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Biomechanics of the Human Temporomandibular Joint during ChewingDepartment of Oral Function, Section of Oral Kinesiology, Academic Center Dentistry Amsterdam (ACTA), Louwesweg 1, 1066 EA, Amsterdam, The Netherlands; Correspondence: *corresponding author, m.naeije{at}acta.nl
Experimental data on the loading of the human temporomandibular joint during chewing are scarce. Coincidence of the opening and closing chewing strokes of the condyles probably indicates compression in the joint during chewing. Using this indication, we studied the loading of the joint during chewing and chopping of a latex-packed food bolus on the left or right side of the mouth. Mandibular movements of ten healthy subjects were recorded. Distances traveled by the condylar kinematic centers were normalized with respect to the distances traveled during maximum opening. We judged coincidence of the opening and closing condylar movement traces without knowing their origin. When subjects chewed, the ipsilateral condyles traveled shorter distances than did the contralateral condyles. During chewing and chopping, all contralateral condyles showed a coincident movement pattern, while a significantly smaller number of ipsilateral condyles did. These results suggest that the ipsilateral joints were less heavily loaded during chewing and chopping than were the contralateral joints.
Key Words: biomechanics • temporomandibular joint • TMJ • chewing • kinematic center
Due to technical limitations, no experimental data are available on the mechanical loading of the human temporomandibular joint (TMJ) during chewing and chopping. As an alternative, investigators estimate joint loading by using biomechanical models (Koolstra and van Eijden, 1997; Langenbach and Hannam, 1999). However, these models also demand experimental verification. A recent study has indicated that when, during chewing, the opening and closing condylar movement traces coincide, there is compression in the TMJ during the closing stroke. However, when the traces do not coincide, the TMJ is not or only slightly under compression during closing (Huddleston Slater et al., 1999). The aim of the present study was to use these observations to study the loading of the TMJ during chewing and chopping tasks.
Participants Ten participants, five women and five men, from 21 to 32 yrs old, participated in the study after giving informed consent. They were free of temporomandibular disorders, joint sounds, systemic diseases, and orthodontic abnormalities such an excessive overbite. The scientific and ethical aspects of this study were reviewed by the review board of the Netherlands Institute of Dental Sciences.
Jaw Movement Recording System
Experimental Protocol During the recording session, each participant performed 4 tasks. First, he/she was instructed to chew or chop (chewing with mainly vertical chewing strokes), for 20 sec, a test food bolus of about 1 cm3 (Fruitella, Van Melle B.V., Breda, The Netherlands) on the left or right side of the mouth. The investigator placed the bolus on the participant’s tongue, and, before chewing, the participant closed the mouth in the intercuspal position to create a reference position for analysis. To keep the consistency of the food bolus constant during chewing, we packed each bolus in latex so that it could not mix with the subject’s saliva. Before we took any recordings, we heated the bolus in hot water (55°C) to soften it and to give it an elasticity modulus of about 4000 N/m. Each task was performed twice, and the recording with the least variation in the frontal movement traces of the lower incisal point was chosen for further analysis (visually checked). The participants also performed 20-second recordings of maximum free opening and closing, of maximum free opening and loaded closing, and of maximum protrusion and retrusion. Loading of the joints during closing was achieved through the application of a small counteracting force to the subject’s chin (± 50 N). On average, 18 movements were performed during each 20-second recording.
Data Analysis
Normally, during free (unloaded) opening and closing movements of the mandible, the closing traces of the condylar kinematic center lie below the opening traces. However, when the joint is loaded during closing—for instance, by a manually applied counteracting force on the chin—the condyle is pressed against the articular eminence, and its closing movement traces will shift upward and coincide with the opening traces (Huddleston Slater et al., 1999) (Fig. 1
For each chewing or chopping task, the type of movement pattern of a joint (coincident or non-coincident) was visually judged by an investigator experienced in the analysis of condylar movements (Fig. 2  ). He was blind to the type of chewing pattern (chewing or chopping) and to the type of condyle (ipsilateral or contralateral). The same investigator judged the movement traces a second time, 6 mos later. Ninety-four percent of the movement patterns received the same classification. The Kappa value for the intra-rater reliability was 0.85 (excellent agreement).
Statistical Analysis Differences between the normalized distances traveled by the ipsilateral and contralateral condyles were analyzed with paired t tests. Differences in the numbers of coincident movement patterns between both sides were analyzed with McNemar tests. Probability levels of p < 0.05 were considered statistically significant. Since each participant chewed on both sides, the number of ipsilateral and contralateral joints was 20. However, one of the participants misunderstood the chopping task and chewed on the left and the right sides alternately, making the number of joints for chopping 18.
When subjects chewed, the distances traveled by the condylar kinematic centers were shorter on the ipsilateral side than on the contralateral side (Table 1 , p < 0.000). We found no differences during the chopping tasks.
During chewing and chopping, the kinematic centers of all contralateral joints showed a coincident movement pattern (Table 2 ). Moreover, the number of joints with a coincident movement pattern was significantly smaller on the ipsilateral side than on the contralateral side, during both chewing (p < 0.000) and chopping (p < 0.001).
Recordings of mandibular movements with 6 degrees of freedom may provide us with more insight into joint loading during chewing (Hannam, 1991; Palla et al., 1997; Huddleston Slater et al., 1999). The new method used in this study relies upon a comparison of the opening and closing condylar movement traces, and its only limitation is that it can recognize only two states of joint loading: compression and no (or only slight) compression. However, the amount of compression in the joint cannot be recorded this way. Like all similar jaw movement recording systems, our system also relies upon the validity of the formulae of rigid body mathematics. However, human bone is not totally rigid during chewing. A combination of sagittal bending, corpus rotation, and transverse bending of the mandible may occur (Van Eijden, 2000). The extent to which this influences the reconstructed movement traces is difficult to establish, and, so far, little can be done to compensate for the possible distortions (Hannam, 1991). To overcome this problem, we were careful that the latex-packed food bolus was soft enough to distort the mandibular bone only minimally, yet was still hard enough to activate the muscles moderately during chewing and chopping. However, the compression in the left joint of the participant shown in Fig. 2B  shifted the closing traces so far upward that they got situated even slightly above the opening traces. This was scored as a "coincident movement pattern" and, in our opinion, illustrates that the assumption of a rigid mandible was not totally valid in this case.
The lower incisal movement traces show considerably more variation than those of the kinematic centers (Fig. 2 Coincidence of the opening and closing kinematic center movement traces was regarded as a sign of compression in the joint during chewing; "no coincidence" was seen as a sign of no (or only slight) compression. However, during chewing, which is an asymmetric task, the condyles may follow slightly different trajectories during opening and closing. This may obscure the interpretation of non-coinciding sagittal opening and closing traces. However, also during chopping, which is a more symmetric task, non-coincident movement patterns were found, suggesting that this observation is not related to the asymmetric nature of chewing. Moreover, all contralateral joints showed coincident movement patterns, during both chewing and chopping. The results of this study are partly in line with the predictions of biomechanical models. Static modeling of tooth clenching on a unilateral molar bite point predicts that joint forces are greater on the contralateral side (Faulkner et al., 1987; Korioth, 1997). However, dynamic modeling of the human jaw suggests that, during chewing, the compressive forces on the ipsilateral side exceed those on the contralateral side (Langenbach and Hannam, 1999), whereas for chopping, the results were similar to those for the static situation, with higher condylar forces on the contralateral side. This confirms the complexity of unilateral chewing and its strong dependency on the co-contraction patterns of the muscles involved. Experiments in macaques (Hylander, 1979) and on human adult mandibles (Mongini et al., 1981) confirm that joint reaction forces are higher on the contralateral side. Analysis of tomograms revealed that the minimum joint space of the contralateral joint was significantly reduced during unilateral molar clenching, whereas no significant change was found on the ipsilateral side (Kuboki et al., 1996). The suggestion that the ipsilateral joint is less heavily loaded during chewing than the contralateral joint may explain why patients with joint pain occasionally report less pain while chewing on the painful side.
This study was supported by the Netherlands Institute of Dental Sciences (IOT). Received for publication May 13, 2002. Revision received December 9, 2002. Accepted for publication March 17, 2003.
Journal of Dental Research, Vol. 82, No. 7,
528-531 (2003) This article has been cited by other articles:
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