Joints

In computer graphics, unless you make a special effort to define a joint differently, all joints are either pivot joints or modifications of pivot joints made by limiting the number of axes the joint will rotate in. As far as your software is concerned, this is not much of a distinction. Real skeletal anatomy has much more variety than this (Fig. 14.1). There are three structural types of joints: fibrous, cartilaginous, and synovial, and several varieties within each of these categories.

The various sections of your skull are bound together along sutures that are fibrous joints. They do not allow movement in adults, but at birth are flexible enough to allow some compression for easier passage through the birth canal. Syndesmosis fibrous joints exist between the long bones of the lower arm and lower leg. They allow greater movement than the sutures of the skull, but less than synovial joints. Gomphosis joints exist between the root of each tooth and the part of the mandible or maxilla it is attached to. These joints allow very little movement.