The abbreviations and terms used in the definition of the variables are as follows:
|
Variables |
Description |
| Age | Estimated age-at-death (in years) |
| Apex | Apex of the dentino-enamel junction |
| CRW | Crown |
| CRWFB | Crown formation beginning (in months after ovulation/fertilisation) |
| CRWFT | Crown formation time (in months) |
| DEJ | Dentino-enamel junction |
| GL | Estimated individual gestation length (in months) |
| H | Height (in mm) |
| HYP | Enamel hypoplastic defect |
| Indiv | Number of the individual |
| Label | Archaeological label |
| N | Number |
| NCK | Neck of the tooth |
| NNL | Neonatal line |
| OP | Occlusal plane |
| T | Thickness (in m m) |
| Wb | Wilson band |
| Wb_Age | Age of formation of the Wilson band (in months) |
List and description of the variables
General variables
Indiv (Field INDIVIDUAL in the database): serial number uniquely identifying the individual in the anthropological record.
Archaeological label (Field ARCH_LABEL in the database): excavation label.
Age (Field AGE in the database): age-at-death.
Tooth variables
Tooth (Field TOOTH in the database).
Side (Field SIDE in the database): side of the tooth (left or right).
CRW H (Field CROWN_H in the database): height (in mm) of the tooth crown measured with digital calipers.
HYP N (Field HYP_N in the database): number of enamel hypoplastic defects detected on the labial surface.
NCK_HYP(n) (Fields NCK_HYP1 to NCK_HYP4 in the database): straight line distance (in mm) from the neck to the n-th enamel hypoplastic defect.
Variables recorded by means of DIP
DEJ-OP (Field DEJ_OP in the database): thickness (in m m) of the enamel measured in unworn teeth as the distance from the apex of the DEJ to the cusp of the tooth (labial cusp in molars).
Wb N (Field WB_N in the database): number of Wilson bands (Wb) occurring on the labial portion of the enamel. To classify Wilson bands, the Condon and Rose (1992) criteria have been used: 1) stria that extends from dentino-enamel junction to surface; 2) presence of an accentuated stria on both buccal and lingual sides; and 3) evidence of prism bending.
The second point could not been always verified, because on the lingual aspect of the tooth the enamel is often thinner than on the buccal aspect, and thus the feature may not be present. However, the first and third points are considered adequate to define a Wilson band (Goodman et al., 1992).
NCK_APX (Field NCK_APX in the database): length (in m m) of the DEJ measured from the DEJ apex to the neck. The measurement was not taken when the bucco-lingual thin section did not coincide satisfactorily with the sagittal plane of the tooth.
NCK_NNL (Field NCK_NNL in the database): length (in m m) of the portion of the DEJ included between the neck and the point where the NNL meets the DEJ.
NCK_Wb(n) (Fields NCK_WB1 to NCK_WB9 in the database): length (in m m) of the portion of the DEJ included between the neck and the point where the n-th Wb meets the DEJ.
NNL_T (Fields NNL_T_1 to NNL_T_3 and NNL_T in the database): thickness (in m m) of the NNL. According to Eli and co-workers (1989), the thickness of the NNL is calculated as the arithmetic mean of three thickness measurements taken at the following three sites: 1) at beginning of the NNL, close to the dentino-enamel junction; 2) in the middle of the crown; and 3) close to the apex. The thickness of the NNL at each site is the mean value of multiple measurements (from three to ten) along the length of the line. The thickness measurements, taken following the orientation of the prisms and not orthogonal to the line itself, primarily represent the portion of the enamel in which the enamel rods were clearly disrupted. Changes in colour, sometimes associated with thinner areas showing changes in the organisation of the rods, were not considered in the measurement. The measurements were not taken when the bucco-lingual thin section did not coincide satisfactorily with the sagittal plane of the tooth.
Derived variables
All the derived variables were calculated according to published data on the age of onset and completion of enamel deposition of primary dentition. Data on the onset of enamel deposition were taken from Sunderland and co-workers (1987, in Smith, 1991); both Lunt and Law (1974, in Schroeder, 1991) and Moorrees and associates (1963) were used for data on the age of crown completion.
The following table is a modified version of the original tables (in Schroeder, 1991: 30; Smith, 1991: 156 and 157). Original figures in weeks after ovulation/fertilisation and years after birth are transformed here into months after ovulation/fertilisation according to a mean length of 4.3 weeks per month and a mean gestation length of 9.4 months (Last, 1983: 283 days divided by 30.4 days per month; see also Friedman, 1995).
|
CRWFB |
CRWFT |
|||
|
Central Value |
Range |
Central Value |
-2s.d. - +2s.d. |
|
| upper central incisor |
3.45a |
2.99-3.91a |
7.46b |
- |
| upper lateral incisor |
3.91a |
3.22-4.38a |
8.20b |
- |
| upper canine |
4.38a |
3.91-4.61a |
12.90c |
9.66-16.18c |
| upper first molar |
3.68a |
3.22-3.91a |
10.20c |
7.60-12.80c |
| upper second molar |
4.38a |
4.14-4.61a |
13.57c |
10.26-16.88c |
| lower central incisor |
3.45a |
2.99-3.91a |
8.46b |
- |
| lower lateral incisor |
3.91a |
3.22-4.38a |
8.70b |
- |
| lower canine |
4.38a |
3.91-4.61a |
12.90c |
9.66-16.18c |
| lower first molar |
3.68a |
3.22-3.91a |
10.20c |
7.60-12.80c |
| lower second molar |
4.38a |
4.14-4.61a |
13.57c |
10.26-16.88c |
a Sunderland et al., 1987; b Lunt and Law, 1974; c Moorrees et al., 1963 (pooled sexes).
Many other standards have been proposed for the primary dentition (for a comprehensive critical review, see Smith, 1991; Skinner and Goodman, 1992).
The NNL_APX, Wb_APX variables used in the formulae defined below are derived variables and are defined only in order to simplify the formulation of the remaining variables; consequently, APX_NNL and Wb_APX are not stored in the database.
NNL_APX: difference between NCK_APX (length of the DEJ measured from the DEJ apex to the neck) and NCK_NNL (length of the portion of DEJ included between the neck and the point where the NNL meets the DEJ):
(1) 
Wb_APX: difference between NCK_APX (length of the DEJ measured from the DEJ apex to the neck) and NCK_Wb(n) (length of the portion of DEJ included between the neck and the point where the n-th Wb meets the DEJ):
(2) 
GL (Field GL in the database): estimated individual gestation length (in months). The crown formation time of the tooth (CRWFT) is multiplied by the ratio of the NNL_APX length and the total DEJ length and added to the crown formation beginning time (CRWFB):
(3) 
If GL can not be computed because one of the variables in (3) is missing, the value of GL = 9.4 (Friedman, 1995) is used as an estimate.
Wb Age (Fields WB_AGE1 to WB_AGE8 in the database): estimated age of Wb formation (in months). Crown formation time (CRWFT) of the tooth multiplied by the ratio of the Wb_APX length and the total DEJ length added to the crown formation beginning time (CRWFB) minus the gestation length estimate (GL):
(4) 
Wb Age can be estimated using (4) for intact teeth only. If the tooth cusp is worn to the dentine or the bucco-lingual thin section does not coincide satisfactorily with the sagittal plane of the tooth, the NCK_APX length can not be measured. In this case, assuming a constant rate of enamel formation and calculating the time needed to complete the crown after birth (CRWFAB) as
the formula becomes:
(5) 
The biological history of the individual as recorded in the enamel
Whenever possible, more than one tooth from each individual was used in the analysis. This, however, created problems related to variability in formation times of the crowns and different thresholds of sensitivity for each tooth (Skinner and Goodman, 1992). It is possible to approximate the standard deviation of the age-at-stress estimate using the figures in column four of the first table in this chapter. The unknown standard deviation for the incisors can be reasonably substituted with the maximum range observed in CRWFB. This approximation is not a true statistical parameter, but the standard deviations listed below can be used as descriptive parameters of the uncertainty in the determination of the age at which a stress resulted in the formation of an enamel defect:
| s.d. (in months) | |
| central incisor | 0.92 |
| lateral incisor | 1.16 |
| canine | 1.63 |
| first molar | 1.30 |
| second molar | 1.65 |
Moreover, variation in the timing of enamel formation influences the synchronicity of dental development, and it is therefore difficult to estimate the amount of deviation from the synchronous pattern expected for each individual in the sample. Consequently, it is problematic to discriminate between events recorded on different teeth within a relatively short span of time. Teeth act as biological tape recorders, reasonably stable in their speed while recording, but the moment when they are turned on and the absolute speed of the recording is quite variable. For example, if a Wb is estimated to have formed on the lateral lower incisor at the age of 4.5± 1.66 months, and a second one on the lower canine of the same individual at 5± 1.63 months, it is difficult to determine if the two Wbs represent a single stress event or two different ones. This can be tested statistically, but the figures in the previous table are only intended to approximate the true variability and nothing is known about the statistical distribution of the events. It is not feasible to fix a reliable threshold of time that can be used to distinguish between Wbs, and the problem becomes even more complex if multiple Wbs are present in different teeth.
