Assessment of the Magnitude and Asymmetry of Micro-Implant-Assisted Rapid Maxillary Expansion
Micro-implant assisted rapid palatal expander such as the Maxillary Skeletal Expander (MSE) have been utilized to achieve skeletal expansion as an alternative to surgically assisted expansion with some success. Previous studies show significant effects on the mid-face, including a degree of asymmetry. The aim of this study is to quantify the magnitude, parallelism, and asymmetry of expansion in non-growing patients and to explore possible factors that can predict the pattern of asymmetry of expansion.
We examined orthodontic non-growing patients (n=31) with an average age of 20.4 years old, with Cone Beam Computed Tomography (CBCT) images taken before and right after expansion using MSE. Those images were superimposed, and expansion was analyzed utilizing the Mid-Sagittal Plane (MSP) as a reference plane. Average magnitude of total expansion was 5 mm at the Anterior Nasal Spine (ANS), and 4.77 mm at the Posterior Nasal Spine (PNS) which showed statistical significance using a paired t-test with p<0.01. Expansion was parallel in the antero-posterior dimension where expansion at the PNS was 95% of that at the ANS. The sample was evaluated for asymmetry and divided into two groups; symmetric (n=15) and asymmetric (n=16), with 16 out of 31 patients exhibiting statistically significant asymmetry. The asymmetric group showed statistical significance asymmetry at p<0.05 when comparing expansion at both sides.
In order to validate any possible predicting factors, correlation of the asymmetry of expansion and multiple measurements of predicting factors was performed to both compare the direction of asymmetry, as well as the total magnitude of asymmetry.
MSE achieved highly parallel expansion in the sagittal plane in non-growing patients with an average magnitude of 5 mm at the ANS. However, transverse asymmetry of expansion was noted in 51% of the patients where one side expanded more than the other by at least 1.1 mm.
Lack of correlation was noted between the magnitude of asymmetry and both the initial asymmetry of the mid-face, as well as the difference in density values bilaterally. However, the direction of asymmetry of expansion at ANS was moderately associated with the initial asymmetry of the frontozygomatic suture.