“Diving into the Gray Zone”: A Case Report of a 19-Year-Old Patient Treated with Tooth-Borne Rapid Maxillary Expansion

<b>Background:</b> This case report aimed to quantify dental, alveolar, and skeletal changes, periodontal health, and sleep quality after treatment with a tooth-borne rapid palatal expander (RPE) in a young adult with bilateral posterior crossbite due to transverse maxillary deficiency. Tooth-borne RPE is typically indicated during the prepubertal or pubertal growth phases; however, some post-pubertal or young adult patients may still present with incomplete maturation of the midpalatal suture-the so-called "gray zone." In clinical practice, treatment decisions should ideally consider multiple skeletal resistance areas (the zygomaticomaxillary buttress, the pterygomaxillary junction, the nasal aperture pillars), although midpalatal suture assessment often remains central to case selection. <b>Methods:</b> A 19-year-old male patient presented with a skeletal Class III tendency, dental crowding, and anterior and bilateral posterior crossbites, accompanied by snoring and breathing difficulties. The patient declined surgical- and miniscrew-assisted RPE. Cone-beam computed tomography (CBCT) scan revealed incomplete midpalatal suture maturation. Based on periodontal evaluation, a conventional tooth-borne RPE was chosen. Pre- and post-expansion CBCT scans were used to evaluate dental, skeletal, and periodontal outcomes. <b>Results:</b> After one year of treatment, bilateral posterior crossbite was successfully corrected. Buccal bone thickness showed a slight reduction only on the upper left first molar (from 1.2 mm to 0.9 mm), without evidence of dehiscence or fenestration. A 2° increase in the dental tipping angle (DTA) was observed on both molars, and the palatal alveolar angle (PAA) increased by 3°. Sutural separation expanded from 0.32 mm to 7.82 mm. The Midpalatal Opening Related to Expander Opening (MORE) factor was 0.54, indicating a predominantly skeletal response. Periodontal health remained stable, and CBCT analysis confirmed increases in intermolar width (from 36.08 mm to 50.02 mm) and palatal maxillary width (from 28.04 mm to 34.5 mm). A reduction in the Pittsburgh Sleep Quality Index (PSQI) from 7 to 3 was observed, though this finding should be interpreted cautiously due to its subjective nature and the absence of objective airway measurements. <b>Conclusions:</b> The present case report suggests that tooth-borne RPE may represent a viable and minimally invasive option for correcting posterior crossbite in carefully selected young adults with incomplete midpalatal suture maturation. However, the findings are limited to a single case with short follow-up and should be regarded as hypothesis-generating rather than conclusive.