Biology Reference
In-Depth Information
FIGURE 14.1
Landmark and semilandmark placement on three-dimensional scans of a cast made of a human
dentition. Semilandmarks were placed along the occlusal surfaces of the six anterior teeth in the maxilla and
mandible.
while landmarks were being placed so changes in three-dimensional perspective could be
used to verify accurate placement. This resulted in a total of 60 points along the incisal
edges of the anterior teeth of both the maxillary and mandibular arches.
To determine the measurement resolution, the scans of three maxillary and three man-
dibular specimens were digitized ten times by the same operator, and the scatter of
Procrustes and Procrustes-SP distances for each specimen about the mean for that speci-
men was determined. This is an underestimate of the actual measurement error because it
does not include estimates of the contributions from the creation of the dental casts and
three-dimensional scanning. From the data of these distances, the average root mean
square (RMS) scatter of repeated measurement specimens was calculated. The RMS scatter
is analogous to a standard deviation, although it is not measured in the same way, nor
does it have the same statistical properties. However, experiments have found that
93
96% of repeatedly digitized specimens lie within twice the RMS scatter level.
In Procrustes units the RMS scatter in this data was 0.02 for both the maxillary and
mandibular dentition, so the matching criteria were twice this, 0.04. Because landmarks
are scaled to the centroid size, this measurement indicates that our typical error in measur-
ing all 60 landmarks was about 2% of the total size of the dentition. When using
Procrustes-SP methods, the RMS scatter per landmark point was roughly 0.2 mm, still
about 2% of centroid size, which is consistent with an approximate 1 mm width of the
