Three-dimensional Procrustes analysis of modern human craniofacial form.

The objective of this study was to analyze modern human craniofacial form using 3D Procrustes superimposition in order to establish a reference model and validate it on computed tomography (CT). The sample consists of 136 specimens from five modern human regional groups. Thirty-three craniofacial landmark coordinates have been recorded using a Microscribe and calculated on CT scans for five crania from the sample. Procrustes superimposition has been performed to calculate the mean shape, and a discriminant analysis has also been carried out to estimate the variability of shape. The results show that the repeatability of measurements made on CT and on Microscribe is excellent (R = 0.99). There is no major distinctiveness in the craniofacial shape; however, discriminant function 1 separates out the European crania from the others, especially African and American. It includes the width and the length of the face, the flatness of the upper face, the prognathism of the maxilla, as well as the length and the inclination of the palate. The width of the maxilla and the palate do not show a great variability. This may be the common invariant feature responsible for the alignment of the teeth in all specimens. It may correspond to functional patterns related to masticatory constraints manifested by the important interproximal and occlusal dental wear in all specimens. This study confirms the high accuracy of measurements made on CT scan and the importance of geometric morphometrics, which provides an accurate characterization of the overall craniofacial shape and its variation within the entire population.

[Computed tomography and cranial paleoanthropology]. / Scanner à rayons X et paléoanthropologie crânienne.

Since its invention in 1972, computed tomography (C.T.) has significantly evolved. With the advent of multi-slice detectors (500 times more sensitive than conventional radiography) and high-powered computer programs, medical applications have also improved. CT is now contributing to paleoanthropological research. Its non-destructive nature is the biggest advantage for studying fossil skulls. The second advantage is the possibility of image analysis, storage, and transmission. Potential disadvantages include the possible loss of files and the need to keep up with rapid technological advances. Our experience since the late 1970s, and a recent PhD thesis, led us to describe routine applications of this method. The main contributions of CT to cranial paleoanthropology are five-fold: --Numerical anatomy with rapid acquisition and high spatial resolution (helicoidal and multidetector CT) offering digital storage and stereolithography (3D printing). --Numerical biometry (2D and 3D) can be used to create "normograms" such as the 3D craniofacial reference model used in maxillofacial surgery. --Numerical analysis offers thorough characterization of the specimen and its state of conservation and/or restoration. --From "surrealism" to virtual imaging, anatomical structures can be reconstructed, providing access to hidden or dangerous zones. --The time dimension (4D imaging) confers movement and the possibility for endoscopic simulation and internal navigation (see Iconography). New technical developments will focus on data processing and networking. It remains our duty to deal respectfully with human fossils.

Eruption of third molars: relationship to inclination of adjacent molars.

The purpose of this study was to determine the relationship between the sagittal inclination of the first and second maxillary molars and the eruption of the third molars. The sample consisted of 2 groups. The subjects in the first group (n = 28) had complete normal dentitions including third molars; those in the second group (n = 32) had impacted right and left third molars. The sagittal inclinations to the palatal plane of the first and second maxillary molars were measured on computed tomography sagittal images obtained with multiplanar reconstructions. The Mann-Whitney U test was used to compare mean angular values between the 2 groups. Spearman correlation coefficients were calculated to assess the relationship with age. A multivariate analysis was used to evaluate the relationship between the eruption of the third molars and the sagittal inclination of the first molar, the second molar, sex, and age. Maxillary first and second molars were more mesially inclined in the first group, particularly in the younger subjects (16-25 years). This inclination increased with age. The logistic regression showed that the sagittal inclination of the first molar is a predictor of the eruption of the third molar. This finding suggests that a more mesially inclined maxillary dentition is likely to be associated with third molar eruption. The absence of data on space requirements in the maxillary arch and interarch relationships warrants further exploration in an orthodontic population.