Vitamin D–Dependent Rickets, Vitamin D–Resistant Rickets and Hypophosphatasia
DNB Orthopaedics • October 2024 • Paper I • 10 Marks
Introduction
Rickets is a disorder of the growing skeleton caused by defective mineralisation of osteoid and growth-plate cartilage before physeal closure. The characteristic abnormalities include widening and disorganisation of the physis, metaphyseal deformity and progressive angular deformity of the limbs.
In an exam question comparing these disorders, the key is to distinguish them by the level of metabolic defect: vitamin D metabolism or action in vitamin D–dependent rickets, renal phosphate wasting in most vitamin D–resistant rickets, and defective alkaline phosphatase activity in hypophosphatasia.
Core distinction
Vitamin D–dependent rickets: defect in vitamin D activation or receptor action.
Vitamin D–resistant rickets: usually renal phosphate wasting, classically X-linked hypophosphataemia.
Hypophosphatasia: deficiency of tissue-nonspecific alkaline phosphatase.
Definition of Rickets
Rickets is defective mineralisation of newly formed osteoid and growth-plate cartilage in a growing child, leading to failure of normal endochondral ossification and characteristic skeletal deformity.
The corresponding disorder after skeletal maturity is termed osteomalacia.
Relevant Classification of Hereditary Rickets
| Disorder | Basic Defect |
|---|---|
| Vitamin D–dependent rickets type 1A | Defective renal 1α-hydroxylase → impaired conversion of 25-OH vitamin D to 1,25-(OH)₂ vitamin D |
| Vitamin D–dependent… |