7.1 Bone Physiology, Fracture Healing & Bone Pathology

Bone Cells and the Remodeling Unit

Bone continuously turns over. Three cell types dominate board-level physiology.

Osteoblasts derive from mesenchymal stem cells. They synthesize type I collagen–rich osteoid and promote mineralization. Markers include alkaline phosphatase (ALP) and osteocalcin. Osteoblasts express RANKL (receptor activator of NF-κB ligand) and OPG (osteoprotegerin). When osteoblasts finish their job, some become lining cells, some undergo apoptosis, and some embed in matrix as osteocytes.

Osteoclasts are multinucleated cells of monocyte–macrophage lineage. They attach to bone via a sealing zone, acidify the resorption lacuna with a proton pump (H⁺-ATPase), and digest organic matrix with cathepsin K and other enzymes. Markers include TRAP (tartrate-resistant acid phosphatase). Osteoclast differentiation requires M-CSF and RANKL binding to RANK on osteoclast precursors. OPG is a decoy receptor that binds RANKL and blocks RANK activation—raising the OPG/RANKL ratio decreases resorption.

Osteocytes are long-lived former osteoblasts entombed in lacunae and connected by canaliculi. They are the primary mechanosensors. Under load, they reduce sclerostin and modulate RANKL/OPG signaling to favor formation; disuse and glucocorticoid excess increase sclerostin and favor net resorption. Sclerostin (from SOST) inhibits Wnt/β-catenin osteoblast anabolism—hence sclerostin antibodies can increase bone formation in therapeutic contexts, but the physiologic point for exams is osteocyte control of remodeling.

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RANKL/OPG Axis and Clinical Hooks

Anything that increases RANKL relative to OPG increases osteoclastogenesis: continuous PTH excess, estrogen deficiency (postmenopausal osteoporosis), inflammatory cytokines (IL-1, IL-6, TNF-α in RA and chronic inflammation), and some malignancy-related factors. Estrogen supports OPG and suppresses RANKL-driven resorption—loss of estrogen accelerates trabecular and cortical bone loss. Denosumab (mentioned conceptually) is a RANKL-blocking monoclonal antibody used clinically to reduce resorption; on exams, map it back to the RANKL–RANK–OPG triangle rather than memorizing brand pharmacology lists.

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PTH, Vitamin D, and Calcitonin on Bone and Mineral

Parathyroid hormone (PTH) raises serum Ca²⁺ by: (1) stimulating renal 1α-hydroxylase → more active 1,25-dihydroxyvitamin D; (2) increasing renal Ca²⁺ reabsorption and phosphate excretion; (3) increasing osteoclast-mediated bone resorption indirectly via osteoblast/osteocyte RANKL. Continuous high PTH (primary hyperparathyroidism) favors net resorption and high-turnover bone disease; intermittent PTH (or teriparatide conceptually) can be anabolic because of preferential stimulation of formation pathways—boards love the continuous vs intermittent contrast.

Vitamin D (active 1,25-(OH)₂D₃ / calcitriol) increases intestinal absorption of Ca²⁺ and phosphate, supports mineralization of osteoid, and participates in feedback with PTH. Deficiency → inadequate mineral supply → rickets (children) or osteomalacia (adults).

Calcitonin (from thyroid C cells) opposes osteoclast activity and lowers serum Ca²⁺; it is less dominant in daily adult Ca²⁺ homeostasis than PTH/vitamin D but appears in pharmacologic and medullary thyroid carcinoma (calcitonin marker) contexts.

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Woven vs Lamellar Bone

Woven bone has irregular collagen orientation, high cellularity, and rapid formation. It appears in the fetus, fracture callus, Paget disease, and aggressive bone-forming tumors. Lamellar bone has parallel collagen layers aligned to stress; it is mechanically efficient and constitutes mature cortical and trabecular bone after remodeling. Pathologists and radiologists use “woven vs lamellar” as a maturity/stress-response clue: immature or pathologic rapid formation → woven; physiologic remodeling → lamellar.

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Fracture Healing Stages

Healing of a typical fracture follows overlapping biologic phases:

1. Hematoma and inflammation (hours–days): vessel disruption, clot, cytokine influx, recruitment of mesenchymal progenitors and inflammatory cells. Soft tissue injury and periosteal blood supply critically influence outcome.
2. Soft (cartilaginous) callus (days–weeks): granulation tissue and fibrocartilage bridge the gap; mechanical stability remains limited.
3. Hard (bony) callus (weeks): endochondral ossification converts cartilage to woven bone; periosteal intramembranous bone may form as well.
4. Remodeling (months–years): woven bone remodeled to lamellar bone along stress lines (Wolff’s law); medullary cavity reconstitutes.

Inadequate reduction/stability, infection, poor blood supply (e.g., scaphoid, femoral neck, talus), smoking, and malnutrition predispose to delayed union or nonunion. Rigid fixation and biology together determine whether primary (direct) or secondary (callus) healing predominates—most exam vignettes emphasize the staged secondary pathway above.

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Osteoporosis vs Osteomalacia vs Osteopetrosis

These three are classic “quantity vs quality vs remodeling failure” contrasts.

Osteoporosis: decreased bone mass and microarchitectural deterioration with normal mineralization of remaining matrix. Bone is porous and fracture-prone (vertebrae, hip, distal radius). Primary forms include postmenopausal (high turnover, trabecular loss) and senile/age-related (cortical and trabecular loss). Secondary causes include glucocorticoids, hyperthyroidism, hyperparathyroidism, malabsorption, hypogonadism, immobilization, and alcohol. Labs are often normal (Ca, PO₄, PTH, ALP typically not diagnostic); diagnosis is clinical/DXA conceptual. Fractures from low-energy trauma are the clinical event.

Osteomalacia / rickets: defective mineralization of osteoid. Children develop rickets (growth plate abnormalities, rachitic rosary, bowing, delayed fontanelle closure); adults develop osteomalacia (bone pain, proximal weakness, Looser zones/pseudofractures). Causes include vitamin D deficiency, phosphate wasting, and mineralization inhibitors. Labs often show low/low-normal Ca, low PO₄, elevated PTH (secondary hyperparathyroidism), and elevated ALP.

Osteopetrosis (“marble bone disease”): osteoclast dysfunction (e.g., carbonic anhydrase II deficiency in one form; other genetic defects in acidification machinery) → failed resorption → dense, brittle bone that obliterates marrow space. Clinical themes include fractures despite density, cranial nerve entrapment, and pancytopenia from marrow crowding; extramedullary hematopoiesis may occur. Contrast: too little bone (osteoporosis), soft poorly mineralized bone (osteomalacia), versus too dense but poorly remodeled bone (osteopetrosis).

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Osteomyelitis: Organisms and Patterns

Acute hematogenous osteomyelitis is most common in children (metaphysis of long bones). Staphylococcus aureus is the leading organism overall across ages. In sickle cell disease, infarcts and functional asplenia predispose to osteomyelitis; Salmonella is classically emphasized (though S. aureus remains common—boards still test the Salmonella association). Neonates may involve group B Streptococcus and others; puncture wounds through footwear raise concern for Pseudomonas; vertebral osteomyelitis in adults may be hematogenous with S. aureus or, in risk groups, TB (Pott disease). Chronic osteomyelitis features sequestrum (necrotic bone) and involucrum (reactive new bone). Contiguous spread from ulcers (diabetic foot) is a major adult pathway.

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Primary Bone Tumors: Age, Location, Mechanism

Primary bone tumors are less common than metastases in adults, but age–site–radiograph patterns are heavily tested.

Osteosarcoma: most common primary malignant bone tumor of adolescents/young adults; metaphysis of long bones (distal femur, proximal tibia, proximal humerus). Malignant osteoid production by mesenchymal cells; RB and TP53 pathway associations (retinoblastoma survivors, Li–Fraumeni). Codman triangle and sunburst periosteal reaction are classic radiographic teaching signs. Metastasizes hematogenously, especially to lung.

Ewing sarcoma: children/adolescents; often diaphysis of long bones or flat bones (pelvis). t(11;22) → EWS–FLI1 fusion. Small round blue cells; onion-skin periosteal reaction. Can mimic osteomyelitis clinically (fever, elevated ESR).

Giant cell tumor: typically 20–40 years; epiphysis of long bones after growth plate closure (distal femur, proximal tibia). Multinucleated giant cells (osteoclast-like) in a mononuclear stromal background; locally aggressive; soap-bubble appearance on imaging teaching files.

Chondrosarcoma: older adults; pelvis, proximal femur, shoulder girdle, or other flat/axial bones; malignant cartilage-producing tumor; may arise de novo or in enchondroma/osteochondroma settings. Less chemo-sensitive than osteosarcoma/Ewing—surgery-centered conceptual management.

Benign anchors (brief): osteoid osteoma (nidus, night pain relieved by NSAIDs—PGE2), osteochondroma (cartilage-capped bony projection continuous with marrow), enchondroma (medullary cartilage, hands/feet), fibrous dysplasia (GNAS, woven bone in fibrous stroma, “Chinese characters”).

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Metastatic Bone Disease Patterns

In adults, metastases >> primary bone sarcoma. Common primaries: breast, prostate, lung, kidney, thyroid (mnemonic teaching lists vary; breast and prostate dominate many series). Osteoblastic metastases classic for prostate (and some breast); osteolytic classic for kidney, thyroid, lung, and many breast lesions—mixed patterns occur. Axial skeleton and proximal limbs are frequent sites. Cord compression, hypercalcemia of malignancy, and pathologic fracture are major complications. Mechanism themes: tumor factors (PTHrP → hypercalcemia and osteolysis; local cytokines recruiting osteoclasts) versus osteoblast-stimulating factors in blastic disease.

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Integrative Clinical Logic

When a vignette mentions bone pain, start by placing the patient on an age–site matrix (child metaphysis infection vs sarcoma; adult axial pain → metastasis until proven otherwise), then layer mineral labs (osteomalacia vs hyperparathyroid high-turnover disease vs normal-lab osteoporosis with fracture), then organism risk (sickle → Salmonella emphasis; overall S. aureus), and finally primary tumor molecular/radiographic signatures. Remodeling cell biology (RANKL/OPG, PTH tempo, vitamin D mineralization) explains both metabolic bone disease and why inflammatory arthritis and estrogen loss accelerate fracture risk.