The sequence and timing of dental growth are of special importance to the dental anthropologist. Estimation of age is an important task of physical anthropology when ancient remains are examined.
Dental development provides some of the best evidence for the age of death in children. The most widely used standard is one familiar to dentists: the Schour and Massler/Ubelaker chart. A version of it is available from the American Dental Association. A copy of the Schour and Massler chart hung in my own orthodontic office for 27 years. Many pictorial displays of dental development have been published since the first one by Legros and Magitot in 1880. Few tell what they are based upon. It has proven difficult to reconstruct the source of data that they used.
The Ubelaker update in 1978 was initially developed for studies of Native American Indians, but has become a universally recognized world standard. The Schour and Massler pictorial chart was first published in 1941. They were preceded by Logan and Kronsfeld of Loyola in 1933.
There are several dental development-based ageing methods. G. V. Black published a dental development chart over a century ago; how it was developed was not specified. Many methods are summarized at the end of the chapter, pp 146-147.
The Schour and Massler chart appears to have been based on development seen in terminally ill children. This is mentioned here NOT to discredit the chart, but to illustrate this point: many standards we take for granted in dentistry are based on studies where the sample population is unspecified. We need to know about those samples. Some of the cephalometric diagnostic analyses taken for granted are based on specific populations standards that may or may not fairly reflect our patient populations.
(My note: I have two other concerns about staging from dental development charts. One is this: it is accepted that dental age correlates poorly with skeletal age. This is why many orthodontists like hand-wrist films. They are somewhat better predictors of remaining growth than dental development or standing height. We will discuss hand-wrist standards late in this course.)
(My other concern is that boys and girls are lumped together in the tooth development charts. As quoted on page 125, Garn and others, girls are consistently ahead of boys in timing of different stages of dental development. Lumping them together makes the staging less accurate.)
I. INITIATION AND TIMING OF DENTAL GROWTH pp 118-125
This section is a brief review of the development of teeth before eruption. These following points should be understood by the anthropologist not trained in clinical dentistry.
(1) The development a tooth is an orderly, sequential, age-specific process beginning early in the first trimester.
(2) Formation of each tooth is likewise an orderly process beginning from centers of calcification at the cusp tips or incisal edges that progresses occlusally/incisally toward the apical end of the tooth.
(3) Deciduous tooth eruption, deciduous shedding, and eruption of the permanent dentition are orderly, sequential, and in general--age-specific.
Keep in mind that boys and girls differ in their pace of development. There are differences between ethnic groups, variation between individuals, and the effect of possible environmental disturbances that may delay growth and development of teeth.
Many of the methods of study, aging methods, and assessment of dental eruption will be unfamiliar to the clinical dentist.
II. METHODS OF STUDY pp 125-131
The familiar dental development diagrams were initially developed by Loyola's Logan and Kronfeld, and the University of Illinois' Schour and Massler on Chicago's west side in the years before World War II.
Logan was an oral surgeon who wanted accurate dates of development for treatment of cleft palate. Kronfeld was a histologist. While we associate the charts with radiographs, the pioneering work was the histological study of jaw sections. Schour's legacy is recognition of the neonatal line, a specialized brown line of Retzius formed at birth in developing enamel used for staging. Keep in mind that in this paragraph we are talking about microscopic study of stained jaw or tooth specimens.
Most studies of dental development in living children are based on radiography. Methodology varies, but most are bases on stages familiar to any dentist: initial mineralization, crown completion, and completion of the root apex.
III. AGING METHODS BASED ON ASSESSMENT OF POSTNATAL TOOTH FORMATION pp 131-137
Several methods are discussed. Pay attention to the age determination based on developmental progress of a single tooth. This is quite different from our usual clinical and teaching method of focusing on tooth eruption as the determinant of chronological age. These are based on radiographic profiles such as on pp 128-129 in the text.
Less familiar to clinicians is the method of Gustafson and Koch, pp 134-135 which uses a geometric presentation of first calcification, crown formation, tooth eruption, and root completion. I urge you to study their chart carefully. The legend beneath it is only an incomplete explanation.
The third molar is the most variable tooth in size, shape, formation, timing, and-even being missing at times. Yet, it covers the critical 15 year to early 20s, a time frame for which there are few alternatives. This technique has special importance in forensic dentistry.
One last creative method is to dissect out, clean, and air-dry tooth specimens. These are both weighed, measured, and compared to a standardized chart. This invasive technique has limited application.
IV. THE DEVELOPMENT AND ERUPTION OF TEETH pp 137-145
Eruption is the process of tooth movement through the alveolar bone and emergence into the oral cavity. These events followed by the stages of primary eruption into occlusion, secondary eruption compensating for occlusal wear, and physiological mesial drift are well known to practicing dentists.
While the observed stages of tooth eruption are well known, the mechanisms of tooth eruption are poorly understood. We cover this topic in a separate article later in the course.
The period of eruption is understood in three familiar phases: the deciduous dentition, mixed dentition, and the permanent dentition.
The permanent dentition has been studied more than the deciduous dentition. There is considerable variation in both sequence and timing.
This sequence applies to many populations around the world:
upper M1, I1, I2, P1, C & P2, M2
lower M1, I1, I2, C & P1, P2, M2
Compare this eruption pattern to the tooth to that in apes:
M1, I1, I2, M2, P3, P4, C, M3
The principal difference is that in apes, the canine erupts after the second molar while in humans the canine precedes it.
In apes the molars appear at 3.3, 6.6, and 10.5 years. In humans, this is 'slowed down' to 6, 12, and 18 years.
Many in anthropology believe that early hominids, such as Australopithecines displayed a distinctly apelike pattern of dental development. If ape standards are applied to the 'Taung child', it would have been 3 years old at the time of death instead of 6 years if human standards apply. (These remarks are from Lewin cited below, p 93.)
These dental eruption trends have been reported:
(1) There is strong correlation between equivalent teeth on the right and left side.
(2) There is poor correlation with neighboring teeth (which strikes a blow at the field theory).
(3) Lower teeth emerge before upper teeth.
(4) In a given population, the permanent dentition emerges earlier in girls than in boys.
(5) Children of poorer families show slightly later tooth emergence that the better well off. My comment: we will examine the relationship of economic status to growth later in this course.
Within a population, there may be differences in the eruption pattern for boys and girls. There are differences between populations, such as the late emergence of molars in Europeans.
Modern dental anthropologists and forensic dentists go beyond gingival or alveolar emergence to include the whole development of the dentition seen in radiographs and/or dissections in order to achieve as much accuracy as is possible.
Smith, B. "Standards of Human Tooth Formation and Dental Age Assessment" in Advances in Dental Anthropology New York: Wiley-Liss, 1991.
Lewin, R. Human Evolution An Illustrated Manual Boston: Blackwell Scientific Publications, 1993.