Slow maxillary expansion with nickel titanium.

requires expansion of the palate by a combination of orthopedic and orthodontic tooth movements. Initially, transverse forces will tip the buccal segments laterally.1 With proper appliance design, 3rd-order moments will induce bodily translation.2-5 If the force is strong enough, separation occurs at the maxillary suture. The amount of orthopedic vs. orthodontic change depends greatly on the patient’s age. Normal palatal growth is nearly complete by age 6,6 and increasing interdigitation of the suture makes separation difficult to achieve after puberty.7-15 The rationale for sutural expansion, assuming the problem is diagnosed early enough, is twofold. First, a skeletal discrepancy should be treated by orthopedic correction whenever possible. Second, dental compensation by buccal translation or tipping is undesirable, because the thin layer of bone covering the roots of the buccal segments is prone to penetration. The objective of rapid palatal expansion is to reduce undesirable orthodontic tooth movement and tipping while producing enough force to overcome the tendency of the anchor teeth to move, thereby maximizing the orthopedic response by causing separation at the suture.16-18 RPE appliances require frequent activations and generate heavy forces—as much as 2-5kg per quarter-turn, with accumulated loads of more than 9kg.19 Although sutural separation is relatively easy to obtain with RPE, implant data indicate that sutural expansion accounts for only about 50% of the overall expansion. In more than 30% of Krebs’s sample of 23 patients, sutural expansion was less than one-third of the total expansion.20 Buccal tipping, even of the amount produced by RPE, is highly unstable and prone to relapse quickly.21 According to Haas, fixed retention after RPE can virtually eliminate sutural relapse.22 However, using the same split-palateexpanding appliance, Krebs found that the distance between implants in the hard palate and infrazygomatic ridge decreased during retention by 10-15%.23 This relapse was found to occur as late as four or five years after fixed retention.