Klein et al., in a recent systematic review, reported that the survival rate of implants with a diameter of < 3 mm was higher than 90% with a follow-up time between 1 and 3 years [3]. In another meta-analysis by Ortega-Oller et al., the majority of the analyzed studies (implants less than 3.3 mm in diameter) have also reported a survival/success rate higher than 90% [11]. However, the results of the meta-analysis have shown higher failure rates for implants with a diameter of < 3.3 mm when compared with implants with a diameter of ≥ 3.3 mm. The authors have related this outcome with the fact that NDIs are usually placed in complicated clinical scenario, and they have a higher possibility of fracture.

 

On the one hand, due to the small sample size of this study and moreover, the short follow-up (only 1 year after loading), it would be hazardous to conclude that the placement of NDIs to support fixed prostheses in posterior mandible is a predictable treatment modality. In order to draw more reliable conclusions, we need to wait for longer follow-ups, since it may be possible that after several years of function, NDI implants might start to fail due to the reduced available bone-implant contact area or to reduce resistance to fatigue. The placement in the posterior mandible of 2.75 mm diameter implants, as well as 3.25 mm ones, must always be splinted with a bridge, placing one implant for each missing tooth. The placement of a NDI implant in a single molar crown is not recommended. Splinting multiple implants has been reported to minimize the lateral force on the prosthesis, to enhance force distribution, and to reduce the stress on the implants [10]. Thus, splinting of NDI implants would protect the implants from excessive loading and prevent implant/abutment screw fracture. Necessary measures should be taken to minimize off-axis forces like reduction in occlusal table and cusp inclines.

The main limitation of the present study is the small sample size. In addition, a 1-year follow-up is too short to make definitive statements on the predictability of the treatment option tested. Longer follow-up periods and larger sample size are needed, and this trial is currently ongoing.

Within the limits of this prospective cohort study, narrow-diameter implants (2.75 to 3.25 mm) can be successfully used as a minimally invasive alternative to horizontal bone augmentation in posterior mandible up to 1 year of function. This outcome could be related to the fact that these implants have been all splinted to other implants by a fixed prosthesis. These preliminary results must be confirmed by larger and longer follow-ups of 5 years or more.