Distribution of lip‐seal strength and its relation to oral motor functions

OBJECTIVES: Lip‐seal strength, which represents the muscle strength of the lips, appears to chiefly contribute to mastication and pronunciation. However, the functional characteristics of lip‐seal strength in adults are still undefined. The present study aimed to understand not only the distribution of lip‐seal strength in adult men and women but also the effect of age on this strength and identify oral motor functions correlated with lip‐seal strength. MATERIALS AND METHODS: The subjects included 339 participants (men: 170, age 39.2 ± 18.2 years; women: 169, age 43.1 ± 19.7 years). Oral motor function was evaluated for lip‐seal strength, oral diadochokinesis (ODK), tongue pressure, occlusal force, and masticatory performance. Statistical analyses included the Shapiro–Wilk, Mann–Whitney U, and Jonckheere–Terpstra tests, in addition to the Spearman's correlation analysis and curvilinear regression analysis. RESULTS: Lip‐seal strength did not have a normal distribution (p < 0.001). The mean ± standard deviation and median (first quartile, third quartile) of lip‐seal strength were 11.2 ± 3.4 and 10.9 (8.7, 13.2)N for the whole sample, 12.3 ± 3.4 and 11.9 (9.4, 14.4)N for men, and 10.2 ± 3.0 and 9.9 (8.0, 12.0)N for women. A significant difference was observed in lip‐seal strength between men and women (p < 0.001). Oral motor functions showed a marked correlation with lip‐seal strength, including tongue pressure, occlusal force, and masticatory performance and ODK (/pa/ and /ta/), tongue pressure, and masticatory ability in men and women, respectively. In women, lip‐seal strength declined with increase in age. CONCLUSIONS: Lip‐seal strength was non‐normally distributed in both men and women, and lip‐seal strength was affected by age only in women. Lip‐seal strength and multiple oral motor functions were significantly correlated. Because the indicators of perioral muscle strength and performance were correlated with lip‐seal strength, lip‐seal strength may also partially reflect the condition of the perioral muscles.