Adult human skeleton: 206 bones. Two divisions: Axial skeleton (80 bones): skull (29 bones), vertebral column (26 bones), ribs (24 bones + sternum). Forms the central axis. Appendicular skeleton (126 bones): limb bones (arms, legs) + girdles (pectoral, pelvic). Functions: support and shape, protection of organs, movement (with muscles), haemopoiesis (blood cell production in red marrow), mineral storage (calcium and phosphate). Bone composition: 65% inorganic (hydroxyapatite — calcium phosphate crystals), 35% organic (mainly collagen type I). Bone cells: osteoblasts (bone-forming), osteoclasts (bone-resorbing, from monocytes), osteocytes (mature osteoblasts in lacunae — mechanosensors).

Human skull consists of 29 bones (8 cranial + 14 facial + 7 others including 3 ear ossicles and hyoid). Occipital condyles: rounded projections on either side of foramen magnum (the hole at base of skull through which spinal cord passes). These condyles articulate with the atlas (C1 vertebra) forming the atlantooccipital joint — allows nodding (yes-movement). Dicondylic (2 condyles): Humans (Homo sapiens), all mammals, amphibians (frogs have 2 condyles). Monocondylic (1 condyle): birds and reptiles. One condyle in centre of occipital bone. The single condyle allows more rotational movement in reptiles and birds. Bicondylic provides more stable articulation with greater range of nodding. Key NEET fact: Humans = dicondylic. Birds/reptiles = monocondylic. This is a classic question because students confuse human skull type.

Human vertebral column: 33 vertebrae at birth, 26 in adults (9 fused). Regions: Cervical: 7 (C1-C7). ALL mammals have 7 cervical vertebrae. C1 = Atlas (ring-shaped, no body, supports skull). C2 = Axis (has odontoid process/dens — atlas rotates around it for no-movement). Thoracic: 12 (T1-T12). Ribs articulate here. Lumbar: 5 (L1-L5). Largest vertebrae (weight-bearing). Sacral: 5 fused = sacrum (forms posterior wall of pelvis). Coccygeal: 4 fused = coccyx (vestigial tail). Curves: cervical lordosis, thoracic kyphosis, lumbar lordosis, sacral kyphosis. Kyphosis at birth (single curve). Cervical lordosis develops when baby lifts head. Lumbar lordosis when child walks. Intervertebral discs: fibrocartilaginous cushions between vertebrae. Nucleus pulposus (soft inner) + annulus fibrosus (tough outer ring). Herniated disc: nucleus pulposus protrudes → compresses nerve roots.

12 pairs of ribs (24 total). Classifications: By attachment to sternum: True ribs (1-7): directly connected to sternum by costal cartilage. False ribs (8-10): connected via costal cartilage to rib 7 (not directly to sternum). Floating ribs (11-12): attached only to thoracic vertebrae posteriorly, free anteriorly. By number of heads (bicephalic/monocephalic): Bicephalic (two heads): ribs 1-10. Each has capitulum (head) that articulates with vertebral body AND tuberculum that articulates with transverse process. Monocephalic (one head): ribs 11-12 (floating ribs). Only capitulum, no tuberculum. Sternum: flat bone in anterior chest. Three parts: manubrium (top), body (middle), xiphoid process (bottom). Manubriosternal joint: cartilaginous joint (angle of Louis at T4-T5 level — landmark for counting ribs). First rib articulates with manubrium. Ribs 2-7 with sternal body.

Joints classified by structure and movement: Fibrous (synarthroses — no movement): sutures of skull (interdigitating fibrous joints — immovable in adults but have slight movement in infants). Syndesmosis (tibia-fibula — slight movement). Gomphosis (tooth in socket). Cartilaginous (amphiarthroses — limited movement): Synchondrosis: hyaline cartilage between bones. Growth plates (epiphyseal plates) — allows growth in children. Temporary — replaced by bone at maturity. First costochondral joint. Symphysis: fibrocartilage between bones. Pubic symphysis, intervertebral joints (fibrocartilaginous discs), manubriosternal joint. Stronger, allows more movement than synchondrosis. Synovial (diarthroses — freely movable): most joints. Ball-and-socket (hip, shoulder), hinge (knee, elbow), pivot (atlas-axis), condyloid (wrist), saddle (carpometacarpal of thumb), plane/gliding (intercarpal, intertarsal).

Shoulder joint (glenohumeral joint): ball-and-socket joint. Most mobile joint in body. Head of humerus (ball) fits into glenoid cavity of scapula (socket). Shallow socket → great mobility but less stability → most commonly dislocated joint. Stabilised by: rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis — SITS mnemonic), glenohumeral ligaments, joint capsule. Hip joint (coxofemoral joint): ball-and-socket joint. Head of femur fits into acetabulum of pelvis. Deep socket (acetabulum deepened by labrum) → much more stable than shoulder but less mobile. Weight-bearing. Stabilised by: iliofemoral, pubofemoral, ischiofemoral ligaments. Strongest ligament in body = iliofemoral ligament (Y-ligament of Bigelow). Hip fractures: common in elderly with osteoporosis. Neck of femur most vulnerable. Garden classification I-IV. Treatment: hemiarthroplasty or total hip replacement.

Knee joint: modified hinge joint (primarily hinge + slight rotation). Largest and most complex synovial joint. Between: femur, tibia, patella. Cruciate ligaments: Anterior cruciate ligament (ACL): prevents anterior translation of tibia. Most commonly torn in sports. Posterior cruciate ligament (PCL): prevents posterior translation. Medial and lateral collateral ligaments: prevent valgus and varus stress. Menisci: medial and lateral fibrocartilaginous pads that deepen articular surface, cushion, stabilise, and distribute load. Medial meniscus: less mobile → more frequently torn. Medial collateral ligament (MCL): attached to medial meniscus — if MCL torn, medial meniscus often also torn. Unhappy triad (O'Donoghue's triad): simultaneous ACL + medial meniscus + MCL injury from lateral tackle force. Patella: largest sesamoid bone. Embedded in quadriceps tendon. Protects knee anteriorly. Patellar tendon connects patella to tibial tuberosity.

Fracture: break in continuity of bone. Types: Simple/closed: skin intact. Compound/open: bone fragments break through skin. Comminuted: bone shattered into many fragments. Greenstick: incomplete fracture in children (bone bends, outer cortex breaks). Transverse, oblique, spiral. Stress fracture: repeated microtrauma (march fractures in military). Pathological fracture: through diseased (weak) bone — osteoporosis, tumour. Bone healing stages (approximately 6-8 weeks for simple fractures): Haematoma formation (hours-days): blood clot at fracture site. Soft callus (days-weeks): fibroblasts and chondroblasts form fibrocartilaginous callus. Hard callus (weeks-months): ossification of callus → woven bone. Remodelling (months-years): woven bone → lamellar bone + cortex restoration. Factors that impair healing: infection, diabetes, malnutrition, poor blood supply, smoking, old age, severe displacement, corticosteroids.