Ball Joint: Structure, Function & Diseases

Ball joints are a form of true joint in which the joint head has a spherical shape. The head rests in the socket according to the lock-and-key principle and has four-axis mobility. The most significant diseases of the joints include osteoarthritis and arthritis.

What are ball and socket joints?

The human body has 143 joints. As such, movable connections between bones colliding with each other are called. Real joints have a so-called joint space and can therefore be distinguished from unreal joints without a joint space. Depending on the localization, a joint has different functional requirements. Depending on the functional requirements, the real joints of the human body have different shapes. One form variant of the true joint is the ball and socket joint. In this type of joint, one of the joint surfaces involved bears a spherical shape. The second joint surface is the socket, into which the spherical joint head engages according to the hand-in-glove or key-lock principle. The hand-in-glove or lock-and-key principle refers to the complementary design of the structures involved and is associated with coherent fitting accuracy. Instead of translational movements, only rotational movements are possible in the ball joint. The hip joint is one of the most important ball and socket joints in the human body. The shoulder joint and the ankle joint also correspond to a ball and socket joint.

Anatomy and structure

All so-called diarthroses, or true joints, bear a gap between their articular surfaces known as the joint space, which is covered by cartilage. As a functional unit, the joint and its individual components in true joints lie within a joint capsule, which consists of an outer membrana fibrosa in the form of tight connective tissue and an inner membrana synovialis of epithelium-like connective tissue dressings. The joint capsule brings the joint cavity under complete closure and lies flaccidly against the individual joint bodies. The outer membrana fibrosa of the joint capsule is reinforced by articular or capsular ligaments. Articular ligaments within the joint cavity each carry a layer of membrana synovialis and are thus connected to the joint capsule, which contains so-called synovial fluid or synovia with a viscous consistency. As a true joint, the ball and socket joint fulfills all the joint characteristics mentioned above. The articulation partners of ball joints consist of an approximately spherical joint head and a complementarily shaped socket that envelops the joint head. A special form of the ball joint is the nut joint. In this joint form, the socket envelops the head beyond its equator.

Function and tasks

Joints connect bones to each other in a movable manner. Accordingly, they fulfill various functions. On the one hand, they have a stabilizing effect, and on the other hand, they give bones a certain degree of mobility, which can be performed via one or more axes. In principle, the ball and socket joint is considered a multiaxial joint. This means that its axes of motion encompass at least two planes. This means that at least four different types of motion are possible in most ball joints. As a special form of ball joint, the nut joint differs from the conventional ball joint in terms of its range of motion. The range of motion in the nut joint is less than in a typical ball joint. The amplitude of the hip joint, for example, is limited in mammals because it is a nut joint in the strict sense. In any ball and socket joint, the center of the condyle represents the fulcrum of the joint body. Thus, the ball and socket joint essentially allows three axes of motion for any form of movement. Its three degrees of freedom thus allow the joint to move in all three spatial planes, such as abduction and adduction or extension and flexion. In the anatomy and function of ball and socket joints, there is often talk of unlimited mobility. In practice, however, the ball and socket joint’s ability to move is limited by the surrounding joint capsule and its ligaments (bands), which require some guidance.

Diseases

Ball and socket joints can be impaired in function by both congenital deformities and acquired diseases. For example, in congenital hip dysplasia, the condyle does not fit ideally into the socket. The hand-in-glove principle is thus disturbed, which may be associated with congenital hip dislocation (hip dysplasia) or a tendency to hip dislocation.Osteoarthritis or arthrosis is one of the most significant acquired diseases of ball joints. Due to age, ball joints are affected by a certain amount of wear and tear, which mainly affects the cartilage. If the wear and tear exceeds the age-physiological level, we speak of arthrosis. Particularly in the hip joint, arthrosis is favored by overweight, since the extra weight causes extra stress on the hip every day. However, even people of normal weight can overload their ball joints, for example with competitive sports or other regular overloading through monotonous movements. Other risk factors for osteoarthritis are deformities, such as those present from birth or those that can result from fractures. The cartilage degrades in osteoarthritis until the joint becomes stiff and loses its original range of motion. Many people with osteoarthritis of the shoulder joint, for example, are no longer able to move their arm over their head. As the disease progresses, the ends of the bones that meet rub against each other without any protective layer, thus wearing away and causing pain. Osteoarthritis in the early stages is characterized by load-dependent pain. After a certain time, this load-dependent pain extends to resting phases. Arthritis is to be distinguished from osteoarthritis. This is an inflammation that can in principle affect all types of joints and causes swelling and redness in addition to joint pain. Arthritis in the ball and socket joint can also be caused by overload, but can also be favored by infections. Chronic arthritis favors the later onset of osteoarthritis.