Bone Response to Damage

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Normal structure

• • Damage to periosteum:
    • Invokes a hyperplastic reaction of the inner layer
    • Is painful
    • Exostoses can remodel or remain
  • Lifting of periosteum causes new bone formation below
  • Circumferential incision (e.g. during fracture)
    • Longitudinal bone growth results
    • May be only on one side where periosteum is damaged
      • Used by surgeons to treat angular limb deformities

Physis (Growth plate)

Growth plate (Image sourced from Bristol Biomed Image Archive with permission)

Growth plate magnified(Image sourced from Bristol Biomed Image Archive with permission)

• Originates from the cartilage model that remains only at the junction of the diaphyseal and epiphyseal centres

• Cartilage of metaphyseal growth plate is divided into: (from right to left on the magnified image)
  • - Resting (reserve) zone
  • - Proliferative zone
  • - Hypertrophic zone

• Site of many congenital or nutritional bone diseases in the growing animal
• Open in neonates and growing animals
  • Chondrocyte proliferation balances cell maturation and death
• Closes and ossifies at maturity
  • Regulated by androgens
• If growth teporarily stops -> layer of bone seals the growth plate -> moves into metaphysis when growth resumes -> forms Harris lines

Bone resorption

• Mediated by two hormones:
  • Parathyroid hormone (PTH)
    • Produced by chief cells in the parathyroid glands in response to decreased serum calcium
    • In response, osteoclasts increase in number and resorb mineralised matrix - increase Ca in blood
  • Calcitonin
    • Produced by C-cells in the thyroid glands in response to increased serum calcium
    • Inhibits osteoclasts

Bone dynamics

• Bone growth and maintenance of normal structure are directly related to mechanical forces
• Mechanical forces generate bioelectrical potentials (piezoelectricity)
  • These potentials strengthen bone
  • Inactivity reduces the potentials -> bone loss

• In neonates:
  • Bone growth predominates
  • Modelling is important
• In adults:
  • Formation of bone is balanced by resorption - remodelling
  • Continues throughout life under the influence of hormones and mechanical pressure
  • Bone resorption may exceed formation in pathological states (hormonal, trauma, nutritional) or in old age and disuse

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