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** Fibrous replacement
 
** Fibrous replacement
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==From musculoskeletal==
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*Can arise in a number of ways but single common factor is elevated PTH
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*Results in increased resorption of bone and replacement by fibrous connective tissue
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=====<u>'''Primary hyperparathyroidism'''</u>=====
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*This is increased production of PTH not related to calcium or phosphorus levels
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*Due to parathyroid neoplasia or bilateral idiopathic parathyroid hyperplasia
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*Rare
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=====<u>'''Secondary hyperparathyroidism'''</u>=====
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*Regardless of pathogenesis, the result is:
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**Increased osteoclastic resorption of bone and deposition of fibro-osteoid matrix that fails to mineralise
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**Flat bones of the skull swell, including maxillary and nasal bones
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**Long bones become soft with thin cortices which fracture easily
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[[Image:Renal osteodystrophy.jpg|right|thumb|100px|<small><center>Renal osteodystrophy (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
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*'''Renal hyperparathyroidism'''
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**Pathogenesis:
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***[[Kidney Renal Failure - Pathology#Chronic|Chronic renal failure]]
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****-> Retention of phosphate (due to reduced glomerular filtration) and inadequate production of vitamin D by kidneys
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*****-> [[Kidney Renal Failure - Pathology#Uraemia|Hyperphosphataemia]] and hypocalcaemia (high P depresses Ca)
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******-> Increased PTH output
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*******-> Increased bone resorption
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********-> '''Fibrous osteodystrophy''' - increased osteoclastic resorption of cancellous and cortical bone + proliferation of fibrous tissue
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**Mainly in dogs
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**Affects whole skeleton but mainly skull
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**Bones soft and pliable
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**Canine teeth easily removed - rubber jaw
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**Microscopically - ''Osteodystrophia fibrosa'' (above  = fibrous osteodystrophy) +/- [[Bones Metabolic - Pathology#Osteomalacia|osteomalacia]]
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*'''Nutritional hyperparathyroidism''' (nutritional osteodystrophy)
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**Also called '''fibrous osteodystrophy, “rubber jaw”''' or '''“bran disease”'''
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**More common in young, fast-growing animals
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**Pathogenesis:
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***Low calcium / high phosphate diets
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****-> Decreased calcium levels in serum
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*****-> Parathyroid gland stimulated (may become enlarged)
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******-> Increased PTH
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*******-> Increased bone resorption
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**Caused by poor diet
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***Cattle and sheep - usually mild disease
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***'''Swine''' fed un-supplemented cereal grain, usually mild disease
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***'''Dogs/cats''' fed all-meat or offal diets (Ca:P often as high as 1:20)
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****Few weeks after weaning
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****Provision of calcium alone correct the problem
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****Very brittle bones -> sponataneous fractures
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****Extreme porosity of the whole skeleton on radioghraphs
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***'''Horses''' fed bran
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****Very susceptible to high phosphorus diet
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****Any time after weaning, susceptibility declines after seventh year
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****Early signs:
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*****Mild changes of gait
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*****Stiffness
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*****Transient shifting lameness
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****Advanced signs:
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*****Swelling of mandible and maxilla - 'Big head'
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*****Dyspnoea caused by swelling of nasal and frontal bones
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*****Teeth lost or buried in softened jaw
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*****Fractures from mild trauma
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*****Detached tendons and ligaments
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*****Histologically:
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******Marked loss of bone
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******Replacement by proliferative tissue
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****Often called '''''Osteodystrophia fibrosa'''''
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[[Category:Bones - Metabolic Pathology]]
    
[[Category:Parathyroid Glands - Pathology]]
 
[[Category:Parathyroid Glands - Pathology]]
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[[Category:To Do - Clinical]]
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