There are six classes of synovial joints: plane, hinge, pivot, condyloid, saddle, and ball-and-socket. They differ mainly in the shape of their articulating surfaces, the number of movement axes permitted, and the balance between joint stability and range of motion.
Synovial joints contain a joint cavity filled with synovial fluid, articular cartilage, a fibrous capsule, and reinforcing ligaments. The shape and fit of the articulating surfaces determine which movements are possible, while joint depth, ligaments, capsule tension, and surrounding muscles influence stability.
| Class | Movement and sporting example | Typical stability and range of motion |
|---|---|---|
| Plane (gliding) | Small sliding movements; intercarpal joints during ball handling | Usually limited range of motion; stability varies with ligament support |
| Hinge | Uniaxial flexion and extension; elbow during a biceps curl | Relatively stable because movement is restricted to one main axis |
| Pivot | Uniaxial rotation; atlanto-axial joint when turning the head | Relatively stable, with rotation around one longitudinal axis |
| Condyloid (ellipsoid) | Biaxial flexion-extension and abduction-adduction; wrist during a basketball shot | Moderate mobility and stability; circumduction is possible, but axial rotation is limited |
| Saddle | Biaxial movement; thumb during gripping | Greater mobility than a hinge joint, with moderate stability from reciprocally shaped surfaces |
| Ball-and-socket | Multiaxial movement and rotation; shoulder during swimming or hip during kicking | Greatest range of motion; stability depends strongly on joint structure and soft tissues |
A general mobility-stability trade-off exists: permitting movement in more axes usually reduces inherent stability. However, joint class alone does not determine stability. For example, the shoulder and hip are both ball-and-socket joints, but the hip is more stable because its femoral head fits deeply into the acetabulum; the shoulder’s shallow glenoid cavity permits greater movement but increases dislocation risk.
In an IB SEHS response for subtopic B.1.2, classify the joint, state its permitted movements and axes, and apply a sporting example. Avoid the misconception that every ball-and-socket joint is equally unstable; explain how joint shape, ligaments, and muscles modify stability.