If you’re reading this, I love having you here. It’s nice knowing that I have some company in the learning process.
Despite that, today I’m going to torture you with some basic science. After all, is it really Friday if we don’t talk about the bending stiffness of solid vs hollow nails?
What I learned today
First, some anatomy from a reputable scientific source, HomeDepot.com:
In case it’s not clear, pitch = distance between threads.
Also, lead = distance advanced when you turn the screw once (one revolution).
What is the working distance of a screw vs. plate?
Screw: length of bone traversed by the screw
Plate: length between the 2 screws closest to the fracture on each side of the fracture
If you shorten the working distance of a plate, does that increase or decrease the stiffness?
Increases the stiffness
What is the bending stiffness of a screw, plate, solid or hollow nail proportional to?
Screw: proportional to inner (minor) diameter^3
Plate: proportional to thickness^3
Solid nail: proportional to radius^4
Hollow nail: proportional to radius^3 (approximately, according to some fancy math on Orthobullets, but I also found some OTA slides that say radius^4 because that’s technically what you use to calculate it)
How can you increase the pullout strength of a screw?
Increase outer (major) diameter (proportional ^2)
Decrease inner diameter
Increase thread density
Increase thickness of cortex
Use cortex with more density
What’s that? Not enough? Still dying to learn more?
Don’t be ridiculous. Go enjoy your Friday. I’ll see you back on Sunday for some extra long learnings.
Sources
Image: Home Depot Types of Screws
Orthobullets Orthopaedic Implants
Orthopaedic Trauma Association Biomechanics of Fractures and Fixation