Bone Pathology — Metabolic Bone Disease

Calcium-Phosphate-Vitamin D Axis Dysfunction

• Vitamin D metabolism: Vitamin D (cholecalciferol) undergoes 25-hydroxylation in the liver to form calcidiol (the storage form), then 1α-hydroxylation in the proximal tubule to form calcitriol (1,25-dihydroxyvitamin D₃, the active form). Calcitriol functions as a steroid hormone, binding to vitamin D receptors (VDR) in intestine, bone, and kidney to regulate calcium and phosphate homeostasis. Deficiency impairs intestinal calcium absorption and promotes secondary hyperparathyroidism.

• Secondary hyperparathyroidism compensation: In vitamin D deficiency or hypocalcemia, PTH secretion increases to normalize serum calcium through enhanced renal calcium reabsorption and osteoclastic bone resorption. Chronically elevated PTH causes parathyroid gland hyperplasia and perpetuates bone loss despite increased osteoid formation.

• Mineralization defect: The pathologic hallmark of osteomalacia and rickets is defective osteoid mineralization, where unmineralized bone matrix accumulates due to inadequate calcium and phosphate availability or impaired alkaline phosphatase activity. Normally, mineralization occurs within days to weeks; delays create a thickened osteoid seam visible on histology as the unmineralized border.

Impaired Bone Remodeling in Osteoporosis

• Osteoblast-osteoclast imbalance: In osteoporosis, there is decreased osteoblastic new bone formation relative to osteoclastic resorption, resulting in net bone loss. The microscopic trabecular architecture becomes increasingly perforated ("trabecular perforation"), converting plate-like trabeculae to rod-like remnants that bear less load and fracture more easily.

• Estrogen deficiency mechanism: Estrogen suppresses osteoclast precursor recruitment and IL-6 production by bone marrow stromal cells; its loss (menopause, hypogonadism) unleashes osteoclastogenesis via the RANK-RANKL pathway, accelerating resorption without compensatory formation increase.

• Aging-related changes: With advancing age, osteoblasts show impaired response to anabolic signals (PTH, mechanical stimulation), reduced alkaline phosphatase activity, and decreased bone formation rate. Simultaneously, osteoclast number and activity remain relatively preserved, creating a progressive resorption surplus.

Abnormal Mineralization Front

• The calcification front (interface between mineralized and unmineralized osteoid) normally advances at ~1 μm/day. In rickets and osteomalacia, this front is either absent or markedly delayed, permitting pathologic accumulation of osteoid. Tetracycline labeling studies demonstrate reduced bone formation rate and prolonged osteoid maturation time.

Osteoporosis (Primary)

• Postmenopausal osteoporosis (Type I): Estrogen withdrawal accelerates osteoclastic resorption; affects trabecular bone preferentially (vertebral bodies, femoral neck).
• Age-related osteoporosis (Type II): Progressive osteoblast dysfunction and reduced bone formation; affects both cortical and trabecular bone; occurs in both sexes after age 70.

Osteoporosis (Secondary)

• Endocrine causes: Hyperthyroidism, hypercortisolism (Cushing syndrome), hypogonadism, hyperparathyroidism (increased resorption), diabetes mellitus type 1 (impaired formation).
• Gastrointestinal causes: Malabsorption (celiac disease, cystic fibrosis, inflammatory bowel disease), chronic liver disease (impaired vitamin D activation), post-gastrectomy.
• Medication-induced: Glucocorticoids (inhibit osteoblasts, increase osteoclasts), anticonvulsants (increase vitamin D catabolism), proton pump inhibitors (reduce calcium absorption).
• Immobilization: Paralysis, prolonged bedrest; mechanical unloading suppresses osteoblast activity and increases osteoclast resorption.
• Other: Chronic kidney disease, HIV/AIDS, multiple myeloma (osteolytic lesions), rheumatoid arthritis.

Rickets (Nutritional)

• Vitamin D deficiency: Inadequate sun exposure, dietary insufficiency, malabsorption.
• Dietary calcium deficiency: Primary dietary lack or secondary to phosphate binders.
• Phosphate deficiency: Rare, seen with phosphate binders or severe malnutrition.

Rickets (Non-Nutritional/Genetic)

• Vitamin D-dependent rickets type 1 (VDDR1): Mutations in 1α-hydroxylase (CYP27B1) prevent formation of calcitriol; autosomal recessive.
• Vitamin D-dependent rickets type 2 (VDDR2): Mutations in vitamin D receptor (VDR) cause end-organ resistance; autosomal recessive.
• Hypophosphatemic rickets (X-linked hypophosphatemic rickets, XLH): Loss-of-function mutations in PHEX gene impair FGF23 regulation, leading to renal phosphate wasting and defective osteoid mineralization; X-linked dominant inheritance.
• Renal rickets: Chronic kidney disease reduces 1α-hydroxylase activity and phosphate excretion, causing secondary hyperparathyroidism and mineralization defects.

Osteomalacia (Adult Rickets)

• Nutritional vitamin D deficiency: Malabsorption, inadequate dietary intake, limited sun exposure.
• Chronic kidney disease: Impaired calcitriol synthesis.
• Hypophosphatasia: Inherited deficiency of alkaline phosphatase enzyme activity impairs osteoid mineralization.
• Renal tubular disorders: Type 2 RTA causes chronic acidosis, impairing mineralization.
• Tumor-induced osteomalacia: FGF23-secreting tumors (phosphaturic mesenchymal tumors) cause severe phosphate wasting and osteomalacia.

Osteoporosis

• Asymptomatic early course: Many patients have no symptoms until fracture occurs; disease is often termed "silent epidemic."
• Fragility fractures: Vertebral body compression fractures (may be painless or cause acute thoracic/lumbar pain), femoral neck fractures (hip), Colles fractures (wrist); typically result from minimal trauma.
• Kyphosis and loss of height: Repeated vertebral compression fractures create progressive thoracic kyphosis; patients lose 3-10 cm of height over decades.
• Back pain and functional impairment: Chronic pain from vertebral deformities, restricted mobility, increased fall risk.
• Physical examination: Height loss, thoracic kyphosis, dorsal fat pad ("dowager's hump" or "buffalo hump"), increased spinal flexion.
• Radiographic findings: Decreased radiodensity, apparent increase in cortical thickness relative to medullary cavity (pseudoincreased cortical thickness due to trabeculae loss), "rugger jersey spine" (exaggerated vertebral endplate sclerosis in secondary osteoporosis).
• DEXA scan findings: T-score ≤ −2.5 standard deviations below young adult mean.

Rickets (Pediatric)

• Growth failure and short stature: Severe growth retardation due to metaphyseal damage and systemic effects of secondary hyperparathyroidism.
• Skeletal deformities: Bowing of long bones (varus/valgus angulation), rachitic rosary (beading of costochondral junctions), frontal bossing and delayed fontanelle closure, dental enamel defects.
• Metaphyseal changes: "Widening of growth plate" on radiographs; represents accumulation of hypertrophic chondrocytes that fail to ossify; visible as bright white line on X-ray.
• Muscular weakness and delayed motor milestones: Impaired vitamin D signaling, hypocalcemia, and phosphate depletion weaken muscle; walking delayed, prone to seizures (hypocalcemia).
• Dental problems: Delayed tooth eruption, enamel hypoplasia, increased caries.
• Laboratory features: Elevated alkaline phosphatase (increased bone turnover and osteoblast activity), elevated PTH (secondary hyperparathyroidism), low 25-OH vitamin D (nutritional rickets), hypophosphatemia and hypocalcemia (varies by type).
• Biochemical markers: Elevated bone-specific alkaline phosphatase (BSAP) and P1NP (procollagen type 1 N-terminal propeptide) reflecting high bone turnover.

Osteomalacia (Adult Rickets)

• Bone pain and muscle weakness: Diffuse aching pain in pelvis, spine, and lower extremities; severe proximal muscle weakness (from phosphate depletion and vitamin D deficiency); myalgia.
• Pathologic fractures: Stress fractures (femoral neck, pubic ramus) and pseudofractures (Looser's zones, discussed below).
• Hypocalcemia-related symptoms: Muscle cramps, paresthesias, positive Chvostek and Trousseau signs, potential seizures or tetany.
• Impaired calcium homeostasis: Secondary hyperparathyroidism with persistent hypocalcemia and hyperphosphatasia.
• Radiographic findings: Looser's zones (pseudofractures)—linear lucencies perpendicular to bone cortex, typically symmetrical at femoral neck, pubic rami, and axillary borders of scapulae; represent stress fractures with failed mineralization at fracture edges; pathognomonic for osteomalacia but not always present.

Laboratory Evaluation

Serum calcium and phosphate

• Osteoporosis: Normal
• Rickets/osteomalacia: Hypocalcemia and hypophosphatemia (except secondary hyperparathyroidism may normalize calcium at expense of PTH)

Parathyroid hormone (PTH)

• Osteoporosis: Normal
• Rickets/osteomalacia: Elevated (secondary hyperparathyroidism)

25-hydroxyvitamin D [25(OH)D]

• <20 ng/mL: Severe deficiency
• 20-29 ng/mL: Insufficiency
• Normal: >30 ng/mL
• Osteoporosis: Typically normal, though low-normal increases fracture risk
• Nutritional rickets/osteomalacia: Markedly reduced (<15 ng/mL)

1,25-dihydroxyvitamin D [1,25(OH)₂D]

• Nutritional deficiency: Low-normal or low (appropriately suppressed by PTH feedback)
• VDDR1: Undetectably low (diagnostic finding)
• VDDR2: Markedly elevated (in presence of VDR mutations, end-organ resistance)
• Helpful for distinguishing types but less routinely measured

Alkaline phosphatase

• Osteoporosis: Normal
• Rickets/osteomalacia: Markedly elevated (reflecting high osteoblast activity)

Phosphate and FGF23

• XLH and tumor-induced osteomalacia: Severe hypophosphatemia and elevated FGF23

Bone turnover markers

• P1NP (formation marker): Elevated in rickets/osteomalacia (high turnover); low in osteoporosis with very low turnover
• CTX (resorption marker): Elevated when resorption is high (postmenopausal osteoporosis, rickets)

Radiographic Findings

Osteoporosis

• Generalized osteopenia: Decreased radiodensity ("radiolucency") affecting spine and proximal femur
• Vertebral compression fractures: Height loss, anterior wedging, "codfish vertebrae" (central depression of endplates from hemangioma or severe osteoporosis)
• Loss of trabecular architecture: Fine trabeculae disappear; coarser trabeculae remain
• Cortical thinning: Narrowed cortical bone; exaggerated prominence of trabecular orientation
• DEXA (dual-energy X-ray absorptiometry): Gold standard for diagnosis and monitoring; T-score classification:
• T-score ≥ −1.0: Normal
• T-score −1.0 to −2.5: Osteopenia
• T-score ≤ −2.5: Osteoporosis
• T-score ≤ −2.5 with fracture: Severe osteoporosis

Rickets

• Growth plate widening: Visible as bright white band at metaphysis; represents unossified hypertrophic cartilage
• Metaphyseal cupping and flaring: Widened, irregularly scalloped metaphyseal margins
• Loss of sharp metaphyseal margins: Indistinct zone of provisional calcification
• Cortical thinning and loss of sharp corticomedullary junction
• Bowing deformities: Particularly varus/valgus of femur and tibia
• Rachitic rosary: Beading of costochondral junctions visible on chest X-ray
• Looser's zones: Pseudofractures (in osteomalacia, also visible in severe rickets)

Osteomalacia

• Generalized osteopenia: Less marked than osteoporosis; preservation of endplate definition
• Looser's zones (pseudofractures): Pathognomonic finding—symmetrical bands of lucency perpendicular to cortex, typically at femoral neck, pubic rami, ribs, and scapular borders; represent stress fractures with mineralization defect at fracture margins
• Vertebral "codfish" appearance: Central depression of endplates
• Coxa vara: Valgus deformity of femoral neck

Histopathological Findings

Osteoporosis

• Reduced trabecular bone volume: Quantitatively decreased bone per unit tissue; normal mineralization pattern
• Trabecular perforation and loss: Plate-like trabeculae become disconnected rod-like remnants; marrow spaces enlarged
• Preserved mineralization: Normal osteoid seam width (<10-20 μm) with normal staining for mineralization (von Kossa stain positive in mineralized regions, negative in osteoid)
• Increased osteoclast activity: May see numerous osteoclasts in Howship's lacunae (resorption pits)
• Reduced osteoblast numbers and activity: Flat, inactive osteoblast lining cells; reduced bone formation rate on tetracycline labeling
• Normal osteoid quality and quantity: Unlike osteomalacia, osteoid seams are not thickened

• Thickened osteoid seams: Markedly increased unmineralized bone matrix; normal seam width is 10-20 μm, whereas osteomalacia shows seams >20-40 μm or greater
• Delayed calcification front: Tetracycline labeling shows prolonged osteoid maturation time (normally 10-20 days; in osteomalacia 100+ days)

Immediate stabilisation

• Symptomatic hypocalcemia (tetany, laryngospasm, seizure, prolonged QT): IV calcium gluconate on a cardiac monitor, then oral calcium plus activated vitamin D — a true emergency in severe rickets/osteomalacia and after parathyroidectomy.
• Hypercalcemic crisis (primary hyperparathyroidism with osteitis fibrosa cystica): isotonic saline volume repletion first, then calcitonin for rapid effect and IV zoledronic acid for durable control; avoid loop diuretics until euvolemic.
• Hip fracture: operative fixation, ideally within 24–48 hours (AAOS), with early mobilisation and delirium/VTE prophylaxis.

First-line therapy

• Repletion of substrate first: calcium and vitamin D (cholecalciferol; ergocalciferol 50,000 IU weekly for deficiency) with weight-bearing exercise, smoking/alcohol reduction and fall-risk assessment — the Endocrine Society and the Bone Health and Osteoporosis Foundation treat this as the floor beneath any drug.
• Bisphosphonates (oral alendronate/risedronate, IV zoledronic acid): pyrophosphate analogs that are internalised by osteoclasts and inhibit farnesyl pyrophosphate synthase, causing osteoclast apoptosis. First-line for osteoporosis (T-score ≤ −2.5, prior hip or vertebral fragility fracture, or osteopenia with high FRAX-estimated risk) per Endocrine Society/ACP, for glucocorticoid-induced osteoporosis per ACR, and for symptomatic Paget disease (single IV zoledronic acid infusion, Endocrine Society).

Escalation / second line

• Denosumab: anti-RANKL monoclonal antibody blocking osteoclast differentiation; useful when eGFR is low or bisphosphonates fail.
• Anabolic agents: PTH/PTHrP analogs (teriparatide, abaloparatide) or the sclerostin inhibitor romosozumab for very-high-risk patients; must be followed by an antiresorptive or gains are lost.
• SERM (raloxifene) in younger postmenopausal women with vertebral-predominant risk: reduces vertebral — but not hip or nonvertebral — fracture risk, and lowers invasive breast cancer risk; it worsens vasomotor symptoms.

Disease-specific and definitive management

• Calcitriol for VDDR1 (bypasses absent 1α-hydroxylase); high-dose calcitriol plus calcium for VDDR2 (overcoming receptor resistance).
• CKD-MBD (KDIGO): lower elevated phosphate toward normal with dietary restriction and binders, and correct 25-OH vitamin D deficiency; calcitriol/vitamin D analogs or calcimimetics are reserved for severe, progressive secondary hyperparathyroidism (and are standard options in dialysis patients), not for routine use in non-dialysis CKD.
• Burosumab (anti-FGF23 antibody) for X-linked hypophosphatemic rickets; surgical resection cures tumor-induced osteomalacia; parathyroidectomy is definitive for osteitis fibrosa cystica.

Contraindicated

• Bisphosphonates with eGFR below roughly 30–35 mL/min, uncorrected hypocalcemia or untreated vitamin D deficiency, or esophageal dysmotility/inability to sit upright.
• Raloxifene in women with prior VTE, immobilization, or high stroke risk (boxed warning: venous thromboembolism and fatal stroke).
• Abrupt denosumab discontinuation without bisphosphonate bridging.
• Romosozumab after recent MI or stroke.

Complications of the disease

• Hip fracture: cortical and trabecular loss at the femoral neck; carries substantial excess one-year mortality and loss of independence — the outcome all osteoporosis therapy is aimed at preventing.
• Vertebral compression fractures: progressive kyphosis reduces thoracic volume, producing restrictive lung physiology, early satiety and chronic pain; a new fracture on therapy signals treatment failure or an unrecognised secondary cause.
• Hypocalcemic tetany, laryngospasm and seizure in severe rickets/osteomalacia: emergency — look for Chvostek/Trousseau signs and QT prolongation.
• Permanent skeletal deformity in rickets: bowing, coxa vara and pelvic narrowing persist after biochemical correction if treatment is delayed past growth-plate closure.
• Paget disease sequelae: hypervascular pagetic bone can produce high-output heart failure; skull expansion causes cranial nerve VIII compression with hearing loss; vertebral overgrowth causes spinal stenosis; sudden new pain with a lytic destructive lesion and a rising alkaline phosphatase suggests secondary osteosarcoma — rare but the classic tested complication.
• Osteitis fibrosa cystica: brown tumors (osteoclast-rich, hemosiderin-laden lesions) cause pathologic fracture; accompanying hypercalcemic crisis with volume depletion and altered mental status is an emergency.
• Renal osteodystrophy/CKD-MBD: vascular and soft-tissue calcification; calciphylaxis with painful retiform necrotic skin ulcers is limb- and life-threatening.

Complications of treatment

• Bisphosphonates: pill-induced erosive esophagitis (take upright, fasting, with water); osteonecrosis of the jaw after dental extraction; atypical femoral fracture — heralded by prodromal groin/thigh pain with a transverse subtrochanteric lucency and lateral cortical beaking; an acute-phase flu-like reaction after IV zoledronic acid; hypocalcemia if vitamin D is not repleted first.
• Denosumab: profound hypocalcemia in advanced CKD, and rebound multiple vertebral fractures if a dose is missed or the drug is stopped without bisphosphonate follow-on.
• Calcium/vitamin D or calcitriol excess: hypercalcemia, hypercalciuria, nephrolithiasis.
• Oral phosphate plus calcitriol in XLH: nephrocalcinosis and tertiary hyperparathyroidism.
• Romosozumab: excess cardiovascular events, hence avoidance after recent MI or stroke.

• Normal labs = osteoporosis: calcium, phosphate, PTH and alkaline phosphatase are all normal; density is low but the bone that remains is normally mineralized. If any of those four is abnormal, the diagnosis is not primary osteoporosis — look for osteomalacia, hyperparathyroidism, myeloma or CKD.
• Isolated elevated alkaline phosphatase with normal calcium and phosphate in an older adult is Paget disease until proven otherwise — after confirming a bone source with GGT or bone-specific ALP, since hepatobiliary disease is the more common cause of an isolated ALP rise. Histology shows the mosaic "jigsaw puzzle" pattern of lamellar bone. Classic stems: enlarging hat size, unilateral hearing loss, warm bowed tibia, high-output heart failure.
• Looser zones (pseudofractures) — symmetric lucent bands perpendicular to the cortex at the femoral neck, pubic rami and scapular border — plus proximal muscle weakness and bone pain equals osteomalacia, not osteoporosis.
• Brown tumors and subperiosteal resorption of the radial side of the middle phalanges point to osteitis fibrosa cystica from primary hyperparathyroidism; rugger-jersey spine is the classic film of secondary hyperparathyroidism in CKD. Definitive management is parathyroidectomy, not a bisphosphonate.
• Single best next step, repeated often: before starting any antiresorptive, check and correct 25-OH vitamin D and calcium — giving a bisphosphonate or denosumab into unrecognised deficiency precipitates symptomatic hypocalcemia.
• The one association examiners love: prodromal thigh pain in a patient on long-term bisphosphonate therapy → obtain femur radiographs for an atypical subtrochanteric fracture with lateral cortical beaking.
• Screening (USPSTF): DEXA for all women 65 and older, and for younger postmenopausal women whose fracture risk equals that threshold. Use T-scores in postmenopausal women and men over 50; use Z-scores in children, premenopausal women and younger men.
• Common distractor: a reactive alkaline phosphatase rise after fracture is transient (weeks to a few months); persistent elevation with low bone density points to a mineralization or remodeling disorder (osteomalacia, Paget disease, hyperparathyroidism), not primary osteoporosis.