Everything You Need to Know About Osteoporosis vs osteomalacia vs rickets for Step 1

Osteoporosis, osteomalacia, and rickets all show up as “weak bones” on exams—but Step questions hinge on what’s wrong with the bone: too little bone (quantity), poorly mineralized bone (quality), or defective mineralization in kids. If you can map each disorder to bone matrix vs mineral, adult vs child, and labs + imaging, you’ll crush most MSK stem twists.

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The 10-second framework (Step-style)

First Aid cross-reference: MSK → “Metabolic bone disease” section; Endocrine → Vitamin D/PTH physiology; Repro/Endocrine pharm → bisphosphonates, teriparatide, denosumab.

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Step 1 essentials: normal bone physiology (why these diseases differ)

Bone is made of:

• Osteoid (matrix): mostly type I collagen (laid down by osteoblasts)
• Mineral: hydroxyapatite (calcium + phosphate)

Key hormones:

• Vitamin D: increases intestinal absorption of Ca and PO₄
• PTH: increases serum Ca, decreases serum PO₄, increases bone resorption indirectly (via osteoblast RANKL → osteoclast activation)
• Estrogen: decreases osteoclast activity; loss → more resorption

So:

• Osteoporosis = not enough bone (but what’s there is mineralized normally)
• Osteomalacia/rickets = you lay down osteoid but can’t mineralize it properly

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Osteoporosis

Definition (what Step expects)

A disorder of decreased bone mass and microarchitectural deterioration, leading to increased fragility fractures, with normal mineralization.

Pathophysiology

High yield mechanisms:

• Postmenopausal (Type I): estrogen ↓ → osteoclast activity ↑ (via ↑ IL-1, IL-6, TNF) → trabecular bone loss
• Senile (Type II): age-related decline in osteoblast function + Ca/Vit D insufficiency; cortical + trabecular loss
• Secondary causes: glucocorticoids, hyperthyroidism, hypogonadism, malabsorption, chronic kidney/liver disease, etc.

Key concept: mineralization is fine → labs are typically normal.

Clinical presentation

• Often asymptomatic until fracture
• Fragility fractures: fall from standing height
• Common sites:
  • Vertebral compression fractures → acute back pain, height loss, kyphosis
  • Hip (femoral neck) fractures
  • Distal radius (Colles) fractures

High-yield association list (risk factors)

• “Thin, white, postmenopausal woman” archetype
• Smoking, alcohol use, sedentary lifestyle
• Long-term glucocorticoids
• Low calcium intake, low vitamin D
• Hyperparathyroidism, hyperthyroidism
• Hypogonadism (including aromatase inhibitors, androgen deprivation)

Diagnosis

• DEXA scan (bone mineral density)
  • T-score:
    • Normal: ≥ −1
    • Osteopenia: between −1 and −2.5
    • Osteoporosis: ≤ −2.5
• Labs: typically normal Ca, PO₄, ALP, PTH
• Consider FRAX for fracture risk (commonly referenced clinically; Step may mention “10-year risk”)

Treatment (Step-level)

Lifestyle / prevention

• Weight-bearing exercise
• Calcium + vitamin D supplementation if deficient
• Fall risk reduction

Pharmacology (First Aid favorites)

• Bisphosphonates (alendronate, risedronate, zoledronate)
  • Mechanism: bind hydroxyapatite; inhibit osteoclasts (↓ bone resorption)
  • Adverse effects: esophagitis (oral), atypical femur fracture, osteonecrosis of jaw
• Denosumab
  • Mechanism: monoclonal Ab against RANKL → ↓ osteoclast activity
  • Often used when bisphosphonates not tolerated or in renal impairment (Step may mention)
• Teriparatide/Abaloparatide (PTH analogs)
  • Intermittent PTH → ↑ osteoblast activity > osteoclast
  • Can cause hypercalcemia; avoid in patients at risk for osteosarcoma
• Raloxifene (SERM)
  • Estrogen agonist in bone, antagonist in breast/uterus
  • AE: hot flashes, VTE risk

High-yield:

• Osteoporosis = normal labs + decreased BMD + fragility fractures.

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Osteomalacia (Adults)

Definition

Defective mineralization of bone in adults, usually due to vitamin D deficiency (or impaired vitamin D activation/absorption).

Pathophysiology (link it to labs)

Vitamin D deficiency → ↓ intestinal Ca/PO₄ absorption → serum Ca tends to fall → PTH rises (secondary hyperparathyroidism) → phosphate wasting in urine → low PO₄ + increased bone turnover → high ALP.

Common causes to memorize:

• Low dietary intake or low sun exposure
• Malabsorption (celiac disease, pancreatic insufficiency, bariatric surgery)
• Liver disease (↓ 25-hydroxylation)
• CKD (↓ 1α-hydroxylation → low calcitriol)
• Meds: anticonvulsants (e.g., phenytoin induces P450 → Vit D catabolism)

Clinical presentation

• Bone pain (often diffuse, worse with weight bearing)
• Proximal muscle weakness
• Increased fracture risk
• “Pseudofractures” (Looser zones) on imaging—classically in ribs, pelvis, femoral neck

Diagnosis

Labs (typical board pattern)

• 25(OH) vitamin D: low
• Ca: low or low-normal
• PO₄: low
• ALP: high
• PTH: high (secondary hyperparathyroidism)

Imaging

• Decreased bone density (can mimic osteoporosis) but clinical + lab pattern points to mineralization defect

First Aid tie-in: Distinguish from osteoporosis using ALP and Ca/PO₄ (osteoporosis usually normal).

Treatment

• Vitamin D replacement (cholecalciferol/ergocalciferol)
• Calcium supplementation if needed
• Treat underlying cause (malabsorption, CKD—may need calcitriol in CKD since kidney can’t activate Vit D)

High-yield differentiation:

• Osteomalacia = low Vit D + high ALP + bone pain.

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Rickets (Children)

Definition

Defective mineralization at the growth plate (epiphyseal plate) in children—essentially osteomalacia of the pediatric skeleton.

Pathophysiology

Same vitamin D/calcium/phosphate problem as osteomalacia, but the hallmark is disordered endochondral ossification at growth plates → skeletal deformities.

Clinical presentation (classic Step clues)

• Bowing of legs (genu varum)
• Rachitic rosary: bead-like costochondral junctions
• Craniotabes: soft skull bones
• Delayed closure of fontanelles
• Widened wrists/ankles (metaphyseal enlargement)
• Growth retardation

Diagnosis

Labs (similar to osteomalacia)

• Low 25(OH) vitamin D (often)
• Low Ca, low PO₄, high ALP, high PTH

X-ray

• Widened, irregular epiphyseal plates
• Metaphyseal cupping and fraying

Treatment

• Vitamin D + calcium repletion
• Address nutritional deficiency, malabsorption, lack of sunlight
• If due to CKD (renal rickets): active vitamin D (calcitriol) + manage phosphate

High-yield:

• Rickets = osteomalacia plus growth plate findings (bowing, rachitic rosary, widened epiphyses).

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Rapid-fire: how Step questions try to trick you

“Low bone density” on imaging

• If elderly + fragility fracture + normal labs → osteoporosis
• If bone pain + proximal muscle weakness + high ALP → osteomalacia

“Child with bowed legs”

• Think rickets first; then ask why:
  • Poor diet/no sunlight
  • Malabsorption
  • CKD (can’t activate Vit D)

Labs table (memorize this)

Note: Early vitamin D deficiency can show normal Ca due to PTH compensation, but ALP rises with increased osteoblast activity.

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Classic vignette patterns (mini-stems)

• Osteoporosis: 67F, smoker, BMI 19, vertebral compression fractures; Ca/PO₄/ALP normal; DEXA T-score −2.7.
• Osteomalacia: 52M with chronic diarrhea (celiac), diffuse bone pain, waddling gait; low 25(OH)D, high ALP; Looser zones on X-ray.
• Rickets: 18-month-old with bowed legs, rachitic rosary; low vitamin D, high ALP; widened growth plates.

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Treatment comparisons (what to pick when)

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First Aid cross-references (quick map)

• FA MSK: osteoporosis vs osteomalacia/rickets summary; fracture patterns
• FA Endocrine/Metabolism: vitamin D metabolism (skin → liver 25-OH → kidney 1,25-OH), PTH effects
• FA Pharm: bisphosphonates, denosumab, teriparatide, SERMs adverse effects and mechanisms

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Take-home memory hooks

• Osteoporosis: “Porous bones, normal minerals” → normal labs.
• Osteomalacia: “O’Malacia = soft adult bone” → low Vit D, high ALP, bone pain.
• Rickets: osteomalacia at the growth plate → bowed legs + widened epiphyses.