How to Maximize Orthopedic Screw Pullout Strength
Key Takeaway
This topic focuses on How to Maximize Orthopedic Screw Pullout Strength, Screw pullout strength is the axial force required to remove a screw from bone. It is maximized by increasing the contact surface area between screw threads and bone, such as through a larger outer diameter, a smaller core diameter, increased thread density, or more engaged threads. A finer pitch also enhances pull-out resistance.
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Understanding Biomechanical Concepts of Screws in Orthopedics: FRCS (Tr & Orth) Oral Examination Review
Screw Design
Screw parts and functions
EXAMINER: Can you describe the different parts of a screw and their function?
The different parts of a screw include the head, shank, thread geometry, pitch, depth, shape, lead, countersink, runout, and flutes. The head provides a connection for a screwdriver and prevents sinking of the screw into the bone. The shank provides a smooth link between the head and thread. Thread geometry is usually asymmetrical and provides a wide surface for pulling and little frictional resistance on the underside. The pitch is the distance between adjacent screw threads, and the depth determines the amount of contact with bone for resistance to pull-out. The lead is the linear distance traveled by a screw for one complete turn. The countersink is the undersurface of the head, and the flutes provide a route for removal of bone debris.
Candidates should be able to draw and describe the different parts of a screw, as well as their function in surgical use.
Screw Pull-Out Strength
EXAMINER: What do we mean by a screw's pull-out strength?
A screw's pull-out strength refers to the axial force required to remove a screw from bone.
Candidates should understand the basic definition of screw pull-out strength and how it is important in assessing the stability of bone fixation.
Screw Pull-Out Strength and Biomechanics
EXAMINER: How can you maximize pull-out strength?
The pull-out strength of a screw can be increased by increasing the contact surface area between screw threads and bone, either by increasing the outer diameter, decreasing the core diameter, increasing thread density, or increasing the number of threads engaged in the bone cortex. The use of a locking screw can also create a monobloc effect for greater stability. The 'finer' the pitch, the more turns of the spiral thread engage in a given depth of cortex, creating greater resistance to pull-out.
Candidates should understand the biomechanical principles of screw pull-out strength and how it can be optimized through surgical technique and screw design.
Factors Affecting Screw Pull-Out Strength
EXAMINER: What surgeon factors can reduce screw pull-out strength?
Surgeon factors that can reduce screw pull-out strength include making too large a pilot hole, repeated withdrawal and reintroduction of a screw causing damage to the negative threads in the bone tissue, and wobbling of the screwdriver handle during insertion. Poor technique or technical mistakes can also impact screw pull-out strength.
Candidates should understand the factors that can impact screw pull-out strength and how to avoid them.
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