Narrow diameter implants (NDI) are used in clinical situations including narrow bony ridges as an alternative to bone augmentation procedures and in sites with reduced interdental gap width.

The available literature describes the use of different types of NDI, but it appears generally accepted that a NDI is one with a diameter of ≤3.5 mm.

Category 1: Implants with a diameter of <2.5 mm (“Mini-implants”)

Category 2: Implants with a diameter of 2.5 mm to <3.3 mm

Category 3: Implants with a diameter of 3.3 mm to 3.5 mm

At the present time, most implants of <2.5 mm diameter are one-piece implants. One-piece implants with a diameter of >3.0 mm are rarely described.

Consensus Statement 1: Mean survival rate of Category 1 implants was 94.5% ± 5%

Mean survival rate of Category 1 implants was 94.5% ± 5% (Range 80%–100%) after observation periods of 12–78 months. The most frequently described applications of these implants were for transitional restorations, overdentures, and single anterior tooth replacement. This statement is based on 20 clinical trials (eight RS, 10 PS, and two RCTs) with 1,220 patients and 5,367 implants. The majority of the included papers exhibited a high risk of bias.

Consensus Statement 2: Mean survival rates of Category 2 implants were 97.3% ± 4%

Mean survival rates of Category 2 implants were 97.3% ± 4% (Range 80.5%–100%) after observation periods of 12–63 months. The most frequently described application was for single anterior tooth replacement. This statement is based on 21 clinical trials (10 RS, 9 PS, and 2 RCTs) with 883 patients and 1,207 implants. The majority of the included papers exhibited a high risk of bias. Compared to SDI, Category 2 NDI exhibit comparable survival rates in meta-analysis ([OR], 1.06; [CI], 0.31–3.61). This statement is based on four clinical trials (2 RS, 1 PS, and 1 RCT). The majority of the included papers exhibited a high risk of bias.

Consensus Statement 3: Mean survival rates of Category 3 implants were 97.7% ± 2%

Mean survival rates of Category 3 implants were 97.7% ± 2% (Range 91%–100%) after observation periods of 12–109 months. The applications of these implants were not always precisely defined, but also included the replacement of posterior teeth in either arch. This statement is based on 35 clinical trials (17 RS, 12 PS, and six RCT) with 3,842 patients and 5,612 implants. The majority of the included papers exhibited a high risk of bias. Compared to SDI, Category 3 NDI exhibit comparable survival rates in meta-analysis ([OR], 1.19; [CI], 0.83–1.70). This statement is based on 10 clinical trials (eight RS, and two RCT). The majority of the included papers exhibited a high risk of bias.

Consensus Statement 4: There is insufficient evidence on the success rates for all NDIs.

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 long-term studies are available, resulting in a high risk of bias.

What are the potential advantages of using NDI?

• NDI should be considered when it is important to ensure maintenance of adequate tooth-implant and implant-implant distances in sites with reduced mesio-distal width.
• The use of NDI can be considered to reduce the need or complexity of lateral bone augmentation procedures to reduce morbidity.
• The use of NDI may allow simultaneous rather than staged bone augmentation procedures.
• The use of NDI may provide increased prosthetic flexibility in certain clinical situations.

What are the potential disadvantages of using NDI?

Biological:

• One-piece NDI with ball attachments might be difficult to manage at the onset of dependency.
• The use of NDI may compromise optimal prosthetic designs allowing the maintenance of peri-implant tissue health.

Mechanical:

• Reducing implant diameter brings an increased risk of implant or component fracture.
• Caution is recommended for the use of NDI in patients with parafunctional habits and malocclusions.

Should NDI be splinted?

• Given the reduced implant strength and bone contact offered by NDI, it may be advisable to use splinted restorations based on the individual clinical situation.

What are the indications for each classification of NDI?

Category 1 implants can be considered for:

• Support of definitive complete mandibular overdentures
• Support of interim prostheses, both fixed and removable

Category 2 implants can be considered for:

• Support of definitive complete mandibular overdentures
• Support of single tooth replacement in the anterior zone with narrow interdental width (maxillary lateral incisors and single mandibular incisors)

Category 3 implants can be considered for:

• Support of definitive complete overdentures
• Support of single tooth replacement in sites with reduced interdental and/or buccal-lingual width
• Support of multiple unit restorations

Personalized informed consent should include the possibility of more technical and biological complications.