Effect of diameter of the drill hole on torque of screw insertion and pushout strength for headless tapered compression screws in simulated fractures of the lateral condyle of the equine third metacarpal bone.

This research investigates the impact of the diameter of a drilled hole on the torque effectiveness and pushing capacity of a headless tapered compression screw inserted in standard and oversize holes, using equine third metacarpal bone fracture simulations. The main finding is that larger drilled holes ease the process of tapping but this compromises the strength of the screws, so it is advisable to use holes with standard diameters when possible.

Research Methods and Variables Examined

• The researchers conducted the study using six pair of third metacarpal bones from horse cadavers.
• A model of a lateral condylar fracture was created and stabilized using a headless tapered compression screw which was inserted in a standard hole or an oversize hole.
• The variables measured during the site preparation and screw implantation process included drill time, torque, work, and time for drilling, tapping and screw insertion.
• Different variables such as axial load and displacement were measured during the screw pushout process.
• The researchers utilized Wilcoxon tests to scrutinize the effects of the drilled hole size on these variables for screw insertion and pushout.

Main Results

• The study showed that drilling time was 59% longer for larger holes compared to the standard ones.
• For oversize holes, the variables for tapping (maximum torque, total work, positive work, and time) were lowered by 42%, 70%, 73%, and 58% respectively, compared to standard holes.
• For the screw pushout test, larger holes resulted in decreased measurements for all variables (yield load, failure load, failure displacement, and failure energy) by 40%, 40%, 47%, and 71% respectively when compared to the standard hole size.
• In some instances (one standard hole case and two oversize hole cases), the screws could not be fully inserted.

Conclusions

• The results of the study revealed that while enlarging the diameter of a hole does facilitate tapping, it consequently reduces the overall holding strength of the screws.
• As a result, the researchers recommend the use of standard diameter holes for the implantation of variable pitch screws whenever possible.
• Moreover, precautionary measures should be taken during the implantation process to ensure that the screw threads properly align with the tapped bone threads.