Synovial joints are most evolved and therefore most mobile type of joints. They possess the following characteristic features;
- There articular surfaces are covered with hyaline cartilage. This articular cartilage is avascular, non nervous and elastic. Lubricated with synovial fluid, the cartilage forms slippery surfaces for free movements.
- Between the articular surfaces there is a joint cavity filled with synovial fluid. The cavity may be partially or completely subdivided by an articular disc known as meniscus.
- The joint is surrounded by an articular capsule which is fibrous in nature and is lined by synovial membrane. Because of its rich nerve supply the fibrous capsule is sensitive to stretches imposed by movements.
- The synovial membrane lines the entire joint except the articular surfaces covered by hyaline cartilage. It is this membrane that secretes the slimy fluid called synovial fluid which lubricates the joint and nourishes the articular cartilage.
- Varying degrees of movements are always permitted by the synovial joints.
Types of synovial joints:
Synovial joints are of the following types;
Plane synovial joints:
The articular surfaces of plane synovial joints are more or less plane. These joints permit gliding movements in various directions. Examples are intercarpal joints, intertarsal joints, and joints between the articular processes of vertebrae.
In these joints the articular surfaces are pulley shaped. There are strong collateral ligaments to provide stability to the joint. Movements are permitted in one plane around a transverse axis. Examples are elbow joint, ankle joint, interphaleangeal joint.
Pivot joints are formed by a central bony pivot surrounded by an osteo-ligamentous ring. Movements are permitted in one plane around a vertical axis. Examples of this type are superior and inferior radioulnar joints and the median atlantoaxial joint.
These are also known as bicondylar joints. There articular surfaces consist of two distinct condyles in which one is convex surface (called the male surface) fitting into a concave surface (called the female surface) of the other bone. These joints mainly permit the movement in plane around a transverse axis. Example of this type of joints is knee joint.
In this case the articular surfaces include an oval convex male surface fitting into an ellipsoid female surface. The movements are permitted around two axis; flexion and extension around the transverse axis and adduction and abduction round antero-posterior axis. Combination of these movements produces Circumduction. Typical rotation around a third vertical axis does not occur. Examples of this type of joints are wrist joint, metacarpophalangeal joint and atlanto-occipital joint.
Articular surfaces are reciprocally concavo-convex. Movements are similar to those permitted by ellipsoid joint with addition of some rotation (conjunct rotation (rotation which accompany other movements)) around a third axis which occurs independently. Examples of this type of joints are first carpometacarpal joint, sternoclavicular joint, calcaneocuboid joint.
Ball and socket joints:
These are also called spheroidal joints. There articular surfaces include a globular head fitting into a cup shaped socket. Movement occurs around an indefinite number of axes which have common center. Flexion, extension, abduction, adduction, rotation, Circumduction all occur quite freely. Examples of this type of joints are shoulder joint, hip joint and talo-calcaneonavicular joint.
Blood supply of Synovial Joints:
The articular and epiphyseal branches given off by the neighboring arteries form a peri-articular arterial plexus. Numerous vessels from this plexus pierce the fibrous capsule and form a rich vascular plexus in the deeper part of the synovial membrane. The blood vessels of the synovial membrane terminate around the articular margins in a fringe of looped anastomoses termed the circulus vasculosus (circulus articularis vasculosus). It supplies the capsule, synovial membrane and the epiphyses. The articular cartilage is avascular.
After epiphyseal fusion in growing long bones the communications between the circulosus vasculosus and the end arteries of the metaphysis are established thus minimizing the chances of osteomyelitis in the metaphysis.
Lymphatic drainage of synovial joints:
Lymphatics form a plexus and the subintima of the synovial membrane and drain along the blood vessels to the regional deep nodes.
Stability of synovial joints:
The various factors maintaining the stability at a joint are described below in order of their importance;
- Muscles: The tone of different groups of muscles acting on the joint is the most important and indispensable factor in maintaining the stability. Without muscles, the knee and shoulder would have been unstable and the arches of foot would have collapsed.
- Ligaments: These are important in preventing any over movement and in guarding against sudden accidental stresses. However they do not help against a continuous strain because once stretched, they tend to remain elongated. In this respect the elastic ligaments (ligament flava and the ligaments of the joints of auditory ossicles) are superior to the common type of white fibrous ligaments.
- Bones: They help in maintaining the stability only in firm type of joints like the hip and ankle joints. Otherwise in most of the joints there role is negligible.