Biomechanics of fractures

Question 1

Bone strength

Answer 1

• Fractures occur when force applied exceeds bone strength

- Bone strength depends on bone density, metabolic, structural aspects of bone, and the modality of the force

Question 2

Metabolic factors of bone strength

Answer 2

• Osteomalacia: disturbance in Ca metabolism (increased parathyroid hormone), leads to decreased Ca in matrix, but normal matrix amount
• Osteoporosis: Ca to matrix relationship is normal, matrix amount is low, leads to a decrease in bone quality

Question 3

Physical properties that determine bone strength

Answer 3

• Cross-sectional diameter: small cross-sectional diameter w/ thick walls are strongest
• Age: over time bones loose wall thickness, and diameter increases (makes bones weaker)

Question 4

Forces applied to bone

Answer 4

• Tension: forces separating bone
• Compression: forces collapsing bone
• Bone is weaker in tension than compression, thus bones will break on the OPPOSITE side of impact (tension force on opposite side, compression force on same side)

Question 5

Rate of force application

Answer 5

• Rapid force: fracture

- Slower (relative) force: ligament failure

Question 6

Locations of fractures

Answer 6

• Can be at diaphysis (middle of bone) or metaphysics (near ends of bone- could effect bone growth)
• Soft tissue integrity: open or closed fracture
• Types of fractures: transverse, oblique, butterfly, spiral, comminuted
• Loss of structural integrity (fractures) lead to muscle losing its resting tension and going into spasm

Question 7

Avascular necrosis

Answer 7

• Carpal navicular
• Femoral neck: retrograde flow required (artery w/ ligament of femoral head not sufficient)
• These fractures are surgical emergencies in young, hip replacement in elderly
• Talus: also retrograde flow required

Question 8

Wolf’s law

Answer 8

• Bone constantly remodels itself to resist forces
• If a constant force is applied to a bone over time, the bone will re-organize to resist the force
• During bone healing, initial bone is woven (disorganized)
• Over time this bone becomes lamellar (organized) bone which is stronger

Question 9

Fatigue failure of bone

Answer 9

• Fatigue failure is constant application of a certain movement leading to failure
• Bone is very resistant to this
• But it can fail from this
• Fatigue failure of bone results in stress fracture

Question 10

Factors affecting bone healing

Answer 10

• Biologic (blood supply, metabolic)
• Mechanical: bolts or casts
• Primary bone healing (bolts): eliminates all motion, inflammation+repair stages skipped
• Callous formation (casts): motion required

Question 11

Compartment syndrome

Answer 11

• Accumulation of fluid in a compartment of fixed volume leads to increase in pressure in that compartment
• 5 Ps: pain, pressure, paralysis, pulselessness, pallor

Question 12

Nerve injuries

Answer 12

• Fibular neck fractures can lead to common fibular nerve injury
• Shoulder (scapula) fractures: injury to the axillary nerve
• Humerus fracture: injury to the radial nerve

Question 13

Vascular injuries

Answer 13

• Fractures of the distal femur: superficial femoral artery

- Knee dislocations: popliteal artery

Question 14

Facture Rx

Answer 14

• Goals: achieve union in short time, restore function, maintain alignment, minimize complications
• Casts: less precise method of maintaining alignment, leads to micro movements which generate callous
• Internal fixation: using implants (screws, nails)