Screw Loosening, a Frequent Problem?
Implant screw loosening is a major concern for both dentists and dental technicians. Unfortunately, the literature indicates it is a frequent problem, with loosening rates as high as 12.5%. Today, there are new ways to decrease its frequency.
Forces keeping screws tight include the “friction between the threads, between the head of the screw and the abutment, and between the implant and the abutment. The force that clamps two screw-tightened components together is called the preload and it depends on the composition of the materials, the texture of their surface and their degree of lubrication.”
Some screws have special surface treatments that “reduce the friction coefficient” and increase the preload to keep screws tighter, longer. One example is TorqTite® (Nobel Biocare Holding AG, Balsberg, Kloten, Switzerland), which uses a “diamond-like, carbon” lubricant.
Using the two types of screws described above, Saliba et al, tested the amount of torque required for screw removal that would simulate how and why an entire implant system would loosen, clinically.
Materials and Methods
Testing was performed on 20 Neodent titanium implants (Osteointegráveis, Curitiba, Paraná, Brazil), with 4.1 platforms, similar to the original Branemark design. Their hexagon bases were removed so abutments could be “rotated on the implant platform during the loosening of the fixation screw.”
Ten relied on an abutment held to the implant with non treated M2.0 titanium screws (Neodent). A different set of ten screws were covered with TorqTite® (Nobel Biocare). “The abutment was attached with a screw, first using a hexagonal 1.2 mm digital wrench (Neodent), followed by tightening with a prosthetic ratchet torque wrench (Neodent) to a torque of 32 Ncm.” Then, the screws were unthreaded to record the highest torque values “required to completely loosen the abutment.”
Titanium screws covered with solid lubricants performed better than plain titanium screws in maintaining the prosthetic implant-abutment junction. The results showed that their unscrewing torque value was higher than the torque applied during seating.
Which Screw to Use
Which type of screw, titanium or gold, is another variable worth exploring. In trying to better address loosening, Farina, et al compared gold and titanium screws. Testing for loosening values after simulated mastication included 20 dentures with eight different groups representing passive and vertical misfits with gold or titanium screws. Their results were based on “(1) 6 months of use, torque loosening, re tightening, another 6 months of use, and loosening torque; and (2) 1 year of use followed by loosening torque.”
Their research findings concluded, “After 6 months and another 6 months of clinical use simulation, titanium screws showed higher loosening torque values than did gold screws for the same fit level (P <.05). After 1 year of clinical use simulation, titanium and gold screws in passively fit dentures showed higher loosening torque values than they did in misfit dentures (P <.05). The titanium screws presented a decrease in the loosening torque after 1 year in misfit dentures.”
Above chart and image at top of page:
Biomechanical considerations for the screw of implant prosthesis: A literature review; J Korean Acad Prosthodont. 2010 Jan;48(1):61-68. Korean; Authors, So-Min Im, et al