The effect of number and distribution of unsplinted maxillary implants on the load transfer in implant-retained maxillary overdentures: an in vitro study.

STATEMENT OF PROBLEM There is little information as to how the number and distribution of implants affect the amount of load transmitted to the palate in implant-retained maxillary overdentures. PURPOSE The purpose of this study was to evaluate the effect of the number and distribution of dental implants on the load transmitted to the palate. MATERIAL AND METHODS Eight implant analogues were placed in a replica of an average sized edentulous maxilla corresponding to the position of canines, first and second premolars, and first molars. The anteroposterior distance between the centers of implants in each quadrant was 8 mm. Fifteen denture bases were fabricated to fit the edentulous maxilla analogue. The denture bases were attached to the oral analogue using 6 different configurations of attachments (6 groups): Either no Locator attachments were used (control group), or the 2 most anterior attachments were attached, or 4 implants were engaged with a distance of 8, 16, or 24 mm between the centers of implants on left and right side, and finally, when all 8 attachments were activated. A force-measuring sensor was used to measure the force transmitted to the palate when a static force of 245 N was applied on the occlusal rims of the denture bases. Data (Newtons) were analyzed using 1-way ANOVA and Tukey's HSD test α=.05). RESULTS The mean (SD) amount of force measured on the palate when the overdentures were supported by 4 Locator attachments; [49.84 (26.52) at 8 mm spacing], [24.42 (15.05) at 16 mm spacing], [35.66 (22.94) at 24 mm spacing] was significantly lower than when no attachments [90.98 (20.20), control], or when 2 Locator attachments were used [76.07 (27.63)] (P<.001). When the overdentures were supported by 8 Locator attachments, the force measured on the palate [20.67(16.06) N] was significantly lower than that for the control group (P<.001), overdentures supported by 2 Locator attachments (P<.001), and overdentures supported by 4 Locator attachments when the distance between the anterior and posterior implants was 8 mm P=.006). CONCLUSIONS The distribution of implants had a significant effect on the force measured on the palate of the oral analogue in overdentures retained by Locator attachments. When the distance between the 4 implants was 16 or more mm, the load was not significantly lower than the 8 implant design, suggesting that the palate of a 4 implant-retained overdenture with a distance of 16 mm or more, does not contribute significantly to the load transfer to underlying hard palate in the in vitro analogue evaluated.