Hip joint

Articular surfaces

The hip joint is the articulation between the ellipsoid head of the femur and the hemispherical concavity of the acetabulum located on the lateral aspect of the hip bone. The femoral head is covered with articular (hyaline) cartilage with the exception of a rough central depression, the fovea capitis, which is a surface of attachment for the ligament of the femoral head (ligamentum teres capitis femoris).

The acetabulum is formed by the fusion of the ilium, ischium and pubic bones. It plays a significant role in the stability of the hip joint as it almost entirely encompasses the head of the femur. The acetabulum bears a prominent semilunar region known as the lunate surface that is covered by articular cartilage. The lunate surface forms an incomplete ring that occupies the superior and lateral aspects of the acetabulum; missing its inferior segment. This surface is broadest anterosuperiorly where it bears most of the body weight during standing. The deficient inferior aspect of the acetabulum forms the acetabular notch.

The deep central nonarticular floor of the acetabulum is referred to as the acetabular fossa. This area is devoid of cartilage and is continuous with the acetabular notch. It contains loose connective tissue (fibroelastic fat pad) which is covered by synovial membrane. Attached to the margin of the acetabulum is a fibrocartilaginous collar called the acetabular labrum. This structure deepens the acetabulum by raising the rim of the acetabulum slightly, thereby increasing the acetabular articular area by about 10%. Inferiorly, the acetabular labrum continues as the transverse acetabular ligament, bridging the acetabular notch and transforming the notch into a foramen. 

The superior aspect of the acetabulum and that of the femoral head bear the greatest pressures. These areas generally have the thickest articular cartilage. The concave acetabulum and the rounded femoral head of the hip joint, in addition to the anatomical relationship between the femur and the pelvis, particularly in the upright position, make this joint incongruent. The articular surfaces are most congruent when the hip joint is in a partially flexed and abducted position.

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Joint capsule

The hip joint is enclosed by a strong fibrous capsule and lined internally by synovial membrane. The external fibrous layer of the capsule is attached to the acetabulum proximally, close to the margin of the acetabular rim and to the transverse acetabular ligament. From its acetabular attachment, the fibrous layer extends laterally to its distal attachment on the proximal femur. It attaches to the intertrochanteric line anteriorly, the base of the femoral neck superiorly, about 1cm superomedial to the intertrochanteric crest posteriorly and on the femoral neck close to the lesser trochanter inferiorly. It is worth noting that part of the femoral neck is intracapsular and part is extracapsular.

The capsule of the hip joint is notably thicker anterosuperiorly, which is the area of maximal stress, particularly in the upright position when the hip is extended. Posteroinferiorly, the capsule is relatively thin and loosely attached. The capsule has two major groups of fibers, longitudinal and circular. The external longitudinal fibers of the fibrous capsule generally travel in a spiral manner from the hip bone to the proximal femur. The deeper circular fibers form a collar around the femoral neck, the zona orbicularis (orbicular zone or annular ligament) and have no bony attachments. The capsule of the hip joint is reinforced inferiorly by the pubofemoral ligament and posteriorly by the ischiofemoral ligament.

Test what you've learned about the hip joint so far, by taking our quiz.

Ligaments

The ligaments of the hip joint can be divided into two groups; capsular ligaments and intracapsular ligaments. Capsular ligaments are intrinsic ligaments of the joint capsule.There are three capsular ligaments that play a key role in maintaining the integrity of the joint during various movements: iliofemoral, pubofemoral and ischiofemoral ligaments. The intracapsular ligaments of the hip joint are found inside the capsule and include the transverse ligament of the acetabulum and the ligament of the head of the femur. 

Innervation

The hip joint is innervated by the articular branches of multiple nerves that emerge from the lumbosacral plexus (L2-S1). The nerve supply to a specific region of the joint typically corresponds to the innervation of the muscle that crosses it: 

• The femoral nerve innervates the anterior aspect 
• The obturator nerve supplies the inferior aspect
• The superior gluteal nerve supplies the superior aspect 
• The nerve to the quadratus femoris innervates the posterior aspect. 

It is important to note that pain sensations from the vertebral column can be referred to the hip joint, while primary hip pain may be referred to the knee as they share similar innervation.

Blood supply

The blood supply of the hip joint is from the medial and lateral circumflex femoral arteries (branches of the deep artery of the thigh), the obturator artery and the superior and inferior gluteal arteries. Together, these arteries form a periarticular anastomosis around the hip joint. This anastomotic network gives rise to the retinacular arteries which supply the greatest volume of blood to the head and neck of the femur. Additionally, the obturator artery gives rise to the artery of the head of the femur within the ligament of the head of the femur.

Movements

Being a ball-and-socket joint, the hip joint permits movements in three degrees of freedom: flexion, extension, abduction, adduction, external rotation, internal rotation and circumduction.

Flexion of the hip joint draws the thigh towards the trunk. When the knee is flexed, the hip joint can be fully flexed with the thigh coming in contact with the anterior abdominal wall. The range of movement during passive flexion is about 120o and reaches around 145o during active flexion. Hip flexion is limited by the tension in the hamstrings when the knee is extended. 

Extension of the hip joint moves the thigh away from the trunk. Extension of the joint beyond the vertical is limited to about 30o by the tension of the capsular ligaments and the shape of the articular surfaces.

Abduction and adduction of the hip joint occur in the coronal plane and have a free range of movement of about 45o. With the hip flexed, the range of abduction is far greater than when extended. Abduction of the hip joint is limited by tightness in the adductor muscles and the pubofemoral ligaments. Hip flexion also makes adduction easier. Adduction, on the other hand, is limited by the contralateral limb, tension in the abductor muscles, the lateral part of the iliofemoral ligament and the fascia lata of the thigh.

Internal and external rotation of the hip joint occurs in the horizontal plane about the mechanical axis of the femur rather than the long axis of the femoral shaft. The mechanical axis runs from the head of the femur to the intercondylar notch of the distal femur. During internal rotation, the femoral shaft moves anteriorly, causing the toes to point medially. The reverse occurs in external rotation where the femoral shaft moves posteriorly, causing the toes to point away from the midline. External rotation is much freer and more powerful than internal rotation. For example, the range of internal rotation with the hip extended is about 35o while external rotation is about 45o. Rotation at the hip joint is generally much freer with hip flexion rather than extension. Tightness in the lateral rotators and the ischiofemoral ligament limit internal rotation of the hip joint. Contrarily, external rotation is limited by tightness in the medial rotators of the thigh and the iliofemoral and pubofemoral ligaments.

Muscles acting on the hip joint

The major muscles that produce movements of the hip joint are categorized into functional groups; flexors, extensors, adductors, abductors, lateral rotators and medial rotators. A single muscle may fall under two functional groups. Multiple muscles participate in both flexion and adduction as well as abduction and internal rotation.

The main flexors of the hip joint are the iliopsoas muscle (psoas major and iliacus) and the rectus femoris muscle. The pectineus, tensor fasciae latae and sartorius muscles assist as weak flexors. Also, the adductor longus and brevis can assist with flexion of the hip joint in addition to its adductor function.

The primary extensor of the hip joint is the gluteus maximus muscle, assisted by the hamstring muscles (biceps femoris, semitendinosus, semimembranosus) and the adductor magnus muscle.

The primary abductors of the hip joint are the gluteus medius and the gluteus minimus muscles. The tensor fasciae latae, piriformis and sartorius muscles also assist in hip abduction. The hip abductors play an active role in stabilizing the pelvis during specific phases of the gait cycle. 

The major adductors of the hip joint are the adductors longus, brevis and magnus and the gracilis muscle. These are assisted by pectineus, quadratus femoris and the inferior fibres of gluteus maximus.

The anterior fibres of glutei minimus and medius are the principal muscles responsible for internal rotation of the hip joint. These muscles are assisted by the tensor fasciae latae and most adductor muscles.

External rotation is produced by the gluteus maximus together with a group of 6 small muscles (lateral rotators): piriformis, obturator internus, superior and inferior gemelli, quadratus femoris and obturator externus.

Learn more about the hip joint by exploring our articles, video tutorials, quizzes and labelled diagrams from this study unit.