Biomechanics of orthopaedic screws, are often confusing. They are simple but the way question is set, confuses a lot of us.
Here we present these multiple choice questions (MCQs)
For a maximum pull out strength, a screw should have
a. Maximum outer diameter, small root diameter
b. Small outer and large root diameters
c. Large outer diameter, independent of root diameters
d. Large outer diameter, independent of the root diameters
Okay, root diameter is the narrowest diameter of the screw across the base of the threads. The smaller the root diameter, the greater tendency to shear off, during insertion, removal or loading. Torque during insertion is directly proportional to the cube to the inner diameter.
On the other hand, outer diameter is the diameter across the maximum thread width. The larger it is, the greater is the holding power. Hence, there is greater resistance to pull out. This also is advantageous for the cancellous screw for more resistance to pull out.
Reference: AJ Thakur, Elements of Fracture fixation.
Hence, the answer to this question is option a, maximum outer diameter, small root diameter.
J.D. Thompson et.al. quote that all long thread screws have significantly greater holding power than all short thread screws.
Question 2 about biomechanics of orthopaedic screws
Which of the following statements regarding polymethylmethacrylate (PMMA) bone cement is incorrect?
a. It is strongest in compression.
b. It has poor tensile strength.
c. It exhibits a high Young’s modulus.
d. It exhibits viscoelastic properties.
As you may know that, bone cement has two components. Compression is a virtue of bone cement. However, it has poor tensile and shear strength. There are various generations of cementing techniques. To grade, the modulus of elasticity (E) of bone cement is lower than the cortical bone.
Hence the answer is c, it exhibits a high Young’s modulus.
The pull-out strength of a cortical screw can increase by
a. Changing to a larger core diameter.
b. Changing to a smaller outer diameter.
c. Changing to a finer pitch.
d. Adding a locking thread to the head.
This question is similar to the first one. Pull-out strength is a parameter depicting screw fixation in orthopaedic implants.
The options here are different than earlier. For better pull out strength, screws must have larger outer diameter and small root diameter. Pitch of a screw is the distance between two adjacent threads.
Cortical screws have finer pitch while cancellous have coarse one. A.J. Thakur quotes, fine-pitched screw moves a smaller distance linearly for a given angular rotation and offers greater mechanical advantage. Hence, it has greater leverage than its counter-part.
In an article by , you can find that, pullout strength is significantly greater for screws with a tapered diameter than a constant diameter.
It is also greater for screws with a constant pitch than for a variable pitch.
Hence, the answer in this case is option c; changing to a finer pitch.