Research at the University of Edinburgh raises concerns about the extent of damage caused to the sensitive tissues of the teeth by routine dental rasping.
The past few years has seen a growing interest in equine dental care in the UK. Improved equipment has been developed, such as tungsten carbide blades and power tools, making the task less strenuous. However, work by Dr Sue Kempson and her colleagues at the Royal (Dick) School of Veterinary Studies, questions whether such equipment causes excessive damage to the teeth..
The cheek teeth have a complex structure composed of folds of tough enamel, surrounded by softer dentine and cement. Enamel is the hardest substance in the body. It has a high mineral content, with a small amount of protein (about 2%), and contains no cells. The cells that produce the enamel (ameloblasts) die off after the tooth is formed and so repair of enamel is not possible. Except on the occlusal (grinding) surfaces of the cheek teeth, a layer of cement covers the enamel.
Dentin makes up the bulk of the tooth. It is less mineralised than the enamel (c70%) making it softer and less brittle. It is produced by odontoblast cells that live in the pulp cavity and have long processes that extend into tubules in the dentin. Because of the presence of the odontoblast processes in the tubules, dentin is considered to be a living tissue. In other species, these processes seem able to transmit some pain sensation to the pulp cavity. It is uncertain to what extent this happens in horses.
The third main component of the tooth is the cement (cementum). It is another mineralised substance containing about 65% minerals. Below the level of the gum it contains cells (cementoblasts) that are capable of producing more cement rapidly if required. Once the tooth has erupted the cement loses its blood supply and becomes inert and unable to regenerate.
As the teeth wear, the softer dentine and cement are ground down, exposing ridges of enamel, which are used for grinding the food. The upper jaws are set further apart than the lower jaws. As a result, there tends to be less wear on the inner edge of the lower cheek teeth and outer edge of the upper (maxillary) cheek teeth. This leads to sharp enamel points forming on the outer (labial) edge of the upper cheek teeth and the inner (lingual) edge of the lower teeth.
Rasping (or floating) the cheek teeth to remove the sharp points that arise due to normal wear is an important part of routine equine dental care. The aim is to prevent damage to the cheeks or tongue without removing the enamel folds that allow for grinding the food.
Dr Kempson and her colleagues have been investigating the damage caused to teeth by routine rasping techniques using the modern improved instruments. They used each of three different types of rasp on teeth from the skulls of two horses that had died from problems unrelated to their teeth.
They collected the debris produced during the rasping and examined that and the teeth under an electron microscope.
They found that all three instruments caused damage to the odontoblast tubules with tearing of the odontoblast processes.
A solid carbide electric burr produced a smooth surface. There was a fine deposit of debris covering the tooth. This layer of debris is called the "smear layer" in human dentistry and is thought to seal the ends of opened odontoblast tubules and so prevent the entry of infection. However it carries the risk of removing excessive amounts of dentin, which could cause pain and might lead to uneven wear. Also, unless measures are taken to cool the instrument and the tooth, the heat could damage the pulp.
A hand-operated solid carbide rasp caused extensive damage to the dental tissues. It cut deep grooves in the dentin and enamel and exposed much more dentin than the other techniques. According to the researchers there was little in its favour other than its speed.
Of the three instruments used, a tungsten carbide chip blade gave the best result. The damage to the dentin was intermediate between the two other techniques, and there was a moderate smear layer. Its disadvantage was that it required more effort to use, but this also meant that an excessive amount of dentin was not removed.
Dr Kempson suggests that further work is required to establish the best techniques for removing enamel points.
For more details see: The effect of three types of rasps on the occlusal surface of equine cheek teeth: a scanning electron microscopic study.Susan A Kempson, Mary EB Davidson, Ian T Dacre.J Vet Dent (2003) 20, 19 - 27