Abstract
Objectives: The aim of this study was to investigate the response of the lower lip musculature to incremental advancement, both during swallowing and at rest.
Material and Methods: Intraoral lip pressures were measured using four miniature pressure sensors in ten participants (2 males, 8 females; 20-39 years) who were free of relevant malocclusion. Custom-made acrylic trays were used, with the sensors located to record pressures at the midline and canine on both the labial and lingual surfaces. EMG sensors recorded muscle activity of the orbicularis oris and mentalis muscle at the lower midline. The pressure sensors were calibrated using a pressure chamber and the recorded pressure and EMG signals were analog-to-digital converted at 1kHz, and stored for off-line analyses. For each participant, the lower lip was advanced incrementally by inserting trays with different labial thicknesses (0.5, 2.5 and 4.5mm). Resting lip pressures were assessed three times over a 30-second window with teeth in light contact and apart. Each participant was also asked to complete three on-command saliva swallows at one-minute intervals for each tray. Data were analysed using a mixed linear model, combined with visual analysis of each individual swallow profile.
Results: Resting lip pressure generated at the midline increased as the lower lip was advanced (p<0.001, F=14.7). There was no statistically significant difference in EMG activity when the lip was advanced from 0.5mm to 2.5mm, but activity increased significantly between 2.5mm and 4.5mm of advancement (p<0.001). For the saliva swallow task, peak pressure generated at the canine labial site increased as the lower lip was advanced (p<0.05), while no change in peak pressure was seen at the other three sites (p>0.05). Each individual’s swallow profile was maintained during both the intra- and inter-tray swallows. For pressure, EMG and swallow activity, there was a highly significant tray/subject interaction (pressure p<0.001, F=24.3; EMG p<0.001, F=26.5; swallow p<0.001).
Conclusion: Lip pressure generated at the midline increased as the lower lip was advanced, while EMG activity increased only after the lip was advanced further than 2.5mm. This suggests that the initial pressure increase on the lower incisors was from the inherent viscoelastic properties of the lower lip, while the pressure increase between 2.5 and 4.5mm advancement was due to increased muscle activity as the subject attempted to maintain oral seal. These findings support the proposal that each individual has a signature swallow which is maintained when the anterior oral seal is challenged. It also suggests that each individual responds to lower lip advancement in a different yet subject-specific manner. The individual pressure response to incremental lip advancement needs to be further investigated in future research to understand how this affects the long-term stability of the lower labial segment.