Ultrasound of the Musculoskeletal System
Authors: Bianchi, Stefano, Martinoli, Carlo
The hip joint is best evaluated in both longitudinal and transverse oblique planes obtained over the femoral neck. Longitudinal US images are well suited to demonstrating the anterior synovial recess, in which even small intra-articular effusions may collect. The US appearance of this recess has been extensively described in the literature and correlates well with both anatomic and histologic features (Robben et al. 1999). It lies between the deep fascia of the iliopsoas and the femoral neck and is composed of an anterior and a posterior hyperechoic layer (Fig. 12). The two layers correspond to the anterior joint capsule which, after leaving the anterior border of the acetabulum, extends inferolaterally to reach the intertrochanteric line. At this level, the most superficial fibers of the joint capsule are in continuity with the periosteum, whereas the deep ones reflect and travel upward to insert into the junction between the femoral head and neck, at the distal edge of the articular cartilage. Each layer is composed of thick outer fibrous envelope and a thin inner synovial membrane; the fibrous component, which is histologically composed of collagen fibers, appears as a 2-4 mm thick hyperechoic band, whereas the normal synovial lining is too thin to be revealed with US. The anterior fibrous layer of the joint recess is thicker than the posterior one, probably because the anterior capsule is reinforced at this level by the iliofemoral ligament. In the absence of an intra-articular effusion, the two layers are shrunk and separated by a hyperechoic line representing the collapsed synovial recess. This sign is commonly referred to as the “stripe sign” (Robben et al. 1999).
Fig. 12a-d. Anterior recess of the hip joint. a,b Transverse oblique 12–5 MHz US images with c,d schematic drawing correlation obtained over the hip joint in a healthy subject (a,c) and in a patient with intra-articular effusion (b,d) . In a,c the hypoechoic band of tissue (arrowheads) found between the anterior bony cortex of the femoral neck (FN) and the deep boundary of the iliopsoas muscle (IPs) is related to the sum of the iliofemoral ligament (IFL), the anterior (AC) and posterior (PC) joint capsule and the synovial membrane (SM). Per, periosteum. b,d An intra-articular effusion (asterisk) distends the anterior synovial recess and allows differentiation of two distinct capsular layers (arrowheads). Note that the anterior layer (open arrowheads) is thicker than the deep one (white arrowheads) because of the presence of the iliofemoral ligament. RF, rectus femoris; Sa, sartorius; IPs, iliopsoas; FH, femoral head. In the insert shown in d, an axial oblique fat-suppressed T1-weighted MR-arthrographic image demonstrates the capsular layers (arrowheads) as hypointense linear bands separated by the joint recess (asterisk) ﬁlled with gadolinium contrast.