Association between myotonometric measurement of masseter muscle stiffness and maximum bite force in healthy elders

Background Maximum bite force (MBF) is a common and useful index of masticatory function; it correlates with physical strength in elderly people. Palpation of stiffness in the masseter muscle during forceful biting has been considered to be associated with MBF. However, this assessment method relies on subjective judgments; no study has verified the relationship between MBF and quantitative measurements of masseter muscle stiffness (MMS). Objective We aimed to verify the association between masseter muscle myotonometric assessment results and MBF. Methods In total, 117 community-dwelling >65-year-old individuals from the Tokyo metropolitan area were assessed. MMS on the dominant side during forceful biting was measured with a MyotonPRO device. Masseter muscle thickness (MMT) during rest and forceful biting was measured with an ultrasonic diagnostic apparatus, and the difference in MMT (DMMT) between the rest and forceful biting conditions was determined. MBF data were obtained with a pressure-sensitive sheet and an associated device. To determine the independent variables affecting MBF and MMS, multivariate linear regression analyses with adjustments for age, sex and number of teeth were performed. Results The multivariate analysis revealed that MBF correlated with the number of teeth (beta = .489, P < .001) and MMS (beta = .259, P = .003) (R-2 = .433). MMS correlated with MBF (beta = .308, P = .003) and DMMT (beta = .430, P < .001) (R-2 = .326). Conclusion Masseter muscle stiffness possibly reflects a force generated by the masseter muscle during forceful biting. Therefore, MMS is effective to assess tooth loss as well as an index of masseter muscle strength when evaluating MBF.