Dental measurements

The dental measurements consisted of 1. crown and 2. tooth axis. These two different measurements were performed using a median reference plane and a set of radiographic reference points and lines (Fig. 1, Table 1). The exact protocol for the measurement of crown axes and tooth axes are described in detail below. Tooth and crown axes were measured for canines, first bicuspids, and second bicuspids in both jaws. A section through each tooth measured it at its widest occlusal vestibulo-oral distance and was adjusted on the axial and sagittal split window respectively. All angular measurements were undertaken on the coronal split window of the imaging software using the built in angle measuring tool. Negative values indicate oral inclination of crown or root respectively whereas positive values indicate vestibular inclination of crown or root respectively. Mandibular and maxillary measurements were undertaken in the same way.

Crown axis

The crown axis (i.e. the vestibulo-oral inclination of the crown) was defined and measured as the angle between the median reference plane (“mrp”) and the line through the FA-point according to Andrews [23]. As described by Smith et al. [24] a parallel shift of the connective line between the clearly defined reference points “cej” (cemento-enamel-junction) and “i” (cusp tip) for was used to construct the line through FA.

Tooth axis

The tooth axis (i.e. the vestibulo-oral inclination of the root) was measured as the angle between the median reference plane (“mrp”) and a line passing through the cusp tip (“i”) of the canines (or “cf” of the bicuspids) and the apical reference point (“ap”). In case of root apex dilaceration, the middle third of the root was used as the apical reference point, similar to conventional cephalometry.

Measurement of crown and tooth axes. Measurements were undertaken using different reference points as defined in Table 1. Left upper canine (a) and left bicuspid (b) are shown here. Negative values (-) indicate oral inclination of crown or root whereas positive (+) values indicate vestibular inclination