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Imaging of hip injuries in dancers.

Pain in the hip region in dancers can be related to a variety of conditions, including injury to the acetabular labrum and femoral cartilage as well as the tendons and bursae surrounding the hip. Often there are anatomic anomalies that predispose to pathology. In addition, it may be difficult to clinically differentiate pain originating from the hip from referred pain related to pathology at the groin or lumbar spine. A variety of conditions that can cause a dancer to present with the complaint of hip pain are discussed, and the advantages and disadvantages of the various imaging modalities for their diagnosis are described.

Imaging Modalities X-ray (Radiography)

In most situations radiographs are the best initial test for evaluation of hip pain; cheap, fast, and widely available, they provide an initial global assessment of osseous anatomy. The standard hip exam includes anteroposterior (AP) and frog lateral (abducted, externally rotated) views, as well as an AP view of the pelvis. In primary osseous conditions, such as avulsion or stress fracture, the evaluation may end with this exam. However, early stress injuries are often not seen on radiographs, and most other injuries and sources of pain presenting in dancers have a component of soft tissue pathology that will not be evident on this modality. Therefore, a "normal" x-ray report should not give the physician or therapist a false sense of security. Alternate views such as Judet views (45[degrees] oblique views of the pelvis), cross-table lateral view (also known as a "surgical lateral view"), pelvic inlet and outlet views (with cranial and caudal angulation), and the false profile view (65[degrees] oblique standing view of the hip) can be acquired as needed to better visualize anatomic details if needed. Judet views are particularly useful for evaluating the anterior and posterior columns of the pelvis, as well as the obturator ring; this and inlet and outlet views have been popular for characterizing pelvic fractures (although multidetector CT has replaced this indication in many settings). The false profile view (Fig. 1) can be used to assess anterior acetabular coverage and anterior joint narrowing in cases of suspected femoroacetabular impingement or dysplasia. (1)

Arthrography

Plain radiographic arthrography has mostly been replaced by more advanced modalities. However, when used in combination with computed tomography (CT) or magnetic resonance imaging (MRI) arthrography, evaluation for internal derangement is superior to the advanced modality alone in certain cases, specifically for hip joint injuries such as labral tears and cartilage injuries (Fig. 2). Additionally, one can inject anesthetic at the same time, providing the referring physician, therapist, or trainer with useful information regarding whether pain is originating from within the joint or outside the joint. If pain disappears for 2 to 4 hours following lidocaine injection, it is evidence that it is arising from an abnormality within the hip joint. (2)

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Computed Tomography

Computed tomography of the hips and pelvis employs a relatively large dose of ionizing radiation compared to radiographs and should, therefore, be used only when absolutely necessary, especially in children and young adults. (3-7) The effect of ionizing radiation on an area of anatomy is cumulative; this "lifetime dose" should be taken into account whenever CT or radiographs (in particular "additional views") are requested. (3,4) The recent development of multidetector technology (in which multiple rows of x-ray detectors are exposed at once instead of the single row used in previous generations of CT) has enhanced the diagnostic capability of this modality, but its increased use has also contributed to the overall annual radiation dose to the population. (5) Multidetector CT (MDCT) enables acquisition of very thin cuts, on the order of 0.6 mm. Data can also be acquired using overlapping slices resulting in a block of data with isotropic characteristics (equal length, width, and thickness); isotropic acquisition allows for reconstruction of the dataset in any plane without loss of information. High quality three-dimensional representations can be reconstructed from the data and are often used for visualization of anatomy prior to surgery. Increased photon flux also improves overall quality of the image, with the added benefit of reducing metal artifact. However, use of thinner cuts and overlap of slices (decrease in "pitch") is not without cost, and this further increases radiation dose. (6,7)

Multidetector CT can be combined with arthrography to produce high resolution CT arthrographic images. Computed tomography is superior for imaging of bony anatomy and pathology such as is seen in femoroacetabular impingement, but this benefit must be weighed against the added radiation exposure. The high quality of MDCT has led to increased use of this modality and an increase in radiation dose to the population overall. Magnetic resonance imaging may be adequate for bony imaging in most cases. In general, unless MRI is contraindicated, MRI and MR arthrography is considered superior for most indications.

Magnetic Resonance Imaging

Magnetic resonance imaging has become an important part of the imaging algorithm for many high-level sports injuries. It provides high resolution imaging with excellent osseous and soft tissue detail and uses no ionizing radiation. (8,9) However, relatively sparse availability of subspecialty interpreters and variable image quality can be problematic. In general, when dealing with dancers and other athletes where very subtle injuries can have a significant impact on performance, a high field strength scanner (1.0 Tesla or above) should be sought along with a subspecialty musculoskeletal radiologist reading. (10) On the other hand, every effort should be made by the person writing the prescription to specify the injury suspected, as hip and pelvis protocols can vary tremendously depending on the clinical scenario. (11) Magnetic resonance arthrography is particularly useful for evaluation of labral pathology. (12-14)

Ultrasound

Ultrasound is excellent for problem-solving when a specific diagnosis is suspected, such as bursitis. It has the capability to perform an assessment actively in the position of pain and at the point of tenderness. (15) Ultrasound also has the capability to perform a dynamic assessment of the area of pain during range of motion, which may be beneficial for certain diagnoses such as snapping hip. (16) However, visualization of intra-articular structures such as the labrum, as well as intra-osseous pathology, is limited, (17) and ultrasound cannot provide a global assessment if the clinical presentation is nonspecific. Also, expertise in performance of musculoskeletal ultrasound is highly variable.

Nuclear Medicine

Nuclear medicine exams have limited utility for dance-related injuries, except perhaps Tc-99m-MDP bone scan for evaluation of stress fracture. However, MRI provides greater resolution and better characterizes the extent of osseous injury, in addition to providing assessment of associated soft tissue pathology and is generally considered superior in this circumstance.

Pathology

Stress Injury

Stress injury is common in athletes of all types, and dancers are no exception due to the high impact and repetitive actions associated with practice and performance. If there is underlying osteopenia (which can affect young individuals due to smoking, eating disorders, or metabolic conditions), the dancer can be particularly susceptible. (18-20) When stress injuries occur, the bone requires breakdown and remodeling before it strengthens, resulting in transient weakening of its structure as a repetitive stress is applied (Wolff's law). Muscle training results in a consistent strengthening over the short term without a prolonged breakdown period. This disparity results in a period during which the bone is weaker and the muscle is stronger; the elastic limit of bone is easily overcome, and the repetitive recurrence of stress causes the injury.

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On radiographs bony stress injuries initially may be invisible, although if the injury occurs in an area with periosteal covering a faint periosteal reaction may be visible. In the early stage, MRI will show a curvilinear subcortical rim of edema. As the injury becomes more established, radiographs or CT show a sclerotic line extending from the cortex into the medullary bone perpendicular to the major trabecular lines of stress. Magnetic resonance imaging shows a low signal line corresponding to the sclerotic line (representing trabecular reparative callus) surrounded by marrow edema. (9,21,22) Common locations for stress fracture in young adults include the pubic rami and femoral neck (Figs. 3 and 4). Stress fractures can also develop in the proximal femoral shaft related to adductor insertion avulsive stress, also known as "thigh splints" (Fig. 5). (23)

Stress fractures occasionally occur in the subchondral bone of the femoral head. (24,25) The associated hyperemia results in calcium resorption and can produce the appearance of osteopenia on radiographs and CT. Because of the radiographic appearance, this entity was formerly referred to as "transient osteoporosis of the hip." It is seen on MRI as intense bone marrow edema within the femoral head, extending into the intertrochanteric region (Fig. 6). A subchondral crescent of low signal is seen, which represents the fracture line. The MR imaging appearance is similar to that of avascular necrosis (AVN), but the latter entity generally shows increased bone density on radiographs and CT.

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Muscle Strain and Tendon Avulsion

Muscle strain is perhaps the most common injury observed in the dancer with pain. A variety of muscles can be injured; the pattern depends on the specific movement or mechanism involved. Most muscle strains are diagnosed clinically and treated with physical therapy. For more significant injuries, or those that do not improve with rest and time, an MRI may be obtained. Magnetic resonance imaging provides the best evaluation; it yields large field-of-view images and allows for a global assessment of the injury pattern (Fig. 7). (11) Strains are graded by MRI on a scale from 1 to 3. Grade 1 strain is a minor injury, exhibiting mild edema without disruption of fibers. Grade 2 strain is synonymous with a partial muscle tear where more soft tissue edema is observed along with discontinuity of some fibers (Fig. 8). There may be fascial edema as well, and the disrupted fibers may retract along the central tendon complex arising from the myotendinous junction, resulting in a low signal center with surrounding edema at the injury site that is characteristic of a muscle injury. Grade 3 strain is the same as a complete muscle tear with full discontinuity. With this injury, there is extensive soft tissue edema and often hematoma. (26-31)

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Direct muscle injury (e.g., from traumatic impact) generally occurs in large muscle groups and is centered at the muscle belly. The mid-thigh is a common location (Fig. 9). Muscle damage related to indirect injury (e.g., eccentric contraction where the muscle is contracting as it is forced to stretch) is far more prevalent and most commonly occurs at the myotendinous junction. (32-34)

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Avulsion fractures occur around the pelvis after acute trauma, but in adolescents stress-related apophyseal injuries can occur. (35-37) These are common at the sartorius muscle origin at the anterior-superior iliac spine, at the rectus femoris origin at the anterior-inferior iliac spine (Fig. 10), and at the hamstring origin at the ischial spine (Fig. 11). These can be diagnosed on radiographs but may be confused with changes related to developmental variation. If confirmation is necessary, MRI will show edema at the junction, indicating underlying pathology.

Groin Pain and Athletic Pubalgia (Also Known as "Sports Hernia")

Groin pain is common in dancers, males in particular, and can represent a wide range of pathology from the hip laterally to the pubic symphysis medially and superoinferiorly from the lower abdomen to the upper thigh. Again, if the clinical exam cannot determine the source of pathology, MRI provides an excellent global assessment. This test, in conjunction with a diagnostic anesthetic hip injection (optimally combined with contrast injection in the form of MR arthrography), can delineate most causes of groin pain specifically.

One syndrome that deserves attention is the "sports hernia," since much work has recently been performed on the etiology of the process, which can be a source of acute or chronic groin pain. The name is actually a misnomer, as the pathology rarely represents a hernia. If hip joint pathology can be excluded, the origin of pain is often related to an avulsive injury to the rectus abdominis and adductor common aponeurosis, which attaches to the anteroinferior capsule of the pubic symphysis. (38,39) At this location there is a concentration of forces that can result in a shearing injury and a variety of patterns on MR imaging. (39-41) A small field-of-view specialized MRI protocol (available at www.bone.tju. edu) of the pubic symphysis is recommended. Injury patterns involving the pubic symphysis include acute osteitis pubis (seen as diffuse bone marrow edema around the joint, Fig. 12) and even subchondral stress fracture (with a low signal line in the marrow adjacent to the symphysis with surrounding edema). This injury pattern may be initiated by disruption of the joint capsule and resultant instability of the articulation. With injury to the rectus-adductor aponeurosis, this common attachment can "peel off" the capsule of the pubic symphysis (Fig. 13) and cause severe pain and tenderness over the inguinal ring with limited leg adduction (this is why the incorrect term "sports hernia" was originally applied). Alternatively, the common adductor tendon can tear or avulse and retract. With these lesions (the Zoga lesion), MRI is the best diagnostic modality, with fluid seen under or within the aponeurosis. (39-41) There is a variable degree of associated adductor muscle strain (usually involving the adductor longus).

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Bursitis

Bursitis is usually a clinical diagnosis, and MRI is only obtained after failure of conservative management to rule out other pathology. Bursitis is common at the hip and pelvis and may be a sign of additional underlying pathology. Most tendon origins and insertions about the pelvis are associated with an anatomic bursa, and the bursa can be thought of as a window to tendon pathology. For example, when greater trochanteric bursal fluid is seen on an MRI or ultrasound, attention should be directed to the adjacent gluteus medius tendon, which may be torn. (42) Alternatively, if the patient has a snapping hip, this could be a manifestation of a tight iliotibial band resulting in snapping over the greater trochanter. Similarly, iliopsoas bursitis (Fig. 14) can be related to snapping of the tendon over a ridge at the anterior acetabulum. (43,44) Hamstring bursitis is another common entity and is usually related to tendinosis or tear of the tendon origin.

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Labral Tear and Cartilage Lesions

Acetabular labral tear and unstable cartilage lesions can also result in pain and a snapping sensation at the hip. (45) The labrum is a fibrocartilagenous ring surrounding the anterior, superior, and posterior acetabulum (inferiorly there is no labrum, but rather a transverse ligament that spans the anteroinferior to the posteroinferior acetabulum). It helps stabilize the joint. Tears can result from a twisting injury or from dislocation, but the most common cause is thought to be related to a developmental anatomic variation with dysplasia on one end of the spectrum and impingement on the other. A labral tear is seen as fluid or contrast extending under or into the labral substance anteriorly, superiorly, or posteriorly (Fig. 15); anterior and superior tears are by far the most common. (9-14)

Labral tears are often associated with cartilage lesions; the hyaline cartilage blends with the fibrocartilagenous labrum, and a labral tear often propagates into the hyaline cartilage. In the case of a shearing type injury to the labrum, hyaline cartilage can delaminate, or "peel away." (10) Labral lesions resulting from hip dysplasia or impingement can result in unstable cartilage flaps or more diffuse cartilage loss and early osteoarthritis. (46)

If a labral tear or a cartilage lesion is suspected, MR arthrography is the best diagnostic modality, followed by high resolution MRI, followed by CT arthrography. (11-14) Plain CT and ultrasound cannot adequately image this lesion. Radiographs remain useful to assess the femoral and acetabular morphology with reference to dysplasia and impingement.

Hip Dysplasia and Femoroacetabular Impingement (FAI)

Anatomic variation of the femoral head and acetabulum can contribute to hip labral tear and cartilage loss and eventually lead to premature osteoarthritis. This variation can be thought of as existing along a spectrum of dysplasia to impingement. (47-50) In dysplasia, the acetabulum is relatively small, resulting in undercoverage of the lateral aspect of the femoral head (Fig. 16). During weightbearing, excess stress is placed at the superior (lateral) margin of the acetabulum, as well as the labral and capsular structures and the superior hyaline cartilage. In response to the stress, the labrum initially hypertrophies (Fig. 2) and eventually undergoes degeneration and tearing. (47) Abnormal forces on the hyaline cartilage result in early wear, especially superolaterally. In dysplasia, AP radiographs show undercoverage of the lateral femoral head articular surface and widening of the medial joint. An increased acetabular angle or upturn of the lateral acetabular rim can be seen. Magnetic resonance imaging in early stages can show blunting and enlargement of the superior labrum with diffuse increased signal on all sequences reflecting degeneration. Early secondary arthritis is characterized by superolateral cartilage loss (on radiographs, the superolateral joint appears narrower than the superomedial joint; normally the joint is uniform width along the superior medial and lateral aspect). In later stages subchondral cystic change and bone marrow edema is seen.

At the other end of the spectrum is a developmental variation or acquired deformity resulting in a mismatch of the shape of the femoral head and acetabulum leading to abutment of the osseous surfaces at the joint margins upon the endpoints of hip range of motion. (48-50) This can result from an acetabulum that is too deep and over-covers the femoral head causing "pincer impingement" or from a non-spherical femoral head that abuts the acetabular rim with hip range of motion causing "cam impingement." These conditions may coexist; in some cases both cam and pincer impingement is present. A non-spherical femoral head can be related to a number of different etiologies, including Perthes disease and slipped capital femoral epiphysis; however, most commonly it represents a spectrum of morphological variation. Obviously a wide variety of radiographic appearances can be associated with this condition. However, the most common appearances on radiographs are a "bump" or "hump" at the anterolateral aspect of the femoral head-neck junction (Fig. 17) or a flattening of the lateral femoral neck compared to the medial neck concavity (Fig. 18). (48-50) The osseous prominence should not be presumed to represent an arthritic osteophyte, especially in a young adult without joint narrowing. If this finding is present in a dancer with chronic hip pain, the diagnosis of FAI should be considered. The bump is often not seen well on AP radiograph as it is somewhat anteriorly positioned; the frog lateral view brings the bump into the imaging plane. On MR imaging an oblique axial imaging sequence (aligned along the femoral neck) can be used to measure the "alpha angle" (the angle between the femoral neck and the point at which the femoral head and neck junction leaves a circle drawn around the femoral head, Fig. 19). Notzli and colleagues determined that the angle averages 42[degrees] in individuals without impingement, and 74[degrees] in patients with FAI. (51) However, this measurement has fallen out of favor due to low sensitivity (the technique is limited to bumps that extend to the mid-anterior neck) and low interobserver agreement. (52)

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Variation in acetabular orientation can also lead to impingement. The normal acetabulum is anteverted (tilted forward), which allows for full hip flexion. If the acetabulum is instead tilted posteriorly, it is known as acetabular retroversion. In these cases, there is overhang of the anterior acetabular rim that can result in bony contact between the femoral neck and anterior acetabular rim during hip flexion. (53,54) Retroversion is seen on an AP radiograph of the pelvis as crossover of the anterior and posterior acetabular rim, resulting in a "figure 8" or "infinity" pattern (Fig. 20). (54) On axial CT or MR images retroversion is seen as lack of anterior acetabular orientation on cuts through the upper joint.

A similar pattern analogous to retroversion can occur functionally in individuals with prominent lordosis. In this case, the entire pelvis is tilted anteriorly, resulting in a propensity to impinge the anterior margin of the femoral head-neck junction against the anterior acetabular rim during hip flexion.

Magnetic resonance imaging in later stages generally shows an anterosuperior labral tear at the site of impingement on flexion and internal rotation. Bone proliferation can occur at the acetabular rim in this location, and it is controversial whether an ossicle in this location (os acetabuli) is a normal variation or is associated with this process. Additionally, a synovial herniation pit at the anterior femoral neck has been implicated by some investigators as related to FAI. (55) This is also controversial, since many believe this represents a normal variation. Hyaline cartilage loss can also occur related to the various forms of impingement; the cartilage of the hip joint is difficult to visualize on MRI, as it is typically no more than 1 mm thick. A small field-of-view (e.g., less than 16 cm) T2-weighted or proton density image in the sagittal plane is essential for evaluation, as the labral tear and cartilage injury is usually present anteriorly. The cartilage lesion is often a delamination-type lesion in which the cartilage is lifted off the bone. If nondisplaced, these cartilage lesions are difficult to visualize, even with high quality MR imaging.

Symptoms of hip dysplasia and femoroacetabular impingement include pain or snapping that occurs mainly with flexion and internal rotation or extension and extreme external rotation. In a positive hip impingement test, hip flexion and adduction or internal rotation causes groin pain. However, symptoms may occur after prolonged sitting, with a sensation of painful "locking" of the hip. In general, females are more prone to dysplasia and pincer impingement, whereas cam-type FAI is seen more commonly in males. A standard radiographic evaluation includes an AP pelvis view, and AP and frog-lateral views are best for screening patients for dysplasia and FAI. A faux profile view can be useful for evaluating the anterior acetabular coverage of the femoral head and associated anterior joint narrowing.

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Summary

For the majority of conditions involving the hip (and groin) in dancers, radiographs are the best initial test. A routine radiographic series includes an AP pelvis view, and AP and froglateral views are generally adequate for evaluation of osseous findings, with additional views as needed to resolve ambiguities. However, for soft tissue and osseous injury, the best overall imaging modality is MRI. Magnetic resonance arthrography is optimal for evaluation of hip joint internal derangement and can be combined with anesthetic injection to provide additional clinical information. Computed tomography is useful, particularly for osseous injury, but must be used with caution in young patients due to the relatively high radiation dose. Ultrasound is excellent for answering specific questions but has many limitations and is dependent on operator experience. Hence, for most indications the recommended course includes radiographs followed by MRI if indicated, with ultrasound used for problem solving, depending on available expertise.

Caption: Figure 1: False profile view. A, Patient stands against radiographic plate with feet at 90[degrees] and pelvis angled at 65[degrees] toward the affected hip (arrow shows alignment of x-ray beam). B. This view is useful to evaluate the anterior joint space (arrow) and anterior acetabular coverage of the femoral head.

Caption: Figure 2: MR arthrography. Fluoroscopic image of the hip in a patient with dysplasia, following iodinated contrast injection into the joint. Note possible filling defect at the superolateral aspect (arrow). Corresponding coronal MR image shows filling defect to be a degenerated, hypertrophied acetabular labrum (arrows), a common consequence of dysplastic hip morphology. Note moderate underlying hyaline cartage loss superiorly.

Caption: Figure 3: Stress fractures of the superior pubic rami in an adolescent. Coronal T1W (A), Coronal STIR (B), and axial T2W fat-suppressed (C) images show bone marrow edema at the pubic rami bilaterally, with low signal lines (arrows) characteristic of stress fractures.

Caption: Figure 4: Stress fracture of the femoral neck. A, Anteroposterior radiograph of the hip shows sclerosis representing an incomplete fracture line at the medial cortex of the femoral neck (arrow). B, Coronal T2W fat-suppressed MR image confirms a fracture that has propagated across the entire femoral neck.

Caption: Figure 5: "Thigh splints" in an 18-year-old girl. Coronal T1W (A), Coronal STIR (B), and axial T2W fat-suppressed (C) images show bone marrow at the medial femoral shaft in a subcortical location with adjacent periostitis (arrows) consistent with stress fracture related to adductor insertion avulsive injury.

Caption: Figure 6: Subchondral stress fracture of the femoral head. A, Anteroposterior radiograph of the pelvis shows regional osteopenia of the right hip, better appreciated on comparison with the other side. Coronal T1W (B) and STIR (C) MR images show abnormal signal within the right femoral head extending into the intertrochanteric region, representing bone marrow edema due to the stress fracture centered in the subchondral bone.

Caption: Figure 7: Proximal hamstring tear. Coronal T1W (A) and coronal STIR (B) images show complete discontinuity of the medial and lateral hamstring origin (long arrows) from the ischium with retraction of fibers. Hyperintensity on the T1W image represents surrounding hematoma. Note normal anatomy of the contralateral medial (arrowhead) and lateral (short arrow) hamstring origins on the coronal T1W image (A).

Caption: Figure 8: Grade 2 strain (partial tear) of the biceps femoris muscle at the thigh. Axial T2W fat-suppressed (A) and coronal STIR (B) images of the thigh in a dancer with indirect injury show fluid signal with partial discontinuity of muscle fibers (arrows).

Caption: Figure 9: Muscle contusion and hemorrhage. Axial T2W fat-suppressed (A) and T1W (B) MR images of the upper thigh following a fall and direct blow show diffuse edema at the anterior soft tissues, with muscle edema predominantly in the vastus intermedius muscle. On the T1w image hyperintensity (arrows) indicates presence of acute and subacute blood products.

Caption: Figure 10: Avulsion fractures. A, An avulsed bone fragment (arrow) from the anterior-inferior iliac spine is seen in an adolescent, due to forceful rectus femoris contraction. B, Sagittal T2W fat-suppressed image shows avulsion of the rectus femoris attachment (arrow). C, Coronal STIR image from a different patient shows the avulsed fragment from the anterior-superior iliac spine (arrow) representing sartorius tendon avulsion with underlying marrow edema.

Caption: Figure 11: Ischial apophyseal avulsion in an adolescent. Axial T1W (A), axial T2W fat-suppressed (B), and coronal STIR image (C) show edema at the ischium with separation of the apophysis (arrows).

Caption: Figure 12: Osteitis pubis. Coronal STIR (A) and axial T2W fat-suppressed (B) MR images demonstrate diffuse marrow edema adjacent to the pubic symphysis. The changes are bilateral and symmetric, characteristic of osteitis pubis.

Caption: Figure 13: Avulsive injury of the common rectus abdonimis-adductor aponeurosis. A, Coronal STIR image through the pubis shows avulsive injury at the adductor origin (arrowhead); overlays at the upper margin of the image represent sagittal slices through the right and left pubic bones showing separation of the aponeurosis on the right (labeled "R") with fluid beneath (arrowhead). Compare with the normal left side (labeled "L"). Diagram in the center depicts anatomy of the attachment site of the common rectus abdominis and adductor on the anterior pubis. B, Axial T2W fat-suppressed image acquired superior to the pubis shows edema within the rectus abdominis muscle (arrow); note edema extending toward the external inguinal ring (arrowheads) leading to common clinical misinterpretation of a herniation. C, Axial T2W fat-suppressed image acquired inferior to the pubis shows avulsion and retraction of the adductor origin (arrow), with associated strain of the obturator externus, pectineus and adductor longus muscles (arrowheads).

Caption: Figure 14: Iliopsoas bursitis. A, Transverse ultrasound of the left hip demonstrates a large, hypoechoic, lobulated structure (arrow) anterior to the hip joint. B and C, Coronal and sagittal fluid-sensitive MR images show a large fluid collection (arrow) adjacent to the iliopsoas tendon anterior to the right hip joint, compatible with bursitis.

Caption: Figure 15: Acetabular labral tear on MR arthrography. A, Sagittal image shows detachment of the anterior labrum (arrow), with contrast extending beneath the base. B, Axial oblique image of a different patient demonstrates contrast extending into the anterior acetabular labrum (arrow), representing a labral tear.

Caption: Figure 16: Hip dysplasia. A, AP radiograph of the pelvis in a 26-year-old female with chronic left hip pain shows relative uncovering of the lateral femoral head on the left (arrowheads on overlay image) compared to the right side, with slight increase in the left acetabular angle (depicted by black lines) consistent with left hip dysplasia. B and C, AP radiograph of the pelvis (A) of a different patient shows bilateral, symmetric uncovering of the lateral femoral head (noted by arrow on the left). Coronal T1W MR image (B) of the same patient. Note the shallow acetabular fossae resulting in undercoverage of the lateral aspect of the femoral head (indicated by arrow on the left).

Caption: Figure 17: Anteroposterior radiograph (A) and MR arthrographic image (B) of the right hip in a patient with cam-type femoral-acetabular impingement. Note a bony prominence ("bump" or "hump") at the femoral head-neck junction anterolaterally (arrow).

Caption: Figure 18: Anteroposterior radiograph of the pelvis in a patient with femoral-acetabular impingement. Note lack of concavity at the lateral femoral neck compared to the medial aspect bilaterally. This results in lack of sphericity of the femoral head and can lead to a shape mismatch with the configuration of the acetabular cup on range of motion activities.

Caption: Figure 19: Technique for measurement of the "alpha angle." Inset image at the lower left shows oblique axial plane oriented through the femoral neck used to acquire the image. This results in an elongated appearance of the hip joint and can be useful for evaluation of acetabular labral tears (note anterior labral detachment, arrow, on the MR arthrographic axial oblique image). To measure the alpha angle, a circle is first drawn exactly encompassing the femoral head. Next, a line is drawn from the center of the neck to the center of the circle. Another line is drawn from the center of the circle to the point at which the anterior femoral head-neck junction leaves the circle. The angle between these lines is the alpha angle. If an anterior bump is present, the angle increases.

Caption: Figure 20: Acetabular retroversion. AP radiograph of the pelvis shows "crossover" sign, where the anterior acetabular rim (white line on inset image) crosses over the posterior acetabular rim (black line on inset image). In an anteverted hip, on a well-positioned AP pelvis image, these lines do not cross. CT and MR images above show the posterior tilt of the acetabulum at the upper aspect of the hip joint. Note the "bump" at the femoral head-neck junction, indicating superimposed cam-type FAI. Acetabular and femoral variations leading to dysplasia or impingement often coexist.

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Alex Hung Lit Chow, M.B.B.S., is at the Kwong Wah Hospital, Hong Kong. William B. Morrison, M.D., is Professor of Radiology, at Thomas Jefferson University Hospital, Philadelphia, Pennsylvania.

Correspondence: William B. Morrison, M.D., Thomas Jefferson University Hospital, 132 South 10th Street, Suite 1079a, Philadelphia, Pennsylvania 19107; william.morrison@jefferson.edu.
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Author:Chow, Alex Hung Lit; Morrison, William B.
Publication:Journal of Dance Medicine & Science
Date:Oct 1, 2011
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