Role of Tooth Size, Morphology, and Composition
Introduction
One approach to the prediction of future caries incidence is to study the tooth itself, allowing for the fact that the environment of the tooth will be equally important. The various aspects of tooth resistance then take on greater importance. With this approach, the individual or group of individuals showing resistance to caries can be identified.
Various aspects of the resistance of a tooth to dental caries can be described. The shape and size of the whole tooth may affect the degree of crowding and influence susceptibility to caries. However, other characteristics must also be considered.
Because dental caries is initiated on the enamel surface, physical characteristics, such as a defective or rough enamel surface, and the chemistry of the enamel might also be determinants of tooth resistance.
Physical characteristics of the tooth
Tooth size
Hunter (1967) studied the size of primary teeth in relation to past caries experience.
He found that teeth that had been restored were significantly larger than untreated, caries-free teeth. This implies that the larger teeth were more caries susceptible, and indeed they were found to have been in the mouth for a shorter time because they erupted later than did smaller teeth. Grahnen and Ingervall (1963) had earlier noted a relationship between tooth width and caries resistance, smaller teeth being associated with lower caries incidence. Paynter and Gainger (1962) reported that the overall dimensions of teeth were smaller in areas with water fluoridation. Other studies have also reported an association between smaller teeth and lower caries prevalence: For example, Stern and Curzon (1975) found that size was related to geographic as well as individual caries prevalence. Similar findings have come from studies of naval recruits in the Untied States (Keene, 1971) and of stable island populations such as in Papua New Guinea (Schamschula et al, 1972a).
However, while on a population basis it can be shown that groups of individuals with low caries may have smaller teeth, this is hardly a useful tool for caries prediction in the individual patient because dental caries is such a multifactorial disease: The effect of tooth size would be negligible in comparison with the combined effect of other factors such as the quality of plaque control, use of fluoride, and dietary habits. However, tooth size may lead to extremely prolonged eruption time and subsequent rotation and tipping of teeth, which will increase plaque accumulation, hinder access for mechanical plaque control, and postpone occlusal contact and the beneficial frictional effect derived from chewing fiber-rich food.
Studies by Carvalho et al (1989) have indicated that de novo plaque reaccumulation over 48 hours is about five times greater in the fissures of erupting first molars than in fully erupted molars, particularly in the distal and central fossae. This explains why almost all fissure caries in molars is initiated during the long eruption time (12 to 18 months), while fissure caries in premolars, which have an eruption time of only 1 to 2 months, is rare. A further factor is the high susceptibility of the immature enamel to caries during eruption and the following year, until completion of secondary maturation.
Tooth morphology and cusp and fissure pattern
The pattern of carious and restored tooth surfaces varies significantly even in the
primary dentition. In a toothbrushing population, caries susceptibility in the
permanent dentition may be ranked in the following order:
1. Fissures of the molars.
2. Mesial and distal surfaces of the first molars.
3. Mesial surfaces of the second molars and distal surfaces of the second premolars.
4. Distal and mesial surfaces of the maxillary first premolars.
5. Distal surfaces of the canines and mesial surfaces of the mandibular first premolars.
6. Approximal surfaces of the maxillary incisors.
The pattern of carious and restored surfaces is related to the buccolingual width of the
tooth crown, and to sites where dental plaque stagnates, that is, to toothbrush
accessibility. In nontoothbrushing populations, or in individuals with poor and
irregular oral hygiene habits (precluding regular use of fluoride toothpaste) and a high
intake of sticky sugary food, cervical lesions may also develop on the buccal surfaces
of the maxillary teeth and on the mandibular molars and premolars.
Although tooth morphology is basically similar among the races, some racial
characteristics that predispose particular groups to dental caries have been identified.
Plaque retention is enhanced by the presence of buccal pits, lingual pits on incisors,
deep palatal grooves, or grooves within the Carabelli cusps. The dentition of the
American Indians and Inuit (Eskimo) has shovel-shaped incisors, barrel-shaped
incisors, and deep buccal pits on molars; plaque accumulation from a cariogenic diet
will predispose these areas to caries (Mayhall, 1977). A similar feature predisposing
to caries in the maxillary molars of whites is the higher prevalence of the Carabelli
cusp (Dahlberg, 1961). The fissure pattern of permanent molars also varies with racial
groups: Some teeth have deeply convoluted fissures prone to caries (Taylor, 1978),
whereas in other teeth the fissures are shallow and almost imperceptible. The
distribution of fissure patterns will vary among population groups, and even within a
racial group, rendering teeth more or less susceptible to dental caries. The permanent
first molar of the Inuit demonstrates this characteristic in particular (Mayhall, 1977).
One congenital condition resulting in characteristically small teeth and few molar
fissures is Down’s syndrome (Brown and Schodel, 1976).
The pattern of cusp form and fissure pattern is genetically determined but is probably
of minor importance in caries resistance (or susceptibility). In an individual patient,
tooth morphology may be an aid in deciding whether or not a tooth should receive a
fissure sealant or not. Attempts to classify the shape of fissures so that they might be
assessed as more or less caries prone were made many years ago (Bossert, 1937).
Furthermore, fissure pattern and its relation to structure within the depth of enamel is
highly variable (Mortimer, 1964). However, based on the cross-sectional shape of the
fissures of first molars, it has been found that most of the teeth (almost 90%) have socalled
normal fissures, in cross section they have a relatively wide opening, followed
by a narrow cleft, approximately 1.0 mm deep (width 0.1 mm), reaching almost to the
dentinoenamel junction (Fig 116). The carious lesion usually starts as an enamel
lesion on both sides of the entrance of the fissure, which is visible and accessible to a
probe. However, some atypical fissures (fewer than 10%) with a narrow opening and
a bulbous widening at the base should be regarded as risk fissures (Fig 117), because
a lesion can start at the base as well as at the entrance to the fissure. Fortunately, from
a diagnostic point of view, there is a strong correlation between steep cuspal
inclination and such sticky risk fissures.
However, even extreme risk fissures with irregularities such as “horizontal tunnels”
can be kept free of caries, as shown in Fig 118 (my daughter Eva at the age of 10
years). These first molar fissures were successfully maintained free of caries, without
fissure sealants, by diligent oral hygiene and daily use of fluoride toothpaste (Fig
119). The many genetic variations in the shape of the tooth crown, vertical and
horizontal, will also influence plaque retention.
