How to Radiograph the Erupted (Clinical) Crown of Equine Cheek Teeth

Indications for Radiography of the Erupted Crown 

Diseases of the equine clinical crown are being recognized with increasing frequency.1, 2 These include diastemata, traumatic or idiopathic fractures of the erupted crown, abnormalities of wear such as exaggerated transverse ridges, wave or step mouths, supernumerary or reduced numbers of cheek teeth (polydontia, hypodontia), and the presence of retained and impacted deciduous cheek teeth (CT) remnants.

The erupted crowns of the cheek teeth, and especially the caudal teeth, are difficult to reliably clinically examine in the horse, even with the use of a full-mouth gag (speculum) and instruments such as high-powered light sources, buccal and lingual retractors, dental picks, and mirrors. Thus, ancillary diagnostic techniques such as radiography provide an alternative technique for accurate assessment of the clinical crowns of cheek teeth.3, 4

Open-mouthed oblique (OMO) radiography is particularly useful for diagnosis of diastemata, which tend to be underdiagnosed in equine practice. OMO radiographs of the affected area can be of invaluable assistance in providing a definitive diagnosis of the number of sites, widths of the diastemata, and angulation of the cheek teeth, to plan optimal treatment and allow for a more accurate prognosis to be given. Repeated OMO radiography of younger horses affected by diastemata allows assessment of the progression (or regression) of the condition.

Similarly, other disorders of the cheek teeth, such as abnormalities of eruption and wear and fractured clinical crowns, may be diagnosed by visual examination and digital palpation, but OMO radiographs provide additional information that may assist in planning treatment protocols and assessing prognosis.

Why Open-Mouthed Views Are Necessary 

Conventional (closed-mouthed) radiographic projections of the equine cheek teeth include standing lateral, 30° dorsolateral–lateral oblique (for the maxillary CT), 35-45° ventrolateral–lateral oblique (for the mandibular CT), and dorso-ventral views.5, 6, 7 In the majority of cases, these views adequately image the reserve crowns, dental apices, and alveolar bone, as well as changes in the associated mandible, maxilla, and sino-nasal structures. However, in these standard radiographic views, most or all of the erupted (clinical) crown is obscured by superimposition of the erupted crowns of the opposing cheek teeth row, in all but very aged horses with short reserve crowns.

Technique 

Any x-ray unit, including low-power portable machines, can be used effectively for equine dental radiography. The use of a large (35 × 43 cm), medium speed, rare earth cassette and medium speed film combination enables the radiographer to fit the entire cheek teeth row onto one radiograph and gives good radiographic definition and contrast.

Sedation of the horse is necessary to obtain most radiographs of the equine skull, but particularly so for OMO views as it facilitates separation of the maxillae and hemimandibles. Sedation is achieved by using a combination of alpha-2 agonist with butorphanol, eg, romifidine (0.03-0.05 mg/kg IV) with butorphanol (0.01 mg/kg IV). Once the horse becomes sedated, separation of the jaws can be achieved by using a Butler’s gag (speculum) or a 7- to 11-cm diameter section of PVC pipe, which is placed between the incisors (Fig. 1) and enables the tongue to be held in the normal midline position. A radiolucent rope or canvas halter with a long lead should be used to provide manual restraint of the patient during radiography.

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Figure 1. A sedated horse with Butler’s gag opened between the incisors, being radiographed using an OMO projection for the mandibular CT.

To achieve accurate collimation, the light-beam diaphragm is rotated so that alignment of the x-ray beam conforms to the angle of the head. A cassette holder should be used to avoid exposure of personnel to the primary beam, and suitable radiation protection (gloves, aprons, thyroid guards) should be utilized by handlers (Fig. 1).

OMO radiographs of the affected cheek teeth row(s) are taken with the x-ray beam directed at 15° ventrolateral–lateral for the maxillary cheek teeth and 10° dorsolateral–lateral for the mandibular cheek teeth (Fig. 2). The direction of the x-ray beam is the opposite to that utilized to obtain standard oblique views of the cheek teeth, ie, to image the maxillary cheek teeth apices, a dorsolateral–lateral projection was used, whereas to image the maxillary erupted crowns, a ventrolateral–lateral projection was utilized.

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Figure 2. Transverse section through equine head at the level of CT, showing beam direction (arrows) and cassette position for OMO radiographic projections.

In horses which have a pronounced curve of Spee (natural dorsal curvature of the caudal mandibular cheek teeth, as seen in Arabian horses), two OMO views at 10° and 15° may be required to optimally image the erupted crowns of both the rostral and caudal mandibular cheek teeth.

Interpretation of OMO Radiographs 

Case 1 

A 9-year-old pony mare was presented to the hospital with focal right-sided maxillary swelling, right sided malodorous unilateral nasal discharge and a history of quidding. Standard oblique radiographic views revealed oligodontia and periapical infection of 106 (Fig. 3). OMO views revealed oligodontia, bilateral mandibular diastemata at multiple sites with associated periodontal food pocketing, exaggerated transverse ridges (ETRs) on the maxillary teeth opposing the diastemata, and caudal dental overgrowths of 310, 410 (Fig. 4). 106 was orally extracted, and the oro-nasal fistula plugged with bone cement. The ETRs on 206 and 207 were removed using electric dental tools. The owners were advised that the mare now required regular dental treatment at 6-month intervals to prevent overgrowths from developing.

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Figure 3. 30 ° dorsolateral-lateral closed mouth radiograph of Case 1. Only 5 CT are present in each row. Note the large periapical halo containing abnormal areas of increased radio-opacity dorsal to 106 which are indicative of chronic periapical infection. Most of the erupted crowns are not visualized due to superimpostition of the opposing arcade.

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Figure 4. 10° dorsolateral-lateral OMO radiograph of Case 1 showing oligodontia (5 CT in each row). Also note the diastemata present between 406/407 and 408/409 and corresponding exaggerated transverse ridges on 107 and 108. A large caudal hook is present on 410.

Two years after 106 had been extracted, the mare was re-presented to the hospital. The owners reported that she had recently started to quid very badly and had become reluctant to eat, but admitted that no dental work had been performed for 18 months. Oral examination revealed that the bone cement plug had been displaced and the extraction site of 106 had healed completely. A very large overgrowth had developed on the unopposed 406, which had supererupted as can be seen on the OMO radiograph (Fig. 5). The diastemata which were previously present had closed markedly, and consequently ETRs had not redeveloped.

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Figure 5. 10° dorsolateral-lateral OMO radiograph of Case 1, taken 2 years after oral extraction of 106. Note the very large overgrowth that has developed on 406. The diastemata previously present between 406/407 and 407/408 have markedly narrowed.

Case 2 

A 6-year-old Thoroughbred-cross mare was presented to the hospital with a history of halitosis and quidding. Oral examination revealed that 309 and 409 were laterally displaced with associated buccal ulceration. Multiple diastemata with periodontal food pocketing were present between 307/308, 308/309, 309/310, 408/409, and 409/410. A shear mouth was present on the right side of the mouth, as were many sharp enamel points on all CT rows. OMO radiographs of both mandibular CT rows confirmed the presence of the diastemata (Fig. 6). These radiographs also showed that the rostral and caudal CT were very “upright,” with little caudal or rostral angulation, and therefore the teeth were not being adequately compressed together, resulting in diastemata formation.

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Figure 6. 10° dorsolateral-lateral OMO radiograph of the left mandibular CT of Case 2, taken at initial presentation to the hospital. Diastemata are present between 307/308 and 309/310. There is overlapping of the caudal aspect of 308 and the rostral aspect of 309, which is due to 309 being laterally displaced. Note that rostral and caudal mandibular CT are not well angulated.

Initial treatment consisted of radical reduction of the lateral aspect of the two laterally displaced CT to stop trauma to the buccal mucosa, and removal of all other overgrowths. It was hoped that further eruption of the CT in a relatively young animal would cause compression of the CT in a rostrocaudal direction, hence reducing the size of some diastemata over time, although due to the inadequate angulation of the CT in this particular case, the horse was given a guarded prognosis.

The horse responded well to the initial treatment, but 2.5 years later, it re-presented to the hospital due to worsening of the quidding despite regular rasping. Oral examination revealed that 309 and 409 were still laterally displaced, and that 311 was now also developing a lateral displacement. The widest diastemata, rostral and caudal to 309/409, now had deep periodontal pockets. A diastema burr was used to markedly widen these diastemata, to prevent food becoming trapped in a “valve”-type diastema, thus worsening the periodontal disease. OMO radiographs of the mandibular CT were then repeated and the left side is shown in Fig. 7. Six months later, the owner reported that the mare had improved markedly following the treatment, and the periodontal disease associated with the widened diastemata had improved, although it had not completely resolved. It was recommended that the horse be re-examined at 6-month intervals to assess if further widening of these or other diastemata was deemed necessary, or if extraction of 309/409 was indicated.

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Figure 7. 10° dorsolateral-lateral OMO radiograph of the left mandibular CT of the mare in case 2 taken 2.5 years after initial presentation. The diastemata between 308/309 and 309/310 have been widened using a diastema burr. There is also ‘overlapping’ of 310/311, due to the development of lateral displacement of 311. Note that there is slight rostrocaudal angulation of the x-ray beam in this x-ray which is undesirable.

Case 3 

A 6-year-old Thoroughbred gelding was presented to the hospital after repeated unsuccessful attempts to orally extract 309 (for unknown reasons) by an equine dental technician. Fractured remnants of 309 could be felt on digital palpation. OMO radiography revealed that these remnants were present at the rostral and caudal coronal aspects of this tooth, and also that the caudal aspect of 308 had been iatrogenically fractured. Standard closed mouth radiographs showed little evidence of apical infection of this tooth, but due to the large amount of pulpar exposure associated with iatrogenic fractures and the poor prognosis for long-term survival of this tooth, 308 was repulsed under general anesthesia (Fig. 8).

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Figure 8. Ten degree dorsolateral-lateral OMO radiograph of Case 3 showing fractured occlusal aspect of 309 with remnants of tooth present at the rostral and caudal aspects of this tooth. The caudal part of 308 has also been fractured.