Radiographic and MR Imaging of the Athletic

Derek R. Armfield, MDa'b'*, Jeffrey D. Towers, MDab, Douglas D. Robertson, MD, PhDa'b'c

"Department of Radiology, University of Pittsburgh Medical Center, 200 Lothrop Street, Pittsburgh, PA 15213, USA

b'Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, 200 Lothrop Street, Pittsburgh, PA 15213, USA

cDepartment of Bioengineering, School of Engineering, University of Pittsburgh, 200 Lothrop Street, Pittsburgh, PA 15213, USA

Magnetic resonance imaging (MRI) and radiography are the imaging essentials needed to evaluate intra-articular pathology and extra-articular sources of hip pain. Over the past decade MR imaging has highlighted the detection of labral tears as a source of hip pain, but it is also critical for detecting cartilage defects, capsular/iliofemoral ligament injury, liga-mentum teres tears, and bony findings associated with femoroacetabular impingement (FAI). Despite the central role of MR arthrography for evaluating intra-articular abnormalities, radiography remains essential for the radiologic work up of the athlete with hip pain. A normal hip radiograph has been redefined over the past several years, as relatively normal appearing radiographs may have evidence of subtle acetabular dysplasia (ie, retroversion) or a femoral neck bump that may provide a clue to the presence of intra-articular labral or cartilage injury. One recent study showed that 87% of patients that underwent surgery for labral tears had a structural hip abnormality identified on conventional radiographs [1]. In addition, periacetabular ossicles and synovial hernia-tion pits were once considered normal variants, but we now view them as markers for underlying FAI or labral pathology. A generalized MR of the entire pelvis may be useful for the evaluation of surrounding muscle, tendon, and bone marrow abnormalities; but it is insufficient for evaluating internal derangements of the hips. This article first describes the general approach for the radiologic work up for the athletic hip, followed by MR appearances of labral and nonlabral abnormalities.

* Corresponding author. Department of Radiology, University of Pittsburgh Medical Center, 200 Lothrop Street, Pittsburgh, PA 15213. E-mail address: [email protected] (D.R. Armfield).

0278-5919/06/$ - see front matter © 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.csm.2005.12.009 sportsmed.theclinics.com

RADIOLOGIC EVALUATION AND MODALITY OVERVIEW

Radiography

Plain film radiography remains the primary screening tool because it is widely available, simple, and relatively inexpensive; but it must be done properly. In the past, a typical screening radiographic series for hip pain often included a nonweight-bearing anteroposterior (AP) and frogleg lateral view of the affected hip with or without a supine AP view of the pelvis. The goal of the screening test was to detect obvious sources of pathology such as advanced arthrosis, tumor, fracture, and advanced dysplasia. In addition to an AP and frogleg lateral view of the hip, we currently prefer an AP standing pelvis instead of nonweight-bearing view and add a crosstable lateral view in cases where FAI is suspected. We prefer reviewing film by electronic softcopy using a PACS system, which allows for optimal window and leveling and facilitates measuring.

When screening radiographs are negative, the next useful imaging modality is generally magnetic resonance imaging using unilateral direct MR arthro-graphy of the hip for the evaluation of intra-articular pathology or screening MR of the pelvis for extra-articular sources of pain. However, radiographs or MRI of the lumbar spine, sacroiliac joints, femur/thigh, or knee may be needed to evaluate for referred pain.

Magnetic Resonance Imaging

Not all MRIs are equivalent, and it is important to differentiate between the types of MRI available and whether they are enhanced with contrast. The quality of MR images depends not only upon field strength (>1.5 Tesla considered high), but also coil selection, contrast administration, imaging plane and sequence parameters, and ultimately interpreter experience and familiarity with pathologic processes and surgical interventions. For optimum care, it is important to develop a relationship with your imaging facility to ensure quality and consistency, both technically and interpretively.

We use a generalized screening protocol of the pelvis to evaluate for nonfocal hip pain or suspicion of nonlabral pathology such as avascular necrosis (AVN), stress fracture, tendon avulsion, sports hernia, tumor, pubalgia, and marrow edema syndromes (Fig. 1). This type of MR study uses larger coils (ie, torso or body) and a wider field of view that includes both hips. Consequently, resolution is decreased and this protocol cannot be used to evaluate for labral tears and subtle chondral pathology. This protocol consists of coronal T1 and inversion recovery images; axial T1-weighted and T2-weighted with fat saturation, and sagittal T1-weighted images in the anatomic plane of the patient. Occasionally a large paralabral cyst can be seen, and a labral tear or advanced bone edema may indicate significant chondrosis.

In general, when one thinks of MR for evaluation of intra-articular hip pathology one refers to high-resolution unilateral direct MR arthrography. Direct MR arthrography involves fluoroscopic guided injection of the hip before MR imaging, and should not be confused with indirect MR arthrography, which relies on intravenous injection of gadolinium contrast with synovial uptake and

Fig. 1. Stress fracture of the medial cortex of the proximal femur (A) confirmed on MRI (B) using general screening MR protocol of the pelvis. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

diffusion into the joint, for which supportive widespread literature does not exist, but it can be useful. A prescription for "MRI hip/pelvis with/without contrast" will often get you neither study, and is reserved for detecting enhancement generally for cases of tumor or infection.

The use of direct MR arthrography is critical not only for preoperative assessment and confirming clinical suspicions, but it also provides information regarding surgical planning (ie, repairability of labral tears) and prognosis (as surgical outcomes are associated with degree of chondrosis) [2]. One study has shown that the clinical assessment is useful for detecting intra-articular pathology but not the type or extent of the pathologic process [3]. This same study also showed improved detection of intra-articular pathology with MR arthrography versus nonarthrogram MR. Other researchers show a high positive predictive value of MR arthrography, but suggested a negative study does not obviate the need for arthroscopy to detect pathology [4]. Due to its generalized acceptance and higher sensitivity and accuracy (90% and 91% versus 30% and 36%, respectively) compared with nonarthrogram MR images, we use unilateral direct MR arthrography to evaluate for labral pathology [5]. In our experience MR arthrography may, however, underestimate extent of injury as unpublished data regarding MR hip evaluation in professional golfers that underwent arthroscopic surgery revealed underestimation of average labral tear size (1.5 versus 2.0 cm) and degree of cartilage injury. Dissenting opinion on the use of MR arthrography from one study suggested nonarthrogram unilateral hip MR may accurately detect labral tears and cartilage defects using an "opti mized protocol"; however, one must consider whether this can widely reproduced in the average setting [6].

Nonetheless, we continue to prefer direct MR arthrography as part of our routine evaluation as other advantages exist. We are more confidently able to predict the morphologic appearance of labral tears (ie, degenerated, intrasub-stance, or detached), which guides surgical planning (ie, debridement, intrasub-stance suture banding, or suture anchor reattachment, respectively). We have also shown (unpublished data) MR arthrography helps predict the presence of capsular laxity and partial tears of the ligamentum teres, treatable entities often overlooked. The ability to detect these latter findings is likely influenced by the joint distention that occurs with MR arthrography.

Another advantage of direct MR arthrography is that the incorporation of anesthetic in the injection mixture can provide diagnostic information regarding intra-articular causes of pain. Intra-articular anesthetic has shown to be 90% accurate for detection of intra-articular pathology [3]. Others have shown that lack of response to lidocaine during MR arthrography does not exclude intra-articular pathology [7]. We routinely incorporate anesthetic (lidocaine) in our arthrogram injection mixture.

Although patient anxiety may exist regarding direct MR arthrography, the injection procedure is routine and fairly simple. Interestingly, one study evaluated patient perception of MR arthrography (all joints, not just hip) and found that patients described less pain than anticipated, and were generally willing to undergo the procedure to obtain more useful information [8].

Direct MR Arthrography Technique

Under fluoroscopic guidance, sterile conditions, and local anesthetic, we advance a 22-gauge spinal needle via an anterior or anterolateral approach targeting the mid- to proximal aspect of the femoral neck (Fig. 2). The femoral artery is palpated before injection to avoid injury, but at this level the vessels are usually located more medial. The patient is positioned with the hip internally rotated and knee mildly flexed and supported with a foam pad to expose the femoral neck and increase laxity to the anterior capsule. Sterile extension tubing is used to connect the needle to the syringe to avoid self-exposure of radiation to the operator's hand. Intra-articular positioning is confirmed with small 1- to 2-mL injection of nonionic iodine-based contrast followed by a dilute gadolinium contrast solution (0.2 mmol/L = 0.1 mL of gadolinium contrast in 20 mL solution), which contains lidocaine (5-10 mL) and normal saline for a total injected volume of 10 to 20 mL, depending on the patient. Overdistention is avoided, as a recent study showed blood flow to the femoral head can be diminished with increased intracapsular pressure [9]. Care is taken to avoid leakage of air bubbles into the joint, which can create artifact on the MR images that mimics debris in the nondependent portions of the joint. Alternatively, one can incorporate the nonionic contrast into the total mixture. If the patient is allergic to iodine-based contrast, rather than premedicating with steroids we avoid using the iodine-based contrast. We typically inject dilute gadolinium solution using fluoroscopic

Fig. 2. Fluoroscopic spot film shows normal appearance of intra-articular injection of nonionic iodine-based contrast and dilute gadolinium solution containing lidocaine from an anterior approach. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 2. Fluoroscopic spot film shows normal appearance of intra-articular injection of nonionic iodine-based contrast and dilute gadolinium solution containing lidocaine from an anterior approach. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

guidance and the tactile loss of resistance as the indicator of being intra-articular with good success. Allergic reactions to gadolinium contrasts are far more rare than iodine-based contrast agents allergies [10,11]. Complications of contrast injection including bleeding, infection, soft tissue injury, and allergic reaction are very low. Anecdotally, < 1% of patients may experience severe postprocedural pain thought to be related to reactive synovitis. This is often treated with rest, ice, nonsteroidal anti-inflammatory drugs, and antihistamine agents. Rarely, patients may notice transient numbness in the leg/thigh likely related to extravasations of dilute gadolinium solution containing lidocaine outside the capsule, which may be iatrogenic but most often related to underlying pathologic capsular perforation. We have not experienced any cases of infection or long-term complication of direct MR arthrography in over 500 cases.

After injection, patients are transferred on a stretcher to the MR unit within 30 minutes to minimize chance of extravasation from the joint. All hips are imaged on 1.5 Tesla field strength MRI or higher to allow for sufficient signal and resolution. We use a phased array surface coil centered over the hip [12]. Scout images are checked to ensure proper coverage and signal output. We prefer a smaller field of view (14-16 mm) to enhance resolution and visualization of the labrum. We also use a combination of T1- and T2-weighted sequences with and without fat saturation in the true coronal and sagittal planes, as well as the oblique axial plane, that is directly perpendicular to the anterior acetabu-lum (ie, parallel to femoral neck) (Fig. 3). Our diagnostic checklist includes not only evaluation of the labrum, but a search for cartilage defects, ligamentum teres tears, anterior and posterior capsular injuries, joint debris, iliopsoas and rectus femoris insertional injuries, marrow signal changes, and muscle injury.

Specifics of our pulse sequences for unilateral MR arthrogram is as follows: true coronal T1 fat saturated (repetition time [ TR] 600, echo time [ TE] min, echo

Torn Labrum Mri

Fig. 3. (A) Coronal T1 fat-saturated image with 16-cm field of view demonstrating plane orientation of oblique axial images. (B) Oblique axial T1-weighted image with anterosuperior detached labral tear (short arrow) eventually reattached with suture anchors. Also note tapered appearance of anterior capsule from lateral to mid-portion (long arrow), which correlates with surgical and clinical findings of iliofemoral ligament/capsular laxity. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 3. (A) Coronal T1 fat-saturated image with 16-cm field of view demonstrating plane orientation of oblique axial images. (B) Oblique axial T1-weighted image with anterosuperior detached labral tear (short arrow) eventually reattached with suture anchors. Also note tapered appearance of anterior capsule from lateral to mid-portion (long arrow), which correlates with surgical and clinical findings of iliofemoral ligament/capsular laxity. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

train length 8, frequency and phase matrix 320 x 256, slice thickness 4 mm with 1-mm interslice gap, number of excitations [NEX]=2), true coronal T2 weighted with fat saturation (TR >4000, TE = 68, echo train length 3, frequency and phase matrix 256 x 224, slice thickness 4 mm with 1-mm interslice gap, NEX = 2), oblique axial T1 (TR 600, TE min, echo train length 8, frequency and phase matrix 256 x 224, slice thickness 4 mm with 1-mm interslice gap, NEX = 2), oblique axial T2 with fat saturation perpendicular to the plane of the acetabulum TR >4000, TE = 68, echo train length 3, frequency and phase matrix 256 x 224, slice thickness 4 mm with 1-mm interslice gap, NEX = 2)

and oblique sagittal Tl-weighted images (TR 600, TE min, echo train length 8, frequency and phase matrix 256 x 224, slice thickness 4 mm with 1-mm interslice gap, NEX = 2).

Use of Other Modalities

Fluoroscopy, aside from providing localization for direct arthrography as described above, is not routinely used, but may be used to assess joint laxity by demonstrating translation or presence of vacuum phenomena with mild traction [13]. Fluoroscopy is also used to guide injections of steroid or visco-elastic supplementation.

Computed tomography (CT) has a limited role as well, and is used primarily for evaluation of small joint bodies, traumatic fracture, bony alignment, and osteoid osteoma. Occasionally, with good success we use multidetector CT arthrography of the hip to evaluate labral pathology in patients that cannot undergo MRI procedures (positive metal screening or significant claustrophobia). In general, with CT, radiation doses to the pelvic organs may be substantial, a concern primarily in the pediatric population. The technique should minimize radiation dose, whenever possible [14]. Recently, multidetector/ multislice CT arthrography of the hip was found useful for evaluating the degree of chondrosis in dysplastic hips. There may be a role for CT arthrography in the future (Fig. 4).

Nuclear medicine bone scintigraphy often provides sensitive but nonspecific information with poor spatial resolution, and is not routinely used at our institution for hip pain in the athlete. One study described increase uptake at t t

Fig. 4. Axial image from multidetector CTarthrogram of the hip in a patient with severe claustrophobia showing normal contour of the anterior labrum (black arrow) and a normal variant posterior labral cleft (white arrow). Note the mild diffuse cartilage thinning with this technology involving both sides of the joint (white arrowheads) as well as a hypertrophied and frayed liga-mentum teres (black arrowhead).

Evaluation Piriformis Muscle

Fig. 5. Twenty-two-year-old college runner with piriformis syndrome received good relief with a CT-guided intramuscular lidocaine injection of the piriformis muscle and perineural injection of sciatic nerve with anesthetic and steroid. In this image the needle is within the piriformis muscle (black arrow), but was subsequently advanced for additional perineural injection of the sciatic nerve at the site of potential impingement (white arrow). (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 5. Twenty-two-year-old college runner with piriformis syndrome received good relief with a CT-guided intramuscular lidocaine injection of the piriformis muscle and perineural injection of sciatic nerve with anesthetic and steroid. In this image the needle is within the piriformis muscle (black arrow), but was subsequently advanced for additional perineural injection of the sciatic nerve at the site of potential impingement (white arrow). (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

the anterosuperior rim in cases of FAI. Absence of this finding had a high negative predictive value [15]. However, even for stress fractures of the hip, MRI has supplanted scintigraphy as well [16].

Ultrasound of the hip is widely used in the pediatric population to assess for congenital hip disorders and joint effusions, but is infrequently used in the adult populations to assess intra-articular abnormalities. Due to its ability for real-time dynamic imaging, it does offer potential to detect internal snapping hip syndrome [17]. In general, the role of ultrasound for evaluating labral tears is limited. One study showed poor detection of labral tears, as only one eighth were visualized with ultrasonography [18]. However, one abstract presentation of 20 patients described good visualization of anterior labral tears during ultrasound guided injections of a steroid mixture [19].

Therapeutic injections of the hip and pelvis may provide diagnostic information and possible therapeutic relief. We routinely use CT guidance for accurate injection for sacroiliac (SI) joint pain, osteitis pubis, piriformis syndrome, iliopsoas bursitis or insertional tendonitis, and peritendinous injections of the gluteus medius/minimus and hamstring insertions (Fig. 5). Based on operator preference, fluoroscopic and ultrasound guidance can be used as well.

RADIOGRAPHIC EVALUATION AND MEASUREMENTS OF THE HIP

To identify more recently described findings of FAI, our radiographic hip protocol deviates from common past screening hip radiographs. The AP weight-bearing view of the pelvis and AP view of the hip provides multiple measure ments to assess acetabular coverage and orientation [1,20,21]. Radiographic measurements should be performed after a thorough assessment for subtle fractures or tumors, soft tissue, and intrapelvic anomalies, as well as sacroiliac, pubic symphyseal, and lower lumbar pathologies.

A commonly used measurement to assess for readily apparent acetabular dysplasia is the lateral center edge angle (of Wiberg), which is obtained by drawing a line from the center of the femoral head to the lateral margin of the acetabulum (as its name implies) referenced to a vertical perpendicular line originating from the center of the femoral head (Fig. 6A). Normal values vary, but generally, values less than 20° to 25° are considered abnormal. The anterior lateral edge angle (or false profile view) has also been used, particularly when the center edge angle is abnormal. The horizontal toit externe (THE) angle, also known as acetabular index of the weight-bearing surface, is measured from a line parallel to the weight-bearing surface of acetabulum referenced to a horizontal line (Fig. 6B). Values greater than 10° are considered abnormal. Other measurements such as femoral head extrusion index (with normal values of less than 25%) and acetabular index of depth may also be useful.

However, to evaluate for FAI it is essential to detect more subtle abnormalities of the femoral head-neck junction and acetabulum that are associated with labral tears. Femoroacetabular impingement has been categorized into two basic types [22]. Type 1 is loss of femoral head neck offset, also known as cam-type or pistol-grip deformity, and is best identified on crosstable lateral view (or CT or MRI), but can be appreciated on some AP and frogleg lateral views depending on the severity. On the AP view the femoral head may appear nonspherical [23].

Lateral Center Edge Angle Hip Dysplasia

Fig. 6. AP view of the pelvis in a patient with hip dysplasia shows that the lateral center edge angle is markedly decreased less than 20° (A) and the acetabular index of the weight-bearing surface is increased above 10° (B). Note also substantial lack of coverage of the femoral head, also indicative of a dysplastic hip. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 6. AP view of the pelvis in a patient with hip dysplasia shows that the lateral center edge angle is markedly decreased less than 20° (A) and the acetabular index of the weight-bearing surface is increased above 10° (B). Note also substantial lack of coverage of the femoral head, also indicative of a dysplastic hip. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Type 2, or pincer-type impingement, is associated with acetabular retroversion. A combination of cam and pincer types has been described, as well as impingement associated with a deep acetabular socket and morphologic changes of the proximal femur such as coxa vara [22,24,25].

Femoral head-neck offset can be assessed on plain films using the crosstable lateral view with the leg in neutral position (Fig. 7) [26,27]. This image is obtained by placing the film cassette adjacent to the hip of concern with the patient in the supine position while the opposite knee is bent to allow passage of the X-ray beam. Offset is measured by creating a line along the longitudinal axis of the femoral neck (which may or may not intersect the center of the femoral head). Two other parallel lines are placed at the level of the anterior femoral neck cortex and the most anterior margin of the femoral head. Distance or offset between the two most anterior lines has been shown to be less than 7.2 mm (SD 2.6) in abnormal cases and 11.5 mm (SD 2.2) in asymptomatic normal patients. A ratio can also be determined by dividing by the diameter of the femoral head. The cause of cam-type impingement is unclear but thought to result from physeal injury and extension of physeal scar [26]. However, it is possible that repeated abutment of the femoral head-neck junction may cause bony bump formation [27]. Also, the finding could be related to underlying initial soft tissue injuries such as labral tear or capsular instability, causing altered mechanics and bone remodeling, reminiscent of Fairbank's type changes in the knee. One immunohistologic analysis of perilesional capsular tissue suggested progenitor cells were recruited to this region of the bony bump, which would support the latter hypotheses [28]. The cause however, is likely multifactorial.

Pincer-type FAI is associated with acetabular retroversion. Although acetabu-lar dysplasia is often associated with anteversion, several recent studies estimate that acetabular retroversion can be seen in one sixth to one third cases of acetabular dysplasia, a finding that may influence surgical techniques and approaches (ie, adjustment of acetabular realignment procedures) [20,29]. When the anterior rim abnormally crosses over the posterior rim (usually superiorly) on plain film radiographs, this finding has been termed the crossover sign, and represents a marker of acetabular retroversion (Fig. 8) [30]. The posterior acetabular rim should also lie medial to the center of the femoral head as well (posterior wall sign). These findings must be measured on a well-centered AP view of the pelvis with the distance between the sacrococcygeal joint and pubic

Fig. 7. (A) Thirty-two-year-old professional football player with femoroacetabular impingement seen on crosstable lateral view. (B) There is loss of normal femoral head-neck offset, which is confirmed with an MR arthrogram. MRI also shows a complex intrasubstance predominant anterior labral tear (arrow) treated arthroscopically with intrasubstance suture banding. A normal crosstable lateral view shows good offset between lines B and C. Line A is drawn along the femoral shaft, lines B and C are drawn parallel to line A along the anterior femoral neck cortex and anterior femoral head respectively (C). (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Acetabular Retroversion Measurement

Fig. 8. (A) Acetabular retroversion with positive crossover sign. (B) The same patient with annotations marking crossover sign of the anterior rim (black line) of the acetabulum superiorly over the posterior rim (white line). Note relationship of coccyx with pubic symphysis. This radiograph shows the pelvis is slightly reclined, which can minimize appearance of retroversion. One must account for reclination, inclination, and rotation to properly assess the degree of acetabular retroversion. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 8. (A) Acetabular retroversion with positive crossover sign. (B) The same patient with annotations marking crossover sign of the anterior rim (black line) of the acetabulum superiorly over the posterior rim (white line). Note relationship of coccyx with pubic symphysis. This radiograph shows the pelvis is slightly reclined, which can minimize appearance of retroversion. One must account for reclination, inclination, and rotation to properly assess the degree of acetabular retroversion. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

symphysis measuring about 3 cm to 5 cm (3.2 cm male, 4.7 cm female) [31]. Others consider the radiograph well centered when the coccyx is about 1 cm from the pubic symphysis [29]. Reclination of the pelvis can underestimate the appearance of retroversion (crossover sign), and inclination can overestimate the finding. One must also account for rotation of the film as well (ie, rotation of the pelvis to the right increases appearance of retroversion on right and decreases on left).

Normal Variants or Pathologic Process?

Periacetabular ossicles are often anecdotally considered normal variants of secondary ossification centers of the acetabulum. However, when we see small superolateral periacetabular ossicles we raise suspicion for underlying labral pathology (Fig. 9). "Os acetabuli" have also been described in dysplastic hips with anterior rim syndrome where the labrum was detached along with an avulsed a piece of the acetabular rim [32].

Radiographic findings of the synovial herniation pit of the hip were first described in 1982, and have a characteristic appearance [33]. Despite past considerations as a normal anatomic variant in about 5% of the population, the original description hypothesized that the finding could be a pathologic abnormality in the setting of a painful hip. Subsequent reports suggesting a pathologic nature as well, described soft tissue impingement, enlargement over time, and increased uptake on bone scan [34,35]. Currently when seen, we report herniation pits as compatible with femoroacetabular impingement (Fig. 9).

Synovial Herniation Pit Femoral Head

Fig. 9. Synovial herniation pit with periacetabular ossicle in the setting of cam type 1 FAI on AP view (A) and frogleg lateral view rather than crosstable lateral view (B). Note loss of sphero-city of the femoral head on both views. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

Fig. 9. Synovial herniation pit with periacetabular ossicle in the setting of cam type 1 FAI on AP view (A) and frogleg lateral view rather than crosstable lateral view (B). Note loss of sphero-city of the femoral head on both views. (From Armfield DR. Clinical evaluation of the hip: radiologic evaluation. Oper Tech Orthop 2005;15(3):182-90, with permission.)

A recent publication retrospectively reviewed 117 hips with femoroace-tabular impingement and found fibrocystic changes (ie, synovial herniation pit) on AP radiographs in one third of the cases. Dynamic MR and intraoperative observations of the same patients demonstrated close proximity of the fi-brocystic lesion with area of impingement suggesting a causal relationship [36]. One recent presentation of radiographic analysis of 54 patients with findings suggestive of FAI on frogleg lateral view reported 15% had synovial her-niation pits and 30% had periacetabular ossicles [37].

MR EVALUATION OF THE HIP

Labrum

Acetabular labral tears have become a commonly recognized source of intra-articular hip pain that affects athletes and nonathletes alike. Although strongly associated with athletes performing twisting pelvic motions and rotations of the hip that occur in sports like soccer, golf, football, ballet, and hockey; athletes in all major sports (and even minor ones such as skateboarding and Olympic yachting) have been affected [38]. Many tour-level professional golfers have undergone successful hip surgery for labral pathology with return to previous level of play and sometimes beyond prior performances (Marc J. Philipponm, personal communication). As stated earlier, direct MR arthrography is the best imaging modality for evaluation of underlying intra-articular disorders. Interpretation should not only include labral evaluation, but also evaluation of chon-dral, capsular, bony, ligamentum teres, and adjacent extra-articular (iliopsoas, rectus femoris, pubic symphysis) abnormalities (Fig. 10). However, it is important to also realize that the clinical situation ultimately dictates the need for surgical intervention, as a negative MR arthrogram does not currently obviate arthroscopic evaluation [4].

Intact Labrum Hip
Fig. 10. Oblique axial T2 fat-saturated image of an intact labrum, but there is a partial tear of the undersurface of gluteus minimus tendon insertion (white arrow) with surrounding lateral edema and inflammation (black arrow). It is essential to search for surrounding extra-articular abnormalities.

The labrum is generally considered a triangular-shaped structure with its medial base firmly anchored to the rim of the acetabulum with the apex extending laterally. It extends nearly circumferentially around the horseshoe-shaped acetabulum but blends with the transverse acetabular ligament inferi-orly (Fig. 11).

On the articular side, the labrum merges with the acetabular cartilage over a 1- to 2-mm transition zone [39]. On the capsular side, this transition does not exist. The labrum (like the meniscus) has been shown to contain nerve endings (presumable related to nocioceptive and proprioreceptive function), and is thought to have low intrinsic healing ability due to low vascularity primarily obtained from the capsule [40,41]. Biomechanically, the labrum increases the depth of the acetabular socket and helps maintains negative intra-articular pressure that increases static stability [42,43]. When the labrum is torn, forces on adjacent cartilage increase, suggesting a role in the development of cartilage injury and arthritis [44].

The labrum demonstrates typical MR imaging features of organized collagen elsewhere in the body with decreased low signal intensity on T1- and T2-weighted images. However morphologic (rounded or irregular) and increased intrasubstance signal intensity changes have been seen in asymptomatic individuals with increasing age based on nonarthrogram MR imaging and likely represent areas of degeneration [45-47]. However, in the young athlete undergoing evaluation for labral tear these findings are considered abnormal.

Several confusing issues regarding the MR appearance of the labrum should be addressed and understood (Fig. 12). First, on MR arthrography there is a normal perilabral recess between the capsule of the hip (particularly superiorly on coronal images) and the capsular side of the labrum (Fig. 11B). This recess may not be seen in a nonarthrogram MRI due to lack of capsular distention,

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