A comparison between jerk-cost derived from a jaw-tracking system with that from an accelerometer.

Jerk-cost is an inverse measure of movement smoothness and can be calculated from the first-time derivative of acceleration obtained from a tri-axial piezoelectric accelerometer (TPA), or from the third-time derivative of position obtained from a jaw-tracking device. The aims of this study were to determine, in 10 asymptomatic subjects who are chewing gum, (i) jerk-cost measures derived from displacement/time data obtained from the JAWS3D jaw-tracking device and from acceleration data obtained from a TPA used in the same jaw movement recordings, (ii) whether there was a significant relationship between jerk-cost measures derived from both devices and (iii) the degree of agreement between the two measures. Jerk-cost data were calculated in the opening phase, the closing phase, and over the full chewing cycle as the first-time derivative from acceleration obtained from the TPA, and the third-time derivative from JAWS3D for each of the X-, Y- and Z-direction series. There was a significant correlation between both measures of jerk-cost over the full chewing cycle and during jaw-opening (r = 0·65, 0·75, respectively; P < 0·001). There was no significant correlation in the closing phase (r = -0·02, P = 0·99). The Bland-Altman test showed that jerk-cost derived from the JAWS3D can differ by up to 78% below and 21% above that derived from the TPA. These results suggest that jerk-cost measures derived from a jaw-tracking system cannot substitute for jerk-cost measures derived from an accelerometer.

Measurement of jerk-cost using a triaxial piezoelectric accelerometer for the evaluation of jaw movement smoothness.

Jerk-cost as a measure of jaw movement smoothness has been used to evaluate the masticatory function of patients with tooth loss and malocclusion. Jerk-cost measuring systems have thus far been unable to determine the jerk-cost of each chewing phase over time. The purposes of this study were (i) to develop a new method for measuring momentary jerk-cost of the jaw movement using a small triaxial piezoelectric accelerometer and (ii) to test the hypothesis that the lowest smoothness is seen during jaw-opening phase. The accelerometer was attached to the skin of the mentum of each subject. Vertical jaw displacement, acceleration, the jerk, and the time differential of jerk-cost during gum chewing were analysed as a function of time in five normal dentate subjects (n = 5). The system revealed intra-class correlation coefficients of intra-examiner, inter-examiner, and test-retest consistency of substantially high values (0.80-0.88). In all subjects, the highest jerk-cost was observed in the opening phase of each chewing cycle when the gum was parting from the teeth; the lowest jerk-cost was observed in the intercuspal phase throughout the chewing cycle, thus confirming the hypothesis. Significant differences were observed between the opening, closing, and intercuspal chewing phases (N = 5, P = 0.007). The smoothness measurement system used in this study evaluated the momentary smoothness of each chewing phase in the masticatory cycle. The system may serve as a diagnostic tool to evaluate the smoothness of the jaw movement in general dental practice.