MR Imaging of Normal Hip Anatomy

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Key Points

  • Understanding normal anatomy is important for diagnosing pathology of the hip.

  • MR arthrography is more sensitive for the detection of intra-articular pathology than noncontrast MR imaging.

  • Important components of hip anatomy include the osseous structures, cartilage, muscles and tendons, capsular ligaments, labrum, nerves, and vessels.

Imaging the hip

Hip pain is a common complaint, especially in athletes, and it has a broad differential diagnosis. MR imaging has improved the radiologist's ability to diagnose causes of hip pain, especially soft tissue pathology.1

The hip joint is difficult to image because it is not oriented in the standard axial, coronal, and sagittal planes of the body, and there is significant variation in hip joint orientation from person to person. Acetabular version, for example, can range from −10.8 to +22.1 degrees.2,

Osseous structures

The hip joint is a ball-and-socket joint composed of the femoral head articulating with the acetabulum. The acetabulum is the junction of the three bones of the pelvis: (1) the ilium, (2) the ischium, and (3) the pubis. It is important to evaluate all of these bones and the sacrum in routine hip MR imaging because fractures of any of these bones can present as “hip pain.”11, 12 The acetabulum approximates the surface of two-thirds of a sphere, which is incomplete at the inferior aspect; the

Cartilage

A horseshoe-shaped articular cartilage called the “lunate” lines the acetabulum. A central depression within the acetabulum known as the acetabular fossa lacks this cartilage lining and is instead lined by synovium and filled with fibrofatty tissue.9

In the weight-bearing portion of the superior acetabulum, at approximately the 12-o'clock position, there can be a normal variant called the supra-acetabular fossa. This is an indentation in the acetabular roof that may be filled with cartilage or

Muscles, tendons, and bursae

There are many muscles and tendons that surround the hip and allow it to perform a wide range of motions, including flexion, extension, abduction, adduction, and internal and external rotation. The muscles of the hip and proximal thigh are generally well-delineated from one another by well-defined fascial and fatty planes (Fig. 8). Normal skeletal muscle is intermediate in signal intensity on all pulse sequences.27 T1-weighted MR images are best for evaluating muscle bulk and signal intensity.

Ligaments and synovium/capsule

The iliofemoral, ischiofemoral, and pubofemoral ligaments represent thickenings of the joint capsule that reinforce and stabilize the hip joint. They are named for the bones that they connect. The Y-shaped iliofemoral ligament is one of the strongest ligaments in the body. It reinforces the anterior capsule by limiting anterior translation during extension and external rotation. It originates between the anterior inferior iliac spine and the superior acetabulum and spirals toward the femur,

Labrum

The labrum is a low signal intensity fibrocartilaginous structure that is attached to the acetabular rim. The sensitivity of MR arthrography for the detection of labral tears is at least 90%,24, 42, 47 and its accuracy is 91%,6 compared with conventional noncontrast MR imaging, where the sensitivity and accuracy are 30% and 36%, respectively.6 Byrd and Jones42 found a lower sensitivity (66%) of MR arthrography caused by overinterpretation of labral abnormalities. The specificity of MR

Nerves

On MR imaging, nerves appear round or oval in cross section with a stippled or honeycombed appearance. The stippling represents individual nerve fascicles separated by variable amounts of fat; this appearance is also called a “fascicular pattern.” The fascicles are of similar or slightly higher signal intensity compared with skeletal muscle on T2-weighted MR images, are uniform in size, and give larger nerves, such as the sciatic nerve, a striated appearance when imaged longitudinally.27

The

Vessels

The external iliac artery and vein exit the pelvis with the femoral nerve and become the femoral artery and vein at the level of the inguinal ligament; at the level of the femoral heads, the neurovascular structures from lateral to medial are the femoral nerve, the femoral artery, the femoral vein, and the lymphatics. The femoral artery divides into the superficial femoral artery and the profunda femoris artery as it exits the femoral triangle, approximately 2 to 5 cm below the inguinal

Summary

Imaging evaluation of the hip presents many challenges because of its intrinsically complex biomechanics and multiple anatomic structures, which constitute and surround the hip joint. In addition, there are developmental variations and anatomic variants that can be mistaken for pathology. Awareness of these entities helps the radiologist to make the appropriate diagnosis.

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    Funding Sources: None.

    Conflict of Interest: None.

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