Classification of Diarthrodial Joint Types

• Nonaxial: Movement is limited to gliding rather than rotation around a specific axis.
• Uniaxial: The joint moves around a single axis (1 df).
• Biaxial: The joint moves around two axes (2 df).
• Triaxial: The joint moves around three axes (3 df), providing the highest range of motion.

Classification by Joint Architecture

Diarthrodial joints are categorized into six distinct types based on the shape of their articulating surfaces and their resulting movement capabilities.

1. Gliding (Plane/Arthrodial) Joints

These joints possess nearly flat articulating surfaces. Because the surfaces slide against one another rather than rotating around a central point, they are considered nonaxial.

• Examples: Intercarpal joints (wrist), intermetatarsal joints (foot), and zygapophyseal joints of the vertebrae.

2. Uniaxial Joints: Hinge and Pivot

Uniaxial joints are restricted to movement in a single plane.

• Hinge (Ginglymus) Joints: These consist of a convex surface fitting into a concave surface. Movement is often stabilized by strong collateral ligaments to prevent side-to-side motion.
  • Examples: Humeroulnar joint (elbow) and interphalangeal joints (fingers/toes).
• Pivot (Screw/Trochoid) Joints: These allow for rotation around a single longitudinal axis.
  • Examples: The atlantoaxial joint (neck rotation) and the proximal/distal radioulnar joints.

3. Biaxial Joints: Condyloid and Saddle

Biaxial joints allow for movement across two planes, typically permitting flexion/extension and abduction/adduction.

• Condyloid (Ovoid/Ellipsoidal) Joints: These feature an oval-shaped convex surface that fits into a complementary concave surface.
  • Examples: Radiocarpal joints (wrist) and the 2nd–5th metacarpal joints.
• Saddle (Sellar) Joints: These are shaped like a rider sitting on a horse's saddle. While they are also biaxial, they offer a significantly greater range of movement than condyloid joints.
  • Example: The carpometacarpal joint of the thumb.

4. Triaxial Joints: Ball & Socket

The most mobile joints in the human body are the Ball & Socket (Spheroidal) joints. They feature reciprocally convex and concave surfaces, allowing for movement across three axes (triaxial). This provides the highest possible range of motion, including rotation, flexion/extension, and abduction/adduction.

• Examples: The hip and shoulder joints.

Summary Table of Diarthrodial Joints

To aid in study, the following table summarizes the relationship between architecture and movement:

Clinical Note: Joint stability is a balance between geometry and soft tissue. For example, the hip joint is highly stable due to its deep socket (increasing contact area), whereas the shoulder joint is more mobile but less stable due to its shallow socket.