Validity and reproducibility of a tripoding method in point registration-based 3D superimposition software compared to a conventional cephalometric method

The ability to measure the amount of tooth movement in both horizontal and vertical directions is important when assessing orthodontic treatment outcomes related to molar anchorage loss, incisor retraction, intrusion and the extrusion of teeth. An analysis of orthodontic tooth movement enables the assessment of the magnitude, rate and type of movement as well as the efficiency of treatment intervention and anchorage conservation. Cephalometric analyses such as the Pancherz, Bjork, Steiner, Ricketts and the McNamara analysis have commonly been used to measure dental and skeletal changes related to orthodontic treatment. The Pancherz analysis has two components which are a vertical occlusal (VO) and a sagittal occlusal (SO) assessment which makes it possible to measure vertical and horizontal tooth movements.1,2

The availability and use of 3D scanners in dentistry have provided an alternative method of measuring tooth changes which are non-invasive as it does not subject the patient to radiation exposure. Measurements on digital casts have been validated and are comparable to direct measurements taken on plaster casts.3,4 Furthermore, a 3D cast rendition from most scanners has also been found to be accurate and reliable.3,5 Methods of measuring tooth movement on digitised casts, however, depend on accurately identifying intraoral landmarks for the superimposition of the dental models. Various landmarks and methods of superimposition have been reported and discussed but are still not fully established.6–10

Digitised maxillary study casts are more commonly used for measuring tooth movement due to the suitability of the palatal rugae as stable reference points.6,7 Some researchers have suggested the use of medial points6,9 or the medial two-thirds of the third palatal rugae7,10 as reference landmarks for maxillary cast superimposition. These points have been shown to remain stable even after orthodontic tooth movement.9,11 Additional studies have found using the ‘best fit’ and the regional palatal area just as reliable.12,13 The mandibular arch, however, lacks stable landmarks and requires surface superimposition of digitised study casts on mandibular basal bone structures using a combination of cone beam computed tomography (CBCT) images.8,14

Most of the stable points on the maxillary casts are based on the anterior segments of the palate. To improve roll and pitch control, some authors have recommended additional posterior landmarks.15 This suggested that authors had noted problems with the control of positions of the superimposed casts without the added reference landmarks (Figure 1). However, these methods used regional posterior extensions and cannot be applied when using point registration-based software.6,7,10 Therefore, the present study aimed to assess the validity and reproducibility of measuring maxillary orthodontic tooth movement on 3D digital study casts superimposed using a tripoding method. Tripoding locates three positions that could accurately reposition the 3D casts in a pre-defined spatial orientation. The null hypothesis is that there is no significant difference in tooth movement measurements between the digital superimposition using a tripoding method and the Pancherz cephalometric analysis.

Figure 1.

The intra-operator and inter-operator ICC values, for measurements using the cephalogram, had excellent agreements of 0.92 and 0.97, respectively. For measurements using the 3D cast method, the intra-operator and inter-operator ICC values had excellent agreement of 0.96.

Validity and agreement

The paired t tests showed no statistical differences (p > 0.05) between the horizontal and vertical tooth movements assessed by cephalometric and 3D cast superimposition (Table I). The mean differences between the two methods were 0.41 mm (s.d. 1.30) and 0.51 mm (s.d. 1.90) for the incisal vertical and horizontal measurements, and 0.10 mm (s.d. 1.54) and 0.03 mm (s.d. 1.35) for the molar vertical and horizontal measurements, respectively.

Table I.

Bland-Altman analysis and paired t tests comparing horizontal and vertical tooth movement from 3D and cephalometric superimpositions.

	Paired t test					Bland–Altman
Tooth movement	Mean (SD)	Standard Error mean	95% CI of difference	t (df)	p-value	Mean difference (SD)	Lower Limit	Upper limit
Incisor horizontal
3D Ceph	0.41 (1.30)	0.24	(-0.07,0.89)	1.72 (29)	0.096	0.41 (1.30)	-2.14	2.95
Incisor vertical
3D Ceph	-0.51 (1.90)	0.35	(-1.21,0.20)	-1.46 (29)	0.156	-0.51 (1.90)	-4.23	3.22
Molar horizontal
3D Ceph	0.10 (1.54)	0.28	(-0.47,0.68)	0.37 (29)	0.712	0.10 (1.54)	-2.91	3.13
Molar vertical
3D Ceph	0.03 (1.35)	0.25	(-0.48,0.53)	0.11 (29)	0.916	0.03 (1.35)	-2.62	2.67

Figure 5, illustrating the Bland-Altman plots, generally showed well-distributed differences. In Table I of the Bland–Altman analysis, the 95% limits of agreement were beyond the cutoff points of acceptable clinical difference (greater than 2 mm). The 95% limits of agreement were between -2.14 and 2.69 mm for the horizontal incisal movements and between -4.23 and 3.21 mm for the vertical incisal movements. The 95% limits of agreement were between -2.92 and 3.12 mm for the horizontal molar movements and between -2.62 and 2.68 mm for the vertical molar movements.

Figure 5.

Validation measures how well a new method, in this case 3D superimposition using a tripoding method, performs against a criterion method, which is represented by cephalometric superimposition. The measurement of dental and skeletal changes is valuable to determine the effectiveness of an intervention and to assess overall post-treatment outcome. Several authors have suggested that the clinically relevant differences in measuring anchorage loss are in the range of 1.5 mm19 to 2 mm.17 Therefore, it was proposed that a difference of less than 2 mm was an appropriate threshold to indicate a clinically acceptable difference between the conventional cephalometric and an alternative method for measuring tooth movements.

Measuring software using surface-based registration has the advantage of allowing regional superimposition without the need for manually locating landmarks which, in turn, is less time-consuming.8 It was also considered more accurate because there is less variability compared to a point registration-based method.20 Nonetheless, measuring software that depends on a point registration-base can still be used for superimposition provided that the fiducial points or landmarks are valid.

The medial two-thirds of the third rugae and a small distal area have high anatomical form stability and have been established as a structural reference for maxillary arch superimposition. However, few studies have included posterior extensions to the palatal rugae area for superimposition references.6,7,10 Garib et al.15 added lateral and posterior landmarks to those of stable regions to provide roll and pitch controls, respectively. This suggested that the authors had experienced problems with control of the positions of the superimposed casts without the added reference landmarks. Unlike regional-based software, this issue is particularly relevant in point registration-based software used for superimposition.10 However, past studies have used arbitrary margins as posterior references and not from recognisable anatomical landmarks. The replication of the arbitrary posterior extensions using point registration-based software is subjective and susceptible to errors because there is no specific landmark that can be identified within the recommended distance of the posterior extension to the midpalatal suture.15

The current study introduced a tripoding method using recognisable anatomical landmarks as references for point registration-based software to superimpose digitised maxillary dental casts. It is recommended that posterior reference points are added to avoid cast rotation in the sagittal plane (pitch) during the superimposition. By using three distanced co-ordinates, this method would concurrently allow for control of roll and yaw during superimposition. The current study found that the method showed no significant differences in the conducted measurements when compared to measurements of dental movements based on cephalograms. The method was also found to be reproducible. Therefore, it is a valid process to measure dental changes using 3D casts but the system is currently only applicable to the maxilla because stable references in the mandible, such as the chin, symphysis and the mental foramen, cannot be clinically identified by a 3D surface scanner and require CBCT imaging. Future studies are recommended to compare the validity of measuring orthodontic tooth movement using a 3D method that applied these reference landmarks to cephalograms.14,21

In the present study, the mean differences between both methods for incisal and molar movements were very small, being less than 0.51 mm. The paired t test confirmed that the differences were not significant, indicating that both methods were comparable. Previous studies assessing methods for measuring tooth movement found similar results; regional superimposition of the palate on digitised casts had acceptable validity when compared to direct measurements using references on plaster models22 and to cephalogram methods.23 However in the current study, the 95% limits of agreement for all dental movements were beyond the 2 mm cutoff point of acceptable clinical difference, which supported the findings of Choi et al.23 for the vertical dimensions. Therefore, this should be borne in mind when comparing the results of studies of dental casts and cephalograms as both methods may not be interchangeable because the applied horizontal and vertical reference planes are different.

The ICC and paired t test results showed that both methods were reproducible. The paired t test results indicated that the differences between measurements made at two intervals by both methods were not significant. Nevertheless, the 3D cast measurements fared better, having excellent reproducibility (ICC>0.90) in all measurements compared to the cephalometric method (ICC within 0.75–0.90). This may be due to easier landmark identification on the dental casts compared to the cephalograms. For example, bilateral structures such as the molars required using the cast as a guide to identify the left molar on the cephalogram compared to immediate tooth identification on the dental casts.

There are several limitations related to the present study. The pre- and post-treatment casts were of non-growing subjects less than 5 years apart, which was considered a stable interval to account for palatal rugae changes.15 Although the palatal rugae structures are considered stable, they are also subject to change over a long period, and therefore need to be carefully considered when comparing study casts of more than five years.6,15 Furthermore, although the bilateral depressions of the greater palatine foramen13 can be clinically identified and are reliably used for local anaesthesia blocks, there are no published studies on the stability of these foramen. Therefore, it is recommended that future studies investigate the stability of these posterior landmarks.

The authors declare that there is no conflict of interest.