Three dimensional (3D) landmark data coiiection

20a/b-22a/b

2.2.3 Three dimensional (3D) landmark data coiiection

The aim of here was to collect 3D data across a broad developmental range from the growing mandibles and permanent molar teeth of the primate taxa above. The general methodology was as follows. 3D landmark data was collectible directly from the surface of the mandible. However, the mandibular corpus concealed the developing molar teeth. This made direct data collection from the developing dentition impossible without damage to the jawbone. Thus, to indirectly collect 3D molar landmark data, each mandible was radiographed in two different 2D planes ([x-, y-] and [x-, z-]). Imaging software enabled the collection of 2D co-ordinate data taken from each radiographic plane once the radiographic images had been scanned into a computer. Another software package combined these sets of 2D co-ordinates to create single sets of 3D co ordinates (X -, y-, Z -). The same software facilitated the registration of these molar data with the 3D mandibular data using nine landmarks common to both data sets. This methodology is described in complete detail below.

3D landmarks were designed upon the strengths of their homologies among taxa. Landmarks were equally chosen by how appropriately they visually represented key areas of mandibular and dental developmental change and overall morphology. It was critical that all landmarks were easily recognisable and could thus be correctly and consistently located among specimens. Landmark homology is largely dependent upon the biological reality of the defined point. Three different types of landmarks defined by the strength of their homology are Type I, Type II and Type III landmarks (Bookstein 1991, Marcus et al. 1996, and Slice et al. 2000). Type I landmarks define real biological features such as the coronal suture and have the most robust homology. Type II landmarks define changeable or inconsistent biological features, such as the superior-most point of the skull along this suture. Their homology is arguably less sound. Type III landmarks define features that have no biological significance or mathematical basis, and are defined in relation to Type I and II landmarks. For example, a point directly lateral to the superior- most point along the coronal suture. The homology of Type III landmarks is

very weak. The landmarks included in this study were largely Type I and II landmarks.

Eighty-six 3D landmarks were collected, or digitized, bilaterally from each mandible (Figures 2.4-2.6, Tables 2.4 and 2.5a-f). The first thirty-seven of these landmarks were strictly mandibular. The next forty landmarks were taken from the developing gubernaculae of the permanent canines, premolars and molars. Each gubernaculum was defined by four landmarks taken at four ‘corners’ at the surface of the mandibular corpus. Where no gubernaculum was yet visible, four landmarks were taken in an identical location either directly behind the gubernaculum of a developing permanent tooth or just medial to the position of the deciduous tooth into which the respective permanent tooth would emerge. A subsequent nine landmarks (landmarks 78-84) were taken around the mandible (Figure 2.7, Table 2.6). However, these nine landmarks had no biological significance and were included in both digitized and radiographic data as the common markers by which the 2D radiographic data were combined and registered with the 3D mandibular landmark data. These nine landmarks were made radio-dense (with metal markers) so that they were easily seen in the radiographs. Forty-nine landmarks were derived from the radiographs of the three developing molar crypts and/or crowns (Figures 2.8-2.10, Table 2.7). The permanent first and second molar teeth were each described by sixteen landmarks. Seventeen landmarks described the permanent third molar; an additional landmark was ascribed to the hypoconulid. Finally, six landmarks describing the mandibular canal were derived from the radiographs and taken at the mandibular (landmark 90) and mental foramina (landmark 95) (Figure 2.7, Table 2.7) and at four equidistant intervals between these two foramina (landmarks 91-94).

Table 2.4 List of mandibular landmarks.

LANDMARK LANDMARK LANDMARK DESCRIPTION

NUMBER TYPE

right, left

1,21 Type II Superolateral-most tip of the coronoid process.

2, 22 Type II Inferior-most point of the mandibular notch. 3, 23 Type II Anterior-most tip of the condyle.

4, 24 Type II Lateral-most tip of the condyle. 5, 25 Type II Posterior-most tip of the condyle. 6, 26 Type II Mesial-most tip of the condyle.

7, 27 Type III Superior-most tip on the articular surface of the condyle.

8, 28 Type II Deepest concavity of the posterior border of the ascending ramus.

9, 29 Type III Point opposite landmark 8/28, on the anterior border of the ascending ramus. 10, 30 Type 1 Apex of lingula, or, if lingula is undefinable

or absent, the anterosuperior-most margin of the mandibular foramen.

11,31 Type III Point on the posterior border of the ramus just superior to the blending of the ramus into the gonial angle or the point at which a tangent leaves the posterior border of the ramus.

12, 32 Type III Posteriorly, the point on the gonial angle that is the apex of the (90 degree) angle formed by landmarks 11/31 and 13/33. 13, 33 Type III Inferiorly, the point along the gonial angle

where a tangent leaves the inferior margin of the mandible.

14, 34 Type II Superiorly directed indentation of the inferior border of the mandibular corpus, just anterior to landmark 13/33.

15, 35 Type II Point at which the ascending ramus meets and obscures the corpus, in lateral view along the alveolar bone.

16, 36 Type 1 Mental foramen, midpoint at the level of the surface of the mandibular corpus.

17, 37 Type III Point on the alveolar border of the mandibular corpus directly superior to landmark 16/36.

18 Type II Midpoint between the central incisors at the superior-most tip of the alveolar bone.

19 Type II Anterior-most projection of the subalveolar bone in the mental region along the midline. 20 Type II Symphyseal midpoint of the inferior margin

of the mandibular corpus, directly inferior to the areas of attachment of the geniohyoid and genioglossus muscles, i.e. between the attachment areas for the anterior belly of the digastric muscle.