Review of bone fracture mechanisms Flashcards
What factors determine the location and mode of a fracture?
Geometry and structure of the bone Loading mode (compression, bending, torsion etc) Loading rate
What is the second moment of area?
Measure of resistant a structure is to bending
What does a larger second moment of area mean?
Stronger, stiffer bone
Why do long bones have a high second moment of area?
Much of the bone tissue is distributed at a distance from the neutral axis
What factors affect bone strength and stiffness in torsion?
Cross-sec area
Distribution of bone tissue around a neutral axis
What is the polar moment of area?
Measure of how resitant a structure is to torsion
Why does torsional fracture of the tibia happen more commonly distally?
As although has a smaller cross-sec area more of the bone tissue is distributed away from the neutral axis (higher J)
If the distal tibia is fracture where would the accompanying fibular fracture occur? Why?
Within the proximal third - fibula does not have same geometry as tibia and the fracture will occur at the weakest point of the fibula
Why do supracondylar and tibial plateau fractures occur?
Cancellous bone is significantly weaker under axial compressive loading and will fail before cortical bone
What type of fracture usually occurs in pure bending in adults? Why?
Transverse - convex side is loaded in tension and concave side in compression
Convex side will fail first as adult bone is weaker in tension than compression
What type of fracture usually occurs in pure bending in children? Why?
Transverse - convex side is loaded in tension and concave side in compression
Concave side will fail first in children as childrens bone is weaker in comrpession than tension
What fracture pattern occurs in pure compression?
Oblique
Describe the fracture pattern if bone is subjected to bending superimposed on axial compression
Bending produces transverse crack on tension side
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Compression results in oblique fracture
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“Butterfly segment” on the compressed side of bone
What type of fracture results from pure torsion?
Spiral - fracture line results from failure of the bone in tension, perpendicular to the crack
What angle is the fracture line at in a spiral fracture caused by pure torsion?
About 45 degrees to the axis about which the torque is applied
What fractures result from pure axial compression? Why?
Fractures close to or within the metaphyses - because cancellous bone is weaker than cortical bone
How does loading rate affect bone strength?
Bone is stronger at a higher loading rate than at a lower loading rate
What type of fracture results when a bone is loaded to failure by impact?
High energy fracture
Describe a high energy fracture
Comminuted with severe soft tissue damage
Summarise the fracture process
Energy delivered to the limb
Energy is transferred via the soft tissue to the bone which absorbs the energy
Bone breaks and energy is released back to the soft tissues
Broken bone and soft tissues bleed - haematoma forms
Acute inflammatory response occurs around damaged area leading to pain and starting processes which lead to healing
