Memory tricks for bones, joints, and the skeleton

Support: framework for body — bones provide the rigid scaffold that holds you upright and gives muscles something to pull against. Protection: skull (brain), vertebrae (spinal cord), ribs (heart and lungs), pelvis (reproductive organs). Movement: bones act as levers — muscles attach via tendons and pull on bones to create movement at joints. Storage: calcium reservoir (99% of body's calcium in bones), phosphate storage. Mineral release regulated by PTH and calcitonin. Blood cell production: hematopoiesis in red bone marrow — RBCs, WBCs, platelets. Adults: red marrow in axial skeleton, pelvis, proximal long bone epiphyses.

Diaphysis: the shaft — compact bone surrounding medullary cavity. Epiphysis: the ends — spongy bone covered by articular cartilage. Epiphyseal plate (growth plate): hyaline cartilage in growing bones — site of lengthening. Replaced by epiphyseal line when growth stops. Periosteum: tough fibrous membrane covering outer bone surface (not articular cartilage) — anchors tendons/ligaments (Sharpey's fibers), contains osteoprogenitor cells for repair, blood vessels and nerves. Endosteum: thin membrane lining medullary cavity and spongy bone trabeculae — contains osteoclasts and osteoblasts. Medullary cavity: hollow center — yellow marrow (fat) in adults, red marrow in children.

Osteoprogenitor cells: stem cells in periosteum and endosteum — divide to produce osteoblasts. Activated during growth and fracture repair. Osteoblasts: bone-BUILDING cells — secrete osteoid (unmineralized matrix, mainly collagen) → calcium phosphate deposits → mineralized bone. Become trapped → become osteocytes. Osteocytes: mature bone cells in lacunae (small cavities) — connected by canaliculi (tiny channels). Maintain bone matrix, sense mechanical stress, communicate with osteoblasts and osteoclasts. Most numerous bone cell. Osteoclasts: large multinucleated cells — bone DESTRUCTION. Secrete acid and enzymes → dissolve bone matrix → releases Ca2+ into blood. Stimulated by PTH.

Long bones: longer than wide — humerus, femur, radius. Lever for movement. Contain medullary cavity. Short bones: roughly cube-shaped — carpals (wrist), tarsals (ankle). Provide stability with limited motion. Mostly spongy bone. Flat bones: thin, flat, curved — skull, sternum, ribs, scapula. Protection and large surface for muscle attachment. Sandwich of compact bone around spongy bone (diploë in skull). Irregular bones: complex shapes — vertebrae, hip bones, facial bones. Sesamoid bones: embedded in tendons — patella (kneecap) is largest. Protect tendons from friction and alter mechanical advantage.

Axial skeleton (80 bones): forms the central axis — skull (22), vertebral column (26), thoracic cage (25 ribs + sternum = 37). Protects central nervous system and thoracic organs. Appendicular skeleton (126 bones): bones of the limbs and the girdles that attach them. Pectoral girdle (4 bones: 2 clavicles + 2 scapulae), upper limbs (60), pelvic girdle (2 hip bones = os coxae), lower limbs (60). The pelvic girdle includes two hip bones (each formed by fusion of ilium, ischium, pubis) — forms with sacrum and coccyx to make bony pelvis. Together 206 bones in the adult (300 in newborn — many fuse).

Projections (elevations — bone sticks out): process (any projection), spine (sharp, slender), tubercle (small, round), tuberosity (large, rough), trochanter (very large, femur only), crest (narrow ridge), epicondyle (above condyle). Depressions (bone goes in): fossa (shallow depression), groove/sulcus (narrow channel — for vessels/tendons), fovea (small pit). Openings (holes through bone): foramen (round hole for vessels/nerves), meatus (tube-like passageway), sinus (air-filled cavity in skull — reduces weight). Every marking has a function — muscle attachment, joint surface, or passage for nerves and vessels.

Fibrous joints: bones connected by dense fibrous CT — no joint cavity. Synarthroses (immovable). Sutures (skull), gomphoses (teeth in socket), syndesmoses (interosseous membrane). Cartilaginous joints: connected by cartilage — no joint cavity. Synchondroses (hyaline cartilage — epiphyseal plates, 1st rib to sternum), symphyses (fibrocartilage — pubic symphysis, intervertebral discs). Amphiarthroses (slightly movable). Synovial joints: most common — joint cavity filled with synovial fluid. Diarthroses (freely movable). All have: articular cartilage, joint capsule (fibrous outer + synovial membrane inner), synovial fluid, reinforcing ligaments. Six types: plane, hinge, pivot, condylar, saddle, ball-and-socket.

Bone is continuously remodeled — old bone is constantly removed and new bone deposited. ~10% of skeleton replaced each year in adults. Osteoclasts resorb bone → osteoblasts fill cavity with new bone (takes ~3-4 months). Wolff's Law: bone responds to mechanical stress by increasing density along lines of stress — explains why exercise strengthens bones and immobilization weakens them. Hormonal control: PTH stimulates osteoclasts (raises blood Ca2+), calcitonin stimulates osteoblasts (lowers blood Ca2+), estrogen promotes osteoblast activity (menopause → estrogen drops → osteoporosis). Growth hormone and thyroid hormone also influence remodeling. Fracture repair: hematoma → soft callus → hard callus → bone remodeling.

Flexion: decreases joint angle (bending elbow). Extension: increases joint angle (straightening elbow). Hyperextension: beyond anatomical position. Abduction: moves limb AWAY from midline. Adduction: moves limb TOWARD midline. Rotation: bone rotates around its long axis. Medial/lateral (internal/external). Circumduction: combination movement — limb traces a cone. Dorsiflexion: foot pulled toward shin. Plantar flexion: foot pointed down (standing on tiptoe). Inversion: sole faces medially. Eversion: sole faces laterally. Supination: forearm rotates — palm faces up. Pronation: palm faces down. Opposition: thumb to fingers.