Hypotheses

The usage of larger diameter drills can prevent cement damage during screw insertion. There is only little loss in pull-out resistance using larger drills. A metal rod (diameter: 13 mm) was cemented into a transparent plastic tube (diameter: 25 mm), leaving a homogeneous cement layer of 6mm. Drills of different diameters (4.3 mm, 4.3 mm + tapping, 4.5 mm, 4.8 mm) were used to implant uni- and bicortical locking screws (all 5mm outer and 4.4mm core diameter) into the cement layer. Locking head screws (LHS: Synthes, Switzerland), periprosthetic locking screws (PPLS: Synthes, Switzerland) and NCB mulitidirectional locking screws (NCB: Zimmer, USA) were used.

The onset of cracks was visually monitored during drilling, tapping and screw implantation. Pull-out resistance was measured on each screw. No crack appeared after implantation of any unicortical screw. No cracks appeared after drilling for bicortical screws. Cracks appeared after tapping or inserting bicortical screws (62.5% of the cases). Increasing the drill diameter reduces the risk of cement mantle cracks (to 25%). Bicortical screws had the highest pull-out resistance (median 3015N compared to 1250N for unicortical screws). Screws with a flat tip, smaller flute or double thread showed higher pullout forces.

Unicortical screws can be implanted without damaging the cement. Bicortical screws have higher pull out resistance but bear the risk of cement mantle damage.

For insertion of bicortical screws a 4.5 mm drill should be considered instead the usual 4.3 mm one. New screws should be developed for unicortical fixation of periprosthetic fractures combining favorable design properties. Further studies should follow to investigate crack formation and loosening after cyclic loading.