Results.

Sections of the roots of the maxillary central incisors were mainly circular in shape or slightly triangular; this shape was generally maintained in the middle third of the root. The roots of the maxillary lateral incisors were flattened lengthwise in the mesio-distal direction and the radicular canal was slightly oval in shape. The sections of the maxillary canines displayed roots with a bucco-lingual major diameter; the radicular canals were oval in shape. Most of the roots of the maxillary premolars were flattened in the mesio-distal direction with a deep sulcus along the root. The maxillary premolars with two roots showed only one large chamber canal in the first section in correspondence with the cemento-enamel junction, the two roots in the remaining part were circular. The roots of the mandibular incisors were thin and flattened lengthwise in the mesio-distal direction and the radicular canals were ribbon-like. The roots of the mandibular canines were oval-shaped and the radicular canals were circular. The roots and canals of the mandibular premolars were mostly circular in shape.

The average and standard deviation of the values obtained for each group of teeth are listed in Tables 1-3. The average thicknesses of the radicular wall for each group of teeth are graphically illustrated in Figs 1-8. With the assumption that the centre of the canal is the centre of each drawing, profiles of the sections are drawn into a grid of known dimensions with values expressed in millimetres. The actual measurements are superimposed on one another for each of the three horizontal sections and displayed on the left side of the figures. In the right column of the figures, a schematic representation of the mean values of the measurements for each of the three sections is illustrated. The graphic images represent the average of the measurements obtained.

**Table 2.** Mean thickness and standard deviation (+-SD), in millimetres, for each cut surface of upper canines and lower canines and premolars.

**Table 3.** Mean thickness and standard deviation (+-SD), in millimetres, for each cut surface of upper premolars. In correspondence, the cementoenamel junction of premolars with two roots, surface1, the two canals are still fused together.

**Figure 1.** Maxillary central incisors (MCI): on the left is a superimposition of the graphic representation of actual measurements of all samples of the three horizontal sections. On the right is the average shape with the average wall thickness in millimetres.

In the three-way anova analysis the values of ETAsqd were calculated. ETAsqd is a statistical index that offers the indication of the size of an effect. The value of ETAsqd shows the capacity, expressed in percentages, that has a factor (tooth type, section and location, or a combination of them) to explain the variability in the dependent variable (thickness). Within the significant effects of each of the analyses, one must al ways be conscious of the magnitude of the effects, i.e. only those effects that may explain a good percentage of the variance should be attempted to be interpreted and emphasized. The ETAsqd indicates that despite the fact that the effects and the interaction of tooth type, section and location on wall thickness were all significant (P < 0.05), major importance must be attributed to the location, followed by the section, and minor importance to the tooth type and interaction of the variables.

In the one-way anova results, the post hoc analysis with the Duncan tests, revealed that the mean wall thickness in the buccal position of the first cut surface %