Background: A bisphosphonate coating improves screw fixation 2 weeks after implantation in cancellous bone. This study on rats examined further development of fixation over time for screws inserted in cancellous and cortical bone.
Methods: SS screws were coated with a multiple layer of fibrinogen. Half of the screws were coated further with bisphosphonates, which were linked to the fibrinogen. The screws were inserted in cancellous and cortical bone in rats. The rats were killed after 5 h, 4 days, 1, 2, 4, 8, and 24 weeks, and fixation was evaluated by pullout test.
Results: There was a gradual increase in pull-out force over time in both cancellous and cortical bone. The bisphosphonate coating improved fixation. Moreover, the difference between the bisphosphonate and control groups increased with time. The pull-out force was almost twice that of the controls for screws inserted in cancellous bone at 8 weeks. Energy uptake was increased more than 3-fold.
Discussion: The energy uptake and pull-out force of a screw depends on the bone engaged with the threads. Thus, the presence of bisphosphonates increased the amount or quality of this bone by affecting the resorption/formation in a positive way. The increased effect of the bisphosphonates with time thus suggests that bisphosphonate is retained within the remodeling bone, with a positive effect on its gradual adaptation to the implant.