Synovial joints include most joints in the body. Bone surfaces are covered by articular cartilage and joined by synovial membrane-coated ligaments. The joint can be divided completely or incompletely by a disc or articular meniscus whose periphery continues with the fibrous capsule, while its free faces are covered by synovial membrane.

Functional Classification of Joints:

The movement of the joints depends essentially on the shape of the contacting surfaces and the joining means that can limit it. Depending on these factors, the joints can perform movements of one, two or three axes. This is the criterion adopted to classify them functionally.

 Monoaxial Joint & #8211; When a joint performs movements only around one axis (1 degree of freedom). Joints that only allow flexion and extension, such as the elbow, are monoaxial. There are two varieties in which the movement is uniaxial: the tingle or hinge joint and the trochoid or pivot joint.

Tingling or Hinge Joint: Articular surfaces allow movement in one plane. The joints are held by strong collateral ligaments. Examples: Interphalangeal joints and humeral-ulnar joint.
– Trocoid or Pivot Joint: When the movement is exclusively of rotation. The joint is formed by a pivot-shaped process rotating within a ring or a ring on a pivot. Examples: Proximal and atlantoaxial radioulnar joint.

 Biaxial Joint & #8211; When a joint performs movements around two axes (2 degrees of freedom). Joints that perform extension, flexion, adduction, and abduction, such as the radiocarpal (wrist joint), are biaxial. There are two varieties of biaxial joints: condylar and seal joints.

Condylar Joint: In this type of joint, an ovoid or condylar joint surface is received in an elliptical cavity to allow flexion and extension, adduction and abduction and circumduction movements, ie all joint movements, except axial rotation. Example: Wrist Joint.

Seal Joint: In these joints the bony faces are mutually concave-convex. They allow the same movements of the condylar joints. Example: Carpometacarpal thumb.

Triaxial Joint & #8211; When a joint performs movements around three axes (3 degrees of freedom). Joints that in addition to flexion, extension, abduction and adduction, also allow rotation, are said triaxials, whose typical examples are the shoulder and hip joints. There is a variety where the movement is polyaxial, called a spheroid joint or enarthrosis.

Spheroid Joint or EnarthrosisIt is a form of joint in which the distal bone is able to move around several axes, which has a common center. Examples: Hip and shoulder joints.

There is yet another type of joint called Flat Joint, which allows only sliding movements. Examples: Joints of vertebral bodies and some joints of the carpus and tarsus.

Movable Joint Structures

Ligaments consist of collagen fibers arranged parallel or closely intertwined with one another. They are malleable and flexible to allow perfect freedom of movement, but are very strong, resistant and inelastic (not to easily give in to the action of forces.

Knee Ligaments
Source: SOBOTTA, Johannes. Atlas of Human Anatomy. 21st. Rio de Janeiro, Guanabara Koogan, 2000.

Articular Capsule
It is a connective membrane that surrounds the synovial joints like a cuff.

It has two layers: the fibrous membrane (external) and the synovial membrane (internal).

THE Fibrous Membrane (fibrous capsule) is more resistant and may be reinforced at some points by also fibrous bundles, which constitute the capsular ligaments, intended to increase its resistance. In many synovial joints there are independent ligaments of the joint capsule called extra-capsular or accessory, and in some, such as the knee joint, intra-articular ligaments also appear. Ligaments and joint capsule are intended to maintain union between bones, but in addition , prevent movement in undesirable planes and limit the range of motion considered normal.

THE Synovial Membrane It is the innermost layer of the joint capsule and forms a closed sac called the synovial cavity. It is abundantly vascularized and innervated and in charge of synovial fluid production. It is argued that synovium is a true secretion or an ultrafiltrate of the blood, but it certainly contains hyaluronic acid that gives it the viscosity it needs for its lubricating function.

Synovial Membrane

Discs and Menisci

In several synovial joints, interposed the articular surfaces, there are fibrocartilage formations, the discs and intra-articular menisci, of function discussed: would serve the best adaptation of the articulating surfaces (making them congruent) or would be structures intended to receive violent pressures acting as shock absorbers. Menisci, with their characteristic half moon shape, are found in the knee joint. Example of intra-articular disc we found in the sternoclavicular and TMJ joints.

                         Knee Menisci               ATM disk
Knee Meniscus ATM disk

Synovial Tendon Sheath
They facilitate the sliding of tendons that pass through fibrous and bony tunnels (wrist flexor retinaculum).

      Synovial Sheaths
                Synovial Sheaths
                 Back of the Hand
 Synovial Sheaths - Palm Synovial Sheaths - Back of Hand

Synovial Bags (Bursas)
They are cracks in the connective tissue between muscles, tendons, ligaments, and bones. They are made up of closed synovial lining bags. Facilitate the sliding of muscles or tendons over bony or ligamentous prominences.



Synovial Joints