The performed μCT scans produced 2D images of high anatomical accuracy which allowed a reliable visualization of the dental substances in 3D models of equine teeth . The computerized 3D models were suitable to obtain novel morphometric data of equine incisors. For the first time detailed measurements as well as three-dimensional analyses of different tooth substances and structures (infundibulum, crown, root) became possible. Unfortunately, μCT investigations are not feasible in the living horse due to technical limitations. Therefore, the presented study had to be designed as a cross sectional study obtaining morphometric measurements only at a single point in time from each tooth, which inevitably complicates data analysis and interpretation. However, the morphometric data of 103 incisors suggest typical morphologic and morphodynamic characteristics after statistical analyses.
Maintained length of equine incisors
Equine incisors, pertaining to the high crowned hypsodont teeth, are subjected to continuous dental wear. The loss of dental hard substances at the occlusal surface is compensated by tooth growth . Therefore dental material is added at the apical end of the tooth. Simultaneously, occlusion is maintained by tooth eruption achieved by the periodontal ligament. The separate processes of apical tooth growth and occlusal wear result in changes in the total length of the tooth when not evenly balanced . According to our data, tooth growth exceeds dental wear for two to four years post eruption resulting in an increasing total length of the tooth. After that time, the total length of the tooth remains constant up to a tooth age of 13 to 15 years. In this period of time, tooth growth and dental wear appear to be well balanced. Hitherto, it is largely unknown how this balance is regulated. An attractive hypothesis comes from studies that demonstrated the perception of mechanical stimuli by human periodontal cells [15, 16]. According to these data, mechanical loads activate several signal transducing molecules in periodontal cells. Subsequently cellular processes are initiated maintaining periodontal integrity [15, 16]. Similar mechanisms of mechanotransduction might be also valid for the structures responsible for tooth growth, i.e. a long lasting equine enamel organ and equine epithelial root sheath. Accordingly a perception and translation of mechanical stimuli by odontogenic cells could explain the balanced processes of dental wear and tooth growth. At least the ability for mechanotransduction has been recently proposed for odontoblasts (cells producing dentin) .
Post eruptive growth of equine incisors
The mere observation of post eruptive growth of equine incisors does not answer the question whether the tooth elongation is facilitated by root growth or by growth of the dental crown (i.e. prolonged enamel production). In previous studies crown formation (production of tooth with an enamel covering) and root formation (apical tooth formation without enamel production) has not been distinguished from each other and post eruptive tooth elongation was referred to as root formation [3, 14]. Acquired data of this study permitted to estimate that the crown itself increases for a distinct period of time by post eruptive deposition of enamel. The length of the infundibula, when once formed cannot shorten except of being worn from the occlusal side. Thus, the distance between the apical infundibular limit and the apical edge of the enamel cover might serve as a useful mark to examine the post eruptive increases of the enamel cover. Unfortunately, such measurements would produce valid data only in a longitudinal study. In a cross sectional study presented here, the related data might be inaccurate, because the initial distance between the infundibular limit and the apical edge of the enamel cover might vary in a wide range between different horses and breeds. Nevertheless, the data obtained suggest a prolonged enamel production for several years after first eruption of the tooth. However, further histological investigations are needed to verify the existence of a productive enamel organ in equine incisors after eruption. For equine cheek teeth such a prolonged existence of the enamel organ for approximately five years after first tooth eruption has been already demonstrated [18, 19].
Remarkably, in equine incisors there is no linear demarcation line between the dental crown (covered by enamel) and the dental root (composed of dentin and cementum). The non-uniform shape of the enamel cover displays mesial and distal indentations and therefore both an enamel organ and an epithelial root sheath exist at the same time in close proximity to each other in equine incisors. It is assumed that complete compensation of occlusal wear in incisors can only be accomplished as long as intact epithelial cell formations (enamel organ and epithelial root sheath) persist. The start of the decrease of tooth length with 13 to 15 years suggests a limited persistence of the epithelial formations up to this age. In incisors older than 15 years tooth elongation can only be facilitated by mere apposition of dental cementum and does not outbalance occlusal wear. This assumption is supported by the presented data concerning the relation of dental crown and dental root. In incisors with maintained length, up to an age of approximately 15 years, the percentage of the tooth covered by dental root increases, if at all, only moderately, which is a further indication of the existences of an active enamel organ in combination with a productive epithelial root sheath. In incisors displaying decreasing length, being older than 15 years, the relative length of the dental root increases markedly, indicating the absence of an enamel organ.
However, an alternative - or supplementary - explanation for a decreasing tooth length can be derived from observations of the infundibulum. According to own investigations (data not shown), the infundibulum disappears with 13–20 years. The tooth apical to the apical infundibular limit, that contains no longer infundibular enamel, may wear faster.
The timescale for the changes in incisor length given here are in between the scales which were suggested by studies of Van Foreest (1995)  and Muylle et al. (1999) . Van Foreest  stated that teeth growth and root formation cease with 10–12 years. Muylle et al.  estimated that maximal tooth length was reached 2–3 years after eruption, maintaining until 17 years p.e. These similar but not identical results which were obtained in three different pools of specimens might indicate that large individual variations exist. However, despite the different timescales given in the literature, it has been generally accepted that incisor length remains constant for much more than ten years.
Equine incisors and equine cheek teeth
It has been demonstrated that equine cheek teeth grow by enamel production, i.e. crown formation for approximately five years post eruption [18, 19]. Thereafter, 5–15 years p.e., the epithelial root sheath exists and tooth elongation is facilitated by root formation, i.e. apposition of dentin and cementum. After 15 years, only apposition of dental cementum is observed [18, 19]. Similar to our findings in equine incisors, the existence of the enamel organ in equine cheek teeth (i.e. approx. five years post eruption) seems to be correlated with the ability to fully compensate occlusal wear. Accordingly, it has been shown that equine cheek teeth preserve a constant length for approx. four years post eruption before occlusal wear overbalances the ability for tooth elongation . These findings are further supported by various studies reporting that the cease of tooth elongation occurs in equine cheek teeth earlier than in incisors [20–23].