UCLA

Introduction: The purpose of this study was to evaluate the skeletal changes induced by Maxillary Skeletal Expander (MSE) and Hyrax appliance in the midface with the use of Cone-Beam Computed Tomographic images. A novel methodology to study the skeletal changes was developed. Our hypothesis is that MSE and Hyrax result in a dissimilar expansion pattern and magnitude.

Materials and Methods: A novel methodology was developed that included three main reference planes (maxillary sagittal plane, axial palatal plane and V-coronal plane), three axial CBCT sections through the maxilla and the pterygoid plates (axial palatal section, lower nasal section, and upper nasal section), one coronal section through the zygomatic and maxillary bones (coronal zygomatic section), and one axial section through the zygomatic arch and maxillary bone (axial zygomatic section). Fifteen and six patients were included in the MSE and Hyrax groups respectively. Parametric or non-parametric tests as appropriate were used to evaluate the treatment changes observed in the MSE group and Hyrax group.

Results: In MSE patients, the midpalatal suture split, and the maxilla moved laterally and forward. Skeletal changes were largest in the axial palatal section, smaller in the lower nasal section and smallest in the upper nasal section. Also, skeletal changes were larger in the anterior than the posterior regions of the skull. The pterygoid processes of the sphenoid bone bent laterally with a center of rotation located in proximity of the cranial base. The pterygopalatine suture underwent substantial loosening in all parts. In the lower part, the pyramidal process of the palatine bone was pulled out from the pterygoid plates of the sphenoid bone, leaving detectable openings in 53% of the sutures. In the middle part, the tuberosity of the maxilla slid laterally and anteriorly relatively to the pterygoid processes. In the upper part, the posterior portion of the perpendicular plate of the palatine bone bent medially. In the coronal plane, the zygomaticomaxillary complex rotated outwards with a center of rotation located in proximity of the frontozygomatic suture in both MSE and Hyrax patients. In the horizontal plane, the zygomaticomaxillary complex rotated outwards with a center of rotation located in the proximal part of the zygomatic process of the temporal bone for MSE group. However, the center of rotation for the maxilla was located in proximity of the pterygoid process for Hyrax group.

Several differences were found between the MSE and Hyrax groups. Skeletal changes were considerably larger in MSE than in Hyrax patients for almost all parameters. The pattern of lateral movement of the maxilla was more parallel in both coronal and horizontal planes with MSE. Moreover, in the horizontal plane, the center of rotation of the zygomaticomaxillary complex in MSE group was located more posteriorly and laterally than that of the Hyrax group. Furthermore, MSE was able to split the pterygopalatine suture while Hyrax appliance could not.

Conclusions: MSE induced significantly more skeletal changes in the midpalatal suture, pterygopalatine suture, maxilla and circummaxillary bones in post-pubertal age patients, compared to Hyrax appliance. MSE and Hyrax resulted in dissimilar expansion pattern and varying magnitude.