Understanding Joints

A joint (articulation) is a junction between two or more bones (or rigid parts) of the skeleton. (Babies have cartilaginous tissue until ossification takes place later in life.) Some joints have no movement, some have slight movement and some are freely moveable.

Joint Classes

Synovial Joints
Synovial joints are the most common type of joint and provide free movement between the bones that they join. They are joints of locomotion. Synovial joints are usually reinforced by accessory ligaments that are either separate or are a thickening of a portion of the joint capsule. Some synovial joints have other features such as fibrocartilaginous articular discs or menisci which are present when articulating surfaces of the bones are incongruous (when the articulating surfaces don’t match).
The articulating bones of synovial joints are surrounded by a joint capsule spanning and enclosing an articular cavity. It’s composed of an outer fibrous membrane and lined by a serous synovial membrane. The articular cavity of a synovial joint; such as the knee, contains synovial fluid which is secreted by the synovial membrane. Inside the joint capsule articular cartilage covers the surface of the articulating surfaces of the bones. All the other internal surfaces are covered by synovial membrane.
Fibrous Joints

The articulating bones of fibrous joints are united by fibrous tissue. The amount of movement that occur in these joints depends on the length of the fibers uniting the articulating bones. The sutures of the cranium are examples of this type of joint. These bones are close together, either interlocking along a wavy line or overlapping.
- Syndesmosis
  A syndesmosis type of fibrous joint unites a bone with a sheet of fibrous tissue; either a ligament or membrane. This type of joint is slightly moveable. The interosseous membrane in the forearm is a sheet of fibrous tissue that joint the ulna and radius in a syndesmosis.
- Gomphosis (socket)
  A gomphosis (dentoalveolar syndesmosis) is a fibrous joint in which a peg-like process fits into a socket articulation between the root of the tooth and the alveolar process of the jaw. Movements of this joint (a loose tooth) indicates a pathological state affecting the supporting tissues of the tooth
Cartilaginous Joints
The articulating structures of cartilaginous joints are united by hyaline cartilage or fibrocartilage. In primary cartilaginous joints, or synchondroses, the bones are united by hyaline cartilage which permits slight bending during early life. Primary cartilaginous joints are usually temporary unions such as those present during the development of a long bone where the bony epiphysis and the shaft are joined by an epiphysial plate. Primary cartilaginous joints permit growth in the length of the bone. When full growth is achieved the epiphysial plate converts to bone and the epiphyses fuse with the diaphysis. Secondary cartilaginous joints, or symphyses are strong slightly moveable joints united by fibrocartilage. The fibrous cartilaginous intervertebral discs between the vertebrae consist of binding connective tissue that joins the vertebrae together. Cumulatively these joints provide strength and shock absorption as well as considerable flexibility to the vertebral column.

Six types of synovial joints:

Plane Joints
Plane joints permit sliding and gliding movements of the articular surfaces. The surfaces of the bines are flat or almost flat with limited movement by tight joint capsules. There are many plane joints in the body and the majority of these joints are small. Ie. Acromioclavicular joint between the acromion and the clavicle.
Hinge Joints
Hinge joints only permit flexion and extension. These movements occur in one plane of movement only (sagittal) around a single axis that runs transversely, therefore hinge joints are uniaxial joints. The joint capsule of a hinge joint is thin and lax anteriorly and posteriorly where movement occurs, however, the bones are joined by strong, laterally placed collateral ligaments. Ie. elbow joint The carpometacarpal joint at the base of the thumb (1st digit) is a saddle joint.
Saddle Joints
Saddle joints permit abduction and adduction as well as flexion and extension. These movements occur around two axis at right angles to one another therefore these are biaxial joints that allow movement in two planes; sagittal and frontal. Circumduction in these joints is also possible. The opposing articular surfaces of these joints are shaped like a saddle. Ie Like Pringles chips
Condyloid Joints
Condyloid joints also permit abduction, adduction, flexion and extension therefore they are also biaxial joints. The movement in one plane is usually greater or freer than the other plane. Although circumduction is also possible it is more limited than in a saddle joint. The metacarpophalangeal joints (knuckle joints) are examples.
Ball and Socket Joints
Ball and Socket joints allow movements in multiple planes and axis; flexion, extension, abduction, adduction, medial (internal) and lateral (external) rotation and circumduction. Ball and socket joints are highly mobile, multi-axial joints. The spheroidal surface of one bone moves within the socket of another. Ie The hip joint where the head of the femur rotates within the socket formed by the acetabulum of the hip bone.
Pivot Joints
Pivot joints are uniaxial and only permit rotation around a central axis. In these joints a rounded process of bone rotates within a sleeve or ring. AN example of this is the atlantoaxial joint in which the atlas (C1 vertebra) rotates around a finger like process, the dens (odontoid process) of the axis (C2 vertebra) during head rotation.

Just for fun – The bones of the calvaria (skullcap) of a newborn infant don’t make contact with one another. At these sites the sutures form wide areas of fibrous tissue called fontanelles. The anterior fontanelle that people call the ‘soft spot’ is the most prominent. The fontanelles in a newborn are often felt as ridges because of the overlapping of the cranial bones by molding of the calvaria as it passes through the birth canal. Normally the anterior fontanelle is flat. Pulsations of the fontanelle reflect a pulse while a depressed fontanelle may indicate the baby is dehydrated.