Fracture Biomechanics Flashcards
What are the 4 principle ‘forces’ that we must counteract in order to facilitate healing?
- bending
- torsional (rotational)
- compressional (axial loading)
- distraction
What were the 4 initial AO principles that were adopted in the 1970s that led to many cases of nonunion and infection?
- anatomical reduction
- stable fixation
- preservation of blood supply
- early active movements
Which of the following statements is FALSE about the AO principles that were adopted in the 1970s?
A. these principles put too much emphasis on mechanics
B. these principles neglected soft tissue structures
C. these principles neglected the bone
D. these principles were the best way to avoid necrosis, non-union, and infection
D. these principles were the way way to avoid necrosis, non-union, and infection
These principles placed too much emphasis on fracture mechanics. This led to impeding the blood supply to the healing bone, and subsequently led to necrosis, non-union, or infection.
What are the biggest disadvantages to ‘Non-Locking’ plates?
Non-locking plates rely on bone-to-plate friction (ie the closer you can get the plate to the bone, the more friction, and therefore the stronger/more stable the plate to bone interface is. This can impede blood supply.
This mechanism is very stable early on, but loosens over time and causes micro-motion and fretting (where 2 metals rub against each other).
This mechanism also requires TWO points of fixation which destroys the blood supply.
In summary – destroys the blood supply
Why are ‘locking’ plates BETTER than ‘non-locking’?
The forces are transmitted through the screw and into the plate. There is space left between the screw/plate and the bone, so preservation of blood supply is achieved.
This technique is also monocortical meaning its less traumatic and more versatile.
Healthier bone quickens healing and decreases chance of infection.
T/F: cerclage wire is never to be used as a principle means of fixation.
true – its ancillary.
T/F: pins can be used as a means of fixation for a torsional fracture
false
why are plates considered the #1 means of fixation?
they neutralize all forces.
T/F: more modernally, we know that perfect anatomic reduction is not a requirement of fixation
true. We just need adeqate alignment of the joint above and below.
the closer the fragments are to one another, the faster the healing.
what aids us in determining the length and size of plate that we need as well as pre-contouring the plate.
pre-planning with digital radiography and bone models
what are the 2 disadvantages of monocortical locking?
- compromise torsional stability (weaker)
- risk of screws pulling out in thin cortices or metaphyseal bone.
however, monocortical is good because it reduces vascular damage and is more versatile (ex. can do double plating and plate-rod combination)
How can you calculate ‘stress’ and how can we alter it for better fixation?
stress = force / unit area
Force is unchangeable (60% weight beared in front limbs, 40% in back)
Unit area can be increased to decrease stress.
(Ex. using a plate PLUS A PIN increases the unit area to decrease the stress)
How can you calculate ‘strain’ and how can we alter it for better fixation?
strain = change in length / orig length
Original length does not change.
Length can be decreased to decrease strain.
Ex. At least 1 point of fixation to be as close to the fracture line as possible to decrease strain
how do we calculate ‘bending moment’ and how do we alter it for better fixation?
bending moment = force x distance
force is the animals body weight
distance is the distance from the force to the implant.
Our goal is to DECREASE bending moment. Bending moment is reduced when force is applied over a smaller distance.
