The hip joint is one of the most important joints in the human body. It allows us to walk, run, and jump. It bears our body’s weight and the force of the strong muscles of the hip and leg. Yet the hip joint is also one of our most flexible joints and allows a greater range of motion than all other joints in the body except for the shoulder.

The hip joint is a ball-and-socket synovial joint formed between the os coxa (hip bone) and the femur. A round, cup-shaped structure on the os coax, known as the acetabulum, forms the socket for the hip joint. The rounded head of the femur forms the ball of the joint.

Hyaline cartilage lines both the acetabulum and the head of the femur, providing a smooth surface for the moving bones to glide past each other. Hyaline cartilage also acts as a flexible shock absorber to prevent the collision of the bones during movement. Between the layers of hyaline cartilage, synovial membranes secrete watery synovial fluid to lubricate the joint capsule.

Surrounding the hip joint are many tough ligaments that prevent the dislocation of the joint. The strong muscles of the hip region also help to hold the hip joint together and prevent dislocation.

Functionally, the hip joint enjoys a very high range of motion. The ball-and-socket structure of the joint allows the femur to circumduct freely through a 360-degree circle. The femur may also rotate around its axis about 90 degrees at the hip joint. Only the shoulder joint provides as high of a level of mobility as the hip joint. In addition to being flexible, each hip joint must be capable of supporting half of the body’s weight along with any other forces acting upon the body. During running and jumping, for example, the force of the body’s movements multiplies the force on the hip joint to many times the force exerted by the body’s weight. The hip joint must be able to accommodate these extreme forces repeatedly during intense physical activities.

If a knee or hip joint breaks in an accident or wears out in old age, a surgeon can replace it with a ball-and-socket joint made from metal and plastic and engineered in such a way that it will duplicate the motions of a human joint. Hip replacement was once impossible because, although joints could easily be produced in a laboratory, the human body rejected the materials. Sometimes the pins that held the artificial joint to other bones worked loose and required further surgery. Some joints, especially the artificial knee, didn't work very well because they were designed like hinges that just opened one way. Later, when the designers realized the knee needed to rotate slightly, they produced a joint that would fulfill these movements as well.

Medical pioneers finally overcame bodily rejection by making the joints out of non-irritating, man-made materials. Surgeons have now perfected hip and knee replacement surgeries so that recipients are relieved of pain and can walk at a smoother pace.

Hip joint is technically known as acetabulofemoral joint (occurs between acetabulum and femur). It is a synovial ball and socket joint that occurs between head of femur and acetabulum of hip bone. Its primary function is to make the legs mobile without weakening the ability to support the weight of human body in both static and dynamic postures. Hip joints are most important factors in maintaining balance of body. Any deformity of these joints may result in serious consequences for human posture.

Hip Joint

Articular Surfaces of Hip Joint:

As stated above, hip joint is formed between head of femur and acetabulum of hip bone. The femoral head is spherical while the acetabulum is cup shaped. Thus the articular surfaces are reciprocally curved. The acetabular articular surface is horse-shoe shaped. It is incomplete inferiorly and the region is known as acetabular notch. The deep part of the acetabulum that doesn’t take part in articulation is known as acetabular fossa. It is devoid of articular cartilage but contains fibroelastic fat largely covered by synovial membrane.

Articular Surfaces of Hip joint

Fibrous capsule of hip joint:

The capsule of hip joint is strong and tough. Anteriorly many fibers ascend along the neck as longitudinal retinacula, containing blood vessels for both the femoral head and neck. The capsule of the hip joint is thickest anterosuperiorly, the region of maximal stress. Grossly, the fibrous capsule of hip joint consists of two types of fibers: Circular and Longitudinal. The circular fibers form the internal part while longitudinal fibers form the external part.

Ligaments of hip joint:

Hip joint consists of five major ligaments as described below.

Ligaments of Hip Joint (Anterior View)

Ligaments of Hip Joint (Anterior View)

• Iliofemoral ligament: It is like an inverted “Y” in shape and is very strong. It lies towards the anterior side and is somewhat blended with the capsule of hip joint. The base of the inverted “Y” is attached to anterior inferior iliac spine. The two limbs of the inverted “Y” are attached to the upper and lower parts of intertrochanteric line of femur.
  Role: It prevents overextension during standing.
• Pubofemoral ligament: It is triangular in shape with its base attached to the superior ramus of the pubis. The apex is attached below to the lower part of the intertrochanteric line.
  Role: It limits extension and abduction.
• Ischiofemoral ligament: It is spiral shaped and is attached to the body of ischium near the acetabular margin. The fibers of this ligament pass upward and laterally and are attached to the greater trochanter of femur.
  Role: It limits extension.
• Transverse acetabular ligament: It is formed by the acetabular labrum as it bridges the acetabular notch. Thus the notch is converted into a tunnel through which blood vessels and nerves enter the hip joint.
• Ligamentum Teres (Ligament of head of femur): It is flat and triangular in form and is attached through its apex to the fovea capitis (pit in the head of femur).. The base of this ligament is attached to the transverse acetabular ligament and margins of acetabular notch. This ligament lies within the joint and is ensheathed by synovial membrane.
  Role: It limits adduction and provides a pathway for blood vessels to enter the head of femur.

Synovial membrane of hip joint:

The synovial membrane lines the capsule and is attached to the margins of the articular surfaces. It ensheathes the ligamentum teres and covers the pad of fat contained in the acetabular fossa.

Bursae related to hip joint:

The hip joint may communicate with the psoas bursa through a circular aperture between the pubofemoral and iliofemoral ligaments.

Blood supply of hip joint:

The arterial supply comes from the following arteries.

• Obturator artery
• Medial circumflex femoral artery
• Superior and Inferior gluteal arteries

These arteries form two important anastomoses namely cruciate anastomosis and trochanteric anastomosis.

Lymphatic drainage of hip joint:

The lymph vessels from front aspect drain to the deep inguinal nodes, while those from the posterior and medial aspects run with the gluteal and obturator arteries repectively to reach the internal iliac nodes.

Nerve supply to hip joint:

Hip joint is innervated by the femoral nerve and its muscular branches. Other nerves contributing to the innervation include obturator nerve, accessory obturator nerve, nerve to quadratus femoris and superior gluteal nerve.

Stability of hip joint:

Like the shoulder joint, hip joint belongs to the class of very mobile joints. Mobility comes at the expense of stability, however, in comparison with the shoulder joint, the hip joint is much more stable and much less mobile. The prime factors in maintaining the stability of hip joint are the ligaments and the joint capsule. In addition, the shape of articular surfaces also enhances the stability.

Movements of hip joint:

Hip joint is a ball and socket type of joint, which is very mobile. Its movements can be classified into the following categories:

• Flexion-Extension
• Adduction-Abduction
• Medial and Lateral Rotation
• Circumduction

Muscles producing movements:

• Flexion: The primary muscles of flexion are Psoas major and Iliacus. They are assisted by pectineus, rectus femoris and sartorius. The adductor longus also assists in early flexion from full extension.
• Extension: It is produced by the Gluteus maximus and hamstring muscles. The hamstring muscles, which are powerful flexors of the knee, are equally strong extensors of the hip joint. They largely control the posture of this joint. The Gluteus maximus only becomes active when the thigh is extended against resistnance for instance in climbing.
• Abduction: It is produced primarily by gluteus medius and minimus. Tensor fasciae latae and sartorius assist them.
• Adduction: It is produced by adductor longus, adductor brevis and adductor magnus. The assistors of adduction include pectineus and gracilis.
• Medial rotation: It is produced by tensor fasciae latae and anterior fibers of gluteus minimus and medius. It is relatively weak in strength.
• Lateral rotation: It is produced by the obturator muscles, the two gemelli and quadratus femoris. The assistors include piriformis, gluteus maximus and sartorius. It is much more powerful as compared to medial rotation.
• Cricumduction: It is a combination of above movements with all muscles involved in it.