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Normal rabbit dentition and pathogenesis of dental disease

image of Normal rabbit dentition and pathogenesis of dental disease
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Abstract

The standard description of a tooth that is used by anatomists and dentists is based on human teeth and divides the tooth into three parts: the crown, the neck and the root. This chapter provides information on the structure and identification of teeth, terminology, features of normal lagomorph dentition; uncommon causes of dental abnormalities; progressive syndrome of acquired dental disease; and prevention of dental disease.

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Figures

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24.1 Dental terminology. Human tooth. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission. Rabbit tooth. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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24.2 Features of normal rabbit dentition.
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24.3 This prepared skull of a wild rabbit shows normal dentition and occlusal relationship between the upper and lower arcades.
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24.4 Resting occlusal position of cheek teeth. The maxilla is wider than the mandible, so the lower arcade of cheek teeth rest with their buccal edges just occluding with the lingual edges of the upper arcade. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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24.5 Relationship of upper and lower occlusal surfaces of the cheek teeth. This drawing shows the occlusal surfaces of the upper cheek teeth superimposed on those of the lower cheek teeth (shaded). The dark grey lines show the lines of enamel that are present on the occlusal surfaces. The enamel edges of the teeth occlude against the central enamel ridges on the opposing teeth. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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24.6 Periodic planing movements maintain the shape of the occlusal surfaces of the teeth. To maintain the sharp tip of the lower incisor, the rabbit protrudes the mandible to bring the labial surface of the mandibular incisor in contact with the tip of the first maxillary incisor as the mouth closes.
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24.7 Innervation of the teeth by branches of the trigeminal nerve. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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24.8 Occlusal relationship of upper and lower cheek teeth during chewing. The sharp enamel ridges that run across the centre of each cheek tooth occlude with the sharp enamel edges of the opposing teeth and effectively act as a series of sharp blades to cut the food into small pieces before it is swallowed. Reducing fibrous food to small particles aids digestion because more food enters the caecum to be degraded by the caecal microflora. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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24.9 Relationship of the nasolacrimal duct to the apex of the 1st (large) upper incisor. The bone over the apex of the 1st maxillary incisor and part of the lateral wall of the bony canal that encloses the nasolacrimal duct have been removed from this prepared skull of a wild rabbit. The nasolacrimal duct has survived preparation as a strand of brown tissue that can be seen bending medially beneath the apex of the incisor. (Reproduced from , with permission)
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24.10 Rabbits with congenitally absent teeth or supernumerary teeth are occasionally encountered. This skull radiograph shows the absence of the upper 2nd incisor (peg tooth) (arrow); this was a bilateral abnormality. This skull shows a supernumerary mandibular premolar situated rostral to P1 (arrow). This was also a bilateral abnormality.
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24.11 Congenital prognathism is a hereditary defect that only affects the incisors. It is evident from an early age and is usually due to a shortened maxilla, rather than an elongated mandible. The rabbit shown was a 6-month-old Netherland Dwarf.
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24.12 This rabbit had a fractured lower right incisor, which was broken below the gingival margin. The broken crown remained and displaced the regrowing tooth, which was removed along with the broken fragment.
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24.13 This 12-week-old rabbit had a mandibular symphyseal separation, which had allowed the left hemimandible to rotate inwardly and cause a malocclusion of the cheek teeth. The 1st lower premolar developed a spur that lacerated the tongue.
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24.14 A prepared mandible from a rabbit that was euthanased because it was unable to eat. There was a rapidly growing large solid mass enclosing the teeth, presumed to be an osteosarcoma. Radiology of the chest showed metastases in the lungs.
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24.15 Right mandibular arcade of a rabbit with a large, rapidly growing polycystic mass in the mandible, which was enveloping the cheek teeth and displacing them (arrow). The rabbit was euthanased.
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24.16 Prepared skull of a rabbit with endstage dental disease and dystrophic calcification with ankylosis. The skull shows many typical characteristics of endstage dental disease. The incisors are deformed and maloccluded. The apices of the upper 1st incisors have penetrated the palatine bone. The crowns of the cheek teeth are missing, eroded, deformed or maloccluded. The apices have penetrated the bone and are so ankylosed that it is difficult to distinguish between bone and dental tissue. The palatine bone, the calvarium and the mandible are transparent and osteopenic in comparison with the skull of the wild rabbit shown in Figure 24.3 .
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24.17 Prepared skull with endstage dental disease and osteopenia. The bone is thin and translucent and the few remaining teeth are distorted and deformed.
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24.18 Signs of early dental disease. Apical elongation is the only sign that is present in all cases. Other signs may be seen in some sites in some rabbits.
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24.19 Features of intermediate PSADD. Malocclusion is caused by alteration in the shape of the crowns. It may not be obvious on radiographs because they are two-dimensional images of three-dimensional objects.
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24.20 Signs of late dental disease. Not all of the teeth may be in the same stage of PSADD. Some normal teeth or teeth showing early changes can be interspersed with teeth showing late changes.
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24.21 Signs of endstage dental disease. Endstage dental disease does not mean the end of the rabbit’s life. As in other species, rabbits can survive with absent or non-functional teeth (see Chapter 30).
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24.22 Apical elongation and sites of bone penetration. Apical elongation is one of the first changes to take place during PSADD. The elongated apices deform the bone and can penetrate the cortex at typical sites around the skull. Lateral aspect. Medial aspect.
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24.23 Transverse section through the hemimandible of a rabbit without dental disease, showing the bony canal that encompasses and protects the mandibular alveolar nerve beneath the apex of the tooth (arrow). Transverse section through the hemimandible of a rabbit in the very early stages of PSADD, showing the effect of alveolar bone loss and apical elongation. The mandibular alveolar nerve is no longer protected from pressure by the apex of the tooth (arrow). It is likely that this rabbit would have experienced pain if chewing hard food such as hay. Most owners offer hay to their rabbits but rabbits with the early stages of dental disease refuse to eat it. This is an effect, not a cause, of dental problems.
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24.24 Curvature and spurs on the cheek teeth in prepared mandibles from a wild rabbit and a rabbit with spurs on both mandibular arcades. The cheek teeth of the wild rabbit point vertically. The cheek teeth of the rabbit with spurs are curved but the height of the teeth is similar to those of the wild rabbit. The apices are long and alveolar bone loss and widening of the periapical space is most evident on the left arcade (arrow).
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24.25 Spurs on the upper cheek teeth are due to increased curvature of the teeth rather than elongation of the crowns. Here, a spur on the 2nd upper premolar has penetrated the inside of the cheek. This can be a source of infection that results in an abscess in the cheek (see Chapter 29).
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24.26 In the later stages of dental disease, loss of alveolar bone can allow teeth to rotate within the socket. The lower 4th cheek tooth may do this and result in a sharp spur digging into the cheek, which is painful. These spurs cannot be seen without a general anaesthetic. Even with the rabbit under anaesthesia, they are easily missed. It is important to examine each tooth carefully.
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24.27 Slanting incisors. Rabbits with pain at the caudal end of the cheek teeth tend to chew on the other side of the mouth without closing the upper and lower arcade on the painful side. This leads to uneven wear of the incisors. The longer lower incisor is on the same side as the painful problem, so this rabbit’s problem was on its right. It is often an abnormality of the tiny 5th (last) mandibular cheek tooth (M3). (Reproduced from , with permission)
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24.28 Radiograph of a sagitally sectioned head of a rabbit with healthy dentition. The lines of bone outlining the structures of the skull (the nasal bone, zygoma, mandible and calvarium) are straight and clear. The cortical bone along the ventral border of the mandible is thick. The palatine bone is straight and thick. The lamina dura can be seen outlining each tooth socket. Enamel can be seen outlining the teeth and the central enamel fold is evident in all the cheek teeth. (Reproduced from , with permission)
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24.29 This radiograph of a sagitally sectioned head of a lop rabbit with advanced dental disease shows marked contrasts with Figure 24.28 . Only a few remnants of dental structures remain. The lines of bone outlining the zygoma and mandible are thin, even in areas that are not subjected to any mechanical force during chewing. This skull is osteopenic, which is highly suggestive of metabolic bone disease rather than disuse atrophy, where bone loss would only occur in the parts of the skull that were subjected to mechanical strain during chewing.
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24.30 Muesli mixes such as this are not suitable as rabbit food.
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24.32 The ideal diet for pet rabbits to prevent dental disease.
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