Clinical History: A 22 year old collegiate soccer player presents with bilateral groin pain. MR arthrography was performed bilaterally. Coronal images of both hips (T1-weighted without fat-suppression, top row), oblique axial image of the right hip (T1-weighted with fat suppression, middle row) and sagittal images of right and left hips (T1-weighted with fat suppression, bottom row) are presented. What are the findings? What is your diagnosis?
Cam-type femoroacetabular impingement with bilateral acetabular labral tears and chondral loss.
Femoroacetabular impingement (FAI) is not a disease, but a common pathomechanical process which occurs when the proximal femur comes into contact with the acetabular rim. This can occur in normal hips in extreme flexion and internal rotation or may be from morphologic changes that cause premature contact between the structures. Chronic microtrauma from recurrent impingement can lead to acetabular labral tears and breakdown of the articular cartilage, subsequently resulting in osteoarthritis. FAI from radiographically subtle morphologic abnormalities has been suggested to be the underlying cause of most cases of idiopathic or primary osteoarthritis9. Timely diagnosis is therefore important. Surgical intervention can give immediate pain relief and, if it is performed before cartilage breakdown has begun, it may prevent later cartilage loss and development of osteoarthritis. FAI can occur from abnormal morphology on either the femoral or acetabular side of the joint.
In order for the hip to flex normally without impingement, the femoral head needs to be round and there must be a suitable space at the femoral head-neck junction (aka the femoral waist). If instead the femoral head is aspherical or there is not a suitable femoral head-neck offset, then this area of the proximal femur will abnormally pressurize the cartilage at the acetabular rim during hip flexion, causing chondral and subsequent labral damage. A mechanical cam, such as part of a combustion engine camshaft, is an aspherical structure that rotates and thereby lifts an adjacent structure, hence the term “cam-type FAI.”
In a hip with a cam lesion, rotation of the femoral head brings a broader radius into the acetabular margin, typically anterosuperiorly, compressing the hyaline cartilage at this site and pushing it medially. This initially forms a wavy appearance to the cartilage with separation from the underlying bone, followed by a chondral flap tear, similar to that of a carpet that has been pushed from the side, a “carpet lesion.” Subsequent to chondral injury, pressure at this site also causes the labrum to tear or detach from the hyaline cartilage margin. With labral injury and further progression of chondral damage, the hip may then later develop instability and femoral head cartilage damage.
We have long known that patients with a history of slipped capital femoral epiphysis or Perthes disease that have developed a pistol grip deformity are prone to developing secondary osteoarthritis, but it was not until more recently that FAI has been recognized as the underlying mechanism. Most commonly, an aspherical femoral head or a bump at the femoral head-neck junction creates cam-type FAI. There are several underlying possible causes (Table 1).
Table 1. Causes of Cam Lesions1
- Nonspherical femoral head
- Coxa vara
- Malunited femoral neck fracture
- Post-traumatic retroversion of the femoral head
- Childhood orthopedic condition
- Perthes disease
- Slipped capital femoral epiphysis (SCFE)
- Femoral osteotomy
Since a prominent acetabular rim pressing down upon on the proximal femur resembles the action of a pincer, this has been termed “pincer-type” impingement. An overly prominent acetabular rim is reflected by the presence of acetabular overcoverage of the femoral head.
There are several possible underlying causes of pincer lesions, developmental and acquired, and the abnormality may be local (e.g. acetabular retroversion) or global (e.g. coxa profunda) (Table 2). Labral ossification and osseous proliferation are most frequently seen, which are likely chronic changes secondary to prior labral injury.
Table 2. Causes of Pincer Lesions1
- Retroverted acetabulum
- Coxa profunda
- Os acetabuli
- Protrusio acetabuli
- Chronic residual dysplasia of the acetabulum
- Post-traumatic deformity of the acetabulum
- Overcorrection of retroversion in dysplastic hips
As with cam-type FAI, the initial injury site is typically anterosuperior in position from hip flexion and internal rotation. However, with pincer-type impingement, the initial injury is to the labrum, which develops myxoid degeneration and tearing. To a lesser degree there may be damage to a thin adjacent strip of acetabular cartilage. Later in this process, when dystrophic ossification has developed in the injured labrum, the prominent acetabular rim acts as fulcrum, leveraging the femoral head posteroinferiorly and causing more significant chondral injury at that site.
Most cases of FAI are a combination of both cam and pincer-type mechanisms, with cam-type usually predominating. It is possible that the femoral head-neck junction bump found in cam-type impingement may be initiated by microtrauma from pincer-type impingement at this site. Once formed, both mechanisms may have a role in bump enlargement, as well as development of cystic changes at this site. Additionally, if more advanced osteoarthritis develops, marginal osteophytes at the acetabular rim or ossification of the damaged labrum become pincer lesions and remodeling at the femoral head-neck junction creates an aspherical femoral head that acts as a cam lesion. However, once there is advanced osteoarthritis, noting the underlying FAI biomechanics is probably a moot point, since correcting them has not been shown to reverse or slow disease progression.
Patients with FAI typically present with deep intermittent groin discomfort during or after activities involving repetitive or persistent hip flexion. Groin pain in an athlete is a nonspecific complaint with a large differential diagnosis, including adductor strain, iliopsoas tendinitis, and stress fracture. With FAI, limitation of the range of motion develops long before the onset of pain but may go unnoticed. Additionally mechanical symptoms may be present if there is an associated labral tear. FAI is frequently bilateral but asynchronous, such that presenting symptoms are often unilateral.
The typical patient with cam-type impingement is a young active male in his 20s or 30s. Pincer-type impingement typically presents a decade later in women engaged in activities such as yoga or aerobics. Nevertheless, FAI can be found in other age groups and one recent study found pincer lesions to be very common among both sexes12.
Clinical examination can reveal an “anterior impingement sign,” which is present when pain is elicited with forced internal rotation while the hip is in 90 degree flexion. In more extreme cases, the bone morphology forces the hip to externally rotate during hip flexion, a positive “Drehmann’s sign.” A “posterior impingement sign” (associated with uncommon posterior pincer lesions) is positive when the hip is placed in hyperextension and forced external rotation elicits pain.
If clinical features are suggestive of FAI, radiographs may reveal the underlying femoral and/or acetabular structural abnormality. The AP view may show a typical “pistol grip” deformity (lack of a normal concavity to the femoral waist), coxa vara, an aspherical femoral head, or a femoral head-neck junction cyst. An additional helpful finding that reflects an aspherical femoral head is a “horizontal growth plate sign” (growth plate extending lateral to a best-fit circle of the femoral head)22. A characteristic femoral head-neck junction bump or aspherical femoral head may be visible on the lateral view and alpha angle and head-neck offset ratio can be measured. However, since these structural abnormalities primarily occur superolaterally, standard AP and lateral views may miss them. Some advocate a specially obliqued view (modified Dunn view) for greater sensitivity15.
Acetabular prominence with femoral overcoverage may also be apparent, confirmed on AP radiographs by parameters such as lateral center edge angle >39° (increased femoral head coverage), acetabular index of elevation =0 (aka Tönnis angle; diminished slope of acetabular roof or “sourcil”), or femoral head extrusion index ~0 (diminished amount of femoral head extruding beyond the acetabular margin)22. Acetabular retroversion can be detected with the “cross-over sign” and posterior wall deficiency with the “posterior wall sign.” These assessments are subject to pelvic tilt/positioning, though compensating measurements and calculations can be made. Occasionally, a damaged acetabular labrum may develop ossification. If this becomes large enough it may act as a pincer lesion. If detached, it may be seen as an os acetabuli. Potential pincer lesion mimics include DISH and ankylosing spondylitis, which do not produce impingement. Detailed reviews of the radiographic assessments have been published in the orthopedic4 and radiology22 literature. However, some of the same authors later determined that in practical use, the reliability of these radiographic measurements is limited5.
In addition to demonstrating the osseous structural abnormalities visible on radiographs as above described, including cam-type deformities, acetabular overcoverage, or acetabular retroversion, MRI (or MR arthrography) allows better visualization of anterosuperior lesions and provides an important assessment for labral tears and hyaline cartilage loss.
MRI evaluation is directed to both femoral and acetabular causes of impingement. The proximal femur must be scrutinized in multiple imaging planes with special attention to the anterior and anterosuperior aspects of the femoral head-neck junction.
One or more features (which frequently coexist) are sought for identifying a cam-type deformity:
- focal bony protuberance or bump at the anterior or anterosuperior aspect of the femoral head-neck junction
- aspherical femoral head at the femoral head-neck junction (such as a flattened lateral femoral head margin)
- diminished femoral head-neck offset (aka “femoral waist deficiency,” or “pistol grip deformity” if seen on coronal images)
Additionally, small cysts at the femoral head-neck junction are a useful indicator of FAI. They have been found in 33% of patients with FAI13 and there is a high likelihood of a causal relationship. Since these may contain fibrous or gelatinous material, some refer to them as fibrocysts. A study suggesting that the cysts may not be related to FAI11, was limited in that the only criterion for cam-type FAI was a pistol grip deformity on an AP view. The study thereby had a low sensitivity for an anterior or anterosuperior asphericity or a femoral head-neck bump, the structural abnormality that is most often associated with the cysts. Another study confirmed that radiographs are not adequately sensitive for asphericity of the femoral head7.
With cam-type impingement, the femoral neck has a below normal offset from the femoral head and demonstrates a diminished or lost transition (“femoral waist deficiency”). Several types of MRI measurements have been developed to confirm and quantify these findings. The alpha angle is the most widely used (abnormal > 55 degrees), indicating the angle at which the femoral head departs from its normal spherical outline. Femoral head-neck offset distance (abnormal < 8 mm) and associated ratios can also be used to demonstrate a cam-type structural abnormality.
Measurement of an abnormal alpha angle from an oblique axial image along the femoral neck16 has generally been the standard for confirmation of cam-type lesions. However, a more recent study comparing radial slices to standard oblique axial images showed a significant discrepancy in 54% of subjects, where the alpha angle was normal (less than 55°) on the conventional oblique axial plane images but positive for cam-lesion (55° or greater) on the radial plane images19. This is understandable since an anterosuperiorly positioned femoral head-neck junction bump will often be just above the image plane typically utilized for alpha angle calculation. It may in part explain why a study comparing subjective assessments of alpha angles did not correlate well with measured alpha angles except in cases where an obvious bone deformity was present17. An additional source of measurement error is the placement of the circle, which should be along the subchondral bone cortex, excluding the femoral head cartilage. Slight variations of circle radius or placement can cause significant differences in measured alpha angles. A less frequent problem with the alpha angle measurement can be the definition of the femoral neck axis. Nötzli defined it as a line between the center of the femoral head and the middle of the narrowest portion of the femoral neck. However, in cases of femoral head retroversion such as from SCFE, this method will yield an incorrect femoral neck axis and an erroneously undervalued alpha angle. In cases where the femoral head and neck are not aligned, a better approach appears to be to draw this line parallel to the visible neck axis. See diagrams at http://www.jbjs.org.uk/media/25117/focuson_fai.pdf8.
While radial imaging planes reconstructed from a high resolution 3D gradient echo sequence would be the most ideal for visualizing and measuring predominantly anterosuperior abnormalities, this method may not be available on many MRI units due to technical limitations.
The acetabulum is assessed for pincer-type lesions related to abnormal acetabular depth or version as well as the presence of labral ossification or bony overgrowth at the acetabular rim. Femoral head overcoverage may be localized from retroversion, localized hypertrophy, osteophytes, or an ossified labrum, or it may be global from coxa profunda or protrusio acetabuli. The lateral center edge angle (LCE) appears to be a better measure of overcoverage than the extrusion index, since LCE will not be affected by enlargement of the lateral portion of the femoral head when there is a concomitant cam lesion.
Acetabular depth can be measured between the center of the femoral head and a line connecting the anterior and posterior acetabular rims. One study showed an average depth of 5mm with pincer-type FAI and 1mm with cam-type18. Deficient acetabular coverage (lateral center edge angle < 25 degrees) limited to the anterosuperior portion may instead indicate the presence of developmental dysplasia of the hip (DDH). Differentiating these conditions is important and other features may also be helpful in this regard. Paralabral cysts and labral enlargement from myxoid degeneration are reportedly more common in DDH than with FAI14. Anterosuperior femoral neck cysts are not present in DDH13. Hypertrophy of the pulvinar and of the ligamentum teres are also features of DDH. In both conditions, labral tears are primarily anterosuperior in position.
Labral tears are important abnormalities that can lead to hip instability and faster hyaline cartilage breakdown. They are increased in patients with FAI, particularly anterosuperiorly, with both cam and pincer types of FAI, and are usually best seen on sagittal or oblique sagittal imaging sequences, followed by coronal images. A useful indicator for a labral tear is that of a paralabral cyst. This is frequently more conspicuous than the tear itself and may be very large, very small, or nonexistent. They are usually in the soft tissues adjacent to the labrum, frequently projecting superiorly. They also frequently have an intraosseous component, which may be difficult to distinguish from subchondral cystic changes associated with hyaline cartilage damage. A potential false positive for labral tears is a sublabral sulcus or recess, but these occur posteroinferiorly6 and anteroinferiorly21, away from the usual anterosuperior labral pathology.
Chondral damage occurs first on the acetabular side of the joint with FAI, only involving the femoral head late in the process. It is located anterosuperiorly with cam-type FAI, due to local compression by the cam, beginning with focal delamination (visible arthroscopically as a “wave sign“) and progressing to a chondral flap tear (“carpet lesion“). With pincer-type FAI, chondral injury is more common posteroinferiorly18 or diffusely3. MR arthrography can allow better delineation of open chondral defects than standard MRI, though intraarticular contrast will not outline early delamination lesions. These must be assessed on T2 weighted images, but remain very difficult to identify. A potential false positive pitfall for an acetabular chondral defect is a supra-acetabular fossa, which can be striking, but has a characteristic location, no associated marrow edema, and is often bilateral.
Since FAI occurs at extremes of the range of motion, behavior modification may be enough to avoid pain and subsequent joint injury. For those whose activities require repeated extreme hip flexion, such as serious athletes or carpet layers, surgery is an option. Surgical treatment has shown good results in midterm follow-up for patients that do not have significant chondral loss prior to surgery2. Open and arthroscopic techniques have been utilized and shown to be equally effective1.
For cam lesions, osteoplasty/cheilectomy is performed to remove excess bone and restore the femoral head-neck offset23. For pincer lesions, overcoverage can be reduced focally by bone removal or if acetabular retroversion is present a periacetabular osteotomy can be performed to reorient the entire acetabulum. Chondral lesions are debrided and labral tears repaired.
In most cases, both cam and pincer type lesions are present and require treatment. However, caution must be exercised to not overcorrect a pincer lesion and create instability from a surgically induced acetabular dysplasia.
Additional Words of Caution
Since many of the morphologic findings of FAI can be found in asymptomatic individuals, it is possible that some of the abnormalities reflect anatomic variations that may predispose them to FAI but do not indicate its presence. For example, cam-type deformities have been found in as many as one in four asymptomatic young men and in half of men with decreased internal rotation20. Pistol grip deformities have been found in one in five males and the crossover sign in half of men and women12. Additionally, there are differences in opinion about the significance of an ossicle adjacent to the acetabular rim, some considering it evidence of a pincer lesion and others an incidental normal developmental variant. A diagnosis of FAI should not be based on imaging findings alone. Correlation must be made with the clinical findings.
Additionally, the natural course of osteoarthritis is not entirely clear. While it is likely that early intervention prevents or delays development of osteoarthritis in the long term, a conclusive study is difficult to conduct due to the need for sham surgery controls. A recent retrospective study showed that 82% of patients with FAI that were asymptomatic at the time of diagnosis remained free of osteoarthritis for a mean of 18.5 years follow-up without surgery10. Only the presence of idiopathic osteoarthritis in the contralateral hip was predictive of the development of osteoarthritis on the asymptomatic side. This also reemphasizes that prophylactic surgery should not be undertaken in the absence of symptoms.
- FAI is an important diagnosis to make and is currently underdiagnosed
- diagnosis can lead to appropriate behavior modification or intervention and pain relief
- early intervention may prevent later development of osteoarthritis
- FAI can be diagnosed by characteristic imaging findings in the appropriate clinical setting
- cam-type FAI is characterized by an aspherical femoral head or diminished femoral head-neck offset
- pincer-type FAI is characterized by a prominent acetabular rim
- most cases have components of both, though cam-type typically predominates
- both types of FAI are associated with anterosuperior labral tears
- both types of FAI are associated with chondral damage, typically anterosuperior in cam-type FAI and posteroinferior or diffuse in pincer-type FAI
- alpha angle measurements on oblique axial MRI images and assessments on radiographs have limited sensitivity, since FAI structural abnormalities are primarily anterosuperior in position
- since there is a high prevalence of FAI-type morphologies in asymptomatic individuals, the diagnosis of FAI should not be made in the absence of clinical symptoms
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