The synovial fluid
- Key People:
- Sir Benjamin Collins Brodie, 1st Baronet
- Related Topics:
- knee
- elbow
- wrist
- sacroiliac
- knuckle
- On the Web:
- Better Health Channel - Joints (Mar. 14, 2025)
The main features of synovial fluid are: (1) Chemically, it is a dialyzate (a material subjected to dialysis) of blood plasma—that is, the portion of the plasma that has filtered through a membrane—but it contains a larger amount of hyaluronic acid than other plasma dialyzates. (2) Physically, it is a markedly thixotropic fluid—that is, one that is both viscous and elastic. Its viscosity decreases with an increase in the speed of the fluid when it is in motion. Its elasticity, on the other hand, increases with an increase in the speed of the fluid. Its thixotropy is due to the hyaluronic acid in it. (3) Functionally, it has two parts to play: nutrition and lubrication. It has been established that synovial fluid alone, by virtue of its being a blood-plasma dialyzate, can nourish the articulating parts of the articular cartilages. Its thixotropic properties make it suitable for forming what are called elastohydrodynamic lubricant films between the moving and the fixed conarticular surfaces of any mating pair. The motion of the synovial fluid, referred to earlier in connection with the fatty pads, assists its nutritional function by distributing it over the articular surfaces, from which it slowly passes into the interior of the cartilage. The source of the hyaluronic acid is the synovial lining cells.
Types of synovial joints
Recognition of the bursal nature of synovial joints makes it possible to describe them simply in terms of the bursal wall and to group together a number of types of structures. There are seven types of synovial joints: plane, hinge, pivot, sellar, ellipsoid, spheroidal (ball-and-socket), and bicondylar (two articulating surfaces). This classification is based on the anatomical form of the articular surfaces.
Plane joint
The plane, or arthrodial, joint has mating surfaces that are slightly curved and may be either ovoid or sellar. Only a small amount of gliding movement is found. Examples are the joints between the metacarpal bones of the hand and those between the cuneiform bones of the foot.
Hinge joint
The hinge, or ginglymus, joint is a modified sellar joint with each mating surface ovoid on its right and left sides. This modification reduces movement to a backward-forward swing like that allowed by the hinge of a box or a door. The swing of the joint, however, differs from that of a hinge in that it is accompanied by a slight spin (rotation) of the moving bone around its long axis. This brings the joint either into or out of its close-packed position, which is always that of extension. The joints between the bones of the fingers (phalanges) and that between the ulna (inner bone of the forearm) and the humerus at the elbow are classic examples.
Pivot joint
The pivot, or trochoid, joints are of two forms: in one a pivot rotates within a ring; in the other a ring moves around a pivot. In each case the ring is composed of fibrous tissue, part of which is converted into cartilage to form a female surface; the remainder may be ossified. Similarly, only part of the pivot is covered by a male articular cartilage. Pivot joints are always of the ovoid class; from a functional aspect, they are the ovoid counterparts of hinge joints. The joint between the atlas and the axis (first and second cervical vertebrae), directly under the skull, allows for turning of the head from side to side. Pivot joints also provide for the twisting movement of the bones of the forearm (radius and ulna) against the upper arm, a movement used, for instance, in unscrewing the lid of a jar.
Sellar joint
The sellar joint has already been described in the section Articular cartilage. It has two types of movement, both swings: flexion-extension and abduction-adduction. In addition to these it allows movements combining these two—that is, swings accompanied by rotation of the moving bone. An example of a sellar joint is the carpometacarpal joint of the thumb. The thumb can be swung from side to side or from behind forward, but the most frequent movement is that in which the thumb swings so that it comes “face to face” with one or another of the fingers, as in grasping a needle or a ball. This movement is called opposition (i.e., of thumb to fingers). During opposition the thumb is rotated around its long axis; it has been said that human civilization depends upon the opposition of the thumb.
Ellipsoid joint
The ellipsoid joint also has two types of movement but allows opposition movement only to a small degree. Its surfaces are ovoid and vary in both length and curvature as they are traced from front to back or from side to side, just as the diameter and curvature of an ellipse vary in directions at right angles to each other (hence the name). The joint between the second metacarpal and the first phalanx of the second finger is a good example. It allows the finger to flex and extend, to swing toward or away from its neighbouring finger, and to swing forward with a slight amount of rotation.
Ball-and-socket joint
The ball-and-socket joint, also known as a spheroidal joint, is the only one with three types of movement. It is an ovoid joint the male element of which could be described as a portion of a slightly deformed sphere. The rounded surface of the bone moves within a depression on another bone, thus allowing greater freedom of movement than any other kind of joint. It is most highly developed in the large hip and shoulder joints of mammals, including humans, in which it provides swing for the arms and legs in various directions and also spin of those limbs upon the more stationary bones.
Bicondylar joint
The condylar joint is better called bicondylar, for in it two distinct surfaces on one bone articulate with corresponding distinct surfaces on another bone. The two male surfaces are on one and the same bone and are of the same type (ovoid or sellar). These joints have two types of movement: one is always a swing, and the other is either another swing or a spin. Bicondylar joints are quite common. The largest is the tibiofemoral joint, in which both pairs of mating surfaces are within a single joint. At this joint, flexion and extension are the main movements; but active rotation of the leg on the femur is possible in most people when the leg and thigh are at right angles to each other. Every vertebra of the cervical, thoracic, and lumbar series is connected to (or separated from) the one below it by a pair of synovial joints as well as by an intervertebral disk. This pair of joints constitutes a bicondylar joint, the shape of whose articular surfaces determines the amount of movement permitted between the vertebra. The atlanto-occipital joint, between the skull and the vertebral column, is also a bicondylar joint. Finally, the right and left temporomandibular joints, between the lower jaw and the skull, are really two parts of a bicondylar joint, not only by definition—if the base of the skull is considered as a single bone—but also functionally, for one mandibular condyle cannot move without the other moving also.
Joint ligaments
Any set of collagen fibres joining one bone of an articulating pair to the other is called a ligament. Thus, the articular bursal wall is a ligament, called either the fibrous capsule or the joint capsule.
There are two types of these sets: capsular and noncapsular. Capsular ligaments are simply thickenings of the fibrous capsule itself that take the form of either elongated bands or triangles, the fibres of which radiate from a small area of one articulating bone to a line upon its mating fellow. The iliofemoral ligament of the hip joint is an example of a triangular ligament. Capsular ligaments are found on the outer surface of the capsule. There is one exception to this rule: ligaments of the shoulder joint (glenohumeral ligaments) are found on the inner surface.
Noncapsular ligaments are free from the capsule and are of two kinds: internal and external. The internal type is found in the knee, wrist, and foot. In the knee there are two, both arising from the upper surface of the tibia; each passes to one of the two femoral condyles and lies within the joint cavity, surrounded by synovial membrane. They are called cruciate ligaments because they cross each other X-wise. At the wrist most of the articulations of the carpal bones share a common joint cavity, and neighbouring bones are connected sideways by short internal ligaments. The same is true of the tarsal bones that lie in front of the talus and the calcaneus.
The external noncapsular ligaments are of two kinds: proximate and remote. The proximate ligaments pass over at least two joints and are near the capsules of these joints. They are found only on the outer side of the lower limb. Examples are the outer (fibular) ligament of the knee, which passes from the femur to the upper part of the fibula over both the knee and tibiofibular joints, and the middle part of the outer ligament of the ankle joint, which passes from the lowest part of the fibula to the heel bone. These two ligaments, particularly that passing over the ankle, are especially liable to damage (sprain).
The remote ligaments are so called because they are far from, rather than close to, the joint capsule. A notable example is that of the ligaments that pass between the back parts (spines and laminae) of neighbouring vertebrae in the cervical, thoracic, and lumbar parts of the spinal column. These are the chief ligaments of the pairs of synovial joints between the vertebrae of these regions. Unlike most ligaments, they contain a high proportion of elastic fibres that assist the spinal column to return to its normal shape after it has been bent forward or sideways.
Contrary to the opinion of earlier anatomists, ligaments are not normally responsible for holding joint surfaces together. This is because a set of collagen fibres, like a string, can exert a reactive force only if stretched and tightened by some tensile stress. Normally, the bones at a joint are pressed together (when at rest) by the action of muscles or by gravity. An individual ligament can stop a movement that tightens it. Such a movement will loosen the ligaments that would be tightened by the opposite movement. The one exception to this case is the movement that brings a joint into the close-packed position. This movement is brought about by a combination of a swing with a spin of the moving bone. Experiments show that the combination of movement screws the articular surfaces firmly together so that they cannot be separated by traction and that the capsule and most of the ligaments are in simultaneous maximal tautness.
Nerve supply and blood supply
The nerve and blood supply of synovial joints follows the general rule for the body: “Ubi nervus ibi arteria” (“Where there is a nerve, there also is an artery”).
