Orthopaedics
Metabolic Bone Disorders
Metabolic bone disorders for MBBS and NEET-PG: osteoporosis, osteomalacia, rickets, hyperparathyroidism and Paget disease with their calcium, phosphate and alkaline phosphatase patterns, mapped to NMC code OR7.1.
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Metabolic Bone Disorders
Metabolic bone disorders for MBBS and NEET-PG: osteoporosis, osteomalacia, rickets, hyperparathyroidism and Paget disease with their calcium, phosphate and alkaline phosphatase patterns, mapped to NMC code OR7.1.
This chapter covers the disorders that weaken bone through problems of mass, mineralisation or remodelling. It contrasts osteoporosis, osteomalacia and rickets, hyperparathyroidism and Paget disease, using the pattern of serum calcium, phosphate and alkaline phosphatase to separate them and guide diagnosis.
High-yield: Metabolic Bone Disorders
- Osteoporosis is a reduction in bone mass with normal mineralisation, giving fragile bone and fragility fractures.
- The commonest fragility fracture sites are the vertebrae, the hip and the distal radius.
- Bone mineral density is measured by DEXA, and a T score at or below minus 2.5 defines osteoporosis.
- In osteoporosis the serum calcium, phosphate and alkaline phosphatase are typically normal.
- Osteomalacia is defective mineralisation of bone in adults due to vitamin D deficiency, and rickets is the same process in growing children.
- Osteomalacia shows low or low normal calcium and phosphate with a raised alkaline phosphatase.
- Looser zones, also called pseudofractures, are a characteristic radiological sign of osteomalacia.
- Rickets causes bowing of the legs, widening of the wrists, a rachitic rosary and delayed fontanelle closure.
- Primary hyperparathyroidism raises calcium and lowers phosphate and can cause osteitis fibrosa cystica with brown tumours.
- The mnemonic stones, bones, groans and psychic moans captures the features of hypercalcaemia in hyperparathyroidism.
- Paget disease of bone shows disordered remodelling with a very high alkaline phosphatase but normal calcium and phosphate.
- Paget disease can cause bone pain, deformity, high output cardiac failure and rarely transformation to osteosarcoma.
- Renal osteodystrophy is the bone disease of chronic kidney disease, driven by phosphate retention and reduced active vitamin D.
- Scurvy, from vitamin C deficiency, impairs collagen synthesis and causes subperiosteal haemorrhage and poor bone formation.
Biochemistry pattern by disorder
- **Osteoporosis:** Calcium, phosphate and alkaline phosphatase all normal. T score at or below minus 2.5.
- **Osteomalacia / rickets:** Low or low normal calcium and phosphate, raised alkaline phosphatase, Looser zones.
- **Hyperparathyroidism:** High calcium, low phosphate, raised alkaline phosphatase, osteitis fibrosa cystica.
- **Paget disease:** Normal calcium and phosphate, markedly raised alkaline phosphatase.
NMC competencies in this chapter
- **OR7.1:** Metabolic bone disorders: osteoporosis, osteomalacia, rickets, hyperparathyroidism and Paget disease
Frequently Asked Questions
How is osteoporosis different from osteomalacia?
Osteoporosis is a loss of bone mass with normal mineralisation and normal biochemistry, while osteomalacia is defective mineralisation from vitamin D deficiency with low calcium and phosphate and a raised alkaline phosphatase.
What defines osteoporosis on a DEXA scan?
A T score at or below minus 2.5 standard deviations below the young adult mean defines osteoporosis, while a T score between minus 1 and minus 2.5 is osteopenia.
What are Looser zones?
They are ribbon like areas of unmineralised bone, also called pseudofractures, seen on x-ray in osteomalacia, typically on the concave side of bones.
What is the biochemical clue to Paget disease?
A markedly raised alkaline phosphatase with normal serum calcium and phosphate, reflecting rapid but disordered bone turnover.
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