Disorders of the Groin and Hip: Anterior

Joint Effusion

The probe is placed in an oblique longitudinal plane along the line of the femoral neck. Joint fluid is identified deep to the echogenic joint capsule, and may appear from hypoechoic to anechoic depending on the nature of the fluid ( Fig. 18.1 ). In adults, a bone to capsule distance of 7 mm and an asymmetrical distension of the anterior recess of more than 2 mm compared to opposite side is diagnostic of joint effusion. However, Ultrasound is nonspecific, and it may be difficult to differentiate simple fluid, septic arthritis and synovial thickening.

Internal echoes may be seen within an exudative effusion, and there may be associated thickening of the joint capsule. However, the absence of internal echoes does not exclude infection, and ultrasound-guided aspiration is indicated to avoid delay in diagnosis. Ultrasound-guided hip aspiration or injection in adults is performed in the transverse plane with the probe over the femoral head or neck and a 22 G spinal needle introduced from a lateral approach. This enables the operator to keep the needle parallel to the probe face for optimal visualization.

Conversely, a negative ultrasound examination reliably excludes joint effusion and septic arthritis, and may be used to avoid unnecessary arthrocentesis. Osteomyelitis, however, is not excluded.

Synovitis

In inflammatory arthritis, synovial hypertrophy and hyperaemia occurs with distension of the joint capsule anteriorly. Simple effusions may also be present. Differentiating fluid from synovitis by evaluation of the echogenicity of fluid is unreliable, and the use of sonopalpation to displace fluid is less reliable than in small joints. Synovitis is not always associated with hyperaemia on Doppler imaging.

Marginal erosions may be detected in the periphery of the femoral head before they are visible on the plain radiographs. They appear as irregular cortical defects filled with hypoechoic, hypervascular pannus.

Proliferative Synovial Disorders

Synovial osteochondromatosis is a neoplastic condition of the synovial membrane. It presents with joint pain, recurrent swelling and intermittent locking. In the early stage of disease, there is hypertrophy of the synovium, with formation of chondral bodies that are released in the joint. In the final stage these bodies may calcify or even ossify. A thickened echogenic synovium may be demonstrated on ultrasound in the early stages, with areas of low echogenicity that represent chondral nodules that may not be visible on radiography. After mineralization, these nodules become echogenic and produce distal acoustic shadowing ( Fig. 18.2 ).

Other proliferative synovial disorders such as pigmented villonodular synovitis may be impossible to distinguish from simple synovitis, but should always be considered in patients with monoarthritis.

Acetabular Labrum

Labral tears most commonly occur in the anterosuperior labrum and this area is amenable to assessment by ultrasound. A labral detachment is identified by separation of the echobright fibrocartilagnous labrum from the acetabular rim by a hypoechoic line. Associated femoroacetabular impingement may be seen during internal rotation on a dynamic examination.

Labral tears are more apparent in the presence of para­labral cysts, which are analogous to meniscal cysts in the knee. Paralabral cysts are hypoechoic lobulated lesions and may have internal septations ( Fig. 18.3 ). They are generally noncompressible. Most cysts are small in size compared to the iliopsoas bursa and may have a thick wall. Uncommonly, large cysts may extend deep to the iliopsoas muscles and compress the femoral neurovascular bundle. These can rarely present as a groin mass.

Ultrasound demonstration of a labral tear or a cyst is often a fortuitous finding as part of a global examination of groin pain. When a labral tear and intraarticular pathology are suspected from clinical examination, MRI is the investigation of choice to evaluate the entire labrum, articular cartilage and other intraarticular structures.