The acetabulum, or hip socket, is a bony structure surrounded by a joint capsule found on the pelvis of vertebrates with developed hind extremities. Due to its cup-shaped form, it not only accommodates the head of the femur, but also allows its multidirectional movement by contraction of corresponding muscles. Diseases of the acetabulum always lead to impaired locomotion or freedom of movement.
What is the acetabulum?
The acetabulum is formed as a hemispherical depression in the lateral part of each half of an individual’s pelvis and serves to accommodate the head of the femur. The acetabulum is thus an important component of the hip joint (articulatio coxae), through which the pelvis (Pelvis) is connected to the femur (Femur) in a movable manner. The inner surface of the socket is covered by a layer of cartilage a few millimeters thick, which separates the two elements and allows them to move smoothly. Due to its ball-like shape, the hip joint is classified together with the shoulder joint and the metacarpophalangeal joints of the fingers (with the exception of the thumb) under the collective term “ball and socket joint”. Since the acetabulum or the fibrocartilaginous lip (labrum acetabuli) surrounding it encloses the head of the femur across its equator, this particular case is also referred to as a nut joint. In mammals, the articulatio coxae is the second largest joint. Depending on size and weight, the diameter of the socket in humans is about 2.7 cm.
Anatomy and structure
Those three bones that are also responsible for the formation of the entire pelvis are involved in the construction of the acetabulum. They meet in the middle of the acetabulum and form a cartilaginous joint in the shape of a “Y”, which, however, ossifies during ontogeny. These are the ilium (ilium), ischium (ischii), and pubis (pubis). While the ischium and pubis provide the anterior and posterior portions in the lower region, the os ilium forms the roof of the acetabulum. The os ischium is the bone that contributes most to the structure of the acetabulum. The acetabulum is recessed into the pelvis in the form of a cup, with its edges cratered from the surrounding bone of the pelvis. The circumferential, uneven, thickened edge of the bone (limbus acetabuli) serves as an attachment site for the arcuate fibrocartilaginous lip, which reduces the opening of the acetabulum and stabilizes the femoral head in the acetabulum. Near the pubic bone hole (foramen obturatum), the wall of the acetabulum is interrupted by a gap (incisura acetabuli) that gives the acetabulum the shape of a crescent. However, the recessed area is encompassed by the ligamentum transversum acetabuli. In the center of the acetabulum is a shallow pit, the acetabular fossa. The femur is held in the socket by the ligamentum teres femoris, which attaches there and also docks to the femoral head.
Function and Tasks
The primary function of the acetabulum is to provide a mobile connection between the femur and the pelvis. The associated broad range of motion of the femur in relation to the rest of the body provides the basis for the individual’s ability to move. Due to the ball-like shape of the hip joint (articulatio coxae), all three degrees of freedom are accessible to the femur. This results in the possibility of movement of the femur in six different directions. In addition to flexion, extension, adduction and abduction, the femur can also rotate inward (internal rotation) and outward (external rotation). However, the bony, cartilaginous and ligamentous guidance of the hip joint somewhat limit these directions of movement. The femur is moved in one of the six directions via specific muscle groups, some of which work antagonistically. Because of the extensive range of motion, the hip joint is equipped with a very strong ligamentous apparatus.
Diseases and complaints
Diseases of the hip joint can be degenerative, inflammatory, congenital or accidental in nature.One of the most common diseases is coxarthrosis, which is characterized by inflammation and degeneration of the articular cartilage and, in addition to pain, causes a restriction in freedom of movement. The inflammatory clinical pictures of the hip joint also include coxitis fugax, which occurs in children and adolescents. The germ-free inflammation leads to radiating pain in the knee, limping and a restriction in hip rotation. Flattened acetabulum is also less common as a condition, occurring either developmentally in newborns or in old age. Hip dysplasia causes the head of the femur to slip out of the acetabulum, resulting in abnormal loading, increased cartilage wear, and subsequently, early osteoarthritis. Likewise, the genetically manifested protrusion of the acetabulum and femoral head (protrusio acetabuli) brings considerable impairments. A restriction in rotation, adduction and abduction of the thigh becomes apparent, which subsequently also makes flexion and extension of the leg more difficult. However, traumatic events that end in a fracture of the acetabulum due to direct or indirect force also cause considerable discomfort. In addition to a disturbed function of the articulatio coxae, a malposition or shortening of the leg is associated. Due to other accompanying diseases, such as rickets or immobility after muscle paralysis, an axial malposition of the femoral neck occurs in some cases. Depending on the angle, a distinction is made between coxa vara, the “knock-kneed” leg, and coxa valga, the “bow-leg” leg.
