Osteomalacia & Rickets

Osteomalacia and rickets are two faces of the same disease of defective mineralisation of the bone matrix (osteoid). When mineralisation fails before epiphyseal fusion it produces rickets (a disease of the growing skeleton, affecting both growth plate and bone); when it fails after growth plate closure it produces osteomalacia (soft adult bone). The commonest cause worldwide is vitamin D deficiency, and the unifying lab biochemistry and radiology are extremely high-yield for NEET PG.

Definition & basic concept

• Rickets: defective mineralisation of both the growth plate cartilage (physis) and the newly formed osteoid in children. Hence you get both growth-plate widening/cupping AND bone deformities.
• Osteomalacia: defective mineralisation of osteoid only (no growth plate present), seen in adults → accumulation of unmineralised osteoid seams that are soft, painful, and prone to pseudofractures.
• The defect is qualitative (poor mineral content of normal-amount matrix), in contrast to osteoporosis, which is a quantitative loss of normal-quality bone.

High-yield: Osteomalacia = soft bone due to under-mineralised osteoid (qualitative defect). Osteoporosis = reduced quantity of normally mineralised bone (quantitative defect). In osteoporosis the biochemistry (Ca, PO₄, ALP) is normal; in osteomalacia it is deranged (↓Ca, ↓PO₄, ↑ALP).

Vitamin D physiology (must-know for the lab profile)

1. Skin: 7-dehydrocholesterol + UV-B (sunlight) → cholecalciferol (vitamin D₃).
2. Liver: D₃ undergoes 25-hydroxylation (enzyme 25-hydroxylase) → 25-hydroxyvitamin D [25(OH)D] — the major circulating/storage form and the best marker of body vitamin D status.
3. Kidney (proximal tubule): 1α-hydroxylase (stimulated by PTH, low PO₄, low Ca) converts it → 1,25-dihydroxyvitamin D [calcitriol] — the active hormone.

Calcitriol → increases intestinal Ca and PO₄ absorption, promotes bone mineralisation, and (with PTH) maintains serum calcium.

High-yield: Best test to assess vitamin D stores/status = serum 25(OH)D. The active form is 1,25(OH)₂D (calcitriol), made in the kidney by 1α-hydroxylase. Calcitriol is the form deficient in renal failure and in type 1 vitamin D-dependent rickets.

Etiology & classification

Causes can be grouped by the biochemical mechanism. A useful split is calcipenic (vitamin D / calcium pathway) versus phosphopenic (phosphate-wasting).

Category	Examples	Key clue
Vitamin D deficiency (calcipenic)	Poor sun exposure, dietary lack, dark skin, veiling, malabsorption (coeliac, post-bariatric), chronic liver disease	Commonest; ↓25(OH)D
Vitamin D-dependent rickets type 1 (VDDR-1)	1α-hydroxylase deficiency	Low calcitriol; responds to physiological calcitriol
Vitamin D-dependent rickets type 2 (VDDR-2)	Vitamin D receptor (VDR) defect → end-organ resistance	High calcitriol; alopecia classic
X-linked hypophosphataemic rickets (XLH)	PHEX mutation → ↑FGF23 → renal PO₄ wasting	Commonest inherited rickets; normal Ca, low PO₄, normal/low calcitriol
Renal osteodystrophy	Chronic kidney disease → ↓1α-hydroxylation + PO₄ retention	↑PO₄, ↓Ca, ↑↑PTH (secondary hyperparathyroidism)
Renal tubular acidosis / Fanconi	Phosphaturia, acidosis	Hypophosphataemia + acidosis
Oncogenic (tumour-induced) osteomalacia	Mesenchymal tumour secreting FGF23	Acquired hypophosphataemia in adults
Drug / others	Anticonvulsants (phenytoin, phenobarbitone induce hepatic enzymes), hypophosphatasia (↓ALP)	Hypophosphatasia is the exception: low ALP

High-yield: X-linked hypophosphataemic rickets is the commonest inherited (hereditary) form of rickets; mechanism = ↑FGF23 (from PHEX gene mutation) causing renal phosphate wasting. Serum calcium is normal, phosphate is low, and it does not respond to vitamin D alone — needs oral phosphate + calcitriol.

Pathophysiology

• Deficient calcitriol → ↓ intestinal absorption of Ca and PO₄ → hypocalcaemia → triggers secondary hyperparathyroidism.
• PTH restores serum calcium (bone resorption, renal Ca reabsorption) but promotes renal phosphate wasting → hypophosphataemia.
• Net result: low/low-normal calcium, low phosphate, high ALP (osteoblastic overactivity laying down osteoid that cannot mineralise), high PTH.
• Inadequate Ca × PO₄ product → failure of hydroxyapatite deposition → wide unmineralised osteoid seams (osteomalacia) and disorganised, uncalcified growth plate cartilage (rickets).

Clinical features

Rickets (children)

• General: growth retardation, irritability, hypotonia ("rag-doll"), delayed motor milestones and delayed dentition, frontal bossing, delayed closure of fontanelles, craniotabes (soft skull, ping-pong feel).
• Costochondral junctions: rachitic rosary — beading at the costochondral junctions.
• Chest: Harrison's sulcus (groove) — horizontal depression along the lower border of the chest where the softened ribs are pulled in by the diaphragm; pigeon chest (pectus carinatum).
• Wrists/ankles: widening of the metaphyses → double malleoli, broad wrists.
• Lower limbs (weight-bearing toddler): genu varum (bow legs) is classic in younger children; genu valgum (knock knees) may occur in older children. Coxa vara, anterior bowing of tibia.

Osteomalacia (adults)

• Diffuse bone pain and tenderness (often proximal, axial), proximal myopathy → difficulty rising from a chair, waddling gait.
• Pseudofractures and increased fragility fractures.
• Features of hypocalcaemia: paraesthesia, carpopedal spasm, Chvostek and Trousseau signs, tetany, even seizures.

High-yield: Remember the rachitic triad of physical signs commonly asked — rachitic rosary, Harrison's sulcus, and bow legs (genu varum) in children; Looser's zones and proximal myopathy in adults.

Mnemonic for rickets signs — "Ricket's CHILD": Craniotabes, Harrison's sulcus, Irritability/hypotonia, Legs bowed (genu varum), Dental delay + rosary (the "rosary beads" round the neck of the Diagnosis).

Investigations & biochemistry

The biochemical pattern is the single most testable thing. Compare the major patterns:

Condition	Serum Ca	Serum PO₄	ALP	PTH	25(OH)D	1,25(OH)₂D
Vitamin D deficiency	↓ or low-N	↓	↑	↑	↓↓	↓ or N
XLH (FGF23 driven)	Normal	↓↓	↑	Normal	Normal	Low/inappropriately N
VDDR type 1	↓	↓	↑	↑	Normal	↓
VDDR type 2	↓	↓	↑	↑	Normal	↑ (resistance)
Renal osteodystrophy (CKD)	↓	↑	↑	↑↑	variable	↓
Osteoporosis	N	N	N	N	N	N
Hypophosphatasia	N/↑	N	↓	N	N	N

High-yield: The classic vitamin D-deficiency osteomalacia/rickets profile = low calcium, low phosphate, HIGH alkaline phosphatase, HIGH PTH. The two great exceptions to remember: renal osteodystrophy has HIGH phosphate, and hypophosphatasia has LOW ALP.

Radiological features

Rickets (look at the growth plate / metaphysis):

• Widening of the growth plate (physis).
• Cupping, splaying and fraying of the metaphysis.
• Loss of the zone of provisional calcification; indistinct, frayed metaphyseal margin.
• Bowing of long bones; generalised osteopenia.
• Best films: wrist (distal radius/ulna) and knee.

Osteomalacia (adults):

• Looser's zones (Milkman's pseudofractures / Looser–Milkman lines): transverse lucent bands perpendicular to the cortex, often bilateral and symmetrical, classically at the medial femoral neck, pubic rami, scapular margin, ribs, and proximal ulna. These are unmineralised osteoid, the radiological hallmark of osteomalacia.
• Generalised reduction in bone density, biconcave ("codfish") vertebrae, triradiate pelvis.

High-yield: Looser's zones (pseudofractures) = pathognomonic radiological sign of osteomalacia in adults. They are NOT true fractures — they are bands of unmineralised osteoid, typically symmetric, perpendicular to the cortex.

Investigation of choice / confirmation:

• Serum 25(OH)D to confirm vitamin D deficiency (status marker).
• Bone biopsy with tetracycline double labelling is the gold standard for definitive diagnosis of osteomalacia (shows wide osteoid seams, prolonged mineralisation lag time) — rarely needed clinically but a favourite exam answer.

Diagnostic flow: Clinical suspicion → bone pain/deformity + biochemistry (↓Ca, ↓PO₄, ↑ALP, ↑PTH) → serum 25(OH)D (confirms deficiency) → X-ray wrist/knee (rickets) or long bones/pelvis (Looser's zones) → if biochemistry atypical (normal Ca, isolated ↓PO₄) → measure FGF23 / urinary phosphate for hypophosphataemic causes → bone biopsy with tetracycline labelling if doubt remains.

Management & drug of choice

Nutritional vitamin D-deficiency rickets/osteomalacia

• Vitamin D (cholecalciferol/ergocalciferol) + calcium supplementation is the mainstay.
• Common regimens: oral vitamin D₃ (e.g. 60,000 IU weekly in deficiency, or stoss therapy as a single large dose) followed by maintenance, with elemental calcium.
• Correct the underlying cause (treat malabsorption, sunlight exposure).
• Expect ALP to normalise and radiological healing over weeks–months; the first lab to respond is a rise in phosphate; the last to normalise is ALP.

Specific forms

• VDDR type 1 (1α-hydroxylase deficiency): give the active hormone calcitriol (1,25(OH)₂D) — bypasses the missing enzyme. Drug of choice.
• VDDR type 2 (receptor resistance): high-dose calcitriol + high-dose calcium (often IV/oral); variable response.
• X-linked hypophosphataemic rickets: oral phosphate (in divided doses) + calcitriol. Newer targeted therapy = burosumab, an anti-FGF23 monoclonal antibody.
• Renal osteodystrophy (CKD-MBD): phosphate binders, active vitamin D analogues (calcitriol/alfacalcidol), dietary phosphate restriction, calcimimetics (cinacalcet) for secondary hyperparathyroidism; parathyroidectomy for refractory tertiary hyperparathyroidism.
• Anticonvulsant-induced: supplemental vitamin D.

High-yield: In renal failure the kidney cannot 1α-hydroxylate, so give the already-active calcitriol (or alfacalcidol), NOT plain cholecalciferol. Similarly, VDDR-1 responds to calcitriol because the defect is the renal enzyme.

Orthopaedic management of deformities

• Most childhood deformities remodel/correct spontaneously once the metabolic cause is treated, especially in younger children.
• Persistent significant deformity after biochemical correction → corrective osteotomy (guided growth/hemiepiphysiodesis or osteotomy). Never operate on uncorrected (active) rickets — first heal the bone biochemically.

Complications

• Permanent skeletal deformities (bow legs, knock knees, short stature).
• Pathological/fragility fractures and pseudofractures.
• Hypocalcaemic tetany, laryngospasm, seizures, and (in infants) hypocalcaemic cardiomyopathy/dilated cardiomyopathy.
• Dental enamel hypoplasia, delayed eruption, caries.
• Secondary and eventually tertiary hyperparathyroidism (in CKD).
• Respiratory compromise from severe chest deformity; basilar invagination from soft skull base.

Key differentials

Differential	Distinguishing point
Osteoporosis	Normal Ca/PO₄/ALP/PTH; quantitative bone loss; no Looser's zones
Osteogenesis imperfecta	Blue sclerae, recurrent fractures, normal biochemistry, family history
Achondroplasia / skeletal dysplasias	Disproportionate short stature, normal metabolic labs
Blount's disease	Pathological tibia vara from medial proximal tibial physis; unilateral or asymmetric, normal biochemistry
Physiological genu varum/valgum	Self-resolving with growth; normal labs and X-rays
Scurvy (vit C deficiency)	Subperiosteal haemorrhage, "white line of Frankel", Wimberger's ring sign; different metaphyseal picture
Hypophosphatasia	Low ALP (unique), elevated urinary phosphoethanolamine
Paget's disease (in adults)	Very high isolated ALP, normal Ca/PO₄, characteristic bony enlargement/lytic-sclerotic X-ray

High-yield: Bow legs (genu varum) — the two great exam differentials are nutritional rickets (bilateral, symmetric, deranged labs) versus Blount's disease (often asymmetric, normal labs, beak-like medial metaphyseal deformity).

Recently asked / exam angle

• "Best indicator of vitamin D status" → serum 25(OH)D (not 1,25). Repeatedly asked.
• "Commonest inherited rickets" → X-linked hypophosphataemic rickets; mediator = FGF23; treatment = phosphate + calcitriol, newest drug burosumab.
• Pseudofractures (Looser's zones) identified on X-ray → answer osteomalacia; know typical sites (femoral neck, pubic rami, scapula, ribs).
• Lab pattern matching: low Ca, low PO₄, high ALP, high PTH → vitamin D-deficient osteomalacia; high PO₄ version → renal osteodystrophy; low ALP version → hypophosphatasia.
• Rachitic rosary vs scorbutic rosary distinction (rickets = costochondral beading from widened cartilage; scurvy = sharp, angular "scorbutic" bead).
• Gold standard for diagnosis of osteomalacia → bone biopsy with tetracycline double labelling.
• VDDR type 2 with alopecia and high calcitriol (receptor resistance) is a classic single-best-answer image/vignette.
• Which form responds to calcitriol but not to cholecalciferol → renal osteodystrophy and VDDR-1.
• Radiograph site of choice in rickets → wrist (distal radius/ulna) showing cupping/fraying/splaying.

Rapid revision

1. Rickets = mineralisation defect before physeal closure (affects growth plate + bone); osteomalacia = same defect after closure (osteoid only).
2. Commonest cause overall = vitamin D deficiency; commonest inherited = X-linked hypophosphataemic rickets (↑FGF23).
3. Classic biochemistry: ↓Ca, ↓PO₄, ↑ALP, ↑PTH; vitamin D status = serum 25(OH)D.
4. Renal osteodystrophy is the exception with high phosphate; hypophosphatasia is the exception with low ALP.
5. Rachitic rosary + Harrison's sulcus + genu varum = childhood signs; Looser's zones + proximal myopathy = adult signs.
6. Looser's zones (Milkman pseudofractures) = radiological hallmark of osteomalacia — symmetric, perpendicular to cortex, femoral neck/pubic rami/scapula.
7. Rickets X-ray = widening, cupping, fraying, splaying of metaphysis; best film = wrist/knee.
8. Calcitriol (active form) is made by renal 1α-hydroxylase (stimulated by PTH, low PO₄/Ca).
9. Treat: vitamin D + calcium for nutritional disease; calcitriol for VDDR-1 and CKD; phosphate + calcitriol (± burosumab) for XLH.
10. First lab to recover with treatment = phosphate rises; last to normalise = ALP.
11. Gold standard diagnosis = bone biopsy with tetracycline double labelling.
12. Correct biochemistry before any osteotomy; most childhood deformities remodel after metabolic cure.