Imaging of Femoral Acetabular Impingement Syndrome

Department of Radiology, University of Colorado Health Sciences Center, 4200 E 9th Avenue, Mailstop A030, Denver, CO 80262, USA

Osteoarthritis of the hip is not limited to the elderly. It can affect both young and middle-aged adults. It is these patients in which early detection and treatment may be particularly beneficial, allowing longer pain-free functionality of the hip and possibly obviating total hip arthroplasty.

Biomechanical factors in development of osteoarthritis (OA) of the hip include concentric or eccentric overload, resulting in cartilage degeneration. This etiology is supported by the example of early development of OA in patients with developmental dysplasia. However, it fails to explain the development of OA in young adults with apparently normal anatomy and intra-artic-ular pressures.

The pathogenesis of such ''idiopathic'' OA has not been well established. Recently, femoroacetabular impingement (FAI) has been implicated as an etiology of both labral tears and adjacent cartilage damage, and through repetative microtrauma at these sites is felt to be a precursor to OA in young adults [1]. FAI represents impingement of the anterior femoral head-neck junction against the adjacent anterosuperior labrum. It may occasionally be a result of unusual stress placing the femoral neck in contact with the anterosuperior labrum, such as in a football punter or carpet layer. However, more frequently FAI results from subtle morphologic abnormalities in the femoral head-neck junction or in the acetabulum. Less stress is required to develop clinical symptoms of FAI when such abnormalities are present.

Recognition of FAI both clinically and radiographically may be difficult. This review serves to demonstrate both the radiographic findings and imaging work-up of FAI, and relates them to early surgical treatment of this syndrome. It is hoped that early recognition will lead to routine early intervention, delaying the onset of end-stage OA in these young patients [2].

CLINICAL SYMPTOMS OF FAI

The most common feature of FAI is that the clinical symptoms seem disproportionate to the radiographic findings. Patients may present with groin pain or

*Corresponding author. E-mail address: [email protected] (B.J. Manaster).

0278-5919/06/$ - see front matter doi:10.1016/j.csm.2006.06.012

© 2006 B.J. Manaster, MD, PhD sportsmed.theclinics.com pain overlying the greater trochanters [1]. They may complain of grinding or popping. The loss of internal rotation is usually out of proportion to other decreased range of motion. Patients report pain with flexion and internal rotation, such as in a sitting position and getting out of a car; the pain is worse after prolonged sitting. The impingement test, elicited by 90° flexion, adduction, and internal rotation of the hip, is almost always positive [2]. This position results in sheer stress or compression on the labrum or adjacent damaged cartilage. The labrum contains proprioceptive and nociceptive nerve fibers [3] that are sensitive to this provocative movement.

Although anterosuperior impingement is most frequent, posteroinferior impingement may rarely occur. The provocative test for this calls for the patient to lie supine with the legs hanging free (creating hip extension); external rotation results in severe deep-seated groin pain [2].

MORPHOLOGIC DEFINITION OF FAI

Morphologically, FAI is defined as a conflict occurring between the anterior femoral head-neck junction and the adjacent anterosuperior labrum and acetab-ular rim. This conflict is caused by an abnormality of either the proximal femur or the acetabulum, or occasionally both [4]. The acetabular cartilage may be focally damaged or delaminated. The histologic features of the damaged labrum include hyperplasia with disorganized cystic matrix, and no inflammatory changes. This suggests the mechanism of damage is chronic irritation consistent with repetative microtrauma [5].

There are two types of FAI, the ''cam'' and ''pincer''; combinations of the two may occur. These types are described based on the pattern and characteristics of chondral and labral injuries observed in situ during surgical dislocation of the hip. There are radiographic patterns that follow these descriptions as well.

Cam Type of FAI

In any position of the femoral neck, one normally can see a head/neck offset (or cutback) (Fig. 1A). FAI with a femoral head/neck abnormality occurs when there is an insufficient femoral head/neck offset. Most frequently this occurs when there is a lateral femoral neck ''bump'' (Fig. 1B). With flexion, adduction, and internal rotation, the nonspherical portion of the femoral head rotates into the acetabular rim (Fig. 1C,D), causing sheer stress on the articular cartilage and a subsequent labral tear or detachment [1,2,4] (Fig. 1E). Because of the offset at the normally rounded femoral head, this has been termed a ''cam'' mechanism. There is surgical evidence supporting the supposition that the principal initial damage in the cam type of FAI is to the cartilage and that the labrum is uninvolved at first. Additionally, all of the labral tears or detachments occur at the articular (not capsular) margin [2]. This further supports the cartilage rather than labrum being the initial site of damage. The cam type of FAI tends to be seen in young active male patients.

Cam Fai OsteoarthritisFemoral Head Neck Offset

Fig. 1. Cam type of femoroacetabular impingement. (A) A normal femoral head/neck offset (arrow). (B) A case of FAI, with a lateral "bump" (arrow) resulting in decreased femoral head/ neck offset. This corresponds to the diagram in (C); with flexion, internal rotation, and adduction (D), the abnormal femoral neck contacts the anterosuperior cartilage first (solid arrow), and secondarily damages the labrum, most frequently as a detachment (dotted arrow). (E) An axial oblique image obtained from an MR arthrogram of the patient in (B), showing a tear in the anterosuperior labrum (arrow).

Fig. 1. Cam type of femoroacetabular impingement. (A) A normal femoral head/neck offset (arrow). (B) A case of FAI, with a lateral "bump" (arrow) resulting in decreased femoral head/ neck offset. This corresponds to the diagram in (C); with flexion, internal rotation, and adduction (D), the abnormal femoral neck contacts the anterosuperior cartilage first (solid arrow), and secondarily damages the labrum, most frequently as a detachment (dotted arrow). (E) An axial oblique image obtained from an MR arthrogram of the patient in (B), showing a tear in the anterosuperior labrum (arrow).

The etiology of the abnormal femoral head/neck offset (or lateral bump, Fig. 2A) is unclear. Currently it is being discussed as a subclinical slipped capital femoral epiphysis [6,7]. Another theory is that a growth disturbance may result in delayed separation or eccentric closure of the common physis between the femoral head and greater trochanter. This would result in an abnormal extension of the femoral head epiphysis and a consequent decrease in the head/ neck offset [6,8].

Coxa Magna

Fig. 2. Cam type of FAI. (A) A subtle lateral "bump" (arrow), of unclear etiology, but the most common type of this disorder. (B) A severe coxa valga hip dysplasia; the motion consisting of flexion, internal rotation, and adduction results in abnormal contact between the femoral head/neck with the anterosuperior acetabulum. (C) An old slipped capital femoral epiphysis, where the femoral neck has outgrown the stabilizing nail. Because the slip is medial, the lateral femoral head/neck offset is lost (arrow), resulting in a Cam-type of morphology.

Fig. 2. Cam type of FAI. (A) A subtle lateral "bump" (arrow), of unclear etiology, but the most common type of this disorder. (B) A severe coxa valga hip dysplasia; the motion consisting of flexion, internal rotation, and adduction results in abnormal contact between the femoral head/neck with the anterosuperior acetabulum. (C) An old slipped capital femoral epiphysis, where the femoral neck has outgrown the stabilizing nail. Because the slip is medial, the lateral femoral head/neck offset is lost (arrow), resulting in a Cam-type of morphology.

There are other, more obvious, etiologies of abnormal femoral head/neck offset. These include retrotorsion of the femoral head, malunited femoral neck fracture, prior femoral neck osteotomy, an elliptical femoral head, severe coxa valga deformity (Fig. 2B), and any etiology of a coxa magna deformity [1,9]. The latter may include prior Legg Perthes, adult avascular necrosis with collapse, and prior slipped capital femoral epiphysis (Fig. 2C).

Pincer Type of FAI

The pincer type of impingement results from any abnormality that results in increased coverage of the anterosuperior portion of the femoral head (Fig. 3A,B). This results in a linear contact between the (normal) anterolateral femoral neck on the prominent acetabular rim (Fig. 3C,D) [1,2]. This impingement results primarily in labral tears and bony proliferation at the acetabular

Large Hip Labrum Ossification Radiology

F

'S

i/ "

* A

Fig. 3. Pincer type of FAI. (A,B) An anteroposterior and false profile view, respectively, of a hip with abnormal acetabular bony rim proliferation. Note that there is an abnormal amount of bone at both the anterior and lateral portions of the rim (arrows). (C) Diagram demonstrating increased acetabular coverage of a normal femoral head and neck. With flexion, internal rotation, and adduction, the normal femoral neck contacts the abnormal acetabular rim and damages the labrum (D). (E,F) Coronal and axial images, respectively, from the MR arthrogram of the patient in (A). The labral tear is not shown in these images, but the fragmented hypertrophic acetabular rim is well demonstrated, with fluid tracking around the rim fragments (arrows in both).

rim (Fig. 3E,F). With chronicity, a focal chondral injury may result. Continued injury may lead to cyst formation from the labral tear or ossification of the acetabular rim, which in turn worsens the condition. Finally, the pincer type of FAI may result in chondral injury in the ''contre-coup'' region of the posteroin-ferior acetabulum [4]. The pincer type of FAI tends to occur more in women and older patient age groups than the cam type.

The acetabular abnormality resulting in the pincer type of FAI may be either local, as in a focal acetabular retroversion (Fig. 4A), or more global (Fig. 4B). Etiologies of the more general abnormal acetabular coverage of the femoral head include coxa profunda (projection of the acetabular fossa medial to the ilioischial line) and protrusio acetabulae (projection of the femoral head medial to the ilioischial line) [7].

Box 1 outlines the major differences between the cam and pincer types of FAI. It is important to note that although these two basic mechanisms of FAI have been described, it is not infrequent to find combined femoral and acetabular abnormalities. It is extremely important to assess all features of FAI, since the corrective surgery might be altered if more than one type coexists. It should also be noted that developmental dysplasia of the hip (DDH) often results in labral and cartilaginous damage, and may be a part of the complex. DDH will be more completely described in a later section.

IMAGING FINDINGS IN FAI: RADIOGRAPHIC, CT, MR, MR ARTHROGRAM

Osseous Abnormalities

The osseous abnormalities found in FAI are similar, whether seen on radiograph, computed tomography (CT), magnetic resonance (MR), or MR arthro-gram. They will be demonstrated in all of these types of imaging throughout the remainder of the review.

Femoral Acetabular Impingement

Fig. 4. Other etiologies of Pincer type FAI. (A) Anterosuperior acetabular retroversion. Both arrows outline the focal region of the acetabulum where the anterior rim overlaps and lies lateral to the posterior rim, resulting in a focal site of abnormal contact between the femoral head/neck and this acetabulum. (B) A case of coxa profunda due to Otto's disease (a hereditary disorder of the acetabulum, possibly related to abnormal fusion at the Y cartilage). The acetabulum projects medial to the ilioischial line (arrow), resulting in a relative overcoverage of the femoral head/neck.

Fig. 4. Other etiologies of Pincer type FAI. (A) Anterosuperior acetabular retroversion. Both arrows outline the focal region of the acetabulum where the anterior rim overlaps and lies lateral to the posterior rim, resulting in a focal site of abnormal contact between the femoral head/neck and this acetabulum. (B) A case of coxa profunda due to Otto's disease (a hereditary disorder of the acetabulum, possibly related to abnormal fusion at the Y cartilage). The acetabulum projects medial to the ilioischial line (arrow), resulting in a relative overcoverage of the femoral head/neck.

Box 1: Cam versus pincer*

• Abnormality is at femoral head/neck junction

• Cartilage damage occurs first

• Labral injury tends to be detachment

• Treatment is femoral neck osteoplasty Pincer

• Abnormality is at acetabular rim

• Cartilage damage is secondary

• Labral injury tends to be a tear

• Treatment is to reduce acetabular rim overcoverage

*Remember that these may present as combinations

Abnormal Lateral Femoral Head/Neck Offset

The most frequently used radiograph is the anterioposterior (AP) view. This is supplemented by various lateral views, including the frog lateral (flexion and external rotation), Dunn lateral (90° flexion and 20° abduction) and groin lateral. If there is abnormal femoral head/neck offset, it should be visible on each of these views, usually as a lateral femoral neck ''bump.'' This configuration has been termed the ''pistol grip'' deformity (Fig. 5A-D) and is typical of cam-type FAI. Although there is usually no difficulty in making this assessment, an abnormal femoral head/neck offset can be measured using the alpha angle. This angle can be measured on any image of the femoral head and neck, but is used most frequently on a lateral radiograph (Fig. 6B) or sagittal oblique (Fig. 6E) or radial MR image. The alpha angle is constructed by the following steps [6]: (a) form a perpendicular line to the femoral neck at its narrowest,

(b) bisect the femoral neck, perpendicular to the line described in (a),

(c) form a best-fit circle on the femoral head, (d) the alpha angle is formed between the line (b) and a line drawn from the center of the head to the point where the neck intersects the circular head. The alpha angle is normally less than 55°.

Os Acetabulae

An os acetabulum is suggestive (although not diagnostic) of FAI, and is often seen in conjunction with a lateral femoral neck bump (Fig. 6). One study of 42 hips with cam-type FAI showed an abnormal alpha angle in 93% and an os acetabulum in 40% [6]. A double rim sign (rim ossification) has also been described in conjunction with FAI.

Fibrocystic Changes (Synovial Herniation Pits)

The relationship between synovial herniation pits (fibrocystic changes in the anterolateral femoral neck) and FAI is not entirely clear. Before the suggested

Herniation Pit

Fig. 5. Abnormal lateral femoral head/neck offset in Cam FAI. (A) A subtle lateral "bump" (arrow), which is also noted on the frog lateral view (B, arrow). Note that the configuration of the femoral head and neck is reminiscent of an old-fashioned pistol; hence the term "pistol grip'' deformity. (C) Sagittal view from the MR arthrogram, showing extensive cartilage delam-ination (arrow), while the coronal view (D) shows the complex labral tear to best advantage (arrow).

association with FAI, these pits were thought to be caused by pressure anteriorly by the iliofemoral ligament on the capsule when the hip is held in full extension, and were considered an incidental finding in 5% of the normal population. However, one study of 117 FAI-affected hips showed fibrocystic changes in 33% of their cases [7]. Examinations using dynamic MR as well as intraoperative observations reveal a close spatial relationship between the region of fibrocystic change and the acetabular rim with the hip in flexion. Therefore, these are considered by some investigators to be a result of repetitive mechanical contact between the femoral head/neck region and the acetabular rim (Fig. 7).

Acetabular Overcoverage, Including Retroversion

Generalized overcoverage, as seen with coxa profunda and protrusio acetabu-lum, is easily diagnosed based on the relationship of the femoral head and ac-etabulum to the ilioischial line. However, focal anterosuperior acetabular retroversion may be more difficult to note. Retroversion can be a result of trauma or prior surgery, but is usually a focal dysplasia. It is seen when the

Femoral Acetabular Impingement Fai Alpha Angle

Fig. 6. Cam type FAI. (A) An anteroposterior view, showing the lateral femoral neck "bump" (dotted arrow) as well as an os acetabulum (so/id arrow). The frog lateral view (B) shows an abnormal alpha angle (see text for description of its construction). The MR arthrogram confirms the suspected labral tear and cartilage damage (C, arrow). (D) A reformatted radial image, showing the lateral bump (dotted arrow) and os acetabulum (so/id arrow). The combination of lateral femoral neck bump, abnormally large alpha angle, labral tear, and cartilage damage is a common theme in cam type FAI. Os acetabulae are frequently seen as well, although less often than these other listed abnormalities. (E) Normal alpha angle on an MR arthrogram of a different patient.

Fig. 6. Cam type FAI. (A) An anteroposterior view, showing the lateral femoral neck "bump" (dotted arrow) as well as an os acetabulum (so/id arrow). The frog lateral view (B) shows an abnormal alpha angle (see text for description of its construction). The MR arthrogram confirms the suspected labral tear and cartilage damage (C, arrow). (D) A reformatted radial image, showing the lateral bump (dotted arrow) and os acetabulum (so/id arrow). The combination of lateral femoral neck bump, abnormally large alpha angle, labral tear, and cartilage damage is a common theme in cam type FAI. Os acetabulae are frequently seen as well, although less often than these other listed abnormalities. (E) Normal alpha angle on an MR arthrogram of a different patient.

Natural Arthritis Pain Remedies

Natural Arthritis Pain Remedies

It's time for a change. Finally A Way to Get Pain Relief for Your Arthritis Without Possibly Risking Your Health in the Process. You may not be aware of this, but taking prescription drugs to get relief for your Arthritis Pain is not the only solution. There are alternative pain relief treatments available.

Get My Free Ebook


Responses

  • Milo
    What is coxa profunda with over coverage of femoral head?
    5 years ago
  • tuomo
    Is fembro acetabular impingement heredetary?
    4 years ago
  • cassandra
    What is insufficient femoral head offset?
    2 years ago

Post a comment