Can Implants Move in Bone? A Longitudinal In-Vivo Micro CT Analysis of Implants Under Constant Forces in Rat Vertebrae

Objectives: Whereas stationary stability of implants has been postulated for decades, recent studies suggested a phenomenon termed implant migration. This describes a change in position of implants as a reaction to applied forces. The present study aims at employing image registration of in-vivo micro CT scans from different time points, and to assess (i) if migration of continuously loaded implants is possible and (ii) migration correlates with the force magnitude. Material & Methods: Two customized machined implants were placed in the dorsal portion of caudal vertebrae in n=61 rats, and exposed to standardized forces (0.5N, 1.0N, 1.5N) applied through a flat nickel titanium contraction spring, or no forces (control). Micro CT scans were performed at 0, 1, 2, 4, 6, and 8 weeks after surgery. The baseline image was registered with the forthcoming scans. Implant migration was measured as the Euclidean distance between implant tips. Bone remodelling was assessed between the baseline and the forthcoming scans. Results: The findings confirmed a positional change of the implants at 2 and 8 weeks of healing, and a linear association between applied force and velocity of movement (anterior implant: X2=12.12, Df=3, p=0.007, posterior implant: X2=20.35, Df=3, p 0.001). Bone apposition was observed around the implants and accompanied by formation of load-bearing trabeculae and a general cortical thickening close and also distant to the implants. Conclusion: The present analysis confirmed that implants can migrate in bone. The applied forces seemed to stimulate bone thickening, which could explain why implants migrate without affecting stability. Keywords: implant migration, animal study, bone to implant interface