Joints (Articulations)

Two classification schemes. Joints are described by what holds them together (structural) and by how much they move (functional), and the two overlap:

• Fibrous joints are connected by dense fibrous connective tissue and are generally immovable (synarthroses) — for example, the sutures of the skull.
• Cartilaginous joints are connected by cartilage and are usually slightly movable (amphiarthroses) — for example, the joints between vertebrae (with their discs) and the pubic symphysis.
• Synovial joints have a fluid-filled cavity and are freely movable (diarthroses) — nearly all the joints of the limbs. These are the joints most people picture and the ones most involved in injury.

Anatomy of a synovial joint. Synovial joints share a set of features designed for smooth, protected movement:

• Articular cartilage (hyaline) caps the bone ends, reducing friction and absorbing shock.
• A joint (articular) capsule encloses the joint; its inner synovial membrane secretes synovial fluid into the joint cavity to lubricate and nourish the cartilage.
• Ligaments (bone to bone) reinforce and stabilize the joint.
• Some joints have extra structures: menisci (cartilage pads, as in the knee) for cushioning and fit, and bursae (fluid-filled sacs) that reduce friction between moving parts.

Types of synovial joints allow different motions, including the ball-and-socket (shoulder, hip — movement in all directions), hinge (elbow, knee — bending in one plane), pivot (between the first two neck vertebrae — rotation), gliding/plane (between wrist bones — sliding), saddle (thumb), and condylar (knuckles) joints.

Common body movements. Synovial joints produce named movements you'll use constantly in assessment:

• Flexion / extension: decreasing / increasing the angle of a joint (bending / straightening the elbow).
• Abduction / adduction: moving away from / toward the body's midline (raising the arm sideways / lowering it).
• Rotation: turning around an axis (shaking the head "no").
• Circumduction: moving a limb in a cone shape (windmilling the arm).
• Specialized moves include pronation/supination (turning the palm down/up), dorsiflexion/plantarflexion (lifting the foot up / pointing the toes), and inversion/eversion (turning the sole inward/outward).

How it works

Classifying a joint:

1. What connects the bones? fibrous, cartilaginous, or synovial (structural).
2. How much does it move? immovable, slightly movable, freely movable (functional).
3. Freely movable? → it's synovial: expect articular cartilage, a capsule with synovial fluid, and ligaments.
4. Name the movement by how the joint angle or position changes (flexion, abduction, rotation, etc.).

Comparisons

Common confusions

• Structural vs functional classification. One describes the material connecting bones; the other describes movement. A joint has both labels.
• Ligament vs tendon. Ligament = bone to bone (stabilizes joints); tendon = muscle to bone.
• Flexion vs extension direction. Flexion decreases the angle (bend); extension increases it (straighten).
• Cartilage in a joint vs cartilaginous joint. Synovial joints contain articular cartilage but are not "cartilaginous joints" (which are held together by cartilage and barely move).

Memory aids

• "Synovial = Slippery, freely moving."
• Abduction = "Abduct = take away (from midline)."
• Flexion = "flex the bicep = bend."
• Ball-and-socket = "all-around" motion.

Quick review

• Joints are classified structurally (fibrous, cartilaginous, synovial) and functionally (immovable, slightly movable, freely movable).
• Synovial joints (freely movable) have articular cartilage, a capsule with synovial fluid, and ligaments; types include ball-and-socket, hinge, pivot, and more.
• Common movements: flexion/extension, abduction/adduction, rotation, circumduction, pronation/supination.
• Synovial joints are the main sites of sprains, dislocations, and arthritis.