Abstract
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.
