Joints and bone health
Joints connect bones and allow movement. Keeping bones healthy matters throughout life.
Classifying joints by structure
Fibrous joints are held by dense connective tissue and barely move, like skull sutures. Cartilaginous joints are joined by cartilage and move slightly, like the joints between vertebrae and the pubic symphysis. Synovial joints have a fluid-filled cavity and move freely; most limb joints are synovial.
Synovial joint structure
A synovial joint has articular cartilage covering bone ends, a joint capsule enclosing the cavity, a synovial membrane secreting lubricating synovial fluid, and ligaments for stability. Some, like the knee, have extra pads of cartilage (menisci) and fluid sacs (bursae) that reduce friction.
Types of synovial joints
Hinge joints (elbow, knee) move in one plane like a door. Ball-and-socket joints (shoulder, hip) move in all directions. Pivot joints (between the first two neck vertebrae) allow rotation, as when shaking your head 'no'. Others include gliding joints in the wrist, condyloid joints at the knuckles, and the saddle joint at the base of the thumb.
Movements
Flexion decreases the angle at a joint (bending the elbow); extension increases it. Abduction moves a limb away from the midline; adduction moves it toward. Rotation turns a bone around its axis. Pronation turns the palm down; supination turns it up, as if holding a bowl of soup.
Fractures and healing
Fractures are breaks. In a simple (closed) fracture the skin is intact; in a compound (open) fracture bone breaks the skin, raising infection risk. Healing goes through stages: a blood clot (haematoma), a soft callus of cartilage, a hard bony callus, then remodelling over months.
Osteoporosis
Osteoporosis is loss of bone density, making bones fragile and prone to fracture, especially in the hip, spine, and wrist. It is most common in older women after menopause, as falling oestrogen speeds bone loss. Weight-bearing exercise, calcium, vitamin D, and some medications help prevent it.
Arthritis
Osteoarthritis, the most common form, is wear-and-tear breakdown of articular cartilage, causing pain and stiffness, often in knees and hips. Rheumatoid arthritis is an autoimmune disease in which the immune system attacks synovial membranes, causing inflammation and joint damage, often symmetrically in hands and feet.
Worked example
Raising your arm sideways to shoulder height is abduction at the shoulder, a ball-and-socket joint. Bending the elbow to bring a cup to your mouth is flexion at a hinge joint. Turning your forearm so your palm faces up to receive change is supination.
Key takeaways
Joints are fibrous, cartilaginous, or synovial; synovial joints move freely with lubricating fluid. Hinge, ball-and-socket, and pivot are common synovial types. Movements include flexion, extension, abduction, adduction, rotation, pronation, and supination. Fractures heal through clot, callus, and remodelling. Osteoporosis thins bone; osteoarthritis wears cartilage; rheumatoid arthritis is autoimmune.
Flashcard
Barely movable
Check your understanding
Bringing your arm back to your side after raising it sideways is: