The knee-joint was formerly described as a ginglymus or hinge-joint, but is really of a much more complicated character. It must be regarded as consisting of three articulations in one: two condyloid joints, one between each condyle of the femur and the corresponding meniscus and condyle of the tibia; and a third between the patella and the femur, partly arthrodial, but not completely so, since the articular surfaces are not mutually adapted to each other, so that the movement is not a simple gliding one. This view of the construction of the knee-joint receives confirmation from the study of the articulation in some of the lower mammals, where, corresponding to these three subdivisions, three synovial cavities are sometimes found, either entirely distinct or only connected together by small communications. This view is further rendered probable by the existence in the middle of the joint of the two cruciate ligaments, which must be regarded as the collateral ligaments of the medial and lateral joints. The existence of the patellar fold of synovial membrane would further indicate a tendency to separation of the synovial cavity into two minor sacs, one corresponding to the lateral and the other to the medial joint. (http://education.yahoo.com/reference/gray/subjects/subject?id=93)
The simplest type of movable joint is the hinge, or ginglymus, such as the articulation between the humerus and ulna at the elbow. In such a joint, the only movement permitted is rotation about a line, represented in mechanical hinges by the center line of the pin which connects the 2 parts of the hinge. When movement takes place in this joint, every point in the moving part must describe an arc of a circle, the center of which lies on this axial line. If the joint is so constructed that some point on the moving part lies on this axis, this point will not be displaced during movement..............
