Week 3 Biomechanics of Fracture

Question 1

Why do fractures occur?

Answer 1

When forces applied to the bone exceeds it’s ultimate strength (which is dependent on bone density)

Question 2

What determines the ultimate strength of bone?

Answer 2

1. Metabolic factors
   - Osteomalacia: decrease in Ca in the matrix but matrix amount is normal, e.g. renal osteodystrophy
   - osteoporosis: Ca to matrix ratio is normal but there is a decrease in bone quantity, matrix amount is below normal
2. Physical properties of bone
   - cross sectional diameter, small cross sec diameter with thick walls are stronger
3. Age Effects
   - bone quality decreases with age, thinner cortex but diameter increases in attempt to maintain strength

Question 3

How do fractures occur due to tension and compression?

Answer 3

• bone is weaker in tension than compression and will always fail first under tension (there is always a compressive and tension component in a force applied)
• fractures occur on tension side (which may be opposite side of contact, e.g. getting hit in the leg with a bat)

Question 4

When would a ligament failure occur vs. a fracture?

Answer 4

• dependent on rate of force application
• rapid force application –>fracture
• slow –>ligament failure

Question 5

Which bones are vulnerable to avascular necrosis due to fracture?

Answer 5

1. carpal navicular
2. femoral neck
   - fracture in the neck causes loss of blood supply to head of femur
   - blood is from reticular arteries that travel form intertrochanteric line to femoral head
3. Talus
   - blood supply is from talar head and flows retrograde to talar body
   - fracture of talar neck causes talar body to loose blood supply

Question 6

Describe Wolf’s law during fracture healing.

Answer 6

• Initial bone that forms during fracture is arranged in a disorganized manner-woven bone
• over time, bone becomes organized-lamellar bone- stronger in planes of force application

Question 7

What are factors affecting bone healing?

Answer 7

1. Mechanical
   - primary bone healing: stabilize fracture very rigidly, bone heals by remodeling
   - callous formation: slight motion allowed such as in cast
2. Metabolic: nutrition, smoking, NSAIDS
3. Local biololgy : blood supply

Question 8

What is compartment syndrome (a complication of fracture)?

Answer 8

• Accumulation of fluid in a compartment of fixed volume, causing an increase in pressure in that compartment
• Characterized by 5 Ps: pain, pressure, paralysis, pulselessness, pallor
• neve injury: tibial fractures to common fibular nerve, shoulder fractures to axillary artery, humerus fractures to radial nerve

Question 9

What are vascular injuries that can occur due to fracture?

Answer 9

1. fractures of distal femur-superficial femoral artery

2. knee dislocations-popliteal artery injury

Question 10

What are the goals of fracture treatment?

Answer 10

• achieve union in shortest time
• restore function to pre injury state
• maintain proper alignment for function
• minimize complications
• restore axial alignment
• restore anatomy