Pull-out forces of headless compression screws in variations of synthetic bone models imitating different types of...

Screw osteosynthesis using headless compression screws has become the accepted gold standard for the surgical treatment of scaphoid fractures. Optimal screw specifications remain controversially discussed. We aimed to investigate the influence of bone model composition on screw stability tests using headless compression screws in different scaphoid fracture models. We conducted pull-out tests using Acutrak2^®mini, HCS^®, HKS^®, HBS^®, Herbert/Whipple^® and Twinfix^® screws. To imitate cortical and cancellous bone, two-layer polyurethane (PU) models with two distinct densities were produced. The cylinders were cut at different positions to replicate fracture localisations at increasing distances. The maximum pull-out force required to achieve up to 1 mm of pull-out distance (N_{to 1 mm}) was measured. Acutrak2^®mini and HCS^® followed by Twinfix^® showed the greatest average pull-out forces. N_{to 1 mm} was, on average, greater in the cortico-cancellous model than in the cancellous cylinder with the Acutrak2^®mini and the Herbert/Whipple^® screws, while it was the least with the HBS^® and the Twinfix^® screws; there were also differences between the HCS^® and HKS^®. There were no differences between the different fracture simulations in the synthesis strength using either the HKS^® or HBS^®. The pull-out forces of the HCS^® and Twinfix^® remained high also in simulations with the smaller screw base fragments. Varying imitations of cancellous and cortico-cancellous bone and fracture localisation reveal important information about the ex vivo strength of screw syntheses. The grip of the cortical structure should be used with the screws that fit more firmly in cortico-cancellous bone.