Effect of implant number on transverse bending moments during simulated unilateral loading of mandibular fixed-detachable prostheses.

High bending moments acting on osseointegrated implants due to transverse forces are believed to be potential contributors to mechanical implant failure. Theoretically, the rigidity of a system comprised of five implants would seem to counter these moments more effectively than one with only three implants. To study this, we built an experimental model comprised of five Brånemark implants embedded in an acrylic mandibular edentulous arch and connected by a metal framework. This lower prosthesis was mounted with an opposing maxillary complete denture in nonbalanced lingualized occlusion on a semiadjustable articulator. Eccentric static bites were simulated by fixing the dentures at 1.5 mm left and right working side (WS) and balancing side (BS) positions, respectively, and loading the upper member of the articulator with 50 N. The distal right implant abutment was transformed into a loadcell by bonding four strain gauges at 90 degrees intervals across its surface. Three 10-second static load ramps were carried out for each of 4 experiments: (1) WS loadcell with five implants, (2) BS loadcell with five implants, (3) WS loadcell with three implants, and (4) BS loadcell with three implants. Transverse bending moments were found to be significantly higher on the WS for the three-implant prosthesis as compared to the five-implant design (1.469 Ncm for five implants vs 2.151 Ncm for three implants; p = 0.001, Student's t-test). This difference was insignificant on the BS (0.532 Ncm for five implants vs 0.521 Ncm for three implants; p = 0.34). These results suggest that a higher number of mandibular implants may decrease the bending moments affecting mandibular fixed-detachable prostheses during unilateral biting tasks.