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[Influence of the interforaminal arch form of edentulous mandibles on design of "All-on-4": preliminary research based on conebeam computed tomography].

OBJECTIVE
To classify the interforaminal arch form of edentulous mandibles by measuring the anterior-posterior distance (A-P distance) of implants planned to be inserted in "All-on-4" protocol using conebeam computed tomography (CBCT) data, and to investigate the influence of the arch form on the "All-on-4" distally tilted implants.


METHODS
Seventy-four CBCT images of edentulous mandibles were collected, including 35 males and 39 females respectively. "All-on-4"implant supported fixed prostheses were designed for these patients based on the CBCT data. The A-P distance was measured in the plane which crossed bilateral mental foramens and was parallel to the occlusal plane. The interforaminal arch form of edentulous mandibles were classified according to the A-P distance. The radian of the jaw arch 7.5 mm mesially to the mental foramen was measured bilaterally, and its correlation with the A-P distance was studied.


RESULTS
The average A-P distance of implant supported fixed prostheses planned in the interforaminal region was (8.5±1.5) mm (minimum 4.5 mm, maximum 11.8 mm). In the study, 12.2% of the subjects' mandibles were classified as square arch form with A-P distances ≤7 mm, 54.0% were classified as ovoid with A-P distances >7 mm and ≤9 mm, 33.8% were classified as tapered with A-P distances >9 mm. Bilaterally, 148 results of the radian of the jaw arch 7.5 mm mesially to the mental foramen were obtained, and the average radian was 15.9°±5.5° (minimum 5.6°, maximum 35.2°). The radian and the A-P distance showed a negative correlation with statistical significance.


CONCLUSION
In this research, the ovoid arch form was the most common type in edentulous mandibles, followed by tapered arch form. The square arch form showed the lowest percentage. As the arch form went squarer, the A-P distance became shorter, the radian of the jaw arch mesially to the mental foramen went greater, and the bone width that distally tilted implants need became bigger. The interforaminal arch form of the edentulous mandible should be analyzed before an implant supported fixed restoration is designed in the interforaminal region. The angle of inclination of distal implants should be reasonable. The bone width of the distal implant site must be adequate. The square arch form contributes negatively to the structure of implant supported fixed prostheses with distal cantilever design.