Mechanical Evaluation of drug delivering bone plugs used in combination with bone screws

After a fracture, fixing a plate to a broken bone can be problematic if the bone suffers of osteoporosis. In order to find a solution to increase bone density around the plate screws, one solution is to insert a drug-delivering bone plug that inhibits bone resorption in that area. The aim of this project is to evaluate the influence of composite bone plugs on the primary stability of bone screws. In this project, different mechanical pullout tests are run in order to compare the maximum pullout force of different types of screws in different types of composite bones designed for experiments, named sawbones. These tests are run with and without bone plugs, which are tube-shaped polymer that is supposed to contain a bone resorption-inhibiting drug, Zoledronate. However, the focus here is only mechanical testing, so the bone plugs tested for this project did not contain any drug. Among the different tests run the plugs also vary in hydroxyapatite and the polymer microspheres’ size. Results show that screw stability depends mostly on the sawbone type and that the impact of the bone plug exists for some of the sawbone tested. Other bone plug types did not show an impact at all. For instance, an osteoporotic-like sawbone does not show difference with or without the presence of a bone plug, whereas a more porous version of sawbone did show an improved stability through maximum pullout force increase. Other factors, such as the screw type and the drug concentration, have only a minor impact on primary stability. The report also brings up other handling related issues that should be avoided regarding the behavior of a bone plug when inserted in the previously drilled hole.