Osteomalacia

Definition

Osteomalacia is a metabolic bone disease characterized by defective mineralization of newly formed osteoid in mature bone. It represents the adult equivalent of rickets, which affects growing bones in children. The hallmark is accumulation of unmineralized or undermineralized osteoid matrix, resulting in soft, deformable bones with increased fracture risk.

Epidemiology

Prevalence:

• Elderly institutionalized: 30-50% have vitamin D deficiency (less than 50 nmol/L)
• Community-dwelling elderly: 10-15% have deficiency
• Post-bariatric surgery: 25-40% develop deficiency without supplementation
• Dark-skinned populations in high latitudes: 5-10 times higher risk

Demographics:

• Female to male ratio: 3:1 (postmenopausal women at highest risk)
• Age: Risk increases with age (reduced sunlight exposure, decreased skin synthesis)
• Geographic variation: Higher prevalence at latitudes greater than 35 degrees (reduced UVB exposure)

Normal bone mineralization requires adequate calcium, phosphate, and alkaline phosphatase. Vitamin D is essential for intestinal calcium absorption and renal phosphate reabsorption. In osteomalacia, deficiency of vitamin D or phosphate leads to:

Molecular cascade:

1. Reduced calcium and phosphate availability
2. Inadequate hydroxyapatite crystal formation in osteoid
3. Accumulation of unmineralized osteoid (increased osteoid volume)
4. Secondary hyperparathyroidism (compensatory response to hypocalcemia)
5. Further phosphate wasting (PTH-induced renal phosphate loss)
6. Progressive bone softening and deformity

Symptoms

Examination Findings

Musculoskeletal:

• Antalgic or waddling gait - due to bone pain and proximal myopathy
• Bone tenderness - sternal pressure, rib compression, pelvic compression
• Skeletal deformities - leg bowing (varus or valgus), kyphosis
• Reduced muscle power - hip flexion (iliopsoas), knee extension (quadriceps) weakness

Neurological:

• Proximal muscle weakness - grade 3-4 out of 5 in hip flexors, shoulder abductors
• Hyporeflexia - reduced or absent deep tendon reflexes
• Tetany signs (if severe hypocalcemia) - Chvostek's sign (facial twitch), Trousseau's sign (carpopedal spasm)

Biochemistry

Vitamin D Thresholds:

• Severe deficiency: less than 25 nmol/L (less than 10 ng/mL)
• Deficiency: 25-50 nmol/L (10-20 ng/mL)
• Insufficiency: 50-75 nmol/L (20-30 ng/mL)
• Optimal for bone health: 75-125 nmol/L (30-50 ng/mL)

Additional Investigations

• Urinary calcium: low (less than 2.5 mmol per 24 hours)
• Urinary phosphate: elevated in renal phosphate wasting
• FGF23 level: elevated in tumor-induced osteomalacia (oncogenic osteomalacia)
• Renal function: to assess for chronic kidney disease (impaired 1-alpha hydroxylation)
• Liver function: to assess for cholestatic disease (impaired vitamin D absorption)

Pathological Fractures

High-risk sites:

• Femoral neck - bilateral, often at sites of Looser zones
• Proximal femur - subtrochanteric, intertrochanteric
• Pelvis - pubic rami, sacrum
• Ribs - multiple, painful
• Vertebrae - compression fractures

Management principles:

1. Optimize medical management FIRST - vitamin D and calcium replacement
2. Prophylactic fixation for impending fractures (Looser zones greater than 50% cortical width, symptomatic)
3. Fracture fixation with caution - bone is soft, screw purchase poor
4. Longer immobilization than normal fractures - delayed healing

Arthroplasty Considerations

Preoperative:

• Screen all arthroplasty candidates for vitamin D deficiency
• Optimize vitamin D greater than 75 nmol/L before elective surgery
• Correct calcium and phosphate abnormalities

Intraoperative:

• Risk of periprosthetic fracture during insertion (especially press-fit stems)
• Poor bone quality may favor cemented fixation
• Careful reaming and broaching to avoid fracture

Postoperative:

• Delayed osseointegration of uncemented implants
• Risk of aseptic loosening if vitamin D not repleted
• Periprosthetic fracture risk with minimal trauma
• Continue vitamin D and calcium indefinitely

Expected Response to Treatment

Biochemical:

• Calcium and phosphate normalize by 4-12 weeks
• PTH decreases by 3-6 months (may take longer if severe)
• Alkaline phosphatase declines by 6-12 months (may initially rise as bone heals)

Clinical:

• Bone pain improves by 6-12 weeks
• Muscle weakness reverses by 3-6 months
• Looser zones heal by 6-12 months (radiographic evidence of mineralization)
• Fracture risk decreases once vitamin D greater than 50 nmol/L

Poor prognostic factors:

• Severe, prolonged deficiency - may have permanent skeletal deformities
• Uncontrolled underlying cause (malabsorption, chronic kidney disease)
• Non-compliance with supplementation
• Oncogenic osteomalacia with unresectable tumor

Global Epidemiology

• Vitamin D deficiency is one of the most prevalent micronutrient deficiencies worldwide, but its consequences vary by latitude, skin pigmentation, diet, sun exposure and food-fortification policy.
• Nutritional osteomalacia remains common in South Asia, the Middle East and North Africa, driven by limited effective UVB exposure (concealing dress, high latitude in winter, air pollution) and low dietary calcium/vitamin D.
• In high-income countries, frank osteomalacia is now mostly seen in the institutionalized elderly, malabsorption (coeliac disease, IBD, post-bariatric surgery), chronic kidney/liver disease, and dark-skinned migrants at higher latitudes.

Side-by-Side Guideline Comparison

• Key controversy: the Endocrine Society favours a higher individual target (above 75 nmol/L) for at-risk patients, whereas the IOM and several national bodies regard about 50 nmol/L as adequate for the general population. For symptomatic/biopsy-proven osteomalacia all bodies agree on therapeutic replacement plus calcium.

Registry & Outcome Notes

• There is no dedicated osteomalacia registry; relevant arthroplasty registries (NJR, AJRR, AOANJRR, Swedish/Norwegian) track revision and periprosthetic fracture, where poor bone quality and metabolic bone disease are recognised contributors to early failure and periprosthetic fracture.

High- vs Limited-Resource Practice

• High-resource: routine 25-OH vitamin D, PTH, FGF23 assays; functional imaging (68Ga-DOTATATE PET/CT, whole-body MRI) for TIO; access to burosumab and calcitriol.
• Limited-resource: diagnosis often clinical/radiographic and biochemical (calcium, phosphate, ALP); calcium plus vitamin D remains the cost-effective cornerstone and the global consensus stresses combined calcium-vitamin D because dietary calcium deficiency alone can cause osteomalacia/rickets even with adequate vitamin D.

Oncogenic Osteomalacia

• Rare FGF23-secreting mesenchymal tumours require functional then anatomical imaging (68Ga-DOTATATE PET/CT, whole-body MRI, selective venous FGF23 sampling) for localisation. Surgical excision is curative; burosumab is the option for unresectable/unlocalised disease.