The fibrous capsule is a thick and strong structure that encircles the proximal femur and the acetabular cup. Proximally, the capsular insertion is located 5 to 6 mm above the acetabular margin, leaving a little space beyond the acetabular labrum called the perilabral sul-cus. Inspection of this area is possible with a 70-degree arthroscope, pulling back the cannula to the edge of the joint.
The point of insertion of the fibrous capsule to the femur varies. Anteriorly it is attached to the in-tertrochanteric line, laterally to the base of the femoral neck, posteriorly 1 cm above the intertrochanteric crest, and medially to the femoral neck near the lesser trochanter. The capsule may be likened to a cylindrical sleeve enclosing the joint and the femoral neck. The capsule is thicker anteriorly than posteriorly and consists of two sets of fibers, circular and longitudinal.
The circular fibers are named zona orbicularis (Figure 8.22) and are of arthroscopic interest because they form a ring around the neck of the femur. This ring has no direct attachment to the bone and is seen arthroscopically as a capsular condensation projecting into the synovial lining, embracing the distracted femoral head. External rotation of the limb during the procedure allows the zona orbicularis to relax, whereas internal rotation tightens it around the femoral neck. The maneuver can be used to differentiate the zona orbicularis from the acetabular labrum.
The longitudinal fibers are greatest in number and strength but have less arthoscopic interest. Rarely, it is possible to identify the iliofemoral ligament, or ligament of Bigelow. It has a triangular shape, its apex being attached between the anterior inferior iliac spine and the acetabular rim, its base to the intertrochanteric line. Because the central part is thinner, it is often referred to as being Y shaped. Other ligaments reinforcing the capsule are the pubofemoral and the ischiofemoral ligaments. None is usually seen with the arthroscope. The latter has some arthroscopic importance because it thickens the back of the capsule and is perforated using the rare posterior approach.
Dvorak and coauthors30 have reported that the Weit-brecht's retinacula can be seen on the posterosuperior aspect of the femoral neck when looking anteriorly from a posterior paratrochanteric portal. This is a flattened band reflecting from the fibrous capsule of the hip joint to the head and neck of the femur, present in 94.8% of male and 92.5% of female subjects. Nutrient arteries for the femoral head run through the retinacula. Noriyasu and coauthors31 have reported that there are two types of retinacula: a complete band shape and a posterior membranous shape.
A circular aperture between the pubofemoral and il-iofemoral ligaments sometimes joins the articular cavity with the subtendinous psoas (iliac) bursa. This structure separates the capsule from the iliopsoas muscle and is lined with synovium.
The inner surface of the capsule is extensively lined with a highly vascularized pink layer of synovial membrane (Figure 8.23). This tissue also covers part of the neck contained within the joint, both surfaces of the acetabular labrum, the ligamentum teres, and the fat within the acetabular fossa. At the femoral attachment of the joint capsule, the synovial membrane is reflected up toward the head as far as the articular margin. The synovium may have a hemor-rhagic appearance (Figure 8.24), and an occasional vascular papillary projection of a polypoid nature can be seen. These findings must not be mistaken for a synovitis.
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