Joints may be classified from different ways.
- From an anatomical point of view, with regard to the substances and the arrangement of the substances by which the constituent parts are united.
- From a physiological standpoint, with regard to the greater or smaller mobility at the seat of union,
- From a physical standpoint, either the shapes of the portions in contact being mainly considered or the axes round which movement can occur.
- From a combination of the preceding methods may be adopted, and this is the plan most generally followed. None of the classi- fications hitherto used is quite satisfactory, but perhaps, on the whole, that suggested by Prof. Alex. Macalister is the least open to objection, and therefore with slight modification it is utilised here.
There are three chief groups of joints:
- Synarthroses. In joints of this class the bones are united by fibrous tissue.
- Synchondroses. Or joints in which the uniting substance intervening be- tween the bones is cartilage.
- Diarthroses. The constituent parts of joints of this class are (a) two or more bones each covered by articular hyaline cartilage ; (b) a fibrous capsule uniting the bones, and (c) a synovial membrane which lines the fibrous capsule and covers any part of bone enclosed in the capsule and not covered with articular cartilage. An interarticular plate of cartilage may or may not be present.
- Sutures or immovable joints, in which the' fibrous tissue between the bones is too small in amount to allow movement.
- Harmonic. The edges of the bones are comparatively smooth and are in even apposition, e. g., vertical plate of palate and maxilla.
- Squamous. The margin of one bone overlaps the other, e. g., temporal and parietal.
- Serrate. The opposed edges interlock by processes tapering to a point.
- Dentate. The opposed edges are dovetailed, e. g., occipital and parietal.
- Limbous. The opposed edges alternately overlap, e. g., parietal and frontal.
- Schindylesis. A ridge or flattened process is received into a corresponding socket, e. g., rostrum of sphenoid and vomer.
- Gotnphosis. A peg-like process is lodged in a corresponding socket, e. g., the fangs of the teeth.
- Syndesmoses. Movable joints in which the fibrous tissue between bones or carti- lages is sufficiently lax to allow movement between the connected parts, e. g., thyreo-hyoid membrane. Interosseous membranes of forearm and leg.
In all synchondroses a certain amount of movement is possible, and they are often called amphiarthroses.
- True synchondroses. The cartilage connecting the bones is the remains of the bar in which the bones were ossified, e. g., occipito-sphenoidal joint.
- False synchondroses. The plate of cartilage intervening between and connecting the bones is fibro-cartilage and is not part of the cartilage in which the bones were ossified, but is developed separately, e. g., intervertebral joint and pubic sym- physis. The articular end of each bone may be covered with hyaUne cartilage and there may be a more or less well-marked cavity in the intervening plate of fibro-cartilage.
In diarthrodial joints the surfaces in contact may be equal and similar or unequal and dissimilar. In the former case the joints are homomorphic; in the latter, heteromorphic.
- Plane or arlhrodial. Flat surfaces, admitting gliding movement, e. g., intercarpal and acromio-clavicular joints.
- Ephippial. Saddle-shaped surfaces placed at right angles to each other, admitting free movement in all directions, e. g., metacarpo-phalangeal joint of thumb.
- Enarlhrodial. Ball-and-socket, allowing the most free movement, e. g., hip and shoulder-joints.
- Condylarlhroses. The convex surface is ellipsoidal, and fits into a corresponding concavity, e. g., wrist and metacarpo-phalangeal joints,
- Ginglymi. One surface consists of two conjoined condyles or of a segment of a cone or cylinder, and the opposite surface has a reciprocal contour. In these joints movement is only permitted round one axis, which may be transverse; e. g., elbow, ankle; or it may be vertical, in which case the joint is trochoid; e. g., odontoid process of axis with atlas, radius with ulna.
Such a classification should be considered as being purely academic and the student must always remember that it is not enough to discuss a joint by assigning it to a particular class in any scheme; for he must be familiar with the actual conditions present in every joint. No classification, however perfect, must be taken as final, and each joint should be studied as a separate thing altogether apart from any general systematic arrangement.