Objectives: While stability and reproducibility of the intercuspal tooth contact can be improved by increasing clenching efforts, forceful clenching has a potential to cause excessive displacements of the mandible by a class III lever effect of jaw closing muscles. These displacements are expected to be exaggerated in patients without molar tooth support. The aim of this study was to clarify the effect of clenching force level and the absence of the posterior occlusal support on the mandibular displacement. Methods: Thirteen subjects with unilateral edentulous area posterior to the first premolar (all female, 59.1 +/- 8.1 years of age) participated in this study after giving informed consent. They were asked to perform intercuspal clenching at 6 different force levels and three-dimensional mandibular displacements were measured with the aid of Gnasthohexagraph system (Ono Sokki Co., Tokyo). The effects of eclenching force level' and eside of molar tooth loss' on the displacements of bilateral condylar and second molar points were analyzed by means of ANOVA and Tukey's multiple comparison tests. Results: These analyses revealed that the effect of the clenching level on the condylar elevations was significant and clenching level as hard as 70% maximal voluntary contraction (MVC) and more caused significantly larger elevations (p<0.05). In regard with the molar points, 50%MVC level and more caused consistent tippings of the mandible with a larger elevation on the edentulous side (p<0.05). Conclusions: These study results suggest that clenching level should not to be as hard as 50% MVC level for the registration of the intercuspal position in Kennedy Class II edentulous patients.

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