Extra-short (< 7 mm) and extra-narrow diameter (< 3.5 mm) implants: a meta-analytic literature review.

Aim: To review available evidence in scientific literature on oral implants of severely reduced length or diameter.
Materials and Methods: Electronic and hand searches up to May 2017 were performed in order to identify clinical investigations providing implant survival and/or marginal bone resorption data for extra-short implants < 7.0 mm in length and extra-narrow implants < 3.5 mm in diameter (excluding one-piece mini-implants).
Results: A total of 2929 extra-short implants and 3048 extra-narrow diameter implants were investigated in 53 and 29 clinical studies, respectively. Shorter implants between 4.0 mm and 5.4 mm in length showed comparable results to implant lengths of 5.5 mm to 6.5 mm (95.1% vs. 96.4%, P = 0.121) and no difference regarding marginal bone resorption (0.7 mm vs 0.5 mm, P = 0.086). Implant lengths of 5.5 mm to 6.5 mm, however, performed significantly better in the mandible compared with the maxilla (P = 0.010). Smaller diameters between 3.0 mm and 3.25 mm yielded a significantly lower survival rate of 94.3% than wider implants of 3.3 mm to 3.4 mm diameter (97.7%, P < 0.001), while marginal bone resorption did not differ (0.4 mm vs 0.5 mm, P = 0.447).
Conclusions: The results of the present literature review suggest that extra-short and extra-narrow-diameter implants show satisfactory survival rates of around 95% and little marginal bone resorption of around 0.5 mm after a mean follow-up of 3 years. However, implant lengths < 7 mm in the maxilla and < 5.5 mm in the mandible as well as diameters < 3.3 mm may increase early failure rates.