Superior Lateral Joint Space Of The

images along the margin of the osteonecrotic focus. The osteonecrotic focus demonstrates signal intensities on MRI as follows: class A, signal intensity isointense to fat on Tl-weighted and T2-weighted images (Figure 4.7A-C); class B, signal intensity isointense to blood, high signal intensity on Tl- and T2-weighted images; class C, signal intensity isointense to fluid, low signal on Tl-weighted images and high signal on T2-weighted images; class D, signal intensity isointense to fibrous tissue, low signal on Tl- and T2-weighted images. On T2-weighted images, the transition between normal and necrotic bone may appear as a double line composed of an inner band of high signal corresponding to granulation tissue adjacent to an outer band of low signal thought to represent sclerosis from bone repair.28 In advanced osteonecrosis, sub-chondral fracture of the femoral head may be seen on MRI as a curvilinear area of low signal on Tl- or T2-weighted images immediately beneath and parallel to the articular surface.

A diffuse bone marrow edema pattern on MRI involving the femoral head and extending to the in-tertrochanteric region has been described.50 Distinction of this pattern of osteonecrosis from transient osteoporosis of the hip is important if core decompression is contemplated. Several authors recommend a careful search for a subchondral focal lesion indicating osteonecrosis.29,46,47 Correlation with plain radiography may be helpful as well as transient osteoporosis typically demonstrates osteopenia on radiographs 4 to 8 weeks after the initial onset of symp-toms.51 Ultimately, clinical and imaging follow-up may be necessary to distinguish these two entities.

Evidence exists that MRI can be helpful in predicting future collapse of the articular surface45-47 in osteonecrosis. Beltran et al. reported a correlation in future collapse of the femoral head after core decompression with the preoperative size of the osteonecrotic focus.45 In this series, femoral head collapse occurred in 87% of cases involving greater than 50% of the weight-bearing articular surface (WBAS), 43% of cases involving 25% to 50% WBAS, and in no cases involving less than 25% WBAS. Shimizu et al.47 reported similar observations in which collapse did not occur in patients with an osteonecrotic focus compromising less than 25% of the diameter of the femoral head. In their series, collapse occurred in 74% of patients with lesions greater than 25% of the femoral head diameter and involving greater than 67% WBAS.47 In a prospective study of patients with steroid-induced osteonecrosis, Iida et al.46 found that marrow edema seen on MRI correlated well with pain and subsequent collapse of the femoral head.

Osteoarthritis

Osteoarthritis is the most common arthropathy encountered in adults and can be primary or secondary. Secondary osteoarthritis occurs from a number of underlying conditions including trauma, infection, crystal deposition disease, osteonecrosis, and acetabular dysplasia. The initial plain radiographic finding of osteoarthritis is nonuniform superolateral joint space narrowing that results from cartilage loss (Figure 4.8). This pattern is easily distinguished from the uniform joint space narrowing seen in inflammatory arthro-pathies that typically occurs in an axial direction. As joint space loss progresses in osteoarthritis, reactive bone formation (subchondral sclerosis and osteophy-tosis) occurs in response to altered mechanical stresses. Osteophytes may develop along the medial femoral head and lateral acetabulum, and new bone formation may occur along the medial femoral neck. Subchon-dral cyst formation develops primarily in the sub-chondral bone of the lateral acetabulum and femoral head.52 More advanced osteoarthritis leads to remodeling of the femoral head and acetabulum.

Although not often used in the evaluation of os-teoarthritis, CT may be helpful in the localization of paraarticular calcification or ossification and in the detection of loose bodies. Lack of contrast in the joint, however, can limit its ability to distinguish intraar-ticular and extraarticular calcifications. In cases of end-stage osteoarthritis, CT can be used for preoper-

Subchondral Cyst Femoral Head

FIGURE 4.8. Osteoarthritis of the left hip. Plain radiograph shows superolateral joint space narrowing, subchondral sclerosis, and subchondral cyst formation. Early osteophytes are noted on the lateral acetabulum and medial femoral head.

Superior Lateral Joint Space The Hip

FIGURE 4.9. Osteoarthritis. Noncontrasted coronal proton density fat-suppressed image of the left hip demonstrates a full-thickness defect in the articular surface cartilage of the superior femoral head with a loose cartilaginous fragment in the inferior joint recess. Note the cartilage demonstrates intermediate signal (brighter than subjacent cortical bone and less intense than joint fluid).

FIGURE 4.8. Osteoarthritis of the left hip. Plain radiograph shows superolateral joint space narrowing, subchondral sclerosis, and subchondral cyst formation. Early osteophytes are noted on the lateral acetabulum and medial femoral head.

FIGURE 4.9. Osteoarthritis. Noncontrasted coronal proton density fat-suppressed image of the left hip demonstrates a full-thickness defect in the articular surface cartilage of the superior femoral head with a loose cartilaginous fragment in the inferior joint recess. Note the cartilage demonstrates intermediate signal (brighter than subjacent cortical bone and less intense than joint fluid).

ative planning and prosthesis fitting for total hip arthroplasty.

Conventional MRI of the hip has had modest to poor success with evaluating cartilage thickness37 and demonstrating cartilage abnormalities of the femoral head and acetabulum. The detection of cartilage abnormalities of the hip with MRI is hindered by the configuration of the articular surfaces and the close apposition of the femoral and acetabular cartilage. In an early investigation of cadaver hips, Hodler et al. reported that conventional MRI with fat-suppressed Tl-weighted images was not accurate in determining cartilage thickness.37 A relatively recent study using a 3D spoiled gradient echo (SPGR) pulse sequence and hip traction reported success in detecting discontinuity of cartilage at the junction of normal and necrotic foci of the femoral head in os-teonecrosis.53 Based on our clinical success with depicting cartilaginous abnormalities of the knee and ankle with fat-suppressed proton density FSE imaging, we employ this sequence in our investigation of the hip to assess cartilage and labral pathology. Normal cartilage on this sequence appears as an intermediate signal structure that has higher signal intensity than subjacent bone and less signal intensity than joint fluid (Figure 4.9). In addition, this sequence demonstrates a distinct junction of the acetabular cartilage and labrum where the labrum is lower in signal than the adjacent cartilage.

In the absence of a joint effusion, conventional MRI is limited in its ability to demonstrate loose bodies. The presence of a joint effusion or introduction of in-traarticular fluid with MR arthrography improves the sensitivity of detecting intraarticular cartilage and os-

Normal Hip Joint Arthrogram
FIGURE 4.10. Loose body in osteoarthritis. Coronal T2-weighted fat-suppressed image of both hips demonstrates a small loose body (arrow) in the medial right hip joint outlined by joint fluid.

seous fragments by outlining these structures with fluid (see Figures 4.9, 4.10). Subchondral cysts in the femoral head and acetabulum demonstrate low signal on T1-weighted images and high signal on T2-weighted images and can fill in with contrast on MR arthrography.

Labral Pathology

Injury of the acetabular labrum is now recognized as an important cause of mechanical hip pain that can be surgically treated with resultant symptomatic relief. Recent interest and efforts directed toward improving our ability to diagnose labral tears has resulted in a growing knowledge of the MRI and MR arthro-graphic appearance of the normal and abnormal labrum.4,5,7-10,12-15,54 Knowledge of the anatomy of the hip capsule, acetabular labrum, and articular cartilage and an understanding of the location and appearance of labral tears are essential to proper diagnosis of labral pathology. The acetabular labrum is a horseshoe-shaped fibrocartilaginous structure attached to the periphery of the acetabulum that adds depth to the hip joint.55 Inferiorly the labrum continues as the transverse ligament from the anterior margin to the posterior margin of the acetabulum. In cross section on MRI, the labrum typically has a triangular configuration. Variation in the shape and intrasub-stance signal of the labrum and even absence of the labrum on conventional MRI in asymptomatic individuals have been reported.54-56 In a group of patients with asymptomatic hips, Lecouvet et al. noted that the presence of increased intrasubstance signal in the labrum and absence of the labrum on T1-weighted images increased in incidence with age.55 Cotton et al., in a study of asymptomatic volunteers, also reported that increased intrasubstance signal in the acetabular labrum on T1- and T2-weighted images and absence of the labrum was frequently observed.9 It must be noted, however, that these observations54,55 regarding absent labra are based on conventional MRI sequences and often lack distension of the perilabral sulcus achieved with MR arthrography, which aids in defining the presence, morphology, and articular surface contour of the labrum. Although debated in the radi-ologic community, the possibility of a normal sub-labral sulcus occurring at the junction of the acetab-ular cartilage and labrum has been reported57 and can be problematic when evaluating for labral tears. Although variation exists in the location of labral pathology in documented series,58,59 it is generally considered that most labral injuries occur in the anterior or anterosuperior portion of the labrum. Lage et al.59 have classified labral tears with respect to etiology (traumatic, degenerative, idiopathic, and congenital) and morphology (radial flap, radial fibrillated, longitudinal peripheral, and unstable).

Although conventional MRI has not proven successful in detecting labral injuries,5 MR arthrography has recently been shown to be sensitive (ranging from 57% to 91%) for demonstrating tears and detachment of the labrum.4,5,9,12,14 Czerny et al. have proposed an imaging classification of labral pathology of the hip based on the morphology of the labrum and labral-capsular relationship, intrinsic MRI signal, and the presence of fluid tracking into the junction of the ac-etabular cartilage and labrum.4,5 This classification is based on the severity of injury and the shape of the labrum and configuration of the labral-capsular junction. One caveat to this classification is the assumption that a sublabral sulcus does not exist. To date no report has correlated the MRI or MR arthrography appearance of the morphologic subtypes of labral tears proposed by Lage et al.59 The most definitive evidence for a labral tear on MR arthrography is the presence of contrast tracking into the substance of the labrum (Figure 4.11). Most authors consider the presence of contrast extending into the junction of the labrum and acetabular cartilage as evidence for a labral tear or de-tachment.4,5,7,9,13-15 This author considers that this finding (Figure 4.12) can represent a longitudinal tear, partial labral detachment, or a sublabral sulcus. Although the distinction between a normal sublabral sulcus and injury to the labrum cannot always be made, subjective assessment of the size of the lesion10 and correlation with the clinical response to intraar-ticular anesthetic may be helpful. Additional MR arthrographic findings that suggest the presence of a labral tear include a blunted or absent labrum or irregularity of the labral surface.13 Enlargement of the labrum with increased intrinsic signal on T2-weighted images without leakage of contrast into the labrum is likely the result of intrasubstance degeneration. Par-

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Responses

  • Ulisse Trevisani
    What is subchondral cystic formation?
    4 years ago
  • ethan milne
    What is superior hip joint space?
    9 months ago
  • leonie
    What does moderate joint space narrowing superior laterally?
    4 months ago
  • Fethawit Biniam
    What is focally granular surface of femoral head in hip joint?
    27 days ago

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