Biomechanical evaluation of a new system to improve screw fixation in osteoporotic bones

In this paper an experimental analysis is undertaken of the affect a new screw-to-bone fixation system has on the stiffness of fixation systems of osteoporotic fractures based on osteosynthesis plates. The proposed system, which we have named the screw locking element (SLE), is made with elements manufactured from a biocompatible polymer material known as polyetheretherketon (PEEK) which act like a lock nut, holding the end of the threaded screw shank after this has passed through both bone corticals. Seventy-two osteoporotic synbone simulated fracture models were instrumented with one of four constructs: locking compression plate with 6 locking screws (LCP), dynamic compression plate with 6 cortical screws (DCP), DCP with 2 SLEs or DCP with 6 SLEs (DCP+6SLEs). Each group of 18 simulated fracture models were further split into 3 subgroups of 6. One subgroup was tested under cyclic cantilever bending, another under cyclic compression and the third under cyclic torsion. Loss of stiffness was determined in each test every 1,000 load cycles, between 0 and 30,000 cycles. Regardless of the load type, it was seen that the DCP system had the highest stiffness loss percentages of all the tested systems. The inclusion of SLEs significantly decreased the stiffness loss of the DCP system. Unlike the cyclic compression loads, where the LCP performed slightly better, on terminating the cantilever bending and torsion load cycles no statistically significant difference was noted (Tukey test, p>0.05) between the percentage stiffness loss of the DCP+6SLEs system and the LCP system. It is concluded that the proposed SLEs enable DCPs to lower the high failure rate that these exhibit in osteoporotic fracture repairs, at significantly lower costs than those resulting from the use of LCPs.