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The initial part of the lecture involved showing the audience the various structures of the equine mouth. The bit sits in a part of the horse's mouth called the diastema, which is a section devoid of teeth that lies between the front incisors and the back pre-molars and molars. The ratio of the size and length of the diastema to the length of the horse's lips, can add an additional bit-fitting element, as Dr. Clayton feels a horse with short lips can be more challenging to fit. If viewed from above, the horse's tongue lies between the two branches of the horse's face, and if viewed from the side, the tongue lies right up against and into the arch of the horse's hard palate, which Dr. Clayton finds is often a surprise to many people. "There is an assumption of space there, but in real life there is no space between the tongue and palate, so the bit has to squash something - usually either the tongue, or the palate," she said. The hard palate separates the oral and sinus cavity, and is a layer of skin, muscle and mucosa which protects the bony section of the mouth. "The first thing we were interested in looking at was the conformation of the mouth," said Dr. Clayton. "We wanted to make absolute measurements and compare between horses using x-rays." However, since x-rays can have a magnifying effect on images, they taped small screws to the horses' face at an exact distance, and then used the comparison of the true measurement to the magnified x-ray measurement to determine the ratio for finding the true measurement of the horses' mouths. The first measurements they took were the length of the hard palate, from the first premolar to the last incisor. From among their test subjects, they found a range of 71 to 86 millimeters. They also measured the height of the hard palate, and found a range of 33 to 44 millimeters. They also measured the height of the roof of the mouth - the soft covering of the hard palate--and came up with a range of 20-32 millimeters. In general they found that it wasn't possible to predict the size of a horse's mouth by the size of the horse. There does appear to be somewhat of a trend, although a statistically insignificant one, between horse size and palate length. However, there is no correlation between palate height and horse size. "Clearly we cannot make external assumptions," said Dr. Clayton. "Horses differ greatly in their oral cavity, and that difference is not proportional to height at the withers." Once these measurements had been taken, and basic data compiled, it was time to discover how the bits works in the horse's mouth. The technique used to achieve this was fluoroscopy, or moving x-rays. Horses were placed in stocks wearing a bridle with a flash noseband and surcingle. The noseband was adjusted in the standard manner, and the bits adjusted such that two small wrinkles were visible at the corners of the horse's mouth. Images and data were taken with both no contact on the horses' mouth, and with side reins that applied a standard pressure of five pounds as measured by strain gages. To calculate the amount pressure, a professional rider was put up on a horse, with strain gages attached between the bit and the mouth. The resultant info showed that the pressure on the horse's mouth ranges from 1 to five pounds of pressure, and that the oscillation of contact takes two full spikes for every full stride of trot. Dr. Clayton showed a tracing of the oscillating nature of pressure on the reins, even in experienced hands, and thus what may feel like steady contact to the rider, is in fact continual motion. The study group of horses was comprised of 8 horses, four Thoroughbreds and four Warmbloods, of which five were geldings and three were mares. The horses were trained horses that were ridden daily and understood how to flex and not fight the side reins. The reins were placed on the surcingle in the location that would most mimic the placement of reins held by a rider's hands. When looking at the images, the metallic bites were very visible, as were the teeth and bones, as they are very dense structures. The horse's tongue is less dense, but is still visible in the images. She also added that she has not done any studies of so-called Happy Mouth or other plastic bits, because they are not dense enough to show up on the fluoroscope. Due to the sensitive nature of the fluoroscopic equipment, the images could not be taken while the horses were in motion, and Dr. Clayton acknowledges that this does not provide a complete picture of the workings of the bit during work. "Certainly static conditions like these are different than real riding," she said. The study used six bits for evaluation: a plain single jointed, loose-ring snaffle, a loose ring KK Ultra bit, a single jointed Boucher bit (which has a hanging cheek piece), and three Myler bits, each one representing a different level of Mylers. The Level I Myler was a comfort snaffle with the curved mouthpiece, and the roller/sleeve over the central section. The Level II Myler was the Port, which was similar to the comfort snaffle, but with a higher central port with more room for the tongue. The Level III Myler used was the Correctional Port, which swivels and has an extra joint which allows it to move back and forth. It should be mentioned that one of the purposes of this study was to study how Mylers function. As these are newer bits, the USEA was interested to learn more about them when considering approving them for use in the dressage phase of eventing. Of the three Mylers used in the study, none of them are currently legal for competitions in dressage or eventing, although the Level I Myler is legal when it does not feature or utilize the slots on the bit ring for the reins and check piece. In this study, the slots were used. Of the three other bits used, all are currently legal for dressage and eventing competitions. Before the fluoroscopic images were taken, regular x-rays were taken to determine some static measurements in the horses' mouths with the bits in place, both with and without pressure on the reins. The first measurement was the distance from the mouthpiece to the palate, in millimeters.
The KK Ultra featured to most distance from bit to palate without tension, while the Myler I featured the most distance with tension. The second measurement was the distance of the bit from the premolars, in millimeters.
This finding showed that all the bits moved closer to the teeth when tension was applied, and that the KK Ultra provided the most space without tension, while the plain snaffle and Boucher provided the most with tension. The final measurement was the angle of the mouthpiece to the cheekbone, with and without tension, in degrees.
In the ported bits, there was a marked increase in angle with tension, but it was the Boucher that showed something very different. Because of the way in which the hanging cheek piece functions, it angles up sharply into the mouth, creating negative angles - a very different sort of action than the other bits. In Part Two, we will discuss the findings of the fluoroscopy, and the recommendations for people trying to find the perfect bit for their horse. Part II will publish on Monday, Dec. 29.
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