Osteodental Biology of the People of Portus Romae (Necropolis of Isola Sacra, 2nd-3rd Cent. AD).I.
Enamel Microstructure and Developmental Defect of the Primary Dentition
by Paola Francesca Rossi, Luca Bondioli, Giorgio Geusa, Roberto Macchiarelli
Soprintendenza Speciale al Museo Nazionale Preistorico Etnografico 'L. Pigorini', Roma, Italy

3.3. The Neonatal Line

The birth process leaves its mark on human enamel in the form of a birth ring, called the neonatal line (Rushton, 1933). This line separates the enamel formed in intrauterine life from that formed after birth, and is a very prominent incremental line which appears in enamel and occasionally in dentine (Skinner, 1992). It is normally observable in individuals who survive at least 15 days after birth (Whittaker and Richards, 1978).

The neonatal line is usually present in all crowns forming at birth, that is, all the elements of the primary dentition and the first permanent molars (mesial cusp).

Occasionally, a macroscopic hypoplasia can be detected in correspondence with the neonatal line, but it is usually worn away (Massler et al., 1941; Moss-Salentijn and Hendricks-Klyvert, 1985; Schroeder, 1991; Skinner, 1992).

The neonatal line can be clearly recognised because of its characteristic location on the tooth, specific for each tooth class, and due to the difference in quality between pre- and postnatal enamel (Skinner, 1992; Skinner and Dupras, 1993; Rossi et al., 1997a, b). In incisors, the line extends from the dentino-enamel junction into the cervical part of the crown and out to the surface, leaving only a small portion of postnatally formed enamel. In canines and molars, it is located more towards the incisal/occlusal part of the enamel, with only a small portion of prenatally formed enamel present (Teivens et al., 1996).

Histologically, the neonatal line is distinguishable from a Wilson band because it is more prominent and, at high magnification, displays a more clearly discernible prism disturbance. Investigations based on scanning electron microscopy reveal that it is produced by a very abrupt change in prism orientation and by a structural change within each prism (Whittaker and Richards, 1978).

It has not yet been definitely established how much time is needed for the neonatal line to form, but Levine and co-workers (1979) observed that in subjects who survived birth by more than a few days, the majority of primary teeth showed a neonatal line. Similarly, Whittaker and Richards (1978) found that a period of about two weeks after birth is necessary to form a neonatal line.

A further problem concerns the factors responsible for the formation of the line itself. At first, its formation was assumed to be related only to environmental and dietary changes, without considering the potential role played by the birth process (Massler et al., 1941). Bouyssou and co-workers (1958, cit. by Wilson and Shroff, 1970) were probably the first to hypothesise that systemic influences at birth could sometimes affect the neonatal line. However, Godt (1963, cit. by Wilson and Shroff, 1970) found no definite relationship between the width of the birthline and striae related to systemic disturbances.

Examining the developing dentitions of three infants who died at birth, 65 days, and 70 days after birth, respectively, Weber and Eisenmann (1971) found neonatal lines of 20-30 mm in width only in the last two individuals. More recently, Eli and co-authors (1989) carefully investigated the relationship between method of delivery and neonatal line thickness in a sample of 147 children of known birth history. Their results showed that: a) operative delivery was always associated with wider lines (13-24 mm); b) normal delivery resulted in lower values (7-17 mm); and c) cesarean sections resulted in very thin lines (6-9 mm). This evidence thus confirms the hypothesis that both environmental changes and the birth process itself contribute to the growth disruption responsible for the formation of the neonatal line.

The location of the neonatal line on the tooth is also an indirect indicator of individual gestation length. By regressing the distance from the neonatal line to the neck and expressing gestation length as a deviation in days from birth, Skinner (1992) showed that a premature birth will shift the line occlusally. This effect, previously hypothesised by Kronfeld and Schour (1939, cit. by Skinner, 1992), has also been confirmed by Skinner and Dupras (1993).