1. The dynamics of synovial joints depends upon the geometry of the articular surfaces, which are always curved. 2. There are two types of articular curvature: ovoid (synclastic) and sellar (anticlastic). 3. The sellar type is mechanically more suited to movements in which sliding is combined with medial or lateral rotation (monodal conjunct rotation). 4. The movement of any hinge-joint is accompanied by a monodal conjunct rotation. This carries the moving part along a path that approximates (at least) to a path of minimal wear. The same is true of the paths of habitual effort-movement (ergodes) of other kinds of joint. 5. Evidence for the foregoing statements is drawn from both normal and abnormal joints, and a clinical application is suggested.
1. All articulating cartilages are fibrocartilages. 2. The articular cartilages of the synovial joints are largely composed of collagen fibres. 3. These fibres form a dense network, the fibres of which run obliquely between the articular surface and the bone. 4. This network is operative when the parts are at rest and in contact under pressure. It takes the tensile component of the resultant shear stress, and is a postural mechanism of the joint. 5. The articular cartilage is most heavily chondrified at its centre, between the juxta-synovial and juxta-osseous parts. 6. The technique for demonstrating the fibrous structure is described.
1. Synovial fluid acts mechanically by forming a convergent lubricant film between the fixed and the moving joint surfaces. This term and the underlying theory are explained. 2. The fatty pads assist lubrication by reducing the "mechanical curvature" in joints with more highly curved surfaces. 3. The intra-articular discs and menisci increase the "mechanical curvature" in joints with surfaces of small curvature. 4. Sesamoid bones exert a "bow-string pressure" upon the bones with which they articulate. This is a corollary from the theory of lubrication.
1. Muscles acting upon any joint can be divided into two principal groups: muscles of displacement or spurt muscles, and muscles of stabilisation or shunt muscles. 2. Muscles which arise far from the joint are spurt muscles; those which arise near it are shunt muscles. 3. The fibrous tendon sheaths are joint-stabiising mechanisms. 4. The lumbrical and interosseous muscles are muscles of stabilisation of the digits. 5. The arrangement of the musculature is such as to ensure a constant pressure across the joint cavity during rest or uniform movement. The necessary centripetal force during movement is supplied mainly by the shunt muscles. 6. Experiments are described to illustrate these observations.
1. Two successive movements at a joint, if not in one and the same plane, constitute a diadochal movement. 2. Diadochal movements impose conjunct rotation upon the bone which has been moved. This may be countered by a rotation of opposite sense. 3. All muscles of a given joint are, therefore, rotators in some degree. 4. Upon the basis of these principles diagnostic and therapeutic suggestions are made.
