Holt et al. 2012.1 4 Brief Communication: Premolar Enamel Formation: Completion of Figures for Aging LEH Defects in Permanent Dentition Sarah A. Holt 1* , Donald J. Reid 2 , and Debbie Guatelli-Steinberg 1 1Department of Anthropology, The Ohio State University, Columbus, OH 43210, USA 2Department of Oral Biology, School of Dental Sciences, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4BW, UK Key Words: Cross striations; Striae of Retzius; Enamel formation; Tooth growth; Dental development Variation in enamel formation provides many avenues of inquiry for those interested in com- parisons of developmental life histories and ag- ing enamel defects. For example, assessing the age of formation of linear enamel hypoplasias provides one line of evidence to estimate rates of childhood morbidity (e.g., Goodman and Song, 1998). Due to the nonlinear nature of enamel growth, linear regression formulas utilizing total crown height cannot be used to accurately esti- mate age of LEH formation (Reid and Dean, 2000; Martin et al., 2008; Ritzman et al., 2008). Data on population specific variation in crown formation times based on percentages of total growth are therefore of great use for accurately estimating LEH formation times. The Reid and Dean (2006) methodology for histological growth assessment reflects the vari- ability of enamel growth rates along the length of the crown and incorporates standard deviations reflecting inter-individual variation, avoiding the problem of applying linear statistical models to a nonlinear growth pattern. Studies completed us- ing other methods, including dental radiographs of living children, suggested high levels of varia- tion in crown formation time among human populations (e.g., Tompkins, 1996). The implica- tion is that new research on LEH formation times conducted without the histological growth as- sessment method should draw on crown forma- tion timing data specific to the population being studied (Reid and Dean, 2000; Reid et al., 2006; Martin et al., 2008). However, recent work in which incremental growth is assessed histologi- cally has shown the anterior teeth and molars to be less variable between populations than previ- ously reported (Reid and Dean, 2006). Reid and Dean first published research com- paring enamel formation times in populations from northern Europe (2000, 2006) and southern Africa (2006) that included tables presenting age in days for enamel formation at the completion of each decile of crown height for anterior teeth and molars (2006: 334-35). Figures depicting these data provided a visual guide for estimating LEH (2006: 343-44). These figures include an estimated age at mineralization based on previous histo- logical studies (Reid et al., 1998; Reid and Dean, 2000; Antoine, 2001; Dean and Reid, 2001) but the authors note these initiation times are highly variable, as much as a full year in the M3 (Reid and Dean, 2006). A follow-up study (Reid et al., 2008) presented premolar data from these same populations, but did not provide charts of growth by decile of crown height or figures in- cluding initiation estimations. ABSTRACT Variation in enamel formation has become increasingly important in comparative studies of dental development. Previously pub- lished work on the development of human enamel in groups from southern Africa and northern Europe has allowed for more accurate estimation of formation timing of linear enamel hypoplasias. Currently, although data for all tooth types has been published, charts of enamel growth by decile useful in this type of estimation have been limited to molars and anterior teeth. This paper completes this series with a table and figure of mean formation times of human premo- lars for each decile of crown development using previously published histological data of daily enamel growth. *Correspondence to: Sarah Holt, Department of Anthropology, The Ohio State University 4034 Smith Laboratory. 174 W. 18th Ave. Columbus, Ohio 43210 USA Email: holt.249@osu.edu TEL: (614)330-6930 5 TABLE 1. Age (in days) for enamel formation at each decile of crown height for molar teeth in each sample, +/-1 standard deviation Southern African premolar tooth crown formation times Northern European Crown Formation Times LP3 n=33 LP4 n=28 UP3 n=45 UP4 n=41 Initiation 675 967 675 967 Cusp completion 902 +/- 27 1234 +/- 32 910 +/- 36 1211 +/- 42 10% complete 943 +/- 29 1270 +/- 34 949 +/- 36 1250 +/- 42 20% complete 995 +/- 33 1310 +/- 38 993 +/- 34 1292 +/- 42 30% complete 1053 +/- 37 1356 +/- 48 1039 +/- 34 1335 +/- 42 40% complete 1116 +/- 48 1408 +/- 53 1092 +/- 37 1382 +/- 46 50% complete 1187 +/- 61 1472 +/- 60 1157 +/- 44 1444 +/- 46 60%complete 1266 +/- 75 1548 +/- 74 1239 +/- 52 1526 +/- 50 70% complete 1356 +/- 92 1638 +/- 92 1336 +/- 55 1627 +/- 58 80% complete 1452 +/- 109 1741 +/- 110 1443 +/- 62 1737 +/- 70 90% complete 1558 +/- 124 1869 +/- 117 1570 +/- 71 1854 +/- 77 Crown completion 1665 +/- 141 1986 +/- 124 1703 +/- 76 1974 +/- 82 LP3 n=33 LP4 n=22 UP3 n=34 UP4 n=44 Initiation 675 967 675 967 Cusp completion 891 +/-44 1231 +/-36 952 +/-65 1225 +/-63 10% complete 952 +/-54 1276 +/-41 1010 +/-61 1284 +/-52 20% complete 1018 +/-68 1328 +/-43 1068 +/-52 1338 +/-48 30% complete 1088 +/-80 1381 +/-52 1132 +/-50 1390 +/-44 40% complete 1164 +/-96 1441 +/-65 1203 +/-49 1448 +/-48 50% complete 1256 +/-113 1509 +/-76 1285 +/-53 1521 +/-52 60%complete 1359 +/-135 1594 +/-84 1389 +/-60 1613 +/-62 70% complete 1481 +/-164 1697 +/-98 1518 +/-66 1724 +/-71 80% complete 1614 +/-194 1817 +/-122 1663 +/-78 1856 +/-85 90% complete 1766 +/-224 1948 +/-143 1838 +/-84 1998 +/-98 Crown completion 1908 +/-253 2071 +/-162 2011 +/-92 2134 +/-110 Currently, therefore, although enamel growth patterns of these populations have been established, the series of charts and figures pro- viding a visual guide for estimating population specific LEH formation times has lacked the pub- lished information on premolars. This communi- cation completes the publication of this series of figures by presenting premolar enamel growth by decile for populations from southern Africa and northern Europe (Table 1, Fig. 1). MATERIALS AND METHODS Data from 147 premolars collected from two populations, southern Africa and Newcastle, England, (northern Europe), were used to create tables of enamel formation (Reid et al., 2008). The premolars were originally collected after extrac- tion during oral surgery and histological thin sec- tions were prepared for polarized light micros- copy (Reid and Dean, 2006). Individual periodic- ity for each tooth was established by counting daily cross-striations in enamel, and formation times for each decile of crown height was then recorded using measurements of long-period striations corresponding to the perikymata on the external crown surface. Initiation ages of crown mineralization for both samples were estimated from a third, French sample (Reid et al., 1998). By adding these decile data to age at initiation of crown mineralization, formation times for both cuspal and lateral enamel formation in days was determined (see Reid et al., 2008 and Reid and Dean, 1998 for full discussion of methodology). 6 Although extremely preliminary research by one of the authors on mineralization initiation times specific to these populations supports the current expectation that mineralization timing will vary, conclusive data from large scale studies remains unavailable. Given the absence of data, the significance of future publications in this area on the chart presented here would be speculative, however, the authors intend to update the avail- able charts as new data are available. DISCUSSION Although total crown formation time was found to be significantly different between the populations (p<0.00) (see Reid et al., 2008 for full discussion of statistical methods), as with the an- terior and molar teeth, premolar formation time between populations is more similar than radio- graphic studies once suggested (e.g., Tompkins, 1996). While the southern African sample crowns formed consistently more quickly, the means of both samples range only from 0.3 years difference in the lower P4 to 0.8 years difference in the up- per P3. Thus, the small amount of variation may not be particularly meaningful in comparisons of human populations (Reid and Dean, 2006; Martin et al., 2008). Because this method presents formation time by decile, the estimation of age during LEH formation can be independent from any variation in length of the crown. This more accurate method has already been widely used for the an- terior teeth and molars (e.g., Reid and Dean, 2000; 2006). The addition of premolar data will allow for comparisons of LEH within the denti- tion of individuals, specifically to match LEH manifestations of a single stress event across pre- molars and other teeth within an individual. De- spite the fact that enamel growth is not linear, Martin et al. (2008) found no difference between a linear and nonlinear interpolation for ages that fall between the established deciles. They con- clude that a linear interpolation is sufficient to age defects that fall within a decile. While we await further histological data from other popu- lations, the charts presented here (Table 1, Fig. 1) make it possible to provide estimates of LEH for- mation times that utilize the population specific information currently available from two geo- graphically distant populations. CONCLUSION Recent studies applying histological methods to establish enamel formation timing have shown less variation in modern human premolar forma- tion between populations than previous method- ologies; however, statistically significant differ- ences have been documented between a large sample of northern European and southern Afri- can individuals (Reid and Dean, 1998; Reid et al., 2008). Previously published data on histological timing of each decile of premolar tooth crowns can be used to estimate timing of LEH formation without destructive histological sectioning. This paper presents a summary of the previously pub- lished data on crown formation variation and presents a graphic diagram of the premolar for- mation times by decile drawn from histological analysis (Table 1, Fig. 1). In addition to providing previously unpublished data on the mineraliza- tion initiation estimates used to create these dec- ile formation charts, it is the hope of the authors that including a visual representation of premolar crown formation by decile to the existing charts for other tooth types will allow for more practical application of the known formation timing to analysis of the external enamel of premolars. Fig. 1. Mean estimates for the chronological ages of enamel formation in premolars for each decile of crown length rounded up or down to 0.1 year for the southern African sample vs. the northern European sample. Both initiation and cuspal enamel formation are included in these estimates. 7 LITERATURE CITED Antoine, D. 2001. Evaluating the periodicity of in- cremental structures in dental enamel as a means of studying growth in children from past populations. Ph.D. Dissertation, Univer- sity of London. Dean, MC, Reid, DJ. 2001. 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