Predisposing factors

General

• The maxillary arcade of cheek teeth is 23-30% wider than the mandibular arcade at any point (anisognathism). 
• An anatomic survey has reported the range of the normal occlusal angle for the cheek teeth to be much more variable than was originally thought. Individual horses may be predisposed to this condition due to their jaw and dental conformation.
• Although this condition is often thought to be more common in stabled horses, recent reports have confirmed free-living horses have as many enamel points as those that are stabled.

Specific

• Lack of fiber diet and increased diet of concentrates.
• Poor and/or infrequent routine dental care Teeth: rasping (floating).
• Dental disease Teeth: dental disease - overview.

Pathophysiology

• As the horse masticates, the mandible is lowered and is then raised in a dorsal movement, during which it circumducts from the buccal to lingual aspects.
• The mandibular teeth occlude in an axial direction and a grinding action occurs as their occlusal surfaces shear over each other. This is known as the “power-stroke”.
• Due to the anisognathism in the equine mouth, only one molar arcade is in occlusion during each masticatory stroke.
• The temporomandibular joint permits free lateral movement and a degree of rostro-caudal movement. As the horse lowers its head to graze, the mandible drops and moves rostrally, and at this point normal occlusion would be expected.
• Pathology that limits lateral jaw movement can result in a gradual increase in the table angle of occlusion. Over time this may lead to dysmastication and further uneven wear on the occlusal surfaces. Horses eating concentrated or pellet feeds have a reduced grinding action compared to forage feeds (such as grass or hay) Resulting in a shorter lateral to midline stride of mandibular to maxillary occlusion.
• Overgrowths on the lateral buccal cingulae of the maxillary arcades can become particularly sharp leading to ulceration to the oral mucosa of the cheeks and elsewhere with pain on mastication.
• Further development of the overgrowths will worsen the occlusal angles to 45-50 degrees when the condition is termed shear mouth. In this situation, the lateral overgrowths stop any lateral masticatory movement, only a scissor-like action is possible, and this exacerbates the problem further.
• Shear mouth can lead to widespread periodontal disease Teeth: periodontal disease and wave mouth.