Horse

Modern Horse Dentistry

Fossil samples dating back as far as 55 million years ago can help us determine how long horses (on average) lived. 55 million years ago the average longevity of the hyracotherium was 5 years of age. 20 million years ago during the Miocene, analysis indicates longevity of up to 15 years. 10 million yeas ago during the Pliocene period Equus population was thought to live up to 25 years.

Horses are herbivores and there teeth are designed for breaking down the hard structures like cellulose found commonly in the horses diet. They have what are known as hypsodont teeth, meaning continuous eruption of the reserve crown of the tooth. This matches the loss of tooth from the grinding down caused during mastication. Numbers of teeth Horses have 24 deciduous teeth (non permanent) and 36 to 44 permanent. The numbers of permanent teeth can vary mainly depending on gender. Male horses normally have 4 canine teeth; mares are often seen without any. 4 wolf teeth can sometimes be seen in horses although 2 on each arcade on the upper jaw are most common. Male and female horses alike can show wolf teeth.

Types of teeth
There are 5 different teeth that can be found within horses' mouths. INCISORS These are the teeth situated at the very front of the horses' mouth. They are used in a pincer like action for nipping biting and defence. There are 12 in total, 6 on the top jaw 6 on the lower jaw. Incisors are used to age horses. The occlusal surface of each tooth changes in appearance dependant on how old the horse is. Initially these teeth are more oval in shape but as the horse ages the shape of the incisors become triangular. The Galvaynes Groove is seen on the corner incisor teeth. This is a longitudinal line that appears also used when ageing horses

CANINES
Canine teeth are situated caudally to the incisors. There can be 4 in total. They are curved in shape with most of the tooth still under the gum line. They can be up to 7cm in length. They are relatively simple teeth that the ancestors of today's horses' would have used for defence.

FIRST PREMOLAR (WOLF TOOTH)
The wolf tooth is a small simple brachydont tooth, (short crown) although it can range in size from 1-25cm. There can be 4 in total. The roots of this tooth can vary from being non existent to being up to 30mm in length. These teeth can sometimes be found to erupt in varying places throughout the horse's mouth although more commonly they are situated just in front on the first cheek tooth.

PREMOLARS AND MOLARS (CHEEK TEETH)
Cheek teeth form 4 rows of 6 teeth that are accommodated in the maxillary (top jaw) and mandibular (bottom jaw) bones. These teeth are more rectangular in shape when a cross section is taken (down the transverse plane). The teeth on the maxillary arcades (rows) are wider and squarer than teeth on the mandibular arcades which are narrower and more rectangular. Ridges are seen on the buccal (outside) edges of the maxillary arcades in particular. Many dental overgrowths are a common occurrence here.

Tooth growth is seen on average at 2-4mm per year. The occlusal surfaces of these teeth are ridged to increase the amount of surface area for breaking down food. These teeth are used to grind foodstuffs in a circular sideways action.

Mastication (the chewing cycle) The horses head is Anisognathic, (a-nee-so-nay-thic). Basically the top jaw is wider than the lower jaw. Mastication begins using the lips and incisors to nip the e.g. grass. The horse whilst grinding the grass between its cheek teeth uses its muscular tongue and ridges on the upper pallet to gradually work the food to the back of its mouth in a circular (spiral) motion and then swallows.

Horses can only chew on one side of their mouth at a time, changing from one side to the other would mean they would drop the food. A horse should be comfortable to eat on both sides of their mouth. A horse has a great amount of lateral excursion (sideways movement) within their jaw. When eating lush feeds there is a greater amount of movement than when the horse eats dry feeds.

The temporomandibular joint This is the joint joining the lower jaw to the head. It enables the jaw to move and laterally has a great range of movement; up and down the movement is limited. Unlike carnivores horses have a transverse power stroke in a lingual direction (towards the tongue), associated with their mastication cycle. Joints should wear evenly. If horses' teeth wear unevenly, it can cause pain within this joint due to uneven pressures being placed on it.

Structure of equine teeth ENAMEL Enamel is the hardest and most dense substance in the body. It has a very high (96 - 98 %) mineral content making it almost translucent. Due to the absence of cellular inclusions (unlike dentine or cement) enamel can be regarded as dead tissue. It has no ability to repair itself once its ameloblasts die off. Enamel varies in thickness up to 3 times throughout areas of the tooth parallel to the long axis of the jaw but remains constant throughout the length of the tooth. Invaginated folds on the occlusal surface give strength to the tooth where the softer dentine becomes depressed.

DENTINE
The bulk of the tooth is made up of dentine; a cream coloured softer tissue comprising of approximately 70% minerals, 30% organic compounds and water. The type of tooth (shape and size) along with the compressibility and percentages of different organic components contributes to its overall strength.

The presence of dentine and cement dispersed between the hard enamel folds forms a very strong durable structure suitable for its purpose. Odontoblasts can synthesize dentine throughout their lives. This prevents the occlusal surface of the tooth from exposing the pulp during normal attrition.

There is a close working relationship between dentine and pulp with some of the structures of each working through each other. This sometimes leads to them acting as a single unit. Dentine is considered a sensitive living tissue.

Young tooth before eruption. Note the presence of cement and enamel covered by the dental sac and the large pulp chamber.

PULP Pulp is soft tissue within the tooth that contains a connective tissue skeleton consisting of fibroblasts, thick collagen, connective tissue cells i.e. Odontoblasts, numerous blood vessels, allowing for continuous dentine deposition and nerves. Pulp is found in large quantities in and around developing teeth. With age more secondary dentine is laid down as development of the tooth, requiring large quantities of pulp, ends. This makes them stronger and more solid.

Later in the tooth development the pulp chamber has formed two horns due to the laying down of dentine within the pulp chamber.

CEMENT
Cement / cementum are a cream coloured calcified dental tissue characteristically similar to bone. Its mineral and inorganic compound make up are similar to dentine and give it its flexibility. The extensive collagen fibres found within the inorganic component of cement are what attach the cement to the alveolar bone, stabilising the tooth. Cement is a living tissue nourished by the vasculature of the periodontal ligament (attaches cement on tooth to socket).