Chapter 41: Internal Fixation Flashcards
316L stainless steel means what?
Low carbon, 0.03% carbon maximum
Weakest point of the orthopedic wire?
Method by which it is secured (knot)
Name 4 ways to secure/knot an orthopedic wire?
twist
loop
double loop
cable and crimp cerclage
How many twists are necessary for the cerclage knot to be secure?
Only 1 to maintain tension in twist cerclage
(The 1.5 is how many turns of the tensioner in a loop cerclage)
For a single loop, how many twists of the tensioner need to be in the loop?
At least 1.5 twists (of the tensioner (it’s a distance thing))
Twist knot loosens by what?
Loop knot loosens by what?
(Think basic)
Untwisting
Unbending of the folded arm
What method of securing an orthopedic cerclage wire is the best?
Double loop - generates more tension and resists greater load before being classified as loose
Pushing a twist knot over to lie flat can result in what % of resting load drop?
45-90% better to just cut it short
vibration at cutting 10%
Resting tension drops below 30N after a collapse of only 1% of diameter of structure
Name the principles of cerclage wire use?
- at least 2
- oblique fracture
- Fracture length 2.5-3x bone diameter
- Space half a bone diameter apart
- Circumference of bone must be completely restored (column)
Pins and screws AMI?
Radius to the fourth power
Plate AMI?
Plate thickness to third power
IM pins used alone should fill how much of the medullary canal?
70%
2 advantages of not reaming with ILN?
Reduced infection and fat embolisation
Ideal % of medullary canal filling for nail for angle stable ILN?
75%
Incidence of missed distal locking bolt in angle stable ILN?
1% (up to 28% can be missed in normal nails)
Name sections of a screw?
Head, shaft, shank, tip
Screw pull-out is determined by which four factors?
Outer diameter
strength of material
core diameter (bending strength of the screw)
AMI (r4)
Optimal tightness is what % of the stripping torque?
70%
Stripping torque is determined by what?
Thickness and quality of bone engaged and design of screw threads
What is the longitudinal and transverse angulation for screws in an LCP?
40 degrees longitudinal and 7 degrees transverse screw angulation
In an LCP combi hole, a screw in neutral position moves how much?
In compression?
Neutral: 0.1mm
Compression: 1mm
If the opposite cortex can be reconstructed, what is improved?
The AMI of the construct
3 Advantages of scalloping edges?
Improved vascularity, reduced AMI of solid section of plate and there is little stress concentration at the screw holes, a bend can be distributed over the whole length of the plate and not just at the screw holes
- Screw hole is also more open allowing for greater angulation of a screw
Because the tightest screw experiences the largest load, the strength of construct can increased (up to a point) by increasing what?
Screw torque
Plate loosening of a conventional plate will occur if axial load exceeds what?
Frictional force
Failure of locking screws requires failure of large areas of bone through _____ force rather than failure of a single screw by axial pullout?
compressive force - cortical bone is much stronger against compressive loads than shear loads
You can decrease plate strain and likelihood of plastic deformation and plate failure by doing what (4 things)?
Spanning long segments of bone >3 x length of fractured segment
- Limiting screw to hole ratio of <0.5
Limiting distance between bone and plate to 2mm
- Leaving 2-3 empty holes over bone defect
Why does the smaller head of locking screws not matter vs. cortical screws?
Minimal impact on construct strength because axial pull out is less of a concern for locking screws than for cortical screws
What are the polyaxial locking systems?
PAX
PLS
EvoLox
Eickloxx
Taragon ILN should not be used in which cat bone?
Humerus (fissures/fractures most common followed by neurovascular damage)
The star drive of an LCP screw allows for what % greater insertional torque?
65%
Describe a neutralization implant:
Implant used after rebuilding the bone column. Neutralizes the forces on bone. Used in simple fractures or articular where reconstruction is vital. Can damage the blood supply (must be contoured against bone)
Describe a bridging implant:
Attaches to the major portions of the proximal and distal fragments to bear the load. Maintains the fracture hematoma usually.
Describe a buttress implant:
Supports a fracture near a joint.
Fracture planning: What is step 1? Step 2?
Step 1: Can it be reconstructed? (if not, must bridge)
Step 2: Should it be reconstructed? (If not, must bridge)
