40 Chapter 2 mouth (U = 4466, p = 0.006, r = − 0.16). CODS item 7 (glassy appearance of oral mucosa especially palate) was associated with more severe oral dryness of the anterior and posterior palate (U = 7058, p = 0.000, r = − 0.27 and U = 6541, p = 0.000, r = − 0.31 respectively anterior and posterior palate). There were no significant relations between CODS item 3 (tongue shows loss of papillae) and item 8 (debris on palate and perceived oral dryness of the corresponding anatomical regions). All the reported significant associations can be considered robust to distributional violations as the bootstrapped 95% confidence interval did not exceed 0. DISCUSSION The results of this study demonstrated intra-oral differences in perceived mouth dryness between different locations in the mouth by using the RODI, a recently developed xerostomia questionnaire. The perceived dryness was considered the highest for the posterior palate and the lowest for the floor of the mouth. The highest correlations were found between regions in the RODI and corresponding related individual items of the XI and CODS. As described in the introduction, the saliva film on intraoral tissue has local differences. The saliva film is thinnest at the anterior hard palate (~ 10 μm), while the saliva film at the anterior dorsal area of the tongue is much thicker (~ 54 μm) [18]. This pattern of different saliva film thickness at various intra-oral locations has been confirmed by other studies, where the palate is considered most dry, and tongue and floor of the mouth are considered as most wet, which explains the high MUC5 concentration on the palate [17, 19–21]. Several factors make the hard palate more susceptible to oral dryness than other intra-oral locations: paucity of (hard) palatal glands, gravity, and evaporation [1, 19, 33]. Gravity forces part of the whole saliva to pool in the floor of the mouth between swallowing episodes. As a consequence, the palate can be insufficiently moistened, especially in case of hyposalivation [20]. Furthermore, the palate is more prone to saliva evaporation, especially during speaking and breathing; and during speech air passes more or less continuously from the lungs over the mucosa of the palate [19]. The advantage of the tongue is that it is located near the opening fromWharton’s ducts [17, 19, 20]. Here, saliva from themany minor glands in this region and the nasopalatine glands as well as the secretions of the submandibular and sublingual glands is collected [20]. This pattern of saliva thickness on the various mucosal sites
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