In horses, which have a hypsodont dentition (high-crowned teeth that continue to erupt throughout life), age can be estimated by the eruption times and general appearance of the (lower incisor) teeth. In species with brachydont dentition (low-crowned teeth), such as cattle and dogs, age determination is less accurate and is based mostly on dental eruption times and, subjectively, on evaluation of root canal width as noted on dental imaging. Aging of teeth becomes important to determine useful working life, when purchasing an animal, for insurance purposes, to help identify an animal, and to determine the prognosis of disease.
Dentition in Horses
In horses, which have hypsodont dentition (characterized by reserve crown length with continual eruption), relative increase in the height of the teeth makes up for occlusal wear, which occurs at a rate of approximately 2.5 mm/year until approximately 17 years of age. The most appropriate teeth to estimate age in horses are the (lower) incisors. With continuous eruption, exposure of occlusal dentin and cementum is inevitable, leading to the presence of three alternate calcified tissues on the occlusal surface (see "Lower incisors, mare"). Radiographic assessment of cheek teeth for root morphology in young horses can also aid in age determination. Dental appearances are subject to individual and breed variations and to differences in environmental conditions. Tooth morphology differs between the deciduous and permanent dentition. The deciduous incisors are smaller than the permanent ones, and the surfaces of their crowns are whiter and have several small longitudinal ridges and grooves. Eruption times are listed in Eruption of the Teeth a Eruption of the Teeth a . Permanent incisors are larger and more rectangular in shape. Their crown surfaces are largely covered with cement and have a yellowish appearance. The upper incisors have two distinct longitudinal grooves on their labial surface, whereas the lower incisors have only one.
Equine incisor teeth develop certain wear-related macroscopic features traditionally used to estimate age. The dental star consists of yellowish brown secondary dentin that fills up the pulp cavity and appears at the occlusal surface as the tooth wears. Its shape and position, as well as the appearance of the white spot in its center, are related to age. The shape, size, and time of disappearance of both the infundibula or cups (funnel-like infoldings in the occlusal surface) and the marks (enamel infundibular bottoms) are additional but more variable indicators of age. Progressive dental wear causes an alteration of the occlusal shape of the incisors. The occlusal surfaces of recently erupted incisors are elliptical; with age, however, they subsequently become trapezoid, round, and then triangular, with the apex toward the lingual side. The curvature of the dental arch formed by the lower incisive tables is also age related. In young horses, this arch is semicircular, whereas in older horses, it forms a straight line. Additionally, the arch formed by the incisors of the opposing jaws (as they meet) changes as the teeth advance from their alveoli and undergo attrition. In young horses, the upper and lower incisors are positioned in a straight line. With increasing age, the angle between upper and lower incisors becomes more acute. The Galvayne’s groove and the 7-year hook, which have traditionally been used as age indicators, are variable, inconsistent, and thus of little value for age determination in horses. The more useful signs are arranged chronologically in the following list:
5 years: The corners are erupting. Dental star in the central incisors.
6 years: Dental star in the middle incisors. Cups gone from the central incisors.
7 years: Dental star in the corners.
8 years: Central incisors trapezoidal. White spot in the dental star.
9 years: Middle incisors trapezoidal. White spot in the dental star.
10 years: Cups gone from the middle incisors. Marks on the central incisors are oval-triangular.
11 years: White spot in the dental star on the corners. Both the central and middle incisors have a lingual apex. Corners are triangular with a labial apex.
12 years: Cups gone from all lower incisors.
14 years: Marks on the central and middle incisors are small and round.
18 years: Marks disappear from the central incisors.
20 years: Marks are gone from the middle incisors and the corners.
Dentition in Cattle
Eruption times of incisors are the most reliable feature for age determination in cattle ( Eruption of the Teeth a Eruption of the Teeth a ). As cattle continue to age, the teeth wear shorter and more neck becomes visible; they loosen in the sockets and eventually drop out. Although breed-related, eruption dates are more reliable for estimation of age than signs of wear, because macroscopic age-related dental features are scarce (dental stars) or absent (cups and marks) and because rate of wear is largely influenced by nutrition.
Dentition in Dogs
Patterns of dental wear in canine patients can vary greatly. Dogs with malocclusions leading to attrition, as well as those with heavy chewing behaviors or those offered harder toys or treats (abrasion), will exhibit wear at a younger age. Variation in root canal width may be pathologic rather than part of the normal process of root maturation and present in teeth affected by endodontic disease.