OR7.1 | Metabolic Bone Disorders — Glossary

A membrane-bound enzyme produced by osteoblasts during bone matrix synthesis; markedly elevated in Paget's disease (reflecting intense osteoblastic activity) and in osteomalacia/rickets (reflecting excessive osteoblast effort to deposit unmineralised osteoid); normal in osteoporosis.

A class of anti-resorptive drugs (e.g. alendronate, zoledronic acid, risedronate) that inhibit osteoclastic activity by incorporating into bone matrix and blocking the mevalonate pathway in osteoclasts; used as first-line treatment for both osteoporosis and Paget's disease.

The biologically active form of vitamin D — 1,25-dihydroxyvitamin D3, synthesised in the kidney by 1-alpha-hydroxylase from 25-hydroxyvitamin D; it promotes intestinal calcium and phosphate absorption and is the key hormone deficient in vitamin D deficiency osteomalacia and rickets.

The pathognomonic radiological appearance of Paget's disease affecting the skull: a heterogeneous mixture of lytic and sclerotic areas producing a mottled appearance resembling cotton wool, caused by alternating foci of osteoclastic resorption and disorganised osteoblastic new bone.

An early sign of rickets in infants: softening and thinning of the occipital and posterior parietal skull bones producing a ping-pong ball feel on gentle pressure; it results from demineralisation of the rapidly growing infantile skull.

A bone densitometry technique using two X-ray beams of differing energies to measure bone mineral density at the lumbar spine and hip; the gold-standard investigation for diagnosing osteoporosis and monitoring treatment response.

A fracture resulting from a mechanical force insufficient to fracture a healthy bone, such as a fall from standing height or less; its occurrence defines established osteoporosis regardless of the DEXA T-score.

Bilateral valgus (knock knee) deformity in which the knees are angulated inward and the ankles separated; characteristic of rickets in older children, or arising as the genu varum of infancy spontaneously overcorrects during normal developmental catch-up.

Bilateral varus (bow leg) deformity of the knees, characteristic of rickets in infants and toddlers who bear weight during the active phase of disease; the outward bowing results from bone softening and the combined forces of gravity and muscle pull on demineralised metaphyses.

A guided growth surgical technique for angular limb deformity correction in children with adequate remaining growth; a physeal tether placed across the convex physis slows growth on that side, gradually correcting angulation over 12-18 months without formal osteotomy.

A complication of widespread polyostotic Paget's disease in which the markedly vascular pagetic skeleton acts as a large arteriovenous fistula, increasing cardiac output demands; seen in patients with extensive disease, particularly those with pre-existing cardiac dysfunction.

A pathognomonic radiological sign of osteomalacia: a narrow ribbon-like radiolucent band perpendicular to the bone cortex representing an incomplete fracture through unmineralised osteoid, classically bilateral and symmetric at the medial femoral neck, pubic rami, and rib margins.

A bone-forming cell derived from mesenchymal stem cells that synthesises and secretes type I collagen (osteoid) and orchestrates its mineralisation; marked osteoblastic hyperactivity is the source of elevated ALP in osteomalacia and Paget's disease.

A large multinucleated cell derived from the monocyte-macrophage lineage that resorbs bone by secreting acid and proteolytic enzymes into a sealed resorption lacuna; excessive osteoclast activity is central to osteoporosis, Paget's disease, and the secondary hyperparathyroidism of osteomalacia.

A metabolic bone disease in adults characterised by defective mineralisation of osteoid (bone matrix), most commonly caused by vitamin D deficiency, resulting in the accumulation of unmineralised osteoid, soft deformable bones, and pathological fractures.

A systemic skeletal disorder characterised by reduced bone mass and microarchitectural deterioration leading to increased fracture risk, defined by WHO as a DEXA T-score at or below -2.5 standard deviations or by the occurrence of a fragility fracture.

A focal disorder of bone remodelling characterised by disorganised accelerated osteoclastic resorption followed by chaotic osteoblastic bone formation, producing enlarged, structurally weak hypervascular bone; most common in the pelvis, skull, femur, tibia, and lumbar spine.

A radiological sign of Paget's disease affecting a vertebral body: thickening of all four cortical margins with coarsened internal trabeculae, producing a rectangular picture-frame-like density on plain X-ray.

Palpable nodular enlargement at the costochondral junctions producing a row of bony prominences at the anterior chest wall, seen in active rickets; it results from widened irregular cartilage at the costochondral junction due to defective mineralisation.

The paediatric equivalent of osteomalacia — defective mineralisation of the growth plate and metaphyseal bone in a child with open physes, causing widened metaphyses, bone deformities (genu varum/valgum), rachitic rosary, and growth failure; most commonly caused by vitamin D deficiency.

Compensatory excessive secretion of parathyroid hormone (PTH) in response to hypocalcaemia, as occurs in vitamin D deficiency; it raises calcium by stimulating bone resorption and renal phosphate excretion, worsening the hypophosphataemia of osteomalacia.

A statistical measure comparing an individual's bone mineral density (measured by DEXA) to the mean peak bone density of a healthy young adult reference population, expressed in standard deviation units; used to classify osteopenia (-1 to -2.5) and osteoporosis (at or below -2.5).

The most common hereditary form of rickets, caused by loss-of-function mutations in the PHEX gene leading to elevated FGF23 and renal phosphate wasting; unlike nutritional rickets, treatment requires oral phosphate plus calcitriol (not plain vitamin D).