Joints: MDCAT Biology notes
Joints for MDCAT: fibrous, cartilaginous and synovial joints, hinge vs ball and socket vs pivot, sutures, pubic symphysis and synovial fluid explained.
What is a joint?
A joint or articulation is a place where two or more bones meet. Joints are classified by structure (the tissue that holds the bones together) and by function (how much movement they allow). In the FSc textbook these two schemes line up neatly into three groups.
| Structural type | Connecting tissue | Movement | Examples |
|---|---|---|---|
| Fibrous | Dense, tough connective tissue rich in collagen fibres; no cavity | Immovable | Sutures between skull bones |
| Cartilaginous | Cartilage (fibrocartilage pad); no cavity | Slightly movable | Pubic symphysis, intervertebral discs |
| Synovial | Fibrous capsule lined by synovial membrane; cavity with synovial fluid | Freely movable | Elbow, knee, shoulder, hip |
Fibrous joints
The bones are joined by fibrous connective tissue made mostly of collagen. There is no joint cavity, so almost no movement occurs. The classic example is the suture of the skull, where the flat skull bones interlock firmly. Sutures protect the brain by forming a rigid case.
Cartilaginous joints
The bones are united by cartilage and there is no joint cavity. The pubic symphysis (between the two pubic bones at the front of the pelvis) and the intervertebral discs (between vertebral bodies) are joined by pads of fibrocartilage. These joints allow a small amount of movement and absorb shock.
Synovial joints
Most joints of the limbs are synovial. Their features:
- The ends of the bones are covered by smooth articular (hyaline) cartilage.
- A tube-like capsule of dense fibrous tissue surrounds the joint and holds the bones together; ligaments strengthen it.
- The capsule is lined by the synovial membrane, which secretes synovial fluid into the synovial cavity. The fluid lubricates the joint and nourishes the cartilage.
Types of synovial joint
| Type | Movement | Examples |
|---|---|---|
| Hinge | In one plane only (bending and straightening, like a door) | Elbow (humerus with radius and ulna), knee, joints of fingers |
| Ball and socket | In all planes, including rotation | Shoulder (head of humerus in glenoid cavity of scapula), hip (head of femur in acetabulum of pelvic girdle) |
| Pivot | Rotation of one bone around another | Atlas turning on the axis (lets the head turn side to side) |
| Gliding (sliding) | Flat surfaces slide over each other | Between carpals and between tarsals |
Remember: the shoulder and hip are joints between a limb bone and a girdle (pectoral or pelvic), and girdle joints are ball and socket. The elbow and knee are hinges.
Common MDCAT traps
- "Synovial joint" is a broad structural class; if an option says "ball and socket", that is the more specific answer for the hip and shoulder.
- A hinge joint pairs the humerus with the radius and ulna, not the humerus with the pectoral girdle (that is ball and socket).
- The pubic symphysis contains fibrocartilage, not elastic, costal or articular cartilage.
- Rotation is the hallmark of ball and socket and pivot joints; hinge joints cannot rotate.
- The humerus articulates with the scapula, not the clavicle or sternum.
Quick revision
- Articulation = connection between bones.
- Fibrous = immovable (sutures); cartilaginous = slightly movable (pubic symphysis, intervertebral discs); synovial = freely movable.
- Synovial cavity is filled with synovial fluid for lubrication.
- Hip and shoulder are ball and socket; elbow and knee are hinge.
- Atlas on axis is a pivot joint.