Synovial Joints

Structure of Synovial Joints

A synovial joint or diarthrosis occurs at articulating bones to allow movement. It is distinguished by a surrounding synovial capsule.

A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in a mammal's body. Diarthroses are freely movable articulations. In these joints, the contiguous bony surfaces are covered with articular cartilage and connected by ligaments lined by synovial membrane. The joint may be divided, completely or incompletely, by an articular disk or meniscus, the periphery of which is continuous with the fibrous capsule while its free surfaces are covered by synovial membrane.

The articular capsule is fibrous and continuous with the periosteum of articulating bones, surrounding the diarthrosis and uniting the articulating bones. The articular capsule also consists of two layers: (1) the outer fibrous membrane that may contain ligaments and (2) the inner synovial membrane that secretes the lubricating, shock-absorbing, and joint-nourishing synovial fluid. The bones of a synovial joint are covered by a layer of hyaline cartilage that lines the epiphyses of joint ends of bone with a smooth, slippery surface that does not bind them together. This articular cartilage functions to absorb shock and reduce friction during movement.

Synovial Membrane and Components

A synovial membrane (or synovium) is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints. Synovial fluid is the clear, viscid, lubricating fluid secreted by synovial membranes. The morphology of synovial membranes may vary, but it often consists of two layers. The outer layer, or subintima, can be fibrous, fatty, or loosely areolar. The inner layer, or intima, consists of a sheet of cells thinner than a piece of paper.

Where the underlying subintima is loose, the intima sits on a pliable membrane called the synovial membrane. This membrane, together with the cells of the intima, acts like an inner tube, sealing the synovial fluid from the surrounding tissue and effectively stopping the joints from being squeezed dry when subjected to impact (such as when running). As with most other joints, synovial joints achieve movement at the point of contact of the articulating bones. The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).

Synoviocytes

The intimal cells are termed synoviocytes and can be either fibroblastic (type B synoviocytes) and macrophagic (type A synoviocytes). Both types have differences from similar cells in other tissues. The type B synoviocytes manufacture a long-chain sugar polymer called hyaluronan, which combines with a molecule called lubricin to give the synovial fluid a stringy, egg-white consistency. The water component of synovial fluid is effectively trapped in the joint space by the hyaluronan due to its large, highly negatively charged moieties. The macrophages are responsible for the removal of undesirable substances from the synovial fluid.

Structure of Synovium

The surface of a synovium may be flat or covered with finger-like projections (villi) to allow the soft tissue to change shape as the joint surfaces move on one another. Just beneath the intima, most synovia have a dense net of small blood vessels that provide nutrients for the synovia and the avascular cartilage.

In any one position, much of the cartilage is close enough to get nutrition directly from the synovium. Some areas of cartilage have to obtain nutrients indirectly and may do so either from diffusion through cartilage or by the stirring of synovial fluid.

Synovial Bursa

The synovial bursa is a small, fluid-filled sac lined by synovial membrane containing synovial fluid. It provides a cushion between bones and tendons and/or muscles around a joint.

Nerve and Blood Supply

Synovial joints are highly innervated but vascularized indirectly by nearby tissues.

The blood supply of a synovial joint comes from the arteries sharing in anastomosis around the joint. The articular and epiphyseal branches of the neighboring arteries form a periarticular arterial plexus. The articular capsule is highly innervated but avascular (lacking blood and lymph vessels), and receives nutrition from the surrounding blood supply via either the slow process of diffusion or convection, a far more efficient process.

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. After epiphyseal fusion in the growth of long bones, communication between the circulosus vasculosus and the end arteries of the metaphysis is established. This minimizes the chances of osteomyelitis in the metaphysis.

The synovial cartilage in the capsule acts somewhat like a sponge. A sponge will absorb fluid, but it will release little of that fluid unless it is squeezed. Exercising the joint, in effect, squeezes the synovial "sponge," allowing gas exchange to occur and nutrients to flow into the cartilage. Flexing and extending the joint alternately squeezes the sponge and releases it to reabsorb more fluid.

Bursae and Tendon Sheaths

Joints are cushioned by small fluid-filled sacs called bursae and stabilized by tough bands of fibrous connective tissue called tendons.

Synovial joints are made up of five classes of tissues: bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments. The synovial lining in the bursae and tendon sheaths, similar to that within joints, is a slippery, non-adherent surface allowing movement between planes of tissue. Synovial tendon sheaths line tendons only where they pass through narrow passages or retinacula, as in the palm, at the wrist, and around the ankle. Elsewhere, the tendon lies in a bed of loose fibrous tissue.

A tendon or sinew is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen, but a ligament joins one bone to another and fasciae connect muscles to other muscles. Tendons connect muscle to bone and move the bones or structures to which they are attached.

A bursa (plural bursae) is a small, fluid-filled sac lined by synovial membrane with an inner capillary layer of fluid (synovial fluid) with the consistency of raw egg white. It provides a cushion between bones and tendons or muscles around a joint. This helps to reduce friction between the bones and allows for free movement.

Bursae occur at sites of shearing in subcutaneous tissue or between deeper tissues such as muscle groups and fascia. Many bursae develop during growth, but new or adventitious bursae can occur at sites of occupational friction. Bursae are found around most major joints of the body, such as the shoulder and the knee. For example, to protect the knee and reduce friction from the various muscles, tendons, and ligaments that attach to and cross the knee joint, knees are cushioned by 14 different bursae: five in front, four laterally, and five medially.

Stability and Range of Motion at Synovial Joints

Tendons provide stability at joints.

A tendon is a mechanism by which muscles connect to bone and that transmits force. However, over the past two decades, research has also characterized the elastic properties of tendons and their ability to function as springs. This characteristic allows tendons to passively modulate forces during locomotion, thus providing additional stability with no active work. It also allows tendons to store and recover energy with high efficiency.

Effect of Tendon Elasticity

During a human stride, the Achilles (calcaneal) tendon stretches as the ankle joint undergoes dorsiflexion. During the last portion of the stride, as the foot undergoes plantar flexion (pointing the toes downward), the stored elastic energy is released. Because the tendon stretches, the muscle is able to function with less or even no change in length, allowing it to generate greater force.

Joint Stability

Certain joints exhibit special movements including elevation, depression, protraction, retraction, inversion, eversion, dorsiflexion, plantar flexion, supination, pronation, and opposition. A number of factors influence joint stability. These include:

• Shape of articular surfaces (how close they fit)
• Strength and tension of capsule and ligaments (dependent on position)
• Arrangement and tension of muscles
• Contact with soft parts such as adipose tissue
• Hormones
• Disuse, causing decrease in synovial fluid, flexibility of ligaments and tendons, and muscle atrophy
• Gravity and atmospheric pressure.

Typically, the more stable the joint is, the less is its range of motion and vice versa. Aging is another factor that influences motion due to decreased fluid, thinning of cartilage, shortening of ligaments, and loss of flexibility.

Synovial Joint Movements

Synovial joints allow an individual to achieve a wide range of movements.

A synovial joint, also known as a diarthrosis, is the most common and most movable type of joint in the body of a mammal. Synovial joints achieve movement at the point of contact of the articulating bones. Structural and functional differences distinguish synovial joints from cartilaginous joints (synchondroses and symphyses) and fibrous joints (sutures, gomphoses, and syndesmoses). The main structural differences between synovial and fibrous joints are the existence of capsules surrounding the articulating surfaces of a synovial joint and the presence of lubricating synovial fluid within those capsules (synovial cavities).

Several movements may be performed by synovial joints. Abduction is the movement away from the midline of the body. Adduction is the movement toward the middle line of the body. Extension is the straightening of limbs (increase in angle) at a joint. Flexion is bending the limbs (reduction of angle) at a joint. Rotation is a circular movement around a fixed point.

There are six types of synovial joints. Some are relatively immobile but more stable than mobile joints. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The six types of joints include:

• Gliding joints: only allow sliding movement
• Hinge joints: allow flexion and extension in one plane
• Pivot joints: allow bone rotation about another bone
• Condyloid joints: perform flexion, extension, abduction, and adduction movements
• Saddle joints: permit the same movement as condyloid joints and combine with them to form compound joints
• Ball and socket joints: allow all movements except gliding

Types of Synovial Joints

There are six different types of synovial joint based on their shapes, each allowing a different kind of movement.

There are six basic types of synovial joints. Anatomical joints may consist of a combination of two or more joint types. Some synovial joints are relatively immobile but stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury. The types of the synovial joints are based on their shapes and can be classified as plane, hinge, pivot, condyloid, saddle, and ball-and-socket. The following descriptions are in ascending order of mobility:

• The articulating surfaces of the plane joint are usually flat to allow slipping and gliding properties. Examples include the carpals of the wrist and the acromioclavicular joint.
• A hinge joint (ginglymus) is formed when the cylindrical end of a bone fits into a trough-shaped surface of another bone, like that of an elbow joint (between the humerus and the ulna). These joints act as a hinge, allowing flexion and extension in just one plane.
• In a pivot joint, the rounded end of the bone fits into a sleeve or ring of bone. The atlanto-axial joint, proximal radioulnar joint, and distal radioulnar joint are examples of pivot joints.
• The condyloid joint occurs where an egg-shaped surface of a bone fits into a concavity in another bone. Examples include the wrist joint (radiocarpal joint) and the temporomandibular joint. Some classifications make a distinction between condyloid and ellipsoid joints, but both allow flexion, extension, abduction, adduction, and circumduction movements.
• The surface of a saddle joint has both convex and concave areas which resemble a saddle and permit the same movements as the condyloid joints. The carpometacarpal or trapeziometacarpal joint of the thumb (between the metacarpal and carpal, the trapezium) and the sternoclavicular joint are examples of saddle joints.
• A ball-and-socket joint occurs where one bone ends in a spherical head and the other bone has a round socket. This joint creates the ball-and-socket movement found in such places as the hip and shoulder (glenohumeral). This type of joint allows for all movements except gliding.

The knee joint is an example of a compound joint/modified hinge joint where different types of joints combine. In this example, the condyles of the femur join with condyles of tibia and the saddle joint, where the lower end of the femur joins with the patella.