Faculty of Biological Sciences, University of Leeds

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Introductory Anatomy: Joints

Dr. D.R.Johnson, Centre for Human Biology

This is a radiograph of the knee region. Seen in the radiograph are two important kinds of joint with quite different functions. Between the secondary centres of ossification or epiphyses of the bones and the metaphyses are joints where movement is positively discouraged: between the femur and the tibia is the synovial knee joint proper, where movement is positively encouraged.
So where two or more bones come together we find a joint: the range of movement at joints may be zero or a just a little give, or extremely large.

Joints are accordingly classified

  1. fibrous and cartilaginous joints where two bones are separated by a deformable intermediate
  2. synovial joints where one surface slides freely over another.

Fibrous joints. We have already mentioned the joint between the bony shaft and cartilage at the ends of long bones. This is a synchondrosis, a cartilage sandwich with bone on either side: bone and cartilage fit together perfectly and the whole thing is cup shaped. If movement occurs the growing bone will be damaged (slipped epiphysis) and this is countered by putting in a long nail to fix it again.
Sutures: are limited to the skull. They resemble a synchondrosis, but with fibrous tissue instead of cartilage between the bones. Sutures are necessary for skull growth: consequently well marked in the young less so in the adult. The only movement in sutures is at birth when the cranial bones overlap to allow passage through the maternal pelvis. After this movement is discouraged by increasing complexity of the suture, which becomes serrated or denticulate. Later in life, when growth is complete they fuse.
gomphoses: are peg and socket joints as seen between teeth and jaws. The joint is maintained by the periodontal ligament which gives only a little to act as a shock absorber when we bite on a ball bearing.
syndesmosis: only one of these in the body, the inferior tibio-fibular joint. In this type there is a little movement, limited by a tight ligament. Since many joints are limited by ligaments this is probably a special definition we can do without. symphysis: two bones united by cartilage, but designed to give a bit. The symphysis pubis with ligaments and fibrocartilage is normally closed, but opens in childbirth due to hormonal influences.

Synovial joints have different parameters. Joint surfaces almost in contact but discontinuous, as a great range of movement is often possible, and the surfaces slide over each other. The sliding surfaces are covered with a thin layer of cartilage. This gives a coefficient of friction of <0.002. The joint cavity is sealed by a synovial membrane which secretes synovial fluid, a lubricant and nutrient. Around this, in turn, is a tough fibrous joint capsule which keeps the ends of the bones in proper orientation. This is often locally thickened to form joint ligaments. The synovial cavity is very small between articular surfaces but larger round the edges where it may form a bursa, a sack-like extension which may be in contact with the joint cavity.
Various inclusions may be present in the joint cavity: a tendon may pass through, sheathed in synovial membrane. Fat pads may be present, packing the large gaps which occur in some joints between bone ends. Pieces of cartilage are also found, in addition to articular cartilage. These may form
1. a labrum or lip deepening a bony socket
2. menisci - incomplete discs or crescents increasing the size of articular surfaces
3. complete, or nearly complete articular discs of fibrocartilage. This will convert a joint into two in parallel, which can then move in independent directions. The temporomandibular joint of the jaw is a good example of this.

Evolution of synovial joints

Not surprisingly such an efficient mechanism is widespread and found in most vertebrates from lungfish onwards. In lungfish a synovial joint is found only in the jaw, with most other joints being simpler: symphyses with a cartilaginous region between them. We can imagine that this could be made more flexible if the cartilage had fluid filled holes in it, which might join up into a single cavity surrounded by a fibrocartilaginous ring. Large pressures exerted on the bones might then bring the cartilage-covered ends into contact, in conjunction with a developing system of lubrication. This is no more than a good story, but we can find most of the postulated intermediates in lower vertebrates, with the synovial joint coming into its own at about the time of the conquest of land.
Movements in synovial joints. These can be very extensive the shoulder joint being particularly free and able to move around three axes.
Various schemes of classification of synovial joints have been used and will be found in different textbooks.

  1. Complexity Many joints possess only two articular surfaces and are therefore simple. Usually one surface is convex or larger than the other and termed male. Compound joints have more than one pair of articulating surfaces (e.g. the elbow has two male surfaces on the humerus which articulate with female surfaces on radius and ulna) and are thus compound. Complex joints have an intracapsular disc or menisci.
  2. Degrees of freedom A joint which moves substantially in one plane (like an elbow) is uniaxial. One which moves in two planes is biaxial, one which moves in three is triaxial. A ball and socket is multiaxial, but is equivalent to a triaxial as it has three degrees of freedom i.e. all movements can be reduced to XYZ axes. Not a good classification as there are often small but vital movements in other planes (e.g. knee rotation at end of flexing) and cannot take account of sliding movement.
  3. Shape Probably the most widely used classification, but still tries to simplify joint surfaces hinge joints: permit flexion and extension (knee) pivot joints: allow rotation (superior radio-ulnar) plane joints: have flat surfaces and allow gliding in several directions (carpus and tarsus) condylar joints: usually regarded as two hinge joints with separate articulations (TMJ) saddle joints: have surfaces shaped like two saddles - allow movement in two planes at right angles and a little rotation (base of thumb) ball and socket: allows very free movement around any axis through ball (hip) ellipsoid: ball and sockets which are not round : rotation therefore impossible (radiocarpal joint)
  4. Functional approach. This is the best classification as regards understanding what is going on. All above classifications are approximations and have holes in them which fit uneasily. First classify joint movement. This is always made up of:-
    • gliding - of one surface over another- slide
    • angulation - flexion, extension etc. - roll
    • rotation about axis of bone - spin

    Movement always occurs at articular surfaces, which are never planes nor spheres nor cones but always spheroids - egg shapes, either male or female i.e. convex or concave. A point moving between A and B on a surface can take the shortest great circle route a chord, or can take a longer, prettier arc. Any movement can be described by a trigone (a bendy triangle) or three arcs. The imaginary point which traces these movements is the end of the axis of rotation. In the simplest case this is the end of the long axis of the bone: for something like a femur it obviously isn't. Lets try this on a real movement, extending the knee. If we hold the tibia still and move the femur extension has three bits.

    1. the femur rolls on the tibia
    2. the femur slides posteriorly
    3. the femur spins to lock the joint.

    The third of these is most important because it tells us something about how joints work. Take an egg and cut it in half. The resulting curved surface has a variable radius of curvature. If we try to fit this to another spheroid we see that it only fits well at one point. Elsewhere there are wedge-shaped gaps and smaller areas of contact. Joints exploit this: the position of best fit, or close packed position usually occurs at the end of the range of habitual movement. As a joint approaches this position ligaments are stretched and often some spin is imparted by them to screw the joint home. In this position the joint is virtually abolished: in practice it is only fully reached under strain and may damage articular surfaces and pull ligaments. So usually it is approached but not realised. This position is comfortable because it uses little muscular energy and can be maintained for long periods. The loose packed position is also important because it allows

    1. loosely fitting surfaces to spin, roll and slide
    2. a reduced area of contact, so little friction
    3. wedge shaped gaps, continually changing circulate synovial fluid like a peristaltic pump.

    Limitation of movement is also important. Usually achieved by

    1. tension in ligament, which have strain and pain receptors
    2. tension of muscles around a joint - passive resistance to stretch followed by reflex contraction when stimuli from mechanoreceptors becomes critical. These explain Hilton's law: that joints and the muscles acting on them share a nerve supply. Paralysis of muscles thus affects joints. In spastic paralysis muscle tone is increased and movement restricted. In other paralyses joints become lax, flail joints or actually disrupted. Charcot elbow in syphilis.
    3. Running out of articular surface.
    4. approximation of soft parts.

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