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
- fibrous and cartilaginous joints where two bones are separated
by a deformable intermediate
- 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
Various schemes of classification of synovial joints have been
used and will be found in different textbooks.
- 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.
- 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.
- 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)
- 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
- the femur rolls on the tibia
- the femur slides posteriorly
- 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
- loosely fitting surfaces to spin, roll and slide
- a reduced area of contact, so little friction
- wedge shaped gaps, continually changing circulate synovial
fluid like a peristaltic pump.
Limitation of movement is also important. Usually achieved by
- tension in ligament, which have strain and pain receptors
- 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
- Running out of articular surface.
- approximation of soft parts.
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