Deformation of implant retaining screws-Study with stereoscopic microscopy and microCT

The prosthetic screw fixes the prostheses to the implants. Upon osteointegration, the untightening of the prosthetic screw is the most common problem in oral rehabilitation with implants.

To study the deformation of the implant retaining hexagonal screw head.

This investigation used two methods for evaluating the screw head's area of deformation (mm² ): a stereoscopic microscopy and micro computed tomography (microCT). For stereoscopic microscopy, 16 titanium alloy (T) and 16 titanium gold-plated alloy (G) screws of the Zimmer Biomet™ brand were used, divided into eight groups: group 0 (control group), groups T1 and G1 (screws tightened 10 times to 20 Ncm), the groups T2 and G2 (screws tightened 20 times to 20 Ncm) and the groups T3 and G3 (screws tightened 10 times to 30 Ncm). In the study with microCT, one screw was randomly chosen from each of the groups described above to perform the scanning by microCT.

When comparing the type of screw material using stereoscopic microscopy, no statistically significant differences were found (p > 0.05). Contrarily, different number of successive grips and different torque value showed statistically significant differences in the head section of the retaining screws (p < 0.05). The observation by microCT showed the torque applied is crucial to the head deformation in titanium screws. In gold-plated screws the successive tightening appears to be pivotal.

Titanium and gold screws tend to behave similarly. By increasing the tightening cycles and the torque values of protocols greater levels of deformations can be expected. In general, microCT data showed better results for gold-plated titanium alloy.

To control severe screw head deformation and the impossibility of untightening the implant's restoration, clinicians should avoid extreme torque values and prevent surpassing 10 tightening cycles.