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Hip Joint Anatomy


Hip Joint Anatomy
The hip joint is a synovial ball­and­socket joint between the head of the femur and the acetabulum of the hip bone (Fig. 6.66).

Articular surfaces
The femoral head, covered by hyaline cartilage, forms two­thirds of a sphere and has a central pit (fovea; Fig. 6.66) giving attachment to the round ligament (ligamentum teres). The head surmounts the femoral neck, whose base abuts the medial side of the greater trochanter. The acetabulum is a deep socket with a C­shaped articular area covered with hyaline cartilage and a fat­ filled nonarticular area (acetabular fossa), the margins of which give attachment to the base of the ligamentum teres (Fig. 6.67). The acetabulum is deficient inferiorly at the acetabular notch (Fig. 6.66), where blood vessels, bridged by the transverse acetabular ligament, enter the joint. A fibrocartilaginous labrum, attached to the margins of the acetabulum and the transverse ligament, helps to deepen the socket.

Capsule
Medially, the fibrous capsule is attached to the outer margin of the labrum; laterally the capsule attaches to the intertrochanteric
line (Figs 6.66 & 6.68) at the root of the femoral neck and to the femoral shaft just above the lesser trochanter. From the femoral attachment of the capsule, retinacular fibres derived from the deep part of the capsule (Fig. 6.67) are reflected medially over the neck to the margins of the head. Posteriorly, the line of attachment of the capsule is such that only the upper (medial) half of the femoral neck lies within the joint.

Ligaments
The iliofemoral, pubofemoral and ischiofemoral ligaments are capsular thickenings that spiral downwards and laterally from the hip bone to the femur. The strong iliofemoral ligament (Fig. 6.68) is an inverted Y­shape, the stem attaching to the anterior inferior iliac spine and the limbs to the upper and lower ends of the inter­ trochanteric line.
The pubofemoral ligament (Fig. 6.68) passes from the iliopubic eminence to the femoral neck just above the lesser trochanter. The ischiofemoral ligament lies posteriorly (Fig. 6.70) and reaches the root of the greater trochanter.
Within the joint is the ligament of the head of the femur (ligamentum teres femoris) (Figs 6.67 & 6.69), which has the form of a flattened cone, the base attaching to the margins of the acetabular fossa and transverse acetabular ligament and the apex to the fovea on the femoral head.

Synovial membrane and bursae
Synovial membrane lines the interior of the capsule and the non­articular surfaces of the joint, clothes the ligament of the head of the femur and is reflected over the retinacular fibres and the femoral neck as far as the head. The iliopsoas tendon and anterior aspect of the capsule are separated by a large bursa (Fig. 6.68), which often is in communication with the joint cavity.
The tendon of obturator externus is separated from the capsule by a smaller bursa, which may also communicate with the joint.

Movements
The hip joint is multiaxial and permits flexion, extension, abduction, adduction, medial and lateral rotation and circumduction.
Flexion is produced by iliopsoas, assisted by sartorius, rectus femoris and pectineus. Gluteus maximus and the hamstrings are extensors. Abductors of the hip include gluteus medius and minimus, while adduction is produced by adductors longus, brevis and magnus, pectineus and gracilis. Medial rotation is produced by iliopsoas, tensor fasciae latae and the anterior fibres of gluteus minimus and medius. Lateral rotation is produced by piriformis, quadratus femoris, obturator externus and internus and the gemelli.

Stability
The hip joint is very stable, largely because of its bony morphology and the deep fit of the femoral head into the acetabulum. Other important factors include the ligaments and the tone of the muscles crossing the joint. The ilio­, pubo­ and ischiofemoral ligaments all limit extension and medial rotation. The iliofemoral ligament, in particular, prevents hyperextension, especially in the upright posture when body weight acts behind the transverse axis of the hip joint and tilts the pelvis backwards. The ligament of the head of the femur limits adduction of the hip.

Relations
The joint is deeply placed behind the midpoint of the inguinal ligament. Laterally, the greater trochanter covers the neck of the femur and is palpable on the lateral side of the thigh. Medially, only the thin bone of the acetabular fossa (Figs 6.71 & 6.72) separates the head of the femur from structures within the pelvis that are vulnerable following acetabular fracture accompanied by medial displacement of the femoral head. Posteriorly lie structures of the gluteal region (Fig. 6.70), including the sciatic nerve, which may be damaged in posterior dislocation. Anteriorly, the joint is covered by the iliopsoas and the femoral vessels and nerve. Obturator externus and the adductor muscles lie inferiorly (Fig. 6.72), while superiorly are gluteus medius and minimus.

Blood supply
The arterial supply of the hip joint, especially that of the head and neck of the femur, is of particular clinical importance. The joint receives branches from the obturator artery, superior and inferior gluteal arteries, and medial and lateral circumflex femoral arteries, either directly or from the trochanteric anastomosis they form. From this anastomosis (Fig. 6.70), nutrient arteries travel in the retinacular fibres to enter foramina on the upper part of the femoral neck and terminate in the head. As only the upper half of the neck is covered posteriorly by the joint capsule, fractures at this site may be classified as either intra­ or extracapsular. Intra­capsular fractures that tear the retinacular fibres may deprive the head of the femur of much of its blood supply, resulting in avascular necrosis. Additional blood supply comes from a branch of the obturator artery conveyed in the ligament of the head of the femur to the femoral head, and from one of the perforating branches of the profunda femoris artery via a nutrient artery that enters the shaft to supply the femoral neck and head.

Nerve supply
Nerves to the joint include the nerve to rectus femoris from the femoral nerve, branches from the anterior division of the obturator nerve, and the nerve to quadratus femoris from the sacral plexus.

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