Bone Response to Damage

Microscopic bone (Courtesy of RVC Histology images)

Normal progression is from woven bone to lamellar bone, even in pathology, except for canine craniomandibular osteopathy and hypervitaminosis D, where lamellar bone is replaced by woven bone
Patterns of collagen deposition:
- Woven bone:
  - "Random weave" which is only a normal feature in the foetus
  - Coarse collagen fibres
  - Later removed by osteoclasts and replaced by lamellar bone
  - In adults it is a sign of a pathological condition (e.g. fracture, inflammation, neoplasia)
- Lamellar bone:
  - Orderly layers which are much stronger than woven bone
  - Fine collagen fibres in concentric or parallel laminae
  - Two main types:
    - Compact bone (cortical)
      - Forms 80% of total bone mass
      - Consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate
      - Forms the shell of long bone shafts - contain Haversian systems
    - Cancellous bone (spongy or trabecular)
      - Made up of plates, tubes or bars arranged in lines of stress
      - In vertebrae, flat bones and epiphyses of long bones
      - Contains no Haversian systems
- Laminar bone
  - Formed on periosteal surface of diaphysis
  - Accomodates rapid growth of large dogs and farm animals
  - Plates of woven bone from within the periosteum
  - Concentric plates
  - As it forms, it fuses with the bone surface

Periosteum
- Specialised sheath of connective tissue covering bone except at the articular surfaces
- Loosely attached except at tendon insertions and boney prominences (associated with major blood vessels penetrating bone)
- Histologically:
  - Outer layer - fibrous for support
  - Inner layer - osteogenic
    - Contains osteoblasts and osteoprogenitor stem cells in young animals and in adults with fractures or disease
- Rich supply of nerves and lymph vessels
- 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
Blood vessels
- Nutrient, metaphyseal, periosteal arteries
- Normal flow of blood from medulla to periosteum due to higher pressures in medulla
- Young animals have greater blood supply

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 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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