Reliability of novel multidirectional lip-closing force measurement system

This study aimed to quantify the directional specificity of multidirectional lip-closing force (LCF) and evaluate the reliability of multidirectional LCF measurements made using a novel system. In fourteen healthy subjects (seven females, seven males, median age = 28 years), LCFs in eight directions and electromyograms (EMGs) from four parts of the orbicularis oris muscles (OOM) were recorded during voluntary pursing-like lip closure tasks. The quantitative reliability was assessed from repeated measurements of the LCFs in the eight directions and from summed values for all eight directions [total lip-closing force (TLCF)]. The intra- and inter-investigator reliabilities for TLCF were assessed by the interclass correlation of the measurements by the same investigator and two investigators, respectively. Lip-closing forces showed directional specificity in vertical, horizontal and oblique directions but those in oblique and horizontal directions were symmetrical bilaterally. The quantitative reliability of measurements was between 0·735 and 0·948 in the eight directions and that of TLCF was 0·934. Interclass correlations of intra- and inter-investigator reliabilities were 0·96 [lower limit of 95% confidence interval (95% LL), 0·87] and 0·96 (95% LL, 0·91), respectively. The intra- and inter-investigator differences of measurements were randomly distributed in the whole range of measurements. The 95% confidence intervals of these differences were significantly narrower than those of the limits of agreement (mean ± 1·96 s.d.). In 13 subjects, Pearson's correlation coefficients between LCF and EMGs from OOM were above 0·95. We conclude that this system has a reasonable quality and reliability for quantitative measurements of multidirectional LCF for evaluating lip functions.