Review of bone and fracture biology Flashcards
Composition of bone
Structural composite
- 70% organic, 25% inorganic, 5% cells
- 90% organic –> type I collagen
- inorganic component is hydroxyapatite
Collagen is stronger in tension
HA is stronger in compression
Osteoblasts
Mesenchymal cell origins
Express osteoblastic markers (ALP, OC)
Deposit mineralised matrix
Morphology varies from flattened to cuboidal
Mature cells become encased in bone (osteocytes)
Osteoclasts
Formed from circulating mononuclear precursors
Multinucleate cells capable of resorbing bone
Acid phosphatase (TRAP), cathepsin K
Two mechanisms of bone formation
Endochondral ossification - get longer
Intramembranous ossification - get wider
First tissue to be formed is woven bone
Both forms can lead to formation of compact or cancellous bone
Endochondral ossification
For the formation of short and long bones of the appendicular skeleton
Hyaline cartilage model is sequentially resorbed and then replaced with bone
Longitudinal bone growth
Cartilage grows at epiphyseal growth plate
Cartilage replaced by bone
Intramembranous bone formation
The direct conversion of mesenchymal tissue into bone.
Condensation - differentiation - mineralisation
Circumferential bone growth
Appositional bone growth occurs by direct deposition of bone at the periosteal surface
At the same time, bone is resorbed from the inner (endosteal) surface to cause the marrow cavity to increase in size
Gross structure of bone
Compact or cancellous ( sponge like in a hard ‘shell’)
Microscopic structure of bone
Unmineralised (osteoid)
Over time becomes mineralised.
Mineralised bone is either woven (immature), or lamellar bone (mature)
Diaphysis
Shaft of long bone
Epiphysis
End of the long bone, separated by physis
Metaphysis
Region between diaphysis and epithysis
Cancellous bone
Plates of extensively interconnected bone
Increased porosity and increased surface area compared to cortical bone
Increased metabolic activity, so a sentinel for systemic disease
Vascular supply to bones
Nutrient artery - enters at foramen, branches proximal and distal to endosteal surfaces
Metaphyseal vessels - arise from periarticular tissues, primary supply to metaphyseal cancellous bone
Periosteal capillaries - arise from soft tissues and attach to bone
De-vascularising during surgery slows healing
Sequence of bone turnover
Resorption
Osteoblast recruitment
Osteoid recruitment
Osteoid formation
Quiescence
Activation
Osteoclast recruitment
Bone remodelling difference between cortical bone and cancellous bone
Cortical bone - occurs in haversian canal
Cancellous bone - occurs on the trabecular surface
Bone turnover vs age
Skeletally immature animal: formation»resorption
Both longitudinal and appositional growth
Skeletally mature: formation = resorption
Endocrine changes can cause bone loss - earlier in females
What drives bone remodelling
mechanical loading
trauma
microfracture
systemic hormones
Wolff’s law
Bone will align along the line(s) of stress
Stages of fracture healing
Inflammatory, repair, remodelling
Mechanical stability
Optimal - plated osteotomy
Good - external fixator
Acceptable - IM pin, cast
Primary bone healing
Abscence of callus
Contact healing (full apposition with plated repairs) or gap healing (apposed within 1mm)
Secondary healing
Fragments are not close and so heal with a callus
Not necessarily worse - can lead to a stronger repair
Downsides of rigid fixation
Plates and screws decrease cortical vascularity
Plates and screws may need to be removed
Plates affect load transer through bone (‘stress shielding’)
Remodelling can take much longer
Can act as an opening for infection