Determining the medial to lateral slope (angle) of occlusion for cheek teeth has previously been made by actual measurement of one cheek tooth. This paper describes a method of determining the angle of occlusion by measurements made using incisor separation and suggests how this information may be utilized for incisor reduction or incisor leveling. (Rucker 2002).

Incisor reduction is an often used procedure in the USA with little scientific evidence supporting its frequent application in horses of all ages. The judicious use of incisor shortening should be based on evidence of dysmastication, quidding, weight loss, lack of weight gain after floating plus excessive excursion distance to molar contact. As stated previously (Rucker 1996) "10 to 20 per cent of horses presented for routine floating may need some incisor shaping." A complete incisor reduction would be expected in 1 to 2 % of these patients. Examination of recent 730 records showed 6 horses with normally shaped incisor occlusal surfaces requiring incisor reduction, 1 horse with an incisor "wave" requiring incisor reduction and 63 horses requiring some leveling of the incisor because of missing teeth, slight tilt, abnormal wear from cribbing, smile or incisor lip secondary to either partial brachynathia or prognathia. This data shows an incidence of 9.5% of horses needing incisor adjustment and .095% requiring incisor reduction.

Incisors that are normal in shape and occlusion (Figure 1), but are too long from lack of

usual wear, should be shortened to allow additional (normal) molar (cheek teeth)

occlusion. Incisors with abnormally shaped occlusal surfaces; smile, stepped or uneven,

should be restored to as normal an occlusal plane as possible.

The mandible opens and moves laterally and rostrally to initiate molar arcade contact for grinding food. (Baker 1999) Next, the mandible moves medially and the molar arcades remain in contact until the incisors touch. The approximate distance the mandible moves laterally can be divided into percent molar contact and percent incisor contact (Rucker 1995).

As the percent of the lateral excursion distance when incisors remain in contact increases, the percent distance molars are in contact decreases. In some horses, molars remain in contact for such a short distance the horse is unable to adequately grind food.

This percent molar contact has been described as 30 per cent or less. (Rucker 1995).

When cheek teeth erupt the chewing surface is irregular but essentially flat; the

normal occlusal angle is not present. This angle is created by lateral chewing contact and

anisognathic relationship of the upper and lower cheek teeth arcades. The normal angle of

occlusion varies 10 to 15 degrees. (Dixon 1999)

The author’s preferred method for checking molar occlusion is to start with the

mouth in the neutral or resting position and push the mandible laterally until the molars

touch. (Figure 2) Next, I push the mandible laterally an additional 1-2 cm, while listening for cheek teeth grinding and observing incisor arcade separation.

This motion does not represent a true reversal of the normal chewing motion, but does give the practitioner some idea of the length or duration of molar and incisor contact. The lateral excursion distance to molar contact can vary from year to year and may gradually, in a small percentage of horses, increase as the horse ages.

Distance traveled to molar contact may be measured or simply charted on dental records as a mark on the upper incisor arcade (Figure 3). Most horses will have molar

contact by ½ to 1 ¼ the width of the upper central incisors (Triadan # 101,201). The measured distance will vary with age because the width of 101/201 decreases with age. With a four year old horse 101 may be 1.9 cm wide while those of a 22 year old may be 0.9cm wide. Excursion to molar contact (EMC) measurements made on 160 horses, ages 1 to 24 years, showed a EMC range of 5mm to 2.5cm, an average of 1.20cm, median of 1.20cm and standard deviation of 2.9mm. The 6 horses requiring incisor reduction had EMC exceeding the average by 5 to 6 standard deviations.

Mandible has been moved to the left until the left upper and lower cheek teeth touch. Distance is 2/3 of the width of the upper incisor (blue mark). Distance to contact is same for both sides. Center of the mandible is a guide (red arrow).

Figures 4 and 5 show a normal 8 year old horse, with mandible pushed laterally to both

sides until molar contact.

Figures 6 and 7 show the same horse at age 15.

Figures 8 and 9 show the same horse in pictures 4 & 5 with the mandible pushed

about 1 cm further laterally past initial molar contact. This horse is floated annually and

has never had incisors shortened. A (imaginary) line (yellow line Figures 8 & 9 ) is

drawn from the upper incisors, beginning at initial point of molar contact, extending to the center of the mandible. This line represents the occlusal angle of the cheek teeth at one point during cheek teeth contact while doing this test. The occlusal angle of the cheek teeth can vary, from one tooth to another, so the yellow line represents a composite angle.

For better visual estimation of the occlusal angle, a small mark can be made,

using a permanent marker, on the upper incisor where the center of the mandible stops

when initial molar contact is obtained. (see red dot on Figures 8 and 9)

When the mandible is moved 1 cm lateral to the initial point of molar contact the space

between the incisor arcades will vary, from 2 to 6 mm. The millimeters of separation per

1cm lateral excursion will vary with the slope of the cheek teeth; greater the slope or

angle the more separation. (Figure 10)

 Figure 10. Younger horses tend to separate more per 1 cm lateral excursion.

Figure 10 shows the range of incisor separation per 1 cm lateral excursion past initial molar contact. Horses over 4 years of age tend to group around the 3 to 4 mm range.

Horses under age 4 tend to have more incisor separation. This is consistent with the observation that foals and yearlings have very steep cheek teeth occlusal angles (Easley 2000) and as they get older the typical angle is reached. These measurements can be easily made by one person.

These measurements; lateral excursion 1cm, plus incisor separation distance, can be used to calculate the angle of occlusion, by using trigonometric functions for solving for angles and side lengths in a right angle triangle. (Figures 11a & 11b)

Figure 12 shows the calculated angle for each millimeter of incisor separation per 1cm

lateral excursion.

The decision to shorten the length of the incisors should not arbitrary or based solely on how far the mandible moves to molar contact. Body condition, chewing ability (dropping grain or quidding), fecal fiber length, EMC distance and residual molar reserve crown are factors influencing the decision. These clinical findings occur more often in horses with less than 30% molar occlusion (Rucker 1995). When the center of the mandible moves past the center of 102/202, (Figure 13) before molar contact in initiated, and/or the percent molar occlusion is close to 30 percent or less, or the EMC exceeds the average of 1.2 cm by 4 to 5 standard deviations, consideration should be given to shortening incisors. The majority of  horses will never need their incisors shortened. Horses that do not wear their incisors sufficiently to maintain normal molar occlusion will need incisor shortening (usually) only once. Horses requiring incisor shortening are usually over twelve years of age. The removal of a slight unevenness or bump on the incisors is not considered incisor shortening.

Techniques described for shortening incisors include observing the space between the molar arcades and estimating how much incisor length to remove, followed by cutoff wheel removal of excess incisor (Green 1997). Nippers and forceps have been used, sometimes with adverse consequences (Scrutchfield 1999). The disadvantages of these two techniques include removing too much incisor and creating a space between the incisor arcades, opening an incisor pulp cavity or fracturing a tooth. Removing 1 mm at a time and observing the results is a safer method of incisor shortening (Easley 1998).

Neither previously described technique offered the practitioner a method of determining how much molar occlusion would be improved prior to incisor removal. An accurate estimate can quickly be made of how much to shorten incisors and how much molar occlusion will increase, by utilizing the right angle triangle described above. Incisor shortening is the removal of a vertical length of incisor in order to reduce the distance to cheek teeth contact (EMC). (Figure 14 red line and blue line respectively)

The upper molar table angle represents the hypotenuse (C) of a right angle triangle where A and B are the other two sides. This triangle can be diagrammatically inserted into the space between the resting molar arcades (Figure 15). The purpose of incisor shortening is to decrease the distance to molar contact (horizontal line B) by shortening incisor length (vertical line A).

Further studies of the necessity of incisor reduction need to be undertaken. The procedure is commonly employed by lay equine dental technicians in the US, with little rationale for its use. The over reduction of incisors can cause dental pain, dysmiasis, pulpar exposure, and loss of incisors.

The combined surface area of all 24 cheek teeth ranges from 10 to 15 times more than the surface area of the incisors (Dixon at al 1999). Also, cheek are composed of Type 1 enamel which is more resistant to wear than the Type 2 enamel found in the incisors (Dixon et al 1999). Logically, incisor wear should balance cheek teeth wear since incisors function to bite off grass, while cheek teeth do all the grinding.

Clinically, some horses, albeit a low percentage, have a contradictory incisor; cheek teeth relationship. A review of 730 records show an average EMC of 1.23 cm. The reason six horses with normally shaped incisors were given incisor reduction were a follows; 3 (ages 12, 15 and 16) had excessive excursion to molar contact (3cm EMC and were reduced 2mm), 3 (ages 12, 17, 18) secondary to correction of cheek teeth wave (1 to2 mm reduction to restore excursion contact to original distance). The later 3 horses may not have needed incisor reduction if they had no cheek teeth malocclusions.

The practical application of the trigonometry lesson is very valuable to the equine practitioner. It enables a quick method to evaluate cheek table angles without actually measuring (or trying to measure) individual teeth. Determining incisor separation per 1cm lateral excursion lets the practitioner calculate the angle of occlusion for each arcade quickly. Knowing the occlusal angle gives accurate knowledge of the effect that 1mm of incisor removal will have on lateral excursion to molar contact. This information can be utilized for horses with uneven incisors as readily as horses with level incisors.

It is to be expected that horses in the most need of an incisor reduction will have flatter occlusal angles. This is because the cheek teeth are in contact only during the initial phase of the chewing cycle and wear down the taller sides of each arcade.

The following set of pictures (Figures 18, 19 and 20) 1depicts a 17 year old horse that had an incisor reduction of 2mm.