The effects of varying pilot hole size on the holding power of miniscrews and microscrews.

Screw failure during insertion is not an uncommon problem, especially with the micro and mini fixation systems. In this study, the holding power of 1-, 1.5-, and 2-mm screws was tested at various simulated bone thicknesses of 1, 2, 3, 4, and 5 mm with an increasing pilot hole size. This was done to ascertain the effects of pilot hole size on screw failure and any subsequent tradeoff with respect to decrease in holding power. For the 1-mm screws, there was no statistically significant decrease in holding power of the screws with increasing pilot hole size that ranged from 70 to 85 percent of the external diameter of the screw. For the 1.5-mm screw, a critical pilot hole size was found to be 1.24 mm (82 percent of the screw's external diameter), with no statistically significant decrease in holding power in the 1-, 2-, and 3-mm thickness blocks. For the 2-mm screws, the critical pilot hole size was found to be 1.65 mm (83 percent of the screw's external diameter). A statistically significant decrease in holding power was observed after this drill hole size in all thicknesses. Commercially available systems use drill bits that represent approximately 70 to 75 percent of the external diameter of the screw. Our results show that the pilot hole size for the 1-, 1.5- and 2-mm screws can be increased to 80 to 85 percent of the screw's external diameter without losing significant holding power.(ABSTRACT TRUNCATED AT 250 WORDS)