Successful orthodontic treatment depends on consistent anchorage. In certain situations, maximum anchorage is needed for adequate control of teeth movement. Orthodontic treatment is easily impeded by anchorage loss that occurs with limited intro-oral anchorage or when extra-oral appliances are not readily accepted by the patient. In the 1960s, Brånemark and co-workers introduced the titanium implant to dentistry. Success rates for such implants are more than 90% in the maxilla and mandible. Dental implants have been accepted as a reliable and useful treatment for oral rehabilitation.

Orthodontists have recently become interested in skeletal anchorage using mini-implants. Mini-implant anchorages provide more stable anchorage and improve the quality of orthodontic treatment. They enhance the efficiency of orthodontic procedures and confirm the treatment outcome. In Asia, orthodontists consider mini-implant anchorage an alternative anchorage option in orthodontic treatment. Infrazygomatic mini-implants are a new form of skeletal anchorage in orthodontic treatment that involve the use of a small, 2-mm mini-implant instead of flap surgery. No data have been reported concerning the resistance strength of infrazygomatic mini-implants. This study aimed to determine the adequacy of the anchorage strength of infrazygomatic mini-implants, in vertical and horizontal directions.

Materials and methods

The authors evaluated 30 infrazygomatic mini-implants ( Fig. 1 ) from the following three brands: AbsoAnchor ^® (Dentos Inc., Taegu, Korea), Bioray ^® (Bio-Ray Biotech Corp, Taipei, Taiwan), and Lomas ^® (Mondeal, Tuttlingen, Germany). Ten mini-implants per brand were equally divided for testing vertical and horizontal resistance strengths. The length of the mini-implants (2 mm diameter) was either 12 mm (AbsoAnchor ^® and Bioray ^® ) or 13 mm (Lomas ^® ) ( Fig. 2 ). AbsoAnchor ^® and Lomas ^® were of the cylindrical type, composed of a parallel thread along the whole length of the thread part. Bioray ^® was a taper-type implant composed of increasing inner and outer diameters at the end of the thread part. Instead of animal bone, artificial bone (Sawbones ^® ; Pacific Research Laboratories, Vashon, WA, USA) was used for the experiments ( Fig. 3 ). To simulate the infrazygomatic crest, a 40-pcf (0.64 g/cm ³ ) cellular rigid polyurethane sheet (cortical bone; 2 mm thickness) was attached to a 20-pcf (0.32 g/cm ³ ) block (cancellous bone; 20 mm thickness) with an acrylate bond (Scotch, 3M). A custom-fabricated clamping apparatus was used to hold the artificial bone in place. The infrazygomatic mini-implants were placed perpendicular to the artificial bone and self-drilled to a depth of 5 mm into the bone. The insertion torque was measured with a digital torque metre (Lutron, Taiwan) ( Fig. 4 ). Vertical and horizontal ( Fig. 5 ) resistance tests were performed using a material testing machine (Lloyd, USA). An orthodontic wire (0.018 in.) was passed through the hole of the implant and tied to the pulling apparatus. Five power chains (Ormco, Glendora, USA) were used to estimate the peak breaking force ( Fig. 6 ). The Kruskal–Wallis test was used to evaluate differences between the implant types. Statistical significance was set at P < 0.05. Pearson’s correlation coefficient was used to predict the relationship between the insertion torque and resistance strength for all the mini-implants.

Bioray ^® (2 × 12 mm) inserted into the right infrazygomatic crest.

From left to right: Bioray ^® (2 × 12 mm), AbsoAnchor ^® (2 × 12 mm), and Lomas ^® (2 × 13 mm) mini-implants.

Lomas ^® (2 × 13 mm) manually driven into Sawbone ^® to a depth of 5 mm.

Digital torque metre (Lutron, Taipei, Taiwan).

The vertical resistance test machine (Lloyd, Pennsylvania, USA). An orthodontic wire (0.018 in.) passed through the hole of the mini-implant and tied to the material testing machine.

Power chain (Ormco, Glendora, USA) breaking test.