Gender Differences in Mouth Opening on Temporomandibular Disorder Patients—Implications for Diagnosis

<b>Background/Objectives:</b> Temporomandibular disorder (TMDs) patients often present limited mouth opening (LMO). A key diagnostic cutoff is a mouth opening threshold >40 mm. However, this universal cutoff may not accurately reflect gender anatomical variations. This study investigates gender-specific differences in maximum mouth opening (MMO) to propose revised diagnostic criteria for LMO. <b>Methods:</b> A five-year prospective study was conducted from 1 August 2019 to 1 May 2024 in a Portuguese TMDs department. The patients' gender, MMO, and LMO complaints with clinical validation were recorded. Statistical analyses, including Generalized Additive Models (GAMs) and Generalized Linear Models (GLMs), assessed the relationship between MMO and LMO, with gender-stratified comparisons. <b>Results:</b> In this study 1045 patients were included. The median (accompanied by the interquartile range [25th percentile-75th percentile]) MMO was lower in females (40 mm [34-45]) than in males (44 mm [40-48]). Patients presenting LMO complaints exhibited significantly reduced MMO values compared to those without LMO complaints (<i>p</i> < 0.001). Gender-specific thresholds emerged: for females, LMO was observed when MMO was ≤35 mm, while in males, LMO symptoms appeared when MMO was ≤38 mm. A "gray zone" of diagnostic uncertainty was identified between 36 and 37 mm for females and 38 and 42 mm for males. <b>Conclusions:</b> In this study we observed the gold standard cutoff for diagnosing MMO in female should be <35mm and for male <38mm. These findings suggest that a single LMO threshold does not account for gender-related anatomical differences, potentially leading to underdiagnosis in females and misclassification in males. Revising diagnostic criteria to incorporate gender-specific thresholds could enhance accuracy, improve early diagnosis, and promote personalized treatment strategies for TMDs patients. Further research incorporating additional variables such as age, dental occlusion, craniofacial structure, and body mass index is recommended to refine these diagnostic guidelines.