Expansion with vestibular shields: an experimental test of the periosteal-pull hypothesis

Two mechanisms are said to be responsible for the expansion commonly produced by buccal shields: (1) unbalanced tongue pressure and (2) periosteal pull; that is, periosteal traction on the bone overlying the molar roots. The present study used 44 young albino rats to examine these two alternatives. Half the rats wore buccal shields to produce molar expansion, and half had their maxillary molar crowns ground down to the gingiva to eliminate the effect of the tongue. In the resulting 2 x 2 design, four experimental groups were formed: (1) SM, shields and intact molars; (2) Sm, shields and reduced molars; (3) sM, no shields and intact molars; and (4) sm, no shields and reduced molars. It was hypothesized that if vestibular shields produce expansion through periosteal traction, the presence or absence of molar crowns should make no difference. Conversely, if the expansion is caused by unbalanced tongue pressure, shields should have an effect only in conjunction with intact maxillary molars. Palatal amalgam implants and dorsoventral cephalograms were used to measure the maxillary basal and dental expansion that occurred during the 6 weeks of the experiment. Analysis of variance showed the presence of highly significant interaction between shields and molars: the shields produced an increase in posterior dental expansion, but only when molar crowns were present. In contrast, basal expansion was unaffected by any combination of treatments. At least for the rat, it may be concluded that unbalanced tongue pressure, rather than periosteal traction, is probably responsible for the expansion produced by buccal shields.