"What Reason Weaves, by Passion is Undone" or, Why Pick the Right Screw

Despite seeming obscenely simple, there is a remarkable amount of specificity required in choosing the right bone screws for a case. The variability exists in more than length and thread diameter; the surgeon must chose between entirely or partially threaded, self-tapping or non-self-tapping, solid or cannulated, and cancellous or cortical.

General terms:
Pitch= distance btwn threads
Root= Inner core of screw (Screw minus thread)
Tapping= Cutting grooves in the bone for the threads to insert
       - Decreases torque on the bone, Increases holding strength
Self Tapping= cut their own grooves as they insert

Larger thread diameter vs. Larger Root diameter-
Afraid of screw pulling out (soft bone)? Use wider threads
Afraid of failure d/t fatigue (hard bone)? Use wider root

**LAG SCREWS
GOAL: to transform torque into compression 
Threads should be seated only into the distal fracture part; so the proximal part should have the screws root only. NO threads across the fracture line for optimal compression.... 
UNLESS you want to pre-cut a groove as wide as the screw, which allows you to use a fully threaded screw

Cortical- Narrow thread spacing, narrow thread --> Increase SURFACE AREA of cortical attachment
Cancellous- Wide thread spacing, wide thread --> Increase PURCHASE into cancellous soft bone

Locking Screws: self tapping, give tight fixation to plate
DCS= Dynamic Compressive Screw; think about what youve seen put in an intertroch fx of an elderly lady. With sliding lag screw put through the side plate, there is compression that is CONTROLLED so that the screw isn't pulled up through the osteoporotic femoral head.






References:
Title Quotation- Alexander Pope, The Essay on Man.
Canale and Beaty. Campbell's Operative Orthopaedics. 
Handbook of Fractures