Comparison of the midline stress fields in maxillary and mandibular complete dentures: a pilot study

Complete dentures typically fracture along the midline due to crack initiation and propagation from stressed areas. Therefore, it is important to characterize the magnitude and direction of maximum stresses developed in the midline of dentures.

The purpose of this pilot study was to determine the midline stress field of a mandibular complete denture under different loading conditions, and to compare this with the corresponding field of a maxillary denture to identify whether significant differences in maximum stresses between these 2 stress fields are responsible for the higher rate of failure seen clinically in maxillary dentures.

The identical casts used in this study were fabricated with commercial molds. Two complete acrylic resin dentures (1 maxillary and 1 mandibular) that were used in a previous study were selected as prototype dentures for this study. The mandibular acrylic denture was used to fabricate a 2-piece mold from silicone (upper half) and polymethyl methacrylate (PMMA) sheets (lower half) that was subsequently used to produce 3 identical mandibular complete dentures. The single maxillary acrylic denture was used to load the mandibular dentures. A rosette strain gauge was cemented onto the midline of each of the 3 mandibular dentures. The test conditions, including the fabrication of the casts used to induce loads, simulation of oral mucosa, loading procedure, strain measurement, and stress calculations, were identical to those used in a previous investigation of maxillary complete dentures to allow a direct comparison between maxillary and mandibular dentures. The Mann-Whitney test (alpha=.05) was used to quantify the differences in the stress magnitudes between maxillary and mandibular complete dentures.

The stresses in the mandibular denture differed both qualitatively and quantitatively from those in the maxillary denture. The midline stress field of the mandibular complete denture was characterized by 2 low compressive principal stresses and a low maximum shear stress, whereas the corresponding stress field of the maxillary complete denture was characterized by a high principal tensile stress and a high shear stress. These differences between the stresses of maxillary and mandibular dentures were statistically significant (P<.001).

The differences in stress patterns between mandibular and maxillary complete dentures as determined by this study, which also used data from a previous study, may be the primary reason why maxillary dentures fracture more often than mandibular dentures.