[…] has been reduced to only two basic types: ovoid and sellar surfaces (Fig. 3). The ovoid surface is uniformly convex or concave with the degree of curvature varying at each point along the surface; the sellar surface is convex in one plane and concave in a plane perpendicular to it. Modification of these basic types occur. The knee comprises two ovoid joints within one capsule and is termed a bicondylar joint. The greater multangular-metacarpal joint at the base of the thumb is a sellar joint. The ankle is also a sellar joint in which abduction and adduction is prevented by the limitations of a mortice arrangement. Movement about an ovoid surface requires more ligaments and muscles to maintain stability. The surface of a sellar joint is more congruous and makes the direction of motion more specific and more limited. A sellar joint is more efficient and more easily stabilized. […] Fig. 3. The two types of joints^([sic]) surface, O—ovoid; S—sellar. […] Conjunct rotation in an ovoid joint depends on muscular action but in a sellar joint it is dependent upon the shape of the joint and the congruity of the opposing surfaces. That is why sellar joints are more efficient. They produce conjunct rotation without muscular effort, and prevent adjunct rotation during motion.
Source: wiktionary