The hip joint is a ball and socket synovial joint, formed by an articulation between the pelvic acetabulum and the head of the femur. It forms a connection from the lower limb to the pelvic girdle, allowing for stability and weight-bearing, but sadly only a limited range of movement. In this article, we shall look at the anatomy of the hip joint - its articulating surfaces, ligaments and neurovascular supply.
The main structures of the hip joint include the articulating surfaces, ligaments and neurovascular supply. Let us look at each of these in turn:
The hip joint consists of an articulation between the head of femur and acetabulum of the pelvis. The acetabulum is a cup-like depression located on the inferolateral aspect of the pelvis. Its cavity is deepened by the presence of a fibrocartilaginous collar – the acetabular labrum. The head of femur is hemispherical in shape, and fits completely into the concavity of the acetabulum. This decreases the probability of the head slipping out of the acetabulum (dislocation). Both the acetabulum and head of femur are covered in articular cartilage, which is thicker at the places of weight bearing. The capsule of the hip joint attaches to the edge of the acetabulum proximally. Distally, it attaches to the intertrochanteric line anteriorly and the femoral neck posteriorly.
The ligaments of the hip joint act to increase stability. They can be divided into two groups - intracapsular and extracapsular:
The arterial supply to the hip joint is largely via the medial and lateral circumflex femoral arteries – branches of the profunda femoris artery (deep femoral artery). They anastomose at the base of the femoral neck to form a ring, from which smaller arteries arise to supply the hip joint itself. The medial circumflex femoral artery is responsible for the majority of the arterial supply (the lateral circumflex femoral artery has to penetrate through the thick iliofemoral ligament). Damage to the medial circumflex femoral artery can result in avascular necrosis of the femoral head. The artery to head of femur and the superior/inferior gluteal arteries provides some additional supply.
The hip joint is innervated primarily by the sciatic, femoral and obturator nerves. These same nerves innervate the knee, which explains why pain can be referred to the knee from the hip and vice versa.
The primary function of the hip joint is to weight-bear. There are a number of factors that act to increase stability of the joint. The first structure is the acetabulum. It is deep, and encompasses nearly all of the head of the femur. This aids in forming a stable joint, as the head of femur is unlikely to dislocate from the acetabulum due to its depth, as well as the horseshoe shaped fibrocartilaginous ring - the acetabular labrum - around the acetabulum, which further increases the depth of the joint.
The ligaments of the hip joint also act to help stabilise the joint by limiting excessive movement in certain directions, by reinforcing the hip joint capsule. The strongest of these ligaments is the iliofemoral ligament, which prevents hyperextension of the hip joint. The pubofemoral ligament prevents excessive abduction and extension of the joint, and the ischiofemoral ligament prevents hyperextension and holds the femoral head in the acetabulum.
The arterial supply to the hip joint is largely via the medial and lateral circumflex femoral arteries, both branches of the profunda femoris artery (deep femoral artery). The medial circumflex femoral artery is responsible for the majority of the arterial supply, and damage to it can result in avascular necrosis of the femoral head. The artery to head of femur and the superior/inferior gluteal arteries also provide some additional supply for the hip joint. Lastly, the hip joint is innervated primarily by the sciatic, femoral and obturator nerves.
All these structures - the acetabulum, ligaments, arterial supply and nerves - act together to stabilise the hip joint and allow for it to bear weight and maintain stability. Though it lacks the range of movement of some other joints, the hip joint is well equipped to provide a sturdy and reliable connection between the lower limb and the pelvic girdle.
The hip joint is a ball-and-socket joint, found where the leg meets the pelvis. The head of femur is a rounded bone that fits into the acetabulum, a cup-like depression located on the inferolateral aspect of the pelvis. Both the head of femur and the acetabulum are covered in articular cartilage, which is thicker at the places of weight bearing and provides a larger articular surface. This increased depth further improves the stability of the joint.
The joint is stabilised by a combination of strong ligaments, thickened joint capsule, muscles, and reciprocal action of ligaments and muscles. There are three main extracapsular ligaments providing stability to the joint – the iliofemoral, pubofemoral and ischiofemoral ligaments, each of which are connected to the outer surface of the hip joint capsule. Of these three, the iliofemoral ligament is the strongest and has a ‘Y’ shaped appearance, preventing hyperextension of the hip joint. The pubofemoral ligament has a triangular shape and prevents excessive abduction and extension, while the ischiofemoral ligament has a spiral orientation and helps to hold the femoral head in the acetabulum. In addition to these, the hip joint also contains one intracapsular ligament, which is the ligament of head of femur, running from the acetabular fossa to the fovea of the femur.
The movements that can be carried out at the hip joint are flexion, extension, abduction, adduction, lateral rotation, and medial rotation. Each of these is facilitated by associated muscles, such as iliopsoas, rectus femoris, sartorius, pectineus, gluteus maximus, semimembranosus, semitendinosus, biceps femoris, gluteus medius, gluteus minimus, piriformis, tensor fascia latae, adductors longus, brevis and magnus, pectineus, and gracilis. The degree to which flexion at the hip can occur depends on whether the knee is flexed – this allows the hamstring muscles to relax, and increases the range of flexion. Extension at the hip joint is limited by the joint capsule and the iliofemoral ligament as these structures become taut during extension.
The hip joint is supplied with blood, oxygen and nutrients by the medial and lateral circumflex femoral arteries, branches of the profunda femoris artery (deep femoral artery). Damage to the medial circumflex femoral artery can result in avascular necrosis of the femoral head. The artery to head of femur and the superior/inferior gluteal arteries provide some additional supply. The hip joint is innervated primarily by the sciatic, femoral and obturator nerves.
The hip joint is a complex structure that requires stability in order to function correctly. The muscles and ligaments work in a reciprocal fashion, with the medial flexors located anteriorly and the medial rotators being greater in number and stronger posteriorly. The three main extracapsular ligaments, as well as the relatively small intracapsular ligament, reinforce the joint capsule, disrupt excessive movement, and support the head of the femur in the acetabulum. The arterial supply to the joint is provided by the medial and lateral circumflex femoral arteries, as well as the artery to head of femur and the superior/inferior gluteal arteries. The hip joint is also innervated by the sciatic, femoral, and obturator nerves.
The hip joint is essential for movement and its stability is maintained by muscles, ligaments, the joint capsule, and reciprocal action of these structures. Through the correct functioning of the hip joint, movements such as flexion, extension, abduction, adduction, lateral rotation, and medial rotation can be achieved. Damage to the medial circumflex femoral artery can lead to avascular necrosis of the femoral head, indicating the importance of the arterial supply – in addition to the three main extracapsular ligaments and the intracapsular ligament – in ensuring the health of this critical joint.
The hip joint is an important region of the body which can refer pain to the knee and vice versa. This is because the same nerves innervate both the hip and the knee. It is paramount for the stability of the joint that there are a number of factors which act in harmony to protect it and aide in its role of weight bearing.
The primary structure providing stability for the hip joint is the acetabulum, which is a deep socket that encompasses almost all of the head of the femur. The acetabulum is further reinforced by a horseshoe shaped fibrocartilaginous ring, which is known as the acetabular labrum. This labrum grants the acetabulum an increased depth, allowing for a greater articular surface and thus increased stability.
The hip joint also has a unique feature; the ligaments which reinforce it have a spiral orientation which causes them to become tighter when the joint is extended. These ligaments, known as the iliofemoral, pubofemoral and ischiofemoral ligaments, are extremely strong and along with with the thickened joint capsule, provide an even larger degree of stability.
In addition, the muscles and ligaments work in a reciprocal fashion. The anterior ligaments are the strongest, while the muscles in this location are weaker. On the posterior side, the ligaments are weaker, but the rotatory muscles are plentiful and able to 'pull' the head of the femur into the acetabulum.
The hip joint has a variety of movements associated with it. This allows for more complex actions such as walking and running. Here are the principle movements that can be carried out at the hip joint, along with the main muscles responsible:
The degree to which flexion at the hip can occur is dependent on the knee's position. If the knee is flexed, the hamstring muscles will be relaxed, allowing for an increased range of flexion. On the other hand, extension at the hip joint is limited by the joint capsule and the iliofemoral ligament. These structures become taut during extension in order to prevent any further movement.
Dislocations of the hip joint can be either congenital or acquired. Congenital dislocations occur due to developmental dysplasia of the hip (DDH), which is a shallow acetabulum caused by a failure to develop properly in utero. Clinically, common features can include limited abduction at the hip joint, limb length discrepancy (the affected limb is shorter than the other) and asymmetrical gluteal or thigh skin folds.
DDH is treated with a Pavlik harness, which holds the femoral head in the acetabular fossa and promotes normal development of the hip joint. In the event that the harness treatment is unsuccessful, surgery may be required.
Acquired dislocations of the hip, although rare, can occur due to trauma or as a complication of total hip replacement or hemiarthroplasty. Of the two main types of acquired dislocations, posterior dislocations (90%) are the most common. This occurs when the femoral head is forced posteriorly, causing a tear through the inferior and posterior part of the joint capsule. In such cases, the affected limb appears to be shortened and medially rotated. The sciatic nerve is particularly at risk of injury in these cases, as it runs close to the hip joint and can be injured in 10-20% of cases.
Anterior dislocations (rare) occur as a result of traumatic extension, abduction and lateral rotation. The femoral head is then displaced both anteriorly and (usually) inferiorly in relation to the acetabulum.
Fig 4 - Radiograph showing dislocation of the left hip joint.
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