Shear Mouth
Shear mouth is a common term describing any conformation of the upper or lower jaw that causes dysfunction of the molars. It can be congenital or acquired. Shear mouth results in ________________ wear of the molars between the medial and lateral portions of each tooth. A slope of approximately ______________ degrees is considered normal but any slope on an individual tooth that exceeds twenty degrees will impair the normal mastication patterns and severely limit the lateral range of motion of the mandible.
Two types of shear mouth are recognized:
The first is a congenital conformational defect where the maxilla is notably wider than the mandible causing the upper molars to wear more quickly on the aspect of each tooth and the lower molars to wear more quickly on the later aspect of each tooth. This deformity is heritable and tends to run in families. The second is an acquired condition where abnormal mastication patterns (such as TMJ disease) cause the molars to wear at different rates at the lateral portion compared to the medial portion. This generally causes a pattern where one side of all the molars is worn more than the other. For example a horse may wear all of its molars more on the left side of each tooth than on the right side of each tooth causing the occlusion of the top and bottom jaw to function like the blades on a pair of . This type of shear mouth is less common than the congenital form of the condition and is not heritable. Predispositions include turning in only 1 direction, temporomandibular joint dysfunction & genetics. The clinical signs and symptoms of shear mouth include the maxilla appearing significantly wider than the mandible when viewed , molars with occlusal surfaces that slope more than fifteen degrees, molars with occlusal surfaces all sloping in the direction, laceration of the cheeks or the tongue, temporomandibular joint dysfunction, difficulty eating or masticating, quidding, reluctance to be bridled, resistance to bit pressure which is more distinct in one direction, decreased lateral flexion of the head and neck, resistance to ventral flexion of the occipitoatlantal joint and decreased lateral range of motion of the mandible. Diagnosis of shear mouth can be made through observation and palpation of the clinical signs and symptoms. Range of motion of the mandible should be assessed to evaluate the level of dysfunction caused by the deformity of the teeth. Treatment is the implementation of regular dental care to level the occlusal surfaces of the molars. Any underlying causes of acquired shear mouth should be identified and treated at the same time as the dental care.
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