The Hip

asymmetry in acetabular contour. Acetabular dysplasia is associated with labral tears and early osteoarthritis (OA), a condition known as lateral rim syndrome. Even a mildly abnormal shallow acetabulum predisposes to the development of premature degenerative chondral changes and labral tears. A shallow acetabulum is best visualized on anterior coronal images. Marrow signal throughout the pelvis and femur is often heterogeneous, as the pelvis is a common reservoir for red marrow.

and the ilium is analyzed for marrow-signal abnormalities, tumor, and fractures. The sacroiliac joints are examined for arthritis, posttraumatic changes, and infection. The sacrum is also a common location for both posttraumatic and insufficiency fractures.

or secondary to communication with the hip joint. Trochanteric bursitis suspected on coronal images is confirmed on axial images.

labral tear; to avoid this pitfall, suspected labral tears should be confirmed in all three planes.

100 asymptomatic patients.⁵³ In another study of renal transplant recipients, followed for 22 months using serial radiographs and MR, untreated AVN was shown to have a benign course without progression from Ficat stage 0.⁵⁴ Jiang and Shih reported that the presence of a complete or dense physeal scar on MR scans was associated with a high risk for AVN of the femoral head.⁵⁵ Segmental or incomplete scars in AVN were uncommon. The sealed-off or complete scar was shown to be a risk factor in patients with or without a history of steroid or alcohol abuse (associated lipogenic factors). Because it is possible to identify MR changes of focal osteonecrosis when radionuclide scans are negative and CT and plain film findings are normal,⁵⁶ a limited or modified MR examination could be used as a low-cost screening tool in at-risk populations.

metabolic bone disease do not benefit greatly from osteotomy.

T2-weighted images also are useful for displaying acetabular and femoral head cartilage. Metaphyseal irregularities and subchondral hyperintensity are identified on FS PD FSE coronal images (Fig. 3.94).

mild to moderate slips, the most common procedure is in situ pinning. Many types of hardware are used, including a variety of multiple pin techniques and single screws. With severe slips, some advocate a gentle closed reduction, open reduction, or cuneiform osteotomy.¹⁰⁵ The complication rate is high with internal fixation.¹⁰⁶ Chondrolysis may occur and has been attributed to pin penetration. Unrecognized pin penetration is a major problem. AVN, another serious complication of treatment in SCFE, may follow closed reduction, open reduction, osteotomy, or vascular damage from internal fixation. It is important to remember that this is an iatrogenic complication. Because of the high complication rate with internal fixation, bone graft epiphysiodesis has gained popularity in some centers.¹⁰⁷ In the long term, the resultant biomechanical abnormality predisposes the patient to degenerative arthritis.¹⁰⁸

is caused by compression between the limbus and the ligamentum teres. Constriction by the iliopsoas tendon may block attempts at reduction. Most cases of DDH present with a type 1 hip and positional instability. Failed hip reduction may be secondary to thickening of the ligamentum teres, an unfolded or blunted limbus, and severe deformity of the acetabulum or femoral head.

patient. Unfortunately, the hypertrophic labrum is exposed to greater joint reaction forces and is at an increased risk for symptomatic tearing. The acetabular labrum may also become inverted, entrapped, and subsequently torn. Direct contact between the hypertrophied labrum and the femoral head chondral surface may produce a chondral crease demarcating a femoral head bump formed proximal to the physeal scar. This finding is associated with a lateral acetabular rim or the DDH equivalent of FAI. Anterior coronal MR images evaluated at the level of the anteriormost portion of the femoral head are sensitive to asymmetry in the slope of the acetabulum. The anterior acetabular roof should maintain a relatively horizontal slope and not open up or deviate from the horizontal plane.

habits of the athlete and any recent modifications to that regimen. On physical examination, pain can often be elicited with deep palpation in the area of the musculotendinous junction or the muscle itself. In addition, pain with resistive muscle contraction can localize the traumatized muscle group.

of muscle contusion include superficial capillary rupture, interstitial hemorrhage, edema, and an inflammatory reaction.^132,134

has been associated with myositis ossificans. Post-contrast-enhanced images define the central core and demonstrate the development of a zonal phenomenon. Adjacent muscle edema is often associated with myositis ossificans on FS PD FSE or STIR images and is a normal finding. The final zonal pattern demonstrates peripheral bone signal without surrounding edema. Rim enhancement alone can be seen in liposarcomas, and thus the zonal pattern or susceptibility of peripheral calcifications can be used to increase the specificity of diagnosis.

In addition, the tissues are affected by the presence of oxyhemoglobin, deoxyhemoglobin, and red blood cells.^129,132,136 This results in T1 shortening due to oxidative denaturation of hemoglobin and the production of methemoglobin. The concentration of methemoglobin increases over an 80- to 90-hour period following the injury, and the shortening of the T1 relaxation time of the blood-containing tissue follows the time course of methemoglobin production.^129,132,136

the muscular contraction and the duration of exercise, with intensity appearing to be the more important factor.^{139,140,141,142,143}

between cramp, with its high-frequency action potentials, and contracture is basic (Fig. 3.123).^134,145 Muscle contracture occurs most often in metabolic disorders such as inherited defects of glycolytic enzymes (e.g., myophosphorylase deficiency or McArdle—s disease).^134,145,150 After the contracture, patients typically develop elevated serum creatine kinase levels and, when severe, pigmenturia and acute renal failure. During the contracture, there typically is intense pain. In some diseases, such as phosphofructokinase deficiency, patients may have few or no symptoms and yet have marked rhabdomyolysis. In these patients, MR imaging may show extensive zones of myonecrosis and fatty infiltration.^134,145,150 Although not specific, the coexistence of muscle edema, atrophy, and fatty infiltration is a significant finding and strongly suggestive of a myopathic or neurogenic disorder.^134,145

The origin of the piriformis is anterior to the S2 to S4 vertebrae, the sacrotuberous ligament, and the upper margin of the greater sciatic foramen. It then passes through the greater sciatic notch to insert on the superior aspect of the greater trochanter. Its primary functions are external rotation with the hip in extension and abduction with the hip in flexion. It is innervated by the nerve roots of L5, S1, and S2. When assessing the piriformis it is necessary to remember that there are several developmental variations in the anatomic relationship between the sciatic nerve and piriformis.

hypertrophy and effacement of fat in the greater sciatic foramen with muscle signal intensity. There is loss of muscle striations and a mass effect resulting in displacement of the muscle from anterior to posterior. Diffuse muscle edema is hyperintense on T2-weighted images. FS PD FSE images display edema associated with tumor, abscess, or hematoma. The gluteus minimus and medius and tensor fascia lata are normal, and there may or may not be signal changes within the gluteus maximus. On fat-suppressed or STIR images, gluteal atrophy may be visualized.

pathology such as AVN. In inflammatory and infectious disorders (Fig. 3.169), MR imaging demonstrates heterogeneity of the bursal fluid signal.

and sagittal images and is associated with chondral involvement. Chondral lesions are frequently associated with joint space narrowing and represent a later stage of FAI, usually less responsive to impingement surgery.^187,211

may stabilize symptoms with synovialization of subchondral cysts, full-thickness chondral lesions of the acetabulum accelerate progression of joint space narrowing. Conservative treatment includes short-term non-weight-bearing, anti-inflammatory medications, muscle strengthening to relieve hip joint stress, and emphasis on nonloading activities such as bicycle riding or elliptical exercisers to exercise the hip joint and stimulate articular cartilage healing. Surgical options include arthroscopic débridement of labral tears, femoroacetabular débridement and femoral reshaping (resection of the dysplastic “bump”), and periacetabular osteotomy. As mentioned, advanced cases may require total hip replacement.

demarcation or groove of acetabular articular cartilage separates the labrum and transverse ligament. The capsule of the hip attaches to the osseous acetabulum laterally, creating the perilabral sulcus, a normal finding between the capsule and labrum on coronal images peripheral to the lateral rim of the acetabulum (Fig. 3.208).

dysplasia, FAI (Fig. 3.211), LCP, SCFE, and degenerative hip disease (Fig. 3.212). Traumatic tears may occur along the inner free margin (a radial flap, the most common type) (Fig. 3.213), or they may be unstable and displaced (longitudinal tears) (Fig. 3.214).

its entire anterior-to-posterior extent on a single sagittal image (Fig. 3.223). A bucket-handle tear (Fig. 3.224), which may be seen more commonly in a hypertrophied labrum in mild DDH, is characterized by fluid cleavage separating the longitudinal tear into two separate fragments. A degenerative labrum may be associated with abnormal morphology or absence of the fibrocartilage (see Fig. 3.212). Increased signal intensity can also be seen through the labral base on gradient-echo sequences, including spoiled gradient recalled acquisition in steady state (SPGR), and is attributed to cartilage degeneration, transitional zone fissures (between the labrum and subchondral bone), and partial labral detachment. Labral surface irregularities (hyperintensity) may be associated with labral base degeneration. The limitations of MR imaging in the diagnosis of intrasubstance degeneration may be related to the presence of fibrovascular bundles or an irregular labral insertion producing signal intensity changes without corresponding degeneration.