Joint Types and Characteristics
Joints are where two or more bones meet. Some joints are built for stability (like the sutures fusing your skull together), while others prioritize mobility (like the ball-and-socket joint in your hip). The trade-off between stability and mobility is a recurring theme: the more movable a joint is, the less inherently stable it tends to be.
Joints are classified into three structural categories based on the type of tissue connecting the bones.
Fibrous Joints
Fibrous joints connect bones with dense fibrous connective tissue. They allow little to no movement.
- Sutures are interlocking, immovable joints found between skull bones. The irregular, zigzag edges lock the bones tightly together.
- Syndesmoses are slightly movable joints where bones are connected by ligaments or an interosseous membrane. The distal tibiofibular joint (near your ankle) is a good example.
- Gomphoses are immovable joints found only in tooth sockets, where a tooth is anchored to the jawbone by periodontal ligaments.
Cartilaginous Joints
Cartilaginous joints connect bones with cartilage. They range from immovable to slightly movable.
- Synchondroses are connected by hyaline cartilage and are typically immovable. The epiphyseal (growth) plates in children's long bones are synchondroses that allow bone growth; they ossify (turn to bone) once growth is complete.
- Symphyses are connected by fibrocartilage and are slightly movable. The pubic symphysis and the intervertebral discs between vertebrae are both symphyses. Individually each disc allows only a small amount of movement, but collectively the spine achieves significant flexibility.
Synovial Joints
Synovial joints are the most movable joint type and the most common in the body. They share several defining features:
- A joint capsule encloses the joint space
- The synovial membrane lines the inside of the capsule and secretes synovial fluid, which lubricates the joint and nourishes the articular cartilage
- Articular (hyaline) cartilage covers the ends of the bones, reducing friction and absorbing shock
- Bursae are fluid-filled sacs found around some synovial joints that reduce friction between tendons, ligaments, and bones
Synovial Joint Shapes and Movement
Synovial joints are further classified by the shape of their articular surfaces, which determines what movements they allow.
| Joint Type | Planes of Movement | Movements Allowed | Examples |
|---|---|---|---|
| Hinge | One | Flexion, extension | Elbow, knee, interphalangeal joints |
| Pivot | One (rotational) | Rotation around a central axis | Atlantoaxial joint (head rotation), proximal radioulnar joint |
| Saddle | Two | Flexion, extension, abduction, adduction | Carpometacarpal joint of the thumb |
| Condyloid (ellipsoidal) | Two | Flexion, extension, abduction, adduction | Metacarpophalangeal joints (knuckles), radiocarpal (wrist) joint |
| Plane (gliding) | Limited sliding | Gliding/sliding in various directions | Intercarpal joints, intertarsal joints |
| Ball-and-socket | Three (multiaxial) | Flexion, extension, abduction, adduction, rotation, circumduction | Shoulder, hip |
Ball-and-socket joints allow the greatest range of motion of any joint type. The shoulder is the most mobile joint in the body, but that mobility comes at a cost: it's also one of the most commonly dislocated.
Skeletal Movements at Joints
These are the specific terms for movements that occur at synovial joints. You'll need to know each one precisely.
Angular Movements
Angular movements change the angle between two bones.
- Flexion decreases the joint angle (bending your elbow brings your forearm closer to your upper arm)
- Extension increases the joint angle (straightening your elbow back out)
- Abduction moves a body part away from the midline (raising your arm out to the side)
- Adduction moves a body part toward the midline (lowering that arm back down)
- Circumduction is a combination of flexion, extension, abduction, and adduction performed in sequence, tracing a cone-shaped path. Swinging your arm in a full circle at the shoulder is circumduction.
Rotational Movements
- Rotation turns a bone around its own long axis
- Internal (medial) rotation rotates the anterior surface toward the midline
- External (lateral) rotation rotates the anterior surface away from the midline
Special Movements
These terms apply to specific body regions.
- Supination rotates the forearm so the palm faces anteriorly (or upward if the arm is flexed). Think of holding a bowl of soup: "supination = soup."
- Pronation rotates the forearm so the palm faces posteriorly (or downward).
- Dorsiflexion bends the foot at the ankle, pulling the toes toward the shin (like lifting your foot off a gas pedal).
- Plantar flexion points the toes away from the shin (like pressing a gas pedal).
- Inversion turns the sole of the foot inward (medially).
- Eversion turns the sole of the foot outward (laterally).
Joint Structure and Function
Several structures work together to keep joints stable and functional.
- Ligaments connect bone to bone. They provide joint stability and limit excessive movement. Sprains occur when ligaments are stretched or torn.
- Tendons connect muscle to bone. They transmit the force generated by muscle contraction to move bones. Strains occur when tendons or muscles are stretched or torn.
A quick way to keep these straight: Ligaments link bone to bone. Tendons tie muscle to bone.
Range of motion (ROM) refers to the full extent of movement possible at a joint. It depends on the shape of the articular surfaces, the flexibility of surrounding ligaments and tendons, and the bulk of surrounding soft tissue.