The stellate crease is a focal area devoid of hyaline cartilage, located at the inner margin of the acetabular cartilage, close to the superior aspect of the acetabular fossa ( Figure 4). It is differentiated from a pathological lesion of the acetabulum by its smooth margins, absence of surrounding bone marrow oedema, and absence of cartilage irregularity. A supra-acetabular fossa is a normal variant of the acetabular roof seen in up to 10% of the population, consisting of a pseudodefect covered by cartilage or fibrous tissue at the 12 o’clock position, and should not to be mistaken for an osteochondral defect ( Figures 3 and and4). The non-articular central depression of the acetabulum is known as the acetabular fossa, and is lined by synovium and filled with fibrofatty tissue/pulvinar.
The central to inferior portion of the acetabulum is devoid of articular cartilage. The articular surface of the acetabulum is horseshoe-shaped and lined by hyaline cartilage, known as the lunate. The entire femoral head is lined by articular hyaline cartilage, except for a small central depression known as the fovea capitis, where the ligamentum teres attaches. The femoral neck and intertrochanteric region may have lower signal intensity on T1-weighted sequences due to the presence of red marrow and less fatty marrow content ( Figures 1 and and2 2).Īrticular hyaline cartilage is a complex combination of water, collagen, and proteoglycans, which help to distribute forces, absorb pressure, and enable gliding of the bony structures of the joint. On MRI, the normal femoral head of adults demonstrates homogeneous high signal on T1-weighted sequences because of fatty marrow content. Deviation from the normal morphology with resultant poor osseous congruency is associated with conditions such as congenital hip dysplasia and femoroacetabular impingement (FAI). There is a spectrum of femoral head and acetabular morphologies. The transverse acetabular ligament bridges the acetabular notch and completes the acetabular socket. The bony acetabulum is an incomplete spherical cavity with an inferior depression which is the acetabular notch. It is a highly congruent ball-and-socket joint consisting of the femoral head (ball) articulating with the acetabulum (socket). The hip is a weight-bearing synovial joint that is inherently stable yet allows for a wide range of motion.