Correction of Posterior Crossbite Using Rapid Maxillary Expansion in a Nine-Year-Old Boy: A Case Report.

Maxillary constriction with a Class III malocclusion in the mixed dentition stage presents a unique challenge that requires early and customized orthodontic intervention. This case report highlights the management of a nine-year-old male patient with maxillary deficiency, an edge-to-edge incisor relationship, and a unilateral posterior crossbite using a modified Hyrax-type rapid maxillary expander (RME) with both anterior and posterior expansion capabilities. A fixed, bonded acrylic Hyrax-type RME with anterior extensions was designed to achieve three-dimensional maxillary expansion. Following appliance activation and subsequent breakage of the anterior segment, the posterior component was retained until the desired expansion was completed. A palatal crib appliance was delivered post-expansion to control the tongue thrust habit and stabilize the anterior open bite correction. Cephalometric analysis post-treatment demonstrated favorable skeletal and dentoalveolar changes, including increased SNA (79°-85°), reduced FMA (28°-22°), and improved Jarabak's ratio (66.0-71.28), indicating horizontal growth and effective protraction of the maxilla. Dental changes included increased incisor proclination and improved overjet and occlusion. The interdisciplinary approach resulted in the correction of the crossbite, an improved facial profile, and normalization of oral function. This case emphasizes the significance of early diagnosis and individualized appliance design in managing maxillary constriction and Class III tendencies. The combination of orthopedic expansion and habit correction successfully addressed both skeletal and functional components, demonstrating the potential for long-term occlusal stability and harmonious facial growth.