Distribution and amount of stresses caused by insertion or removal of orthodontic miniscrews into the maxillary bone: A finite element analysis.

Objective: Initial stability of miniscrews is an important factor in their success as orthodontic anchorages. One of the factors affecting this stability is the stresses exerted to the bone by the screw. Since the distribution and extent of stresses and strains produced during insertion or removal of miniscrews had not been measured before, this study used finite element analysis (FEA) to measure these parameters in tapered versus cylindrical screws with or without pilot sockets.

Materials And Methods: An FEA model of maxilla, pilot hole, and tapered/cylindrical miniscrews were created from 875 CT scan data. The bone cortex was considered 2mm thick. The cancellous bone was reconstructed below the cortical bone. Miniscrews were modelled on the basis of commercial titanium tapered and cylindrical miniscrews (1.6mm wide, 8mm long). The diameter and length of the guiding hole were considered to be 1.1 and 1.5mm, respectively. The miniscrews were inserted (and removed) between the maxillary second premolar and first molar. Stress/strain produced in the bones or screws were measured.

Results: During screw insertion, in all setups, the highest stress existed within both the bone and screw, when the screw was in the cortical bone; after insertion into the cancellous bone, the stress suddenly dropped. In cylindrical screws, the highest amount of stress was distributed around the neck which was used for screw driving. In tapered screws, the stress was mostly distributed around the front one-third of the screw. During screw removal, the results of four setups were rather similar with stresses concentrated around screw necks, in the depth of the screw hole, and around the bone surface. The greatest bone stress during insertion was caused by the pilot-less tapered screw (10.18MPa) and the lowest stress was exerted by a pilot-less cylindrical screw (0.74MPa).

Conclusion: Most of the stress and strain is tolerated by the cortical bone and not the cancellous one. Using cylindrical miniscrews might be more bone-friendly. However, all cases had stresses below tolerable thresholds, and hence are safe.