Osteoporosis

Osteoporosis – High‑Yield Study Guide for Medical Students

Definition

Osteoporosis is a systemic skeletal disorder characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to increased bone fragility and susceptibility to fractures, typically with minimal trauma.

Diagnosis is made primarily by bone mineral density (BMD) measurement using dual-energy X-ray absorptiometry (DXA), with osteoporosis defined as a T-score ≤ −2.5 at the lumbar spine, total hip, or femoral neck.

Epidemiology

Osteoporosis is one of the most common metabolic bone diseases and a major cause of morbidity in older adults.

• Prevalence: Affects millions worldwide; prevalence increases markedly with age.
• Sex: More common in women, especially postmenopausal (due to estrogen deficiency). Men are underdiagnosed but account for a substantial proportion of osteoporotic fractures.
• Age: Incidence rises after age 50; hip and vertebral fractures are particularly common in the elderly.
• Race/Ethnicity: Higher risk in Caucasian and Asian populations; however, absolute numbers are significant across all ethnicities.
• Public health impact: Associated with increased fracture risk, loss of independence, chronic pain, and increased mortality, particularly after hip fractures.

Pathophysiology

Normal bone remodeling is a tightly regulated process involving osteoclast-mediated bone resorption and osteoblast-mediated bone formation. In osteoporosis, there is an imbalance favoring resorption over formation, leading to net bone loss and compromised microarchitecture.

Key mechanisms include:

• Estrogen deficiency: After menopause, reduced estrogen increases osteoclast activity and lifespan and decreases osteoprotegerin (OPG), enhancing RANKL-mediated bone resorption.
• Aging: Decreased osteoblast number and function, reduced growth factor levels, impaired osteocyte signaling, and accumulative microdamage.
• Secondary causes: Glucocorticoid therapy, hyperparathyroidism, hyperthyroidism, hypogonadism, malabsorption, chronic liver or kidney disease, multiple myeloma, and other chronic inflammatory states increase bone resorption or decrease formation.
• Calcium and vitamin D deficiency: Low calcium/vitamin D leads to secondary hyperparathyroidism and increased bone resorption.
• Lifestyle factors: Physical inactivity, smoking, excessive alcohol consumption, and low body weight contribute to peak bone mass reduction and accelerated bone loss.

Risk Factors

Recognizing risk factors is crucial for targeted screening and prevention.

• Non-modifiable:
  • Advanced age
  • Female sex, especially postmenopausal
  • Caucasian or Asian ethnicity
  • Positive family history of fragility fracture (particularly hip fracture in a first-degree relative)
  • Low peak bone mass (e.g., early menopause, delayed puberty)
  • History of prior fragility fracture
• Modifiable:
  • Smoking
  • Excessive alcohol intake (typically >3 units/day)
  • Low body mass index (BMI < 20 kg/m²)
  • Sedentary lifestyle, low weight-bearing activity
  • Poor calcium and vitamin D intake
• Medications associated with osteoporosis:
  • Glucocorticoids (e.g., prednisone ≥5 mg/day for ≥3 months)
  • Anticonvulsants (e.g., phenytoin, phenobarbital)
  • Aromatase inhibitors, androgen deprivation therapy
  • Proton pump inhibitors, some SSRIs, heparin (long-term)
• Secondary causes:
  • Endocrine: Hyperthyroidism, hyperparathyroidism, Cushing syndrome, diabetes mellitus, hypogonadism
  • GI: Celiac disease, inflammatory bowel disease, malabsorption, bariatric surgery
  • Renal: Chronic kidney disease, renal tubular acidosis
  • Hematologic: Multiple myeloma, leukemia
  • Other: Rheumatoid arthritis, chronic immobilization

Clinical Presentation

Osteoporosis is often asymptomatic until a fracture occurs, hence the term "silent disease." When symptoms occur, they are usually related to fractures or structural deformity.

• Common fracture sites:
  • Vertebral bodies (thoracic and lumbar)
  • Proximal femur (hip)
  • Distal radius (Colles fracture)
• Clinical manifestations:
  • Acute back pain after minimal trauma, often due to vertebral compression fracture
  • Loss of height over time
  • Kyphosis or "dowager's hump" due to multiple vertebral fractures and wedge deformities
  • Chronic back pain and reduced mobility
  • Fragility fractures (e.g., fracture from a fall from standing height or less)
• Complications of fractures:
  • Hip fractures: high morbidity and mortality, loss of independence
  • Vertebral fractures: chronic pain, restrictive lung disease due to thoracic deformity, reduced quality of life
  • Psychological impact: fear of falling, depression, social isolation

Diagnosis

Diagnosis integrates clinical risk assessment, imaging (primarily DXA), and evaluation for secondary causes.

Bone Mineral Density (DXA)

The gold standard test is dual-energy X-ray absorptiometry (DXA), usually at the lumbar spine, total hip, and femoral neck.

• T-score: Number of standard deviations a patient's BMD differs from the mean BMD of a young healthy reference population (same sex).
• Z-score: Number of standard deviations a patient's BMD differs from age- and sex-matched reference population (useful in premenopausal women, men <50 years, and children).
• WHO DXA-based definitions (for postmenopausal women and men ≥50 years):
  • Normal: T-score ≥ −1.0
  • Osteopenia (low bone mass): T-score between −1.0 and > −2.5
  • Osteoporosis: T-score ≤ −2.5
  • Severe (established) osteoporosis: T-score ≤ −2.5 plus one or more fragility fractures

Clinical Risk Assessment Tools

Risk calculators help integrate BMD and clinical risk factors to estimate fracture risk.

• FRAX (Fracture Risk Assessment Tool):
  • Estimates 10-year probability of major osteoporotic fracture and hip fracture.
  • Includes age, sex, BMI, prior fracture, parental hip fracture, smoking, glucocorticoid use, rheumatoid arthritis, secondary osteoporosis, alcohol intake, and optional femoral neck BMD.
  • Used to guide treatment thresholds.

Laboratory Evaluation

Basic labs are used to exclude secondary causes and to evaluate bone metabolism.

• Baseline investigations:
  • Serum calcium, phosphate, albumin (adjusted calcium)
  • 25-hydroxy vitamin D
  • Renal function (creatinine, eGFR)
  • Liver function tests
  • Thyroid-stimulating hormone (TSH)
  • Complete blood count (CBC)
• Additional tests when secondary osteoporosis suspected:
  • Parathyroid hormone (PTH)
  • Serum protein electrophoresis (for myeloma)
  • Testosterone (in men), LH/FSH, estradiol (in women with suspected hypogonadism)
  • 24-hour urinary calcium
  • Celiac serology
  • Cortisol testing if Cushing syndrome suspected

Imaging Beyond DXA

• Plain radiographs:
  • Used to detect vertebral fractures, deformities, and loss of vertebral height.
  • Not sensitive for early bone loss; significant bone loss (≥30%) required before changes are visible.
• Vertebral fracture assessment (VFA): DXA-based lateral spine imaging to detect asymptomatic vertebral compression fractures.
• Quantitative CT: Provides volumetric BMD and may differentiate cortical vs trabecular bone; higher radiation and less commonly used.

Indications for BMD Testing (Typical Exam Points)

• Women ≥65 years and men ≥70 years, regardless of risk factors.
• Postmenopausal women and men 50–69 years with clinical risk factors (e.g., low body weight, prior fracture, high-risk medications, secondary causes).
• Adults with a fragility fracture after age 50.
• Chronic glucocorticoid therapy (e.g., ≥5 mg prednisone equivalent for ≥3 months).

Management

Management aims to reduce fracture risk by optimizing bone strength and minimizing falls. It includes both non-pharmacologic and pharmacologic strategies.

Non-Pharmacologic Management

• Calcium and Vitamin D:
  • Ensure adequate dietary calcium intake (roughly 1,000–1,200 mg/day total from diet plus supplements, depending on age and sex).
  • Vitamin D supplementation to maintain 25(OH)D levels typically ≥20–30 ng/mL, often 800–1,000 IU/day or higher if deficient.
• Exercise:
  • Regular weight-bearing (walking, jogging, stair climbing) and muscle-strengthening exercises improve BMD and reduce fall risk.
  • Balance and functional training (e.g., Tai Chi) helps prevent falls.
• Lifestyle modification:
  • Smoking cessation.
  • Limit alcohol intake.
  • Maintain healthy BMI (avoid underweight).
• Fall prevention:
  • Home safety assessment (remove loose rugs, improve lighting, install grab bars).
  • Correct visual impairment, review medications causing orthostatic hypotension or sedation.
  • Use of appropriate footwear and assistive devices when indicated.

Pharmacologic Therapy

Pharmacologic treatment is indicated in patients with established osteoporosis or high fracture risk based on BMD and/or clinical risk factors.

• General indications (typical criteria used in guidelines):
  • History of hip or vertebral fragility fracture (clinical or radiographic).
  • DXA T-score ≤ −2.5 at lumbar spine, femoral neck, or total hip.
  • Osteopenia (T-score between −1.0 and −2.5) with elevated FRAX 10-year fracture probability above treatment thresholds (e.g., hip fracture risk ≥3% or major osteoporotic fracture ≥20%, depending on guideline).
  • Chronic glucocorticoid therapy with elevated fracture risk.

First-Line: Anti-Resorptive Agents

• Bisphosphonates (e.g., alendronate, risedronate, ibandronate, zoledronic acid)
  • Mechanism: Bind to hydroxyapatite in bone and inhibit osteoclast-mediated bone resorption, inducing osteoclast apoptosis.
  • Use: First-line for most patients with postmenopausal osteoporosis and in men at high risk.
  • Dosing examples:
    • Alendronate: 70 mg orally weekly.
    • Risedronate: 35 mg orally weekly.
    • Zoledronic acid: 5 mg IV annually (or q2 years in some prevention settings).
  • Administration tips: Take oral agents with a full glass of water, fasting, remain upright for at least 30 minutes to reduce esophageal irritation.
  • Adverse effects: Esophagitis, gastrointestinal upset, acute phase reaction (with IV), rare osteonecrosis of the jaw (ONJ), atypical femoral fractures with long-term use.
  • Treatment duration: Typically reassess after 3–5 years; consider a "drug holiday" in low-to-moderate risk patients after long-term use, while high-risk patients may continue longer with close monitoring.
• Denosumab
  • Mechanism: Human monoclonal antibody against RANKL, inhibiting osteoclast formation and activity.
  • Dosing: 60 mg subcutaneously every 6 months.
  • Use: Alternative first-line agent in high-risk patients or those intolerant of bisphosphonates, and in patients with renal impairment (with caution and calcium/vitamin D repletion).
  • Adverse effects: Hypocalcemia, dermatologic reactions, infections, rare ONJ and atypical femur fractures.
  • Important: Rapid bone loss and rebound vertebral fractures can occur upon discontinuation; transition to another antiresorptive (e.g., bisphosphonate) is recommended when stopping.
• Selective Estrogen Receptor Modulators (SERMs) (e.g., raloxifene)
  • Mechanism: Estrogen agonist on bone and lipid metabolism, antagonist on breast and uterine tissue.
  • Use: Prevention and treatment of osteoporosis in postmenopausal women; reduces vertebral fracture risk, but data on non-vertebral and hip fractures is limited.
  • Adverse effects: Hot flashes, leg cramps, increased risk of venous thromboembolism (VTE).
  • Additional benefit: Reduces risk of estrogen receptor-positive breast cancer.
• Hormone Replacement Therapy (HRT)
  • Mechanism: Repletion of estrogen (with or without progestin) reduces bone resorption in early postmenopause.
  • Use: Considered in symptomatic early postmenopausal women with vasomotor symptoms and elevated fracture risk, after individualized risk–benefit assessment.
  • Risks: Increased risk of breast cancer, VTE, stroke, and cardiovascular disease depending on regimen and patient profile.

Anabolic (Bone-Forming) Agents

• Parathyroid hormone analogs (e.g., teriparatide, abaloparatide)
  • Mechanism: Intermittent PTH receptor stimulation increases osteoblast activity and bone formation.
  • Use: Patients at very high fracture risk (e.g., multiple vertebral fractures, very low BMD) or those who fail/intolerant of antiresorptive therapy.
  • Dosing: Daily subcutaneous injection; treatment duration is limited (commonly up to 2 years).
  • Adverse effects: Hypercalcemia, hypercalciuria, nausea, leg cramps; theoretical risk of osteosarcoma seen in animal studies (hence duration limits).
  • Follow-up: Usually followed by an antiresorptive agent to maintain gains in BMD.
• Sclerostin inhibitors (e.g., romosozumab – where available)
  • Mechanism: Monoclonal antibody against sclerostin, increasing bone formation and reducing resorption.
  • Use: Severe osteoporosis at very high fracture risk.
  • Concerns: Potential cardiovascular risk signal in some trials; not appropriate in patients with recent MI or stroke.

Glucocorticoid-Induced Osteoporosis

Glucocorticoids accelerate bone resorption, inhibit osteoblasts, and reduce calcium absorption, significantly increasing fracture risk.

• Who is at risk: Patients on prolonged systemic glucocorticoids, especially ≥5 mg prednisone equivalent daily for ≥3 months.
• Management principles:
  • Use lowest effective dose and shortest duration of glucocorticoids.
  • Ensure adequate calcium and vitamin D supplementation.
  • Consider early initiation of antiresorptive therapy (usually a bisphosphonate) in at-risk patients.
  • Monitor BMD at baseline and periodically during therapy.

Monitoring and Follow-Up

• BMD monitoring:
  • Typically repeat DXA every 1–2 years after starting therapy, then individualized intervals based on response and risk.
  • Assess for stability or improvement in T-score and identify treatment failure (significant bone loss or new fractures).
• Bone turnover markers:
  • Biochemical markers (e.g., serum CTX, P1NP) can be used in some settings to assess adherence and biologic response, but are not always necessary for routine care.
• Ongoing risk assessment:
  • Re-evaluate falls risk, medications, and secondary causes periodically.

Key Clinical Pearls for Exams

• Fragility fracture = osteoporosis until proven otherwise, especially hip or vertebral fracture after minimal trauma.
• DXA T-score ≤ −2.5 at spine, hip, or femoral neck defines osteoporosis in postmenopausal women and men ≥50.
• Most common fracture sites: vertebral bodies, proximal femur, distal radius.
• Postmenopausal woman with acute back pain and height loss: think vertebral compression fracture due to osteoporosis.
• First-line pharmacologic therapy: oral bisphosphonates (alendronate, risedronate) for most high-risk patients.
• Bisphosphonate administration: Take with water on empty stomach, remain upright ≥30 minutes to prevent esophagitis.
• Long-term bisphosphonate use: associated with rare atypical femoral fractures and osteonecrosis of the jaw; consider periodic therapy review and drug holidays.
• Denosumab: effective antiresorptive, but stopping abruptly can cause rebound bone loss and multiple vertebral fractures; transition to another agent on discontinuation.
• Glucocorticoid therapy: anticipate and prevent bone loss; consider early bisphosphonate therapy.
• Calcium + vitamin D are essential components but not sufficient alone for high-risk osteoporosis; pharmacologic therapy is needed to significantly reduce fracture risk.
• FRAX tool: integrates clinical risk factors with or without BMD to estimate 10-year fracture risk and guide treatment decisions.
• Screen all women ≥65 and younger high-risk individuals; similar approach in men ≥70 or with additional risk factors.

Summary

Osteoporosis is a common, largely preventable and treatable cause of fragility fractures. For medical students, mastering its risk factors, diagnostic criteria (especially DXA T-scores), and first-line treatments (bisphosphonates and other antiresorptives) is essential. Combining lifestyle interventions, adequate calcium and vitamin D, and appropriate pharmacologic therapy significantly reduces fracture risk and improves patient outcomes.