Introduction

Developmental dental disturbances may be inherited, but they may also be acquired or idiopathic. Clinical importance of any defect varies from insignificant and mainly cosmetic, to extremely significant, e.g., leading to abscesses or cysts.

Good-quality radiographs are mandatory for diagnosis: only radiographs will tell the clinician what a tooth, including its root, is truly like.

Variations in Tooth Number

Decrease in Number: Anodontia, Oligodontia, Hypodontia

Anodontia (congenital absence of teeth) and oligodontia (only a few teeth present) are rare conditions, often associated with generalised disorders. Hypodontia (one or a few teeth missing) is a common condition. It is seen most often in the incisor and premolar region.

Hereditary factors are often involved in the congenital absence of teeth, though teeth can also be missing as a result of disturbances such as trauma or infection during initial development.

Radiography is essential to differentiate missing teeth from impacted and embedded teeth.

Clinically hypodontia is mainly a cosmetic issue, though function, especially in working dogs, may be compromised when canine or carnassial teeth are missing. Differentiation between possible hereditary and proven traumatic causes is important for breeding dogs.

Increase in Number: Supernumerary Teeth, Hyperdontia

Supernumerary teeth are seen most often in the incisor and premolar region. The condition may be inherited, but can also be caused by disturbances during tooth development. Supernumerary teeth can, but do not necessarily, have a normal shape and size.

Supernumerary teeth may cause disturbances in eruption, crowding and deviation of adjacent teeth. In these cases extraction needs to be considered. Preoperative radiographs are mandatory. The owner needs to be advised of the possible inheritability of the disorder.

Alterations in Size

Alterations in the size of teeth are of limited clinical importance in dogs and cats. Teeth that are smaller than normal are referred to as microdont. Macrodont teeth are teeth that are larger than normal and these may interfere with normal occlusion.

Alterations in Shape

Gemination, Fusion and Concrescence

Gemination is defined as an attempt to make two teeth from one enamel organ. Occasionally complete cleavage or twinning is seen (two teeth from one enamel organ). Gemination is seen most often in the incisor region.

Fusion is the joining of two tooth germs, resulting in a single large tooth. Fusion may involve the entire length of the teeth, or only the roots, depending on the stage of development of the teeth at the time of the union. The root canal can be shared or separate.

The aetiology of gemination and fusion is unknown, but trauma and a familial tendency have both been suggested as a possible cause. Both can be seen in the deciduous as well as in the permanent dentition.

Concrescence is the fusion of adjacent already formed teeth by cementum. It may take place before or after eruption. It is a form of fusion, where the teeth are united by cementum only. It is thought to arise from trauma or crowding of teeth.

Although fusion and gemination are mainly a cosmetic issue, both conditions may complicate procedures such as exodontic or endodontic treatment in affected teeth.

Dilaceration

Dilaceration refers to a sharp bend or curve or angulation in the root or crown of a tooth. The cause is usually acute mechanical trauma during the development of the tooth. The curve or bend may occur anywhere along the length of the tooth.

A dilacerated crown may be an aesthetic problem. Often the surface of a dilacerated crown is irregular, leading to a highly plaque-retentive surface. Extraction or endodontic treatment may be difficult in case of a dilacerated root. Severely dilacerated teeth may be unable to erupt.

Dens Invaginatus, Dens in Dente

Dens in dente is an uncommon tooth anomaly, with only a few cases described in the veterinary literature. It represents an invagination of enamel and dentine towards the pulp of the tooth. The clinical significance of dens in dente depends on the severity of the lesion, varying from higher caries susceptibility to pulpal necrosis and periapical inflammation.

Supernumerary Roots

Accessory roots can be seen in dogs and cats. Most commonly involved are the upper third premolar in the dog and the upper second (9%) and third premolar (10%) in the cat.

Structural Defects of Enamel

Amelogenesis imperfecta is a hereditary form of enamel defect which affects both dentitions. The incidence in dogs and cats is unknown, but a number of cases have been reported in Standard Poodles. The condition is mainly a cosmetic problem. Environmental enamel hypoplasia is a common structural defect seen in dog's teeth.

Enamel develops in two stages: a secretory stage (matrix production and early mineralisation) and a maturation stage (increase in mineral content by withdrawal of water and protein). In enamel hypoplasia, the enamel is quantitatively defective (insufficient amount but normal hardness). Qualitatively defective enamel (normal amount but hypomineralised) is referred to as enamel hypocalcification. Some disturbances affect both matrix formation and mineralisation.

Aetiological factors may occur locally or systemically. Examples include: vitamin deficiencies (rickets); epitheliotropic viruses (e.g., distemper virus); hypocalcaemia; excessive fluoride ingestion; local infection; localised trauma. Sometimes no apparent cause can be identified. The extent of the defect(s) depends on the intensity of the aetiological factor, the duration of the factor's presence and the time at which the factor occurs during tooth development. Any serious nutritional deficiency or systemic disease is potentially capable of producing enamel hypoplasia in the areas of the teeth that are being formed at that time.

Clinical presentation varies. In the hypoplastic type, the enamel is irregular, pitted or thin and may even be absent, but what is present has a normal hardness and colour. In the hypocalcification type and the hypomaturation type, there is a normal amount of enamel, but calcification is deficient. The enamel is relatively soft, becomes discoloured and wears away readily, and this more rapidly and severely in the hypocalcification type than in the hypomaturation type. The colour of the teeth varies from white opaque to yellow to brown. Affected teeth tend to darken with age, due to exogenous staining. Severely affected teeth may develop pulpitis and pulp necrosis over time. In some cases, enamel defects are associated with serious root abnormalities.

Treatment options include restoration of the defect (in localised forms) and sealing of the dentine tubules to prevent tooth sensitivity and pulp disease. Since affected teeth have a very irregular, plaque-retentive surface, good oral home care (daily toothbrushing) is very important.

Other Structural Defects

Very few reports exist in veterinary literature regarding dentine defects. Two reports mention dentinogenesis imperfecta in dogs, an inherited disorder of the dentine. Affected individuals have blue-brown opalescent teeth, which may break with normal mastication. The prognosis for retention of functionally important teeth is grave.

Regional odontodysplasia affects both dentine and enamel and has been described in the dog. One or several teeth in a localised area are affected and are described as 'ghost teeth'. Because of the poor quality of affected teeth, extraction is the treatment of choice.

Disorders of Eruption and Shedding

Persistent Deciduous Teeth

Persistent deciduous teeth is a common condition in toy and small dog breeds. The condition is likely to be inherited, at least in some breeds. Persistent deciduous teeth may cause or aggravate malocclusion and promote periodontal disease due to crowding, and should therefore be extracted. Whenever a deciduous and a permanent tooth of the same type are present at the same time at the same location, the deciduous tooth should be extracted as soon as possible.

Impacted Teeth

An impacted tooth is a tooth which fails to erupt into its normal position because of some physical barrier in the eruption path. Generally this is an acquired condition but it can be genetic. Impaction can be caused by trauma or simply because the tooth's position in the alveolus is abnormal so that it is unable to erupt into its normal position.

Embedded teeth used to de differentiated from impacted teeth: they were considered to be unerupted because of some failure of the normal eruption mechanism. Recent literature suggests that embedded teeth are usually hindered by less obvious physical barriers and therefore should be considered as 'impacted'.

Impacted teeth need to be differentiated from missing teeth. Radiographs are therefore indicated. Impacted teeth should be treated or at least monitored on a regular basis. Depending on the presenting problem, treatment may consist of removing the physical barrier (e.g., the overlying gingival tissue or operculectomy) or removal of the tooth.

Impacted teeth may cause resorption of the roots of adjacent teeth. In people, periodic pain due to tooth impaction has been described. A dentigerous cyst may develop around the coronal portion of the tooth. Furthermore, cases of ameloblastoma have been reported to develop in the wall of these cysts.