Does the anchorage form and depth influence the pull-out strength of screws from bone cement? An experimental study

The pull-out strengths of cortical screws inserted into soft, unpolymerised Refobacin Palacos bone cement (procedure S) and into hardened polymerised cement into which a hole had been drilled and tapped (procedure P) were compared. Cortical screws 58 mm in length, outer diameter 4.5 mm and inner diameter 2.95 mm were used. Screws were inserted into cement cylinders at 5 mm incremental depths between 10 and 30 mm. At a screw depth of less than 25 mm, the screws pulled out, and at a depth of greater than 25 mm, the screws broke in both procedures. There was no statistically significant difference in pull-out strength leading to burst or break between the two procedures for screws inserted to comparable depths, but there was a statistically significant difference regarding the screwing depth regardless of the procedure of screw insertion chosen. The average material stability (sigma) of the cortical screws used was calculated to be 1191 N/mm2, and the elasticity limit was 5137 N. This study demonstrated that the material stability and not the depth of screw insertion was the limiting parameter in screw anchorage in bone cement while static testing. To avoid screw breakage due to fatigue during continuous alternate loading, the screws should not be loaded above this value.