1. Fracture Mechanisms (Long Bones)
Direct and Indirect
Direct
– Magnitude and area distribution of the force
– Rate at which force acts on the bone
6. Fracture
3 phases
– Inflammation
– Reparation
– Remodeling
Endosteal and Periosteal callus formation which is
replaced by haversian (secondary osteon) remodeling
– Suppressed by rigid fixation
– Excessive motion is also harmful
7. Fracture Fixation Devices
In vitro studies to determine
structural rigidity
– Cadaver
– Synthetic bone
Apply axial, bending, torsion to
determine load-formation or torque
deformation curves
– Axial stiffness
– Bending stiffness
– Torsional stiffness
8. External Fixation
Low axial stiffness
Success is dependent on type of fracture
Mode of reduction
– Whether or not you have bones in contact
– Maximum of about 1mm between them for
successful remodeling
9. External Fixation
– Increased pin diameter
– Increased pin number
– Decreased side-bar separation
– Decreased pin separation
– Increased pin group separation
Think about as an engineering how you would
change the external fixation to make it more stiff, but
not too stiff and ugly to wear or open too much to
infection, (pin wear, stress concentrations)
– Pin geometry and thread design
– Bone thread preparation
– Pin insertion technique
– Pin-bone stress
10. Rigid Compression Plating
Definition:
– Bringing entire fracture surfaces into contact
– Compressing the fracture surfaces
Developed to prevent micromotion (too much
micromotion causes fracture end resorption)
Implementation
– Pre-load between 2 joined bones
– Friction between bone and plate
Disadvantage?
11. Rigid Compression Plating
Disadvantage:
– “stress shielding”-no loading of bone so when
remove the plate, bone refractures- also called
postunion osteopenia.
How can we overcome this disadvantage?
– Optimize plate removal
– Biologically degradable material internally
– Reduce rigidity
13. Experimental Studies
Canine tibial shaft osteotomy
Plate vs. Intramedullary
– Rod-fixed osteotomies healed by periosteal callus
– Plate-fixed healed by endosteal callus
– No differences in final bone porosity
– Differences in torsional stiffness disappeared after 4 months
– Time to normal stiffness and strength was same
14. Experimental Comparisons
Plate versus Ext. Fixation
– Plate more rigid and bone had increased stiffness and max.
torque
– Increased porosity with ext. fixation (increased resorption)
– Same biological union after 4 months
15. Experimental Comparison
External Fixation
– 4 versus 6 pins increased periosteal callus, but also increased
porosity
– Static compression increased rigidity of fixation, but did not
change bone union at 4 months
– Bilateral stiffer and results in bone with less porosity and
stiffer union
16. What you have gained
Be able to list advantages and disadvantages of
various fixation methods, and how to overcome
the disadvantages with engineering
Be aware and understand why 1 type of fixation
doesn’t work for all types of fractures