• Wave mouth. Localized differences in the density of the occlusal surfaces of the molar teeth may also have marked effects upon the occlusal efficiency and the development of abnormal wear patterns. Alternate hard and soft areas in the structure of the cheek teeth or, more commonly, stereotyped chewing behavior, may result in the development of wave mouth in which either a series of waves develops on the occlusal surface or individual teeth wear faster or slower than their neighbors giving a much coarser irregularity of occlusal surfaces.

• Step mouth. In other cases, there may be one (or more) tooth which, often for inexplicable reasons, wears excessively. This results in gross variations in the height of the teeth (step mouth) and necessarily limits the occlusal efficiency. The full extent of the condition may only be apparent from lateral radiographs when gross variations in height and pyramid deformities of the crowns of the molar teeth may be present.

• Shear/scissor mouth (see p. 3). While the loss of lateral grinding movement of the molars, for any reason, will initially induce sharp buccal margins on the upper teeth, this may progress into a severe and debilitating parvinathism (shear/scissor mouth), in which lateral movement, which is essential for normal chewing, is prevented. These unfortunate horses are often noted to have an abnormally narrow lower jaw but, while under these circumstances it is regarded as a developmental deformity, the same dental deformity may develop as a consequence of a sensitive molar tooth (or teeth) in the opposite arcade or from pain associated with the temporomandibular joints. This results in the upper molars becoming bevelled from the inside outwards with the lower molars worn in the opposite fashion. This deformity prevents further lateral movement of the teeth and seriously interferes with mastication. Under these circumstances extensive overgrowth or bizarre dental deformities may be encountered. This may itself induce secondary temporomandibular arthropathy as the animal attempts to chew with the sides of the molar teeth. It is, in most cases, impossible to identify whether the occlusal problem arose first and caused secondary joint degeneration, or whether the occlusal deformity is the result of abnormal jaw movement created by joint inflammation and degeneration. It represents one of the most serious deformities of the horse’s mouth and carries a poor prognosis.

• Sand mouth. In areas where horses have, of necessity, to eat and chew large amounts of sharp sand or other abrasive substances the occlusal surface of the cheek teeth may become completely smooth and therefore ineffective as grinders of food. Failure to masticate efficiently results initially in difficulty with swallowing and slow eating. Weight loss, as a result of ineffective digestion, is commonly present. Under similar circumstances, because these horses are usually grazing very short pasture or having to find food in soil or sand the incisors may become severely worn down.

• Overgrown cheek teeth. Overgrown cheek teeth may arise from the absence of the opposite occlusal tooth. Such defects may follow either from normal old-age shedding, or from failure of normal eruption or, more often, from surgical extraction of one or more of the cheek teeth. Molar teeth with no occlusal pressure are likely to grow faster than normal teeth and in addition have no occlusal abrasion. The resultant loss of normal control of dental growth creates abnormal wear patterns which are usually visible as gross overgrowth. Pyramidal peaks on the tooth opposite the gap are common where the gap created by a missing cheek tooth is narrowed by angulation of the adjacent teeth but leaving a relatively small area of non-occlusion.

    Normal shedding of molar teeth usually begins when the horse is over 25–30 years of age and the first molar tooth, being the oldest, would be expected to be lost first. Under these circumstances the scope for subsequent overgrowth in the opposite occlusal teeth is minimal and dental hooks, overgrowths or pyramids are usually of marginal significance.

    The loss of any of the cheek teeth, except the first and last leaves a gap in the dental arcade, which may narrow significantly (and sometimes completely) with time as the adjacent teeth angle inwards. Although these defects may be less in size than the more dramatic overgrowth encountered in either the first or the last tooth the secondary consequences may be more significant and arise more quickly. Thus, limited overgrowth may result in gross discomfort and inability to chew effectively within months of the onset. Alternatively, overgrowth may continue unabated for years before any clinical effects may become apparent. Overgrowth impinges upon the opposing gingiva causing ulceration, necrosis and possibly infection of the ulcerated area.

• Excessive incisor wear. While severe incisor wear is sometimes encountered where grazing is short and large amounts of sand or other abrasive substances are ingested, the wear pattern of the incisors, in particular, may be influenced by behavioral factors. Crib-biting is a common vice (neurosis) developed by horses showing a characteristic wear pattern on the rostral margin of the upper incisor teeth which is variable in extent according to the severity and duration of the vice, and to some extent upon the structural character of the teeth. The earliest indications of the vice may be gained from close examination of the rostral margin of the upper central incisors where a worn edge will be detected. The persistence and severity of the effort involved in cribbing is often enough, even in young horses, to cause severe wear of the central incisors, often down to gingival level.

    Habitual grinding of the teeth on metal rails or concrete walls results in wear patterns involving, usually, the corner and lateral incisors. The pattern is usually such that it is hard to visualize any normal behavior pattern which could produce them. Usually only one side is affected.