Reliability of Novel Multidirectional Lip-closing Force Measurement System

Objectives: Lip-closing force (LCF) is produced by the perioral tissues around the orifice. The aim of this study was to quantify the multidirectional LCF and to evaluate the total reliability of measurements made by the novel system. Methods: Fourteen healthy subjects (7F:7M; 25-36 y.o.) were recorded for LCF and electromyogram (EMG) from orbicularis oris muscle (OOM) in upper and lower lips during the repeated pursing-like lip closure tasks. Total maximum voluntary lip-closing force (TLCF) (the summed value of the LCFs in eight directions) was used to assess total reliability, intra- and inter- investigator reliabilities. Results: The LCFs in two vertical and four oblique directions accounted for approximately 25% and 12.5% of TLCF, respectively, while those in two horizontal directions were very low. No side difference was noted for the LCFs in oblique and horizontal directions. Total reliability of measurements of TLCF by this system was 0.934 (perfect score: 1). Interclass correlations of intra- and inter- investigator reliability were 0.96 (the low limit of 95% confidence interval[95% LL]: 0.87) and 0.98 (95% LL: 0.91), respectively. The intra- and inter- investigator differences of measurements were randomly distributed in whole range of measurements. The 95% confidence intervals of these differences were significantly narrower than those of the limits of agreement (mean ± 1.96SD). The Pearson's correlation coefficient between TLCF and EMG activity of the OOM was above 0.95 in 13 subjects and 0.88 in one subject only. Conclusions: This system has a reasonable quality of reliability for evaluating the multidirectional LCFs.