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FIGURE 4.11. Longitudinal labral tear. MR arthrogram of the left hip demonstrates direct evidence of a labral tear with contrast extending into the substance of the labrum. (A) Axial Tl-weighted fat-suppressed image shows contrast entering the substance of the anterior labrum (arrow). (B) Sagittal Tl-weighted fat-suppressed image demonstrates contrast extending into the interface of the anterior labrum and acetabular cartilage (arrow).

alabral cysts are considered to be secondary evidence for underlying labral pathology,60 theoretically developing as synovial fluid leaks through a labral defect. These cysts are dark on Tl-weighted images and bright on T2-weighted images and may fill with contrast on MR arthrography (Figure 4.13).

Ligamentum Teres Pathology

The clinical significance of abnormalities of the liga-mentum teres is less well known. The ligament carries a small artery to the fovea of the femoral head but has no known mechanical function. A tear of the ligament with an unstable fragment or enlargement of the ligament theoretically may lead to mechanical hip symptoms. The ligamentum teres is a triangular-

FIGURE 4.12. Detachment of the anterior labrum on MR arthrography. Axial Tl-weighted fat-suppressed image demonstrates a large cleft between the anterior labrum and acetabulum (arrow) consistent with labral detachment. Arthroscopy revealed traumatic detachment of the anterior labrum.

shaped structure with a broad-based attachment to the posteroinferior portion of the cotyloid fossa of the acetabulum and courses cephalad to attach to the fovea.55 Based on arthroscopic findings, injuries of the ligamentum teres result from both major and minor hip trauma.61 Gray and Villar61 proposed an arthro-scopic classification of abnormalities of the ligamen-tum teres as follows: complete tears, partial tears, and degenerate ligamentum teres. On MRI and MR arthrography, the normal ligamentum teres appears as a band-like structure that demonstrates low signal intensity on all sequences and courses from the pos-teroinferior portion of the acetabulum to the fovea (see Figure 4.3). The ligament is best evaluated on the axial and coronal images. A chronic complete tear of the ligament may manifest by absence of this structure on MR arthrography. An acute complete tear may appear as a discontinuity in the ligament fibers or avulsion from the fovea. Partial tears or degenerative ligamentum teres are difficult to discern and typically demonstrate partial thickness tearing, irregularity, or thickening of ligament fibers or increased intrasub-stance signal on T2-weighted images (Figure 4.14).17

Trauma

Plain radiographs remain the initial imaging examination in the evaluation of trauma to the pelvis and hip. Judet views can be helpful in establishing fracture patterns of the acetabulum. Inlet and outlet views are used to better evaluate fractures of the anterior and posterior pelvic ring. Secondary imaging of the hip with CT may be helpful for surgical planning in complex fractures and fracture/dislocations, and for the evaluation of intraarticular fragments (see Figure 4.2) and associated articular surface fractures in hip dislocations (see Figure 4.1).

FIGURE 4.13. Posterior paralabral cyst. (A) Axial proton density pathology. (B) Axial T1-weight fat-suppressed image from an MR fat-suppressed image of the right hip reveals a small posterior par- arthrogram demonstrates contrast filling the paralabral cyst alabral cyst (arrow) that is typically indicative of underlying labral (arrow).

FIGURE 4.13. Posterior paralabral cyst. (A) Axial proton density pathology. (B) Axial T1-weight fat-suppressed image from an MR fat-suppressed image of the right hip reveals a small posterior par- arthrogram demonstrates contrast filling the paralabral cyst alabral cyst (arrow) that is typically indicative of underlying labral (arrow).

Although it is not typically used to evaluate hip fractures in patients with normal bone mineralization, Potter et al.62 reported the ability of MRI to demonstrate occult injuries of the femoral head not seen on CT and injury of the sciatic nerve in patients with ac-etabular fractures. MRI has had an increasing role in the evaluation of hip trauma in the elderly. Nondis-placed femoral neck or intertrochanteric fractures in elderly patients can be radiographically occult or difficult to diagnose. MRI has proven to be the imaging modality of choice to exclude an occult hip fracture (Figure 4.15) in this patient population.32-34,63 Ra-dionuclide bone scanning can be normal in the first 48 hours after a fracture in the elderly and is less sensitive than MRI in detecting occult hip fractures.33 In addition to identifying occult femoral fractures, frac tures of the pelvis and soft tissue injuries of the hip can be detected with MRI23'64 (Figures 4.16, 4.17).

Most protocols used in the evaluation of a potential hip fracture include coronal T1-weighted images and short T1 -weighted inversion recovery (STIR) or T2-weighted fat-suppressed images. T1-weighted images demonstrate decreased signal either diffusely or oriented in a linear fashion at the site of a fracture (see Figure 4.15). STIR and T2-weighted images demonstrate increased signal at the fracture site corresponding to bone marrow edema or hemorrhage. A linear focus of decreased signal corresponding to a fracture line may be seen coursing through the area of marrow edema on these latter two sequences. It is now well recognized that a common fracture pattern in the elderly hip is an incomplete intertrochanteric fracture

FIGURE 4.14. Partial tear of the ligamentum teres on MR arthro- mentum teres (arrow). (B) Coronal Tl-weighted fat-suppressed imgram. (A) Axial Tl-weighted fat-suppressed image reveals irregu- age shows a partial tear of the ligamentum teres at the foveal at-larity of the anterior fibers of the foveal attachment of the liga- tachment (arrow).

FIGURE 4.14. Partial tear of the ligamentum teres on MR arthro- mentum teres (arrow). (B) Coronal Tl-weighted fat-suppressed imgram. (A) Axial Tl-weighted fat-suppressed image reveals irregu- age shows a partial tear of the ligamentum teres at the foveal at-larity of the anterior fibers of the foveal attachment of the liga- tachment (arrow).

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