cementum
dentin
enamel

Cementum is the outer layer of the tooth.  The periodontal ligament is attached to the cementum anchoring the tooth to the surrounding bone.
Dentin makes up most of the tooth substance.  It surrounds the enamel giving strength to the tooth. 
Enamel is the strongest substance in the body, but is brittle when compared to dentine.

Cementum, dentin, and enamel wear at different rates, producing irregular and rough tooth surfaces that are capable of crushing very tough food material. 

Horses do not chew as people do.  They grind their food by sliding their teeth side to side.  The horse opens its mouth slightly and moves the jaw to one side.  As the mouth closes the chewing surfaces shear off or grind their food. They chew on one side of the mouth at a time.  As the jaw returns to the resting position the chewing surfaces separate.  When the front teeth, or incisors are in contact, there should be no contact between the upper and lower molars.

Horses chew in this manner because of the anatomy of their mouth.  The upper molars are set wider apart than the lower molars.  There is little overlap of the teeth surfaces.  Therefore, chewing in the way that humans do, would have little affect on their food.  This grinding motion causes the surface of the teeth wear at a 15 degree angle.

Poor Dental Health

Malocclusion is bad contact between the teeth.  Horses can have incisor and/or molar malocclusions.  Either type may interfere with grasping or chewing food and performance.  Long standing malocclusions usually result in premature loss of teeth.

Exaggerated traverse ridges are tall with sharp points on top.  They interfere with normal chewing and jaw motion front to back.

Canine tooth problems
Tartar, especially around the canines and the incisors' gumline. This should be removed to avoid gum infections. Canines can also get fractured.

Decay and endodontic disease - caries
Decay as we see it in humans rarely occur in the horse. This is probably due to the anatomy of the horses tooth and its ability to produce secondary dentin to fill in cavities.

Failure of enamel development

Fractures/sequestrums

Fractures to skull bones
Oral trauma from bit being pulled too hard or reins being stepped on can cause a part of the jaw bone to be chipped off, this usually won't grow back in place but will instead be sequestered and lie in the same spot to get inflammed and painful, and hard pull on the bit or step on the reins can also seriously damage the tongue. A sequestrum like this should be removed surgically.

Fractures to teeth
Horses falling over, being kicked or banging their heads against something can fracture teeth easily. Clipping of canine teeth and cutting molars can also cause fracture with later infections to the root and surrounding tissues. Wolf teeth being broken off during extraction is something that is not unusal, but it rarely causes any problems as the remains will dissolve and be taken care of by the body.

Maleruption
Due to injury, teeth buds can have moved to abnormal places

Missing teeth
As described in the section of molar and incisor problems, missing teeth can be caused by and/or will lead to dental and periodontal disease in the horse. The loss of a tooth can be caused by trauma, it can be genetic or it might be due to periodontal disease.

Pain in the chewing (masticatory) muscles
There are very powerful muscles that makes it possible for the horse to move its lower jaw sideways and to close the mouth (chew). There are conditons that can make these muscles very sore. Myositis is a condition of inflammation of muscles.

Periodontal disease
Disease in the tissues surrounding and supporting the teeth crown and reserve crown is called periodontal disease. It is caused by infection and inflammation, often because for some reason feed has been allowed to pack into a pocket around the tooth or next to a tooth because of misalignment, missing teeth etc. Periodontal disease can lead to a large number of problems with infections, pain, sinusitis, chewing problems and tooth loss

Problems in the temporo-mandibular joint (TMJ)
Prolapse, arthritis, arthrosis, inflammation and pain
This is a very underestimated problem in the horse and it is very painful. It can cause problems with chewing and being ridden and general behaviour alterations due to the pain. It can be caused by misalignments of the teeth pushing the joint to an unnatural position, it can be caused by a mouth speculum being kept on for too long or in a wrong way, it can be caused by misfitted tack or a rough rider/driver.................. Constant Arthritis can lead to degenerative changes in the joint (artrosis)

Sinusitis
The anatomy of the head makes the connection between the molars and the sinuses quite intimate. An infected root and sinusitis has...........The signs of sinus infection can be swelling and pain of the face, nasal discharge (normally just from one nostril) and ..................It is usually treated with antibiotics, but if this is unsuccessful more serous measures will be made. Making a hole in the sinus from the outside of the scull (trephan...) and flushing the sinuses with saline water is one solution, but if the rooth infection can't be cured in any of these ways, tooth extractions may be a necessity.

Supernumerary teeth
Too many teeth in the mouth is usually caused by the teeth buds splitting in by trauma in the foetal development. It can also be genetic.

Tartar
The buildup of tartar around the teeth by the gumline is a potential problem as it can provide conditions for bacterial growth. In the horse this is mainly a problem around the canine tooth.

Tongue injuries
In some sports, especially harness racing, it is very common to use a tongue tie around the horse's tongue to prevent it from sliding back during racing and preventing airflow. Forgetting to remove a tongue tie after racing may seriously damage the tongue. The horse can also inflict trauma on itself in the mouth ie by getting the tongue in the way of the teeth causing wounds, ulcers or in the worst case I have seen a tounge being bitten off completetly.

Tumours
There are a variety of tumours that can be found in the mouth of the horse and it can originate from different tissues. Some are benign, others malignant.  Some examples are: Melanomas, Squamous cell carcinomas.

Ulcers
Because the equine teeth forms such sharp edges and points, the soft tissues in the mouth

Wolf tooth problems
Apart from the problem the wolf teeth can cause for using being sharp when soft tissue is pressed against it, there are also other problems they can cause. If they are present, but haven't erupted yet (blind), they can cause painful swellings (unerupted) on the bars and/or they can be situated in an abnormal place, all causing bitting problems due to pain. Fractures also occur, but this seldom makes any trouble as the fragements in time will be absorbed.

Other
Other painful dental or oral disorders or systemic disorders manifesting in the mouth. Infections, wounds, inflammation, tumours etc

Dental Care

Equine dentistry is evolving from just floating to performance dentistry.  Interest of horse owners, trainers, and veterinarians have furthered studies on how the mouth functions and how dental problems affect performance.  Simply removing sharp molar points may not eliminate all the sources of oral pain.  Horses experiencing oral pain will not perform to their full ability.

Floating is done to make the surface of the horse's tooth level or smooth.  This is accomplished by the removal of sharp points on the outer edge of upper molars and inner edge of lower molars.

Performance Dentistry includes a floating, comprehensive oral examination, identification and correction of incisor and molar malocclusions, and tooth shaping.  Tooth shaping of certain molars to prevent soft tissue damage, which cause oral pain, is an important part of performance dentistry

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Here are a few poems that are slightly different..

 

Horse's Age by Teeth To tell the age of any horse Inspect the lower jaw of course Two middle nippers you'll behold Before the colt is two weeks old Before six weeks two more will come Twelve months the corners cut the gum At two the middle nippers drop At three the second pair cant stop At four years old the side pair shows At five a full new mouth he grows Black spots will pass from view At six years from the middle two The side two pairs at sever years And eight will find the corners clear The middle nipper upper jaw At nine the black spots will withdraw At ten years old the sides are light Eleven finds the corners white As time goes on horsemen know The oval teeth three sided grow They longer get project before 'Till twenty when we know no more!

THE AGE OF A HORSE To tell the age of any horse Inspect the lower jaw of course; The six front teeth the tale will tell, And every doubt and fear dispel. Two middle nippers you behold Before the colt is two weeks old; Before eight weeks two more will come Eight months: the corners cut the gum. At two the middle "Nippers" drop: At three the second pair can't stop; When four years old the third pair goes, At five a full new set he shows. The deep black spots will pass from view At six years from the middle two; The second pair at seven years; At eight the spot each corner clears. From the middle "Nippers" upper jaw At nine the black spots will withdraw. The second pair at ten are bright; Eleven finds the corners light. As time goes on the horsemen know The oval teeth three-sided grow; Then longer get - project before - Till twenty, when they know no more." Anon.