Six identical CUDs were constructed according to a previous methodology, which is briefly described below [2- 4].

Two commercial molds of edentulous jaws were used: one of the upper jaw and one of the lower jaw (edentulous molds, size 55; Columbia Dentoform, Long Island, New York, USA) [2,3,13].

These molds were used to fabricate two prototype casts from type III dental stone (Hydrock, KerrLab, Orange, California, USA). Using a standardized methodology, two wax denture bases of uniform thickness (2.5 mm) were constructed using wax denture sheets (Tenatex, Associated Dental Products, Wiltshire, UK). Waxed rims were placed on the denture bases so that their long axes coincided with the crests of the residual ridges. Guidelines were drawn on the waxed rims to indicate the position of the ridge crest. The anterior teeth (Upper Anterior, Uhler Dental Supply, Chicago, Illinois, USA) were arranged according to standard procedures [2,3,13,14].

 The posterior teeth (Upper Posterior, 33˚; Uhler Dental Supply) were placed with the central grooves of their occlusal surfaces coinciding with the guidelines, and the occlusal surfaces were arranged according to standard procedures to reproduce the anteroposterior and lateral compensating curves [13,14].

The corresponding wax complete lower denture (CLD) was then constructed with standardized vertical dimensions of occlusion using the corresponding lower teeth (Lower Posterior, 33˚; Uhler Dental Supply).

The waxed CUD was removed from the upper cast of the articulator and used to construct a two-piece mold to reproduce six identical standardized CUDs. A new cast was placed in its respective position in the lower half of the mold (made from Plexiglas), while the upper half of the mold (made with silicone) was fixed in position on the lower half (made from Plexiglas). Then, molten baseplate wax (Tenatex, Associated Dental Products) was poured into the intervening space. This procedure yielded six fully waxed CUDs and a waxed CLD, which were then flasked using an acrylic-resin material in accordance with the manufacturer’s instructions (Paladon 65, Heraeus Kulzer, Hanau, Germany). Thus, six identical CUDs were obtained with teeth made from the same material as was used to construct the denture bases. Artificial teeth were not used for the construction of the identical dentures specimens because if this was the case, the artificial teeth could be placed at a slightly different location, resulting in different loading condition of the acrylic dentures bases. The acrylic dentures were finished according to standard finishing procedures for acrylic-resin denture bases [14].

During the grinding and polishing of the definitive acrylic-resin dentures, the thickness was measured at six points on each of the labial and buccal flanges using an analog thickness gauge with 0.1 mm precision (K series, Schmidt Control Instruments, Waldkraiburg, Germany) to ensure that the bases of all of the denture specimens had the same thickness (3.0 ± 0.1 mm) [2-4].

It is a common view of many researchers that strain gauges are a reliable method for measuring torque [15, 16].

According to the simple theory of torsion on isotropic rectangular cross-section beams, the principal direct strains (ε₁, ε₂) are inclined at ±45˚ to the torsion axis (the longitudinal center line) and are equal in magnitude but opposite in sign [10,12]. In this case the following equations are valid:

where T is the torque, t is the specimen thickness, L is the length of the specimen, θ is the angle of twist, G is the shear modulus, w is the width of the specimen, and β is a known coefficient dependent on the thickness and width of the specimen [10,12].

If a torsion axis exists on the structure, for example the CUD, then by affixing biaxial strain gauges with their sensitive axes oriented at ±45˚ to the longitudinal axis of the structure, it is possible to record the shear strains due to the torsion [11,16].

When a biaxial two elements (also known as “torsion” or “fish bone”) strain gauge is cemented on a specimen which is under torsion, such that its central axis coincided to the axis of torsion and the gridlines of its one element are at 45˚ to this axis, the bridge output is sensitive to the torque but insensitive to any bending or axial loads (Figure 1) [17].

By connecting the two elements of the biaxial strain gauge to a half-bridge circuit it can be shown through mathematical equations that the shearing strain γ_xy can be measured. The function ε₁ - ε₂ will be performed automatically in the bridge, and the output will yield 2γ_xy directly [17].

Five biaxial strain gauges for torsion measurement (type KFG-2-120-D31, Kyowa Electronic Instruments, Tokyo, Japan) were cemented onto each identical upper denture with cyanoacrylate adhesive (type CC-15A, Kyowa Electronic Instruments). The strain gauges were cemented onto the palatal section of the dentures so that their middle axis coincided with the midline axis of the outer surface of the denture. Their centers were positioned at points that were 1, 2, 3, 4, and 5 cm from the contact point of the central incisors (Figure 2). All of the strains were measured using an electronic six-channel data acquisition system (Wheatstone bridge) consisting of a personal computer, a 12 bit PCI A/D card, a terminal board and specific acquisition and control software (Advanced GeniDAQ; American Advantech Corporation, Sunnyvale, California, USA). The three wires of each individual torque gauge were connected to the electronic Wheatstone bridge to produce a two active half-bridge connection for each gauge, which resulted in five halfbridge circuits.

Each upper denture specimen and the selected lower denture were mounted onto die stone casts, made by pouring high-strength type IV dental stone (Vel-Mix, KerrLab, Orange, California, USA) into the two commercial jaw molds, using the methodology of previous studies [2-4, 13].

Before the denture specimens were placed on the load casts, the intaglio surfaces were painted with mediumviscosity silicone (Coltoflax, Coltene/Whaledent, NJ, USA) to simulate the oral mucosa. To ensure that the thickness of the silicone was similar to the average thickness of the masticatory mucosa in vivo, the thicknesses of the die stone were reduced by 2 mm and 1.5 mm in the residual ridges of the maxillary cast and the mandibular cast, respectively, before the placement of the painted specimens [2-4,13].

The upper and lower dentures were positioned in centric occlusion, and their mounted casts were placed between the plates of a hydraulic press (Bego Hydraulic, BEGO, Bremen, Germany) (Figure 3).

Each denture specimen was loaded three times under three notch conditions: condition 1 was loaded with the initial fraenal notch (5 mm in depth), condition 2 was loaded after deepening the notch by 4 mm (to a total notch length of 9 mm) using a cutting wheel (Ceraflex high-cutting-efficiency wheel, Bredent, Germany), and condition 3 was loaded after creating an incisal (midline) diastema by making a 7 mm slot between the two central incisors. This latter condition was considered as representative of the creation of a diastema between the teeth at request of the patient or of the presence of a crack due to insufficient penetration of acrylic resin between the teeth. The loads were applied at time intervals of 2 hours in duration to allow the acrylic base to recover from the previous load [4,18].

The denture specimens were loaded at 100 N. The maximum load of 100 Ν is consistent with a previous study that showed that the force exerted by edentulous patients at the vertical dimension of occlusion ranges from 30 to 110 N [19].

The shear strain was determined for every denture specimen and for each of the three notch conditions. The final mean value of the stress was determined from the

six identical complete-denture specimens and was considered to be the representative value of the strain for every strain gauge in the corresponding notch condition.

The sensitivity of the used biaxial strain gauge in torsion, was measured by cementing a gauge onto an acrylic plate with dimensions of 6 cm ´ 3 cm ´ 3 mm so that its long axis coincided with the middle axis of the long dimension of the plate.

One end of the plate, which could be considered as a wide rectangular beam, was clamped to a bench. By applying a known torque of 1.2 Nm on the other end of the plate (free end) deflections in three orientations were induced, allowing three strains to be measured on the plate.

The orientations producing bending along the long axis of the plate and along the short axis of the plate (with no torsional deflection) induced recorded strains of 215 and 288 μ Strains, respectively. The strain recorded for the third orientation (rotation around the long axis of the plate causing pure torsional deflection) was 3830 μ Strains. From these results it can be concluded that strain values of up to 300 μ Strains, during the loading of the denture specimens, indicate deformations resulting from bending without torsional deformation, while values exceeding 300 μ Strains indicate small to large torsional deformations.

One-way ANOVA was applied using standard statistical software (SPSS version 15.0 for Windows, SPSS, Chicago, Illinois, USA) to estimate the statistical significance of the differences between the mean values of shear strains in the three notch conditions. In addition, a Tukey HSD test was used for multiple comparisons among all possible pairs of the three notch conditions. The significance level was set as P = 0.05.