Narrow (reduced)-diameter implants (NDI) were categorized as follows for the systematic review by Klein et al:

• Category 1: one-piece, < 3.0 mm (mini-implants)
• Category 2: two-piece, 3.00 to 3.25 mm
• Category 3: two-piece, 3.30 to 3.50 mm

Potential benefits of NDIs may include less invasive surgery and reduced need for bone augmentation. However, these issues have not yet been scientifically assessed. Possible confounding factors of the studies evaluated might be available bone quantity and quality, splinting of the suprastructure, loading forces of the opposing dentition, and the biomechanical role of the implant-abutment connection.

• One-piece titanium mini-implants with a diameter of 1.8 to 2.9 mm demonstrated a mean survival rate of 94.3% (91% to 100%) after a mean follow-up time of 3.9 years (1 to 6 years) for the indications of overdenture treatment in the edentulous mandible (four implants) and for an anterior single tooth maxillary lateral incisor, mandibular incisor).
• Two-piece titanium implants with a diameter of 3.0 to 3.3 mm demonstrated a mean survival rate of 98.5% (94% to 100%) after a mean follow-up time of 2.8 years (1 to 5 years) in only a single-tooth treatment (maxillary lateral incisor, mandibular incisor).
• Two-piece titanium implants with a diameter of 3.3 to 3.5 mm demonstrated a mean survival rate 96.9% (89% to 100%) after a mean follow-up time of 4.1 years (1 to 11 years) for all indications including posterior regions.
• There is insufficient evidence on the success rates for all NDIs. Clinical parameters and treatment protocols are often not sufficiently described and no controlled comparative studies are available, resulting in a high risk of bias.

• NDIs might be indicated in situations with reduced mesiodistal space or reduced ridge width, provided that the general positioning rules are followed.
• NDIs have several indications. However, the risk of biomechanical problems (eg, fracture) after longterm loading and the limited knowledge of their clinical behavior should be taken into account.
• In this respect, implant diameter should be the widest possible in relation to the emergence profile and ridge configuration.
• NDIs should have a length of 10 mm or more.