Since it is difficult to record joint loading in vivo, there are no experimental data on the actual joint loading during chewing. Condylar movement recordings with 6 degrees of freedom provide an indirect method to estimate joint loading. Previous studies have shown that when condylar opening and closing traces coincide there is compression in the joint during closing. When closing traces lie under the opening traces no compression in the joint during closing is present. Objectives: the aim of this study is to study temporomandibular joint loading during chewing with different patterns of muscle activity. Methods: seven subjects were recorded with 6 degrees of freedom while chewing a latex packed food bolus, first on the left and then on the right side. In a previous session, subjects were trained to use two patterns of muscle activity: as much as possible equal muscle activity on both sides (balanced chewing) and more muscle activity on the ipsilateral side (ipsilateral chewing). Forces of about 50 N were reached during chewing. Whether the condylar traces indicate compression or not in the joint during chewing was judged blindly. Results: the length of the condylar traces was smaller on the ipsilateral side than on the contralateral side (P=0.000). Differences in loading between the ipsi and contralateral joint were found, both for balanced (P=0.003) and for ipsilateral chewing (P=0.001). In the contralateral joint there was always compression, independent from the pattern of muscle activity. In the ipsilateral joint, compression was found for 8 out of 14 joints during balanced chewing and in only 3 out of 14 joints during ipsilateral chewing. This difference in loading on the ipsilateral side tended to be significant (P=0.06). Conclusions: especially joint loading in the ipsilateral joint depends upon the muscle activity pattern.

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