Metabolic Bone Disease
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Direct answer
Osteoporosis is too little bone of normal quality — the mineral-to-matrix ratio is preserved, and dual-energy X-ray absorptiometry diagnoses it at a T-score of −2.5 or below — whereas osteomalacia and rickets are defective mineralisation of otherwise normal osteoid, from vitamin D deficiency in most Indian cases, producing Looser zones in adults and rachitic deformities in children. Renal osteodystrophy layers secondary hyperparathyroidism, osteomalacia and osteitis fibrosa onto chronic kidney disease, and Paget disease rewrites bone as a mosaic of disorganised lamellae with a grossly raised alkaline phosphatase and a small risk of sarcomatous change.
What you must remember
- Osteoporosis types: type I postmenopausal — oestrogen-deficient trabecular loss, hence distal radius (Colles) and vertebral crush fractures; type II senile — cortical and trabecular, hence hip fracture; Indian women combine lower peak bone mass with widespread vitamin D deficiency, fragility presenting at relatively younger ages.
- Diagnosis and definition: densitometric T-score −2.5 or below at spine or hip; a fragility fracture (from a fall at standing height or lower) is clinically osteoporosis whatever the score; calcium and phosphate are normal, which separates it biochemically from osteomalacia.
- Osteomalacia and rickets: unmineralised osteoid seams; Looser zones — pseudofractures at the scapula, femoral neck and pubic rami; childhood rickets gives rachitic rosary, Harrison sulcus, bowing, widened wrists, and cupped and frayed metaphyses; Indian infant-weaning patterns and low dietary calcium keep both prevalent.
- Secondary osteoporosis screen: corticosteroids, thyrotoxicosis, hyperparathyroidism, hypogonadism, myeloma — an elderly fracture with anaemia and a high erythrocyte sedimentation rate is myeloma until excluded.
- Renal osteodystrophy: chronic kidney disease retains phosphate, starves calcitriol, raises parathyroid hormone — the skeleton then shows osteitis fibrosa cystica with brown tumours, osteomalacia, and in over-suppressed patients adynamic bone disease.
- Paget disease (osteitis deformans): frenzied osteoclasts then disordered osteoblasts produce thickened mosaic bone with cement lines; skull thickening with increasing hat size and eighth nerve deafness, sabre tibia, kyphosis; alkaline phosphatase markedly raised with normal calcium and phosphate; about 1 per cent lifetime risk of osteosarcoma.
- Osteopetrosis: the mirror-image disease — osteoclast failure leaves marble-dense bone that crowds the marrow into pancytopenia and compresses cranial nerves; Erlenmeyer-flask deformities at the femora.
One X-ray panel, three diseases
Read the films as a set, because the biochemistry dictates the radiology. The first patient, a 62-year-old woman with sudden severe back pain, has wedge compression of a mid-thoracic vertebra with thinned cortices and washed-out trabeculae, and her bloods are unremarkable — osteoporosis, where the problem is quantity; the missing bone cannot be seen on biochemistry because what remains is normally mineralised, hence densitometry rather than blood tests makes the diagnosis. The second panel shows a femoral neck with a linear lucency running perpendicular to the cortex — a Looser zone, the painless-but-limiting pseudofracture of osteomalacia — and the biochemistry completes the picture: low vitamin D, raised alkaline phosphatase, low or normal calcium, phosphate low.
The third patient, on dialysis for eight years, brings the composite: brown tumours in the jaw, subperiosteal resorption in the phalanges, and rickets-like changes — renal osteodystrophy, in which the kidney's failure to excrete phosphate and make calcitriol has driven the parathyroids into secondary hyperplasia and the skeleton into high-turnover resorption, with unmineralised osteoid from the same calcitriol lack. The Paget film closes the round: a thickened pelvis with coarsened, Cotton-wool trabeculae and a skull vault approaching 2 centimetres, alkaline phosphatase a multiple of normal — a disease of disorganised remodelling rather than of quantity or mineral, and the one on this panel that can turn malignant.
Where students slip
Every collapsed vertebra in an elderly Indian patient is osteoporosis in the student's answer, while myeloma waits undiagnosed; protein electrophoresis is the cheap step that separates them. The osteoporosis-osteomalacia pair is confused at the level of the basic concept: quantity against mineralisation, and writing "decalcified bone" for osteoporosis exposes it. The Indian paradox is also viva-worthy: abundant sunshine, yet widespread deficiency from skin pigmentation, indoor living, covered clothing and low dietary calcium — a framing that national examinations reward.
Frequently asked questions
What T-score defines osteoporosis?
Minus 2.5 standard deviations or below on dual-energy X-ray absorptiometry at the lumbar spine or hip; a fragility fracture qualifies clinically regardless of score.
What are Looser zones and where do they occur?
Pseudofractures of osteomalacia — symmetric radiolucent lines through the cortex, classically of the scapula, femoral neck and pubic rami.
Which biochemical pattern fits Paget disease?
Markedly raised alkaline phosphatase with normal calcium and phosphate, from disordered osteoblastic activity laying mosaic bone.
Which X-ray sign is specific to hyperparathyroidism?
Subperiosteal resorption of the phalanges, with brown tumours and salt-and-pepper skull completing the hyperparathyroid skeleton.
Why does osteopetrosis cause pancytopenia?
Osteoclast failure leaves marble-dense bone that obliterates the medullary cavity, crowding out haematopoiesis; extramedullary haematopoiesis and cranial nerve compression follow.
What distinguishes osteoporosis from osteomalacia conceptually?
Osteoporosis is reduced bone mass with normal mineralisation of the matrix present; osteomalacia is defective mineralisation of normal osteoid — hence normal biochemistry in the former, abnormal in the latter.