Empty clench and the masticatory-salivary reflex

Objectives: To investigate the absence of a masticatory-salivary reflex in empty clenching by examining whether the presence of material between teeth during chewing is the adequate stimulus for salivary flow. Methods: Salivary flow was recorded in humans (n=8) using Lashley cups and cannulae connected to an instantaneous flow meter. Left and right masseteric EMGs were recorded. Full arch hard bite registration paste (HBRP) indices (Futar D, Kettenbach Dental) were made in three thickness; with back teeth in occlusion (A) and separated by increasing incisal separation by approximately 1 and 2 mm (B & C respectively). Chewing stimuli were provided by base gum, HBRP and empty clenching of teeth. The order of stimuli were ipsilateral gum chewing, empty clench (x2), clench on full arch HBRP (x2) at three thicknesses, clench on ipsilateral half arch HBRP (x2) at two thicknesses (B & C), empty clench (x2), ipsilateral and contralateral gum chewing. Chewing stimuli were applied for 60s at a normal regular rate and repeated following a rest period of 60s. Visual display of EMGs ipsilateral to saliva collection gave feedback to allow subjects to bite with a consistent force. Results: Mean unilateral parotid salivary flow and EMGs were normalised as a percentage of salivary flow and EMGs for ipsilateral gum chewing respectively. No significant difference existed between EMGs (ipsilateral to salivary collection) for each masticatory stimulus (P>0.05, ANOVA). Salivary flow for ipsilateral gum chewing was significantly higher than salivary flow for rest and all other stimuli (P<0.05, Dunnett's method multiple comparison in ANOVA). No significant difference existed in the salivary flow between empty clench and rest and all other stimuli (P>0.05, Dunnett's method multiple comparison in ANOVA). Conclusion: The presence of material between the teeth during chewing alone is not the adequate stimulus for the masticatory-salivary reflex.