threshold value for an abnormal alpha angle should consider the sex of the subject and the location of the alpha angle measurement. The importance of measurement location is illustrated by the significantly higher alpha angles anterosuperiorly compared with anteriorly (p < 0.001). This raises the issue as to whether radial imaging, which is sensitive to the location of the peak of the cam deformity, is more accurate for detection of cam morphology and the diagnosis of potential impingement^15,24. The prevalence of cam morphology approaches 14% in the asymptomatic population and is significantly higher in men than in women, with these individuals having an associated limited internal rotation of the hip. Of note, 79% (158) of the 200 subjects in our study group were white, which may or may not be relevant as current prevalence data suggest that osteoarthritis of the hip is most prevalent in Europeans and North Americans and least prevalent in Asians and Africans³⁰. The high prevalence of cam morphology in asymptomatic individuals is critical information in determining the natural history of femoroacetabular impingement as well as for establishing treatment strategies in patients presenting with prearthritic hip pain and, more importantly, establishing causation with respect to hip osteoarthritis.

Acknowledgements

Note: The authors thank Timothy Ramsey, PhD, and Dean Fergusson, PhD, from the Ottawa Hospital Research Institute for their invaluable comments and guidance. They also thank Anna Fazekas, MA, for her assistance with patient recruitment and data analysis, and Natasha LaRussa, MRT(R)(MR), for imaging all patients.

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Topics

neck ; femur ; hip joint ; vibration ; hip pain ; femoral acetabular impingement

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Kalesha Hack, Gina Di Primio, Kawan Rakhra, Paul E. Beaulé; Prevalence of Cam-Type Femoroacetabular Impingement Morphology in Asymptomatic Volunteers. The Journal of Bone & Joint Surgery. 2010 Oct;92(14):2436-2444.

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