DENTOALVEOLAR EFFECTS DURING MINIMALLY INVASIVE SURGICALLY ASSISTED RAPID PALATAL EXPANSION: A FINITE ELEMENT ANALYSIS

Abstract

Introduction: Maxillary transverse deficiency is a common problem in all types of malocclusion. The treatment differs relative to the age of the patient. In growing patients, the maxillary suture is not yet well inter-digitated; therefore, rapid palatal expansion is the treatment of choice and is usually associated with suture opening and correction of the deficiency. However, in adults, the suture opening and the expansion are less likely to occur, necessitating an adjunct surgical intervention. A minimally invasive surgical protocol has been introduced, yet the dental effects on the supporting dentition and amount of skeletal expansion are yet to be explored.

Aims: 1. Evaluate the stresses on the teeth and define the setup with the least amount of stress and greatest amount of expansion. 2. Evaluate the effects of adding brackets with different wire sizes on the amount of stress on the teeth and the amount of expansion at the level of the bone. 3. Investigate a potential pattern between amount of stress and expansion with respect to the bone thickness variations between the models.

Methods: A 3-dimensional model of the maxilla was adapted, and a total expansion amount of 8 mm was applied on the anchor teeth of the tooth-borne expander namely, the first premolars and first molars, and on the 4 mini-screws of the bone-borne expander. Five different clinical setups of the maxilla were simulated: setup 1: control with only a tooth-borne expander, setup 2: tooth-borne expander with sagittal palatal osteotomy, setup 3: tooth-borne expander with sagittal palatal osteotomy and fixed appliances with 0.016*0.022 inch stainless steel arch wire, setup 4: tooth-borne expander with sagittal palatal osteotomy and fixed appliances with 0.019*0.025 inch stainless steel arch wire, and setup 5: bone-borne expander with sagittal palatal osteotomy. The data derived from 13 cadavers were incorporated into each of the setups to simulate individual variation of cortical bone thicknesses at different regions. The stresses on the teeth and the skeletal expansion were recorded and analyzed through finite element analysis. Statistical methods included t-tests, pairwise comparisons tests, and correlation tests for associations among variables.

Results: The stress was observed in setups 1 and 2 only on the anchor teeth of the RPE. However, in setups 3 and 4, stress was observed on all the teeth, while in setup 5, no stress was observed on the teeth. Moreover, more expansion occurred posteriorly with setups 1, 2, 3, and 4 that decreased gradually towards the incisors. An opposite trend was observed in setup 5. Setup 3 and 4 showed the least amount of stresses at the anchor teeth and the highest amount of expansion posteriorly with only mean differences between the stresses on the central and lateral incisors were statistically significant among all variables. The expansion was statistically significant at the molars and premolars regions with all the setups compared to setup 1 except with setup 5 at the molars region. In the setups with the tooth-borne expander included, the stresses at the first premolars were positively correlated to the stresses at the first molars with higher amount of stresses observed at the first premolars. The stresses on the teeth were negatively correlated to the expansion, while positively correlated to the cortical bone thicknesses. In addition, the expansion was negatively correlated to the cortical bone thicknesses, but no correlation was seen between these two variables in setup 5.

Conclusion: The sagittal palatal osteotomy was minimally invasive and efficient in increasing the simulated expansion in a skeletally mature patient by decreasing the resistance to expansion at the mid-palatal suture. The bone-borne expander led to more bony expansion anteriorly rather than posteriorly, with absent stresses on the teeth. In contrast, the tooth-borne expanders resulted in more posterior than anterior expansion. As the cortical bone thickness increased, the expansion decreased and stresses on the teeth increased. As expansion increased, the stresses on the teeth decreased. The results suggest that the optimal posterior expansion would be achieved with a tooth-borne expander combined with a sagittal palatal osteotomy and fixed rectangular arch wires engaged in the maxillary teeth. The 0.016*0.022 inch stainless steel arch wire produced the same effect as the heavier wire (0.019*0.025 inch stainless steel). However, anterior expansion would be accomplished with a bone-bone expander combined with the sagittal palatal osteotomy. Future research is needed in more diverse facial structures, such as hyper and hypo-divergent facial patterns with variable resistance to expansion from the buttressing bones. Also, time-dependent finite element modeling should better elucidate the progressive response to expansion.