The articular head is one of a total of two joint surfaces. Bones are flexibly connected with the articular head and the associated socket. In dislocations, the articular head slides out of the associated socket by application of force from the outside.
What is the articular head?
There are 143 joints in a person’s body. Articular connections between two bones give the joint some ability to move, making it a critical component of human mobility and motor function. Each joint consists of articular cartilage, the joint space containing synovial fluid, the joint capsule, and a stabilizing ligamentous apparatus. The heart of a joint, however, is the condyle and the glenoid cavity. The glenoid cavity is the concave surface of the joint. It receives the convex-shaped articular head. This type of joint occurs in the human body at countless places where two bones meet. Accordingly, the one bone end of bones that meet directly almost always functions as the condyle. The respective shape of the condyle depends mainly on the shape of the socket and the range of motion realized by the joint. Accordingly, the condyle of ball and socket joints such as the hip or shoulder joint has a different shape and range of motion than the condyle in hinge joints, saddle joints, rotational joints, egg joints, or plane joints.
Anatomy and structure
The condyle is always shaped to fit into its associated socket. Articular sockets are concave in shape. The shape of the condyle is correspondingly convex. The exact anatomy depends mainly on the type of joint. The shoulder and hip joints are ball and socket joints. The shoulder joint has a comparatively small socket and a proportionally relatively large condyle. In contrast, the condyle of the hip joint is largely enclosed by the deep and pit-like socket. Thus, the anatomy of a condyle is characterized by differences even when it is the same type of joint. Hinge joints such as the humeroulnar joint consist of a cylindrical condyle within a hollow cylinder-like socket. Saddle joints consist of concave surfaces. Their articular head sits on top of the saddle-like socket like a rider. The radioulnar joint is a swivel joint and, as such, has a peg-shaped joint head whose socket forms a channel-like short cup. In contrast, the joint head of egg joints is much smaller than the associated socket. Plane joints are a special type of joint. The plane vertebral arch joint, for example, consists of joint surfaces that slide against each other and whose head is not received by a socket in the strict sense.
Function and tasks
In the human body, articular heads almost always rest in the associated glenoid cavity and thus correspond to one of two bony surfaces involved in the joint. The articular head can move within its socket receptacle. This movement acts similar to the movements of mortars within a cup. The exact type of motion of the condyle depends on the joint type in each individual case. For all bones in an articulated joint, the bony surface shape, and thus the shape of the condyle and socket, predetermines the range of motion that is possible in the joint. In ball and socket joints such as the shoulder, the spherical condyle can move in all directions in its socket. In hinged joints such as the ankle joint, the condyle in its socket can only move about a specific axis. Accordingly, the joint is constrained by its natural design, as the cylindrical head can only move in a certain direction in the channel-like socket. Among the saddle joints, the thumb saddle joint allow a greater range of motion and allow the condyle head to move in two directions that are perpendicular to each other. With pivot joints, the rod end can only rotate in its socket. Thus, the rod end has different movements to function. Together with the socket, the condyle connects free bone ends to each other and moves more or less flexibly in this connection. Thus, in the context of motor function and mobility, the articular head performs tasks just as relevant as the muscles or the innervation of the muscles. Through its unity with the socket of a joint, for example, extensions, flexions, approaching movements, spreading movements and external or internal rotations of limbs are possible.
Diseases
Joints can be affected by various injuries caused by passive or active force. In some cases, this causes the joint head to lose contact with the joint socket. When the head of joints does not move in the associated socket but has slipped outward, the condition is referred to as dislocation. In most cases, luxation is the result of a torn joint capsule or torn ligaments. After such a tear, the joint surfaces are no longer sufficiently stabilized and slide apart. Incomplete dislocation is called subluxation. Medicine also distinguishes between direct and indirect luxation. Direct dislocation always occurs when external force acts directly on a joint, causing a torn ligament or capsule that causes the condyle to leave the socket. The indirect type of dislocation occurs when the physiological motor inhibition of the joint is violently overcome. A long bone acts as a lever arm and leverages the head out of the socket. It is not only an articular head outside the socket that causes problems. Deformities of the articular head can also have pathological value, for example in the context of acquired or congenital diseases. One such disease is Legg-Calvé-Perthes disease. In this rare disease, the femoral head becomes necrotic on one or both sides of the body. The processes of osteogenesis attempt to compensate for the dead bone tissue by building bone. However, the newly built femoral head is usually deformed and therefore often no longer fits into the associated socket. Diseases such as arthrosis deformans or inflammation in the joint can also result in deformities of the femoral head.
