Bone

Question 1

Synthesis of Vitamin D

Answer 1

• 7-dehydrocholesterol photolysed by UV light to form cholecalciferol (vitamin D3)
• Cholecalciferol hydroxylated (25-hydroxylase) in liver -> 25 hydroxcholecalciferol (calidiol)
• Caldidiol -> 1,25 dihydrozycholecalciferol (calcitriol) in the kidneys (enzyme 1 alpha hydroxylase)

Question 2

Features of Familial Hypocalciuric Hypercalcaemia

Answer 2

• Benign cause of hypercalcaemia
• AD
• Results from inactivating mutation in the calcium sensing receptor (CaSR)
• The inactivating mutation of the CaSR in FHH makes the parathyroid gland less sensitive to calcium
• This means that a higher than normal serum calcium concentration is required to reduce PTH release (set point raised)
• Clinical findings:
  
  • Positive family history, often asymptomatic
  • Hypercalcaemia
  • Hypocalciuria (24 hour urinary calcium excretion typically <200mg/day)
  • High/normal serum magnesium
  • Normal renal function
  • Normal PTH
  • Normal phosphate

Question 3

What is the mechanism of osteoporosis in long term steroid use

Answer 3

• Increased osteoblast apoptosis

Question 4

Notes on atypical femoral fractures - epidemiology, definition, risks

Answer 4

• Stress fractures originating in lateral shaft of femur, minimal or no trauma
• 1-2/1000 after 6-7 years of continuous bisphosphonate therapy
  
  • Absolute risk low. Asians higher risk
• Distinct pathogenesis:
  
  • Likely a form of stress/insufficiency #
  • Occurs in bones subject to repetitive loading
  • ?Microcracks where tensional stress is high
• Higher risk
  
  • Longer duration treatment
  • Steroid use
  • Relative youth
  • Higher weight, lower weight
• Risk drops rapidly on cessation bisphosphonate

Question 5

Features of atypical femoral fractures

Answer 5

Radiographic - 2 key

1. Fracture line originates at lateral cortex, transverse in orientation and may become oblique as progresses medially
2. Localised periosteal or endosteal thickening/flat cortex at fracture site (beaking or flaring)

Clinical features

• Prodromal pain in 70% (pain then fracture)
• Bilaterality - fractures in 28%, radiological changes present in a higher proportion
• Delayed heaing

Prevention

• Ask about hip and thigh pain in patients on bisphosphonates
• More likely to progress to complete fracture if cortical lucency on xray and thigh pain
• Image CL femur to assess for incomplete atypical femoral fracture