Anatomy & Organs

Acetabulum – structure, function & diseases

Acetabulum

The acetabular cup , or acetabulum , is a bony structure surrounded by a joint capsule found in the pelvis of vertebrates with developed hindlimbs. With its cup-shaped shape, it not only accommodates the head of the thigh, but also allows its multidirectional movement by contracting the corresponding muscles. Diseases of the acetabulum always lead to impairments in locomotion or freedom of movement.

What is the acetabular cup?

The acetabulum is designed as a hemispherical depression in the lateral part of the respective half of the pelvis of an individual and serves to accommodate the femoral head. The acetabulum is therefore an important component of the hip joint ( articulatio coxae ), through which the pelvis ( pelvis ) is connected to the thigh bone ( femur ) in a flexible manner.A few millimeters thick layer of cartilage lies on the inner surface of the joint socket, which separates the two elements from one another and enables sliding movement. Due to its spherical shape, the hip joint is classified together with the shoulder joint and the metatarsophalangeal joints (with the exception of the thumb) under the collective term “spherical joint”.

Since the acetabulum or the fibrous cartilage lip surrounding it ( labrum acetabuli ) encloses the head of the femur across its equator, this special case is also referred to as a socket joint. The articulatio coxae is the second largest joint in mammals. Depending on size and weight, the diameter of the human socket is about 2.7 cm.

Anatomy & Structure

The three bones that are responsible for forming the entire pelvis are involved in the structure 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 in the course of ontogeny. These are the ilium ( Os ilium ), the ischial bone ( Os ischii ) and the pubic bone ( Os pubis ).). While the ischium and the pubic bone form the front and rear parts in the lower area, the ilium forms the roof of the acetabulum. 

The ischium is the bone that contributes most to building the acetabulum. The acetabular cup is sunk into the pelvis in the form of a cup, the edges of which form a crater from the surrounding bone of the pelvis. The circumferential, uneven, thickened bony ridge ( limbus acetabuli ) serves as an attachment point for the arcuate fibrocartilage lip, which reduces the opening of the acetabulum and stabilizes the femoral head in the acetabulum. Near the hole in the pubic bone ( foramen obturatum ), the wall of the acetabulum is separated by a gap ( incisura acetabuli) which gives the acetabulum the shape of a crescent. However, the spared area is encompassed by the transverse acetabular ligament . In the center of the acetabulum is a shallow fossa, the acetabular fossa . The femur is held in the socket by the ligamentum teres femoris , which is attached there and also docks onto the head of the femur.

Function & Tasks

The primary function of the acetabular cup is to allow a flexible connection between the femur and the pelvis. The associated, extensive freedom of movement of the thigh in relation to the rest of the body forms the basis for the individual’s ability to move.

Due to the spherical shape of the hip joint ( articulatio coxae ), all three degrees of freedom for the femur can be accessed. This results in a possibility of movement of the thigh in six different directions. In addition to bending (flexion), stretching (extension), advancing (adduction) and spreading (abduction), turning in (internal rotation) and turning out (external rotation) the femur are also possible.However, the extent of the bony, cartilaginous and ligamentous guidance of the hip joint limits these directions of movement somewhat. The femur is moved in one of the six directions via specific muscle groups, some of which work antagonistically. Due to the extensive range of motion, the hip joint is equipped with a very strong ligament apparatus.

Diseases & Ailments

Diseases of the hip joint can be of a degenerative, inflammatory, congenital or accidental nature. One of the most common diseases is coxarthrosis , which is characterized by inflammation and regression of the articular cartilage and, in addition to pain, causes a restriction in freedom of movement. Coxitis fugax , which occurs in children and adolescents, is also one of the inflammatory diseases of the hip joint . The sterile inflammation leads to radiating pain in the knee, limping and a restriction in hip rotation.

Flattened acetabular cups, which are either developmental in newborns or occur in old age, are also less common as a condition. Due to hip dysplasia , the head of the femur slides out of the acetabulum, which leads to incorrect loading, increased cartilage abrasion and, as a result, to early arthrosis .

Likewise, the genetically manifested bulging of the acetabulum and femoral head ( protrusio acetabuli ) brings with it considerable impairments. There is a restriction in the rotation, adduction and abduction of the thigh, which later also makes bending and extension of the leg more difficult. But also traumatic events, which end in a fracture of the acetabular cup due to direct or indirect violence, cause significant complaints.In addition to a disturbed function of the articulatio coxae, there is also a misalignment or shortening of the leg . Due to other concomitant diseases, such as rickets or immobility after muscle paralysis, in some cases there is an axial misalignment of the femoral neck. Depending on the angle, a distinction is made between coxa vara, the “ knock-knee ”, and coxa valga, the “ bow-leg ”.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.