Reasons for performing study: Cheek teeth (CT) diastemata are a major equine dental disorder that can be treated by mechanically widening the diastemata. There is limited anatomical knowledge of the spatial relationships of the individual pulps to the adjacent interproximal surfaces; on the risks of exposing the 6th pulp horn when performing the clinically unproven ‘bit seating’ procedure on Triadan 06s.
Objectives: To describe the anatomical relationships between the occlusal and interproximal surfaces of CT and the adjacent pulp horns; and between the 6th pulp horn and the occlusal and rostral surfaces of Triadan 06s.
Methods: The CT from 30 skulls of horses subjected to euthanasia for non-dental reasons were sectioned to expose the rostrally and caudally situated pulp horns to allow the anatomical relationships between the pulp horns and the occlusal and interproximal aspects of the CT to be assessed.
Results: Pulp horns were mean ± s.d. of 5.74 ± 1.45 (range 1.3–10.8 mm) from the nearest interproximal surface, with 5.3% of pulp horns being <3.5 mm from the interproximal surface. In contrast to expectations, pulps tended to became closer to the interproximal surface (and also to the occlusal surface) with increasing age. Teeth with physiologically tall clinical crowns, and also those in the Triadan 09 position had pulps that were closer to the interproximal surfaces than the remaining CT. The more caudally situated pulp horns, i.e. in particular, the 4th maxillary and 5th mandibular pulp horns were closer to the interproximal surfaces than the remaining pulp horns and these pulp horns also had the thinnest sub-occlusal secondary dentine. Pulps that were close to the interproximal surface were also found to be close to the occlusal surface of the CT.
Conclusions and potential relevance: While diastema widening is theoretically safe between the majority of CT, a small proportion of pulp horns are only 1.3 mm from an interproximal surface and others lie just 1.6 mm beneath the occlusal surface, and such pulps are at risk of pulpar exposure and to thermal injury during this procedure. The risk of pulpar exposure increases when dental tissue is removed from the caudal aspects of CT.
Equine cheek teeth (CT) diastemata, especially valve diastemata (Carmalt 2003; du Toit et al. 2009) that are narrower on their occlusal than gingival aspects, are a common and often painful dental disorder of horses (Dixon et al. 1999; Collins and Dixon 2005). Diastemata may be termed primary diastemata if, for example, the CT develop too far apart, or there is insufficient angulation of the rostral and caudal CT; or they can be termed secondary diastemata, if they develop resultant to other disorders, such as CT displacements, supernumerary or focally overgrown teeth that become secondarily displaced, e.g. if lying at the periphery of a CT row. The main clinical problem with diastemata occurs when long food fibres become transversely trapped within them, and are later forced through the gingival margin into the periodontal space between adjacent CT. Such periodontal food impaction is very painful and is a major cause of quidding in horses (Dixon and Dacre 2005; Dixon et al. 2008). Secondary infection and progressive extension of the periodontal disease can lead to periapical infection, oromaxillary fistula, paranasal sinusitis, widespread osteomyelitis of the jaws or tooth loss (Collins and Dixon 2005; Hawkes et al. 2008; Dixon 2010).
Management of CT diastemata include feeding finely chopped food, cleaning and filling of periodontal pockets with dental impression material, reduction of overgrowths (if present) on adjacent or opposing CT, or extraction of very displaced causative CT. Mechanical widening of CT diastemata with a motorised dental burr is an increasingly used treatment that clinically improves most cases (Carmalt and Wilson 2004; Rucker and Carmalt 2004; Rucker 2006; Dixon et al. 2008). However, this technique can potentially cause mechanical or thermal injury to pulps of the two adjacent CT (Dixon et al. 2008). A pilot anatomical study concluded that there is a small risk of pulpar exposure with CT diastema widening (Rucker and Carmalt 2004; Rucker 2006), with pulps being a mean distance of 7.1 mm (range 3.5–19.7 mm) from the interproximal space.
Another commonly performed, but scientifically unproven dental technique, used to a marked degree by some operators, is the reduction (grinding down) of the rostral aspects of the 06s, i.e. the so called creation of ‘bit seats’ (‘bit seating’), and there is also a risk of pulpar damage when performing this technique.
The aim of this study was to anatomically examine the positions of the CT pulp horns in relation to the rostral and caudal interproximal margins, and the occlusal surface in order to more accurately assess the risk of pulpar injury occurring during CT diastema widening or creating ‘bit seats’ of the Triadan 06s.
Materials and methods
This study utilised the skulls of 30 horses: 19 females and 11 males; aged 2–25 years, including 16 Thoroughbreds and Thoroughbred crosses, 10 ponies and 4 draught horses, that were subjected to euthanasia on humane grounds for non-dental related reasons at Easter Bush Veterinary Centre. After removal of the heads, the mandible was disarticulated at the temporo-mandibular joint. The skull was then aged using case records; and when the age was unavailable, by utilising incisor examination (Muylle 2005) and by radiographic evaluation of the CT reserve crowns (Dixon and Copeland 1993).
All CT were extracted intact using a hammer and a bone chisel, and were then sectioned in 3 planes using a Dimas TS 230 F water cooled tile saw1 with a 0.81 mm wide lapidary blade2. Pulp horns were identified using the modified CT pulp nomenclature of du Toit et al. (2008) (Fig 1). The pulp horns examined in this study were those that could be damaged during diastema widening (i.e. situated adjacent to an interproximal space) or during ‘bit seating’ (pulp 6 of the 06s). In Triadan 06s, pulp horns 6, 2 and the 4th mandibular or 5th maxillary pulp horns were examined (See Fig 1); in Triadan 07s, 08s, 09s and 10s, pulp horns 1, 2, 3 and the 4th maxillary or 5th mandibular pulp horns were examined, and in Triadan 11s, pulp horns 1 and 3 were examined (Fig 1).
The full CT row was first sectioned in the transverse plane, 3 cm below the occlusal surface in order to expose the pulp horns. A second section was then performed on the individual crowns in a transverse (bucco-palatal/lingual) plane through the middle of the erupted crown (Figs 2, 3) to allow the final sectioning to be performed more precisely. For the third section, each pulp horn to be examined was cut in a sagittal (rostro-caudal) direction (Figs 2, 3). The aim of this final section was to connect the midpoint of the secondary dentine on the occlusal surface to the exposed underlying pulp horn on the apical aspect of the section (that had been exposed with the initial sectioning).
If the pulp horn was not exposed (or fully exposed) on this sagitally directed, third section, further dental tissue was gradually ground away utilising the abrasive action of the side of the saw blade (Fig 4), to more fully expose the most occlusal aspect of the pulp horn in the section. Before obtaining any measurements, each pulp horn was probed in an apical direction with a 23 gauge needle, in order to ensure that the pulp chamber was fully exposed on its occlusal aspect and if not, further dental tissue was gradually ground away until the occlusal aspect of the pulp was fully exposed (Fig 4).
For each selected pulp horn (Figs 1–3), the following 5 measurements were obtained using an electronic digital callipers3 at the sites shown in Figure 4.
• Occlusal surface - pulp horn (OS-PH): the shortest distance between the occlusal surface of the tooth and the occlusal aspect of viable pulp in the pulp horn (i.e. thickness of subocclusal secondary dentine).
• Interproximal space - pulp horn occlusal (IPS-PH): the shortest distance between the interproximal space and occlusal aspect of the pulp horn.
• Interproximal space - pulp horn15 (IPS-PH15): the shortest distance between the interproximal space and the pulp horn at a level of 15 mm beneath the occlusal surface.
• Mean crown height lateral (MCHL): Three measurements of the clinical crown height (distance between occlusal surface and gingival margin), were obtained at random sites on the lateral (buccal) aspect of the tooth and their mean value, termed ‘mean crown height lateral’ (MCHL) was obtained.
• Mean crown height lateral (MCHL): Three measurements of clinical crown height were taken at random sites on the medial (palatal/lingual) side of specimen and their mean value, termed ‘mean crown height medial’ (MCHM) was obtained.
Data were normally distributed. For statistical analyses the population was divided into 6 age groups; aged 2–5 years (n = 5); 6–10 years (n = 5); 11–15 years (n = 10); 16–20 years (n = 4); 21–25 years (n = 4) and >25 years old (n = 2). Some horses in the 2–5 year age group still had deciduous CT, or partially erupted permanent CT, that would not be clinically treated by diastema widening. As the inclusion of data from such teeth could increase the variability of the results, in addition to making the findings less clinically relevant, data were analysed both including and excluding the 2–5 year age group. Correlations between the 5 measured variables were assessed by Pearson's correlation. Using ANOVA, the following effects were examined: OS-PH; IPS- PH and IPS- PH15 with Triadan positions, individual pulp horns and age group.
It was not possible to maintain a standard 3 cm distance below the occlusal surface on the first section for all CT, because some older teeth were <3 cm long. Additionally, not all pulp horns (or their remnants) were identifiable in some older teeth, due to irregular (but physiological) variation in the direction and shape of the pulp horns near the apex and consequently, measurements could not be obtained from these specimens. Occasionally, some younger CT pulp horns were not straight, especially if they ran beside an expansion of an infundibulum, and accurate exposure and thus measurements of such curved pulp horns was not always possible. In some specimens, especially in older teeth, the third sectioning (rostro-caudal direction) fully removed the pulps, that presumably were narrower than the width of the saw blade (0.81 mm), and no measurements could be taken from such sites. Between 994 to 1060 observations were made on each examined pulp horn (Table 1). Occasionally wide pulp horns were found in older CT and conversely, narrow pulp horns were sometimes present in younger CT (Table 1), and such variation was occasionally found even between pulp horns of the same tooth (Fig 5).
Table 1. Values (mm) excluding the youngest age group, for mean crown height lateral (MCHL); mean crown height medial (MCHM); occlusal surface-pulp horn distance (OS-PH); interproximal space to occlusal aspect of pulp horn distance (IPS-PH) and interproximal space to pulp horn at a level of 15 mm below the occlusal surface of tooth (IPS-PH15). No. = number of measurements of each parameter; Min = minimum value; Max = maximum value
Mean ± s.d. (mm)
9.82 ± 3.19
9.00 ± 3.94
9.23 ± 3.00
5.74 ± 1.45
5.39 ± 1.41
The mean values for IPS-PH and IPS-PH15 were 5.74 and 5.39 mm, respectively (Table 1); however, of greater clinical importance is that the minimum value for both of these measurements was just 1.3 mm, with 0.06% of measurements of IPS-PH being <1.5 mm; 0.24% <2.0 mm; 0.59% <2.5 mm; 1.85% <3 mm; 5.3% <3.5 mm and 11.4% being <4 mm.
The mean clinical crown height on the lateral (MCHL) of 9.8 mm and the medial (MCHM) (mean 9.0 mm) aspects of combined maxillary and mandibular CT is of similar value to the mean thickness of subocclusal secondary dentine (OS-PH) (mean value 9.2 mm), indicating that the occlusal aspect of viable pulp is often at gingival level.
The findings in the 6 different age groups (Table 2), show a small, but consistent decrease in OS-PH (subocclusal secondary dentine thickness) values, and for distances between the pulp and interproximal space (both for IPS-PH and IPS-PH15) with increasing age, especially in the 2 oldest age groups.
Table 2. Mean distances (mm) between: occlusal surface of tooth and occlusal aspect of pulp horn (OS-PH); interproximal space and occlusal aspect of pulp horn (IPS-PH); interproximal space and pulp horn 15 mm beneath the occlusal surface of tooth (IPS-PH15)
Both MCHL and MCHM were negatively correlated to IPS-PH (Table 3), indicating that CT with taller clinical crowns have pulp horns lying closer to the interproximal space. MCHL and MCHM were strongly correlated, even though there is much variation between clinical crown height on the medial and lateral aspects of equine CT. OS-PH and IPS-PH were positively correlated, with both values decreasing slightly in older horses (Table 3). Similarly, OS-PH and IPS-PH15 were even more strongly correlated. The very strong positive correlation (R = 0.86) between IPS-PH and IPS-PH15 is because these are similar measurements taken at different levels of the pulp horn (Table 3).
Table 3. Correlations (excluding the 2–5 year old age group) between variables: mean crown height lateral (MCHL); mean crown height medial (MCHM); interproximal space to occlusal aspect of pulp horn distance (IPS-PH); interproximal space to pulp horn distance 15 mm below occlusal surface of tooth (IPS-PH15); distance between occlusal surface of tooth and occlusal aspect of pulp horn (OS-PH)
R = 0.25
R = -0.17
R = -0.15
R = -0.08
R = 0.22
R = 0.38
R = 0.86
Occlusal surface - pulp horn (OS-PH) distance
OS-PH - Triadan position: ANOVA, with or without the youngest age group, showed OS-PH (subocclusal secondary dentine thickness) to be significantly correlated (P<0.0001) to individual Triadan positions, Highest values for this parameter (i.e. thickest subocclusal secondary dentine) (mean value above all measured pulps in individual Triadan positions) were present in mandibular 08s (mean ± s.d. 10.3 ± 3.0; range 3.5–19.8 mm) and 07s (9.0 ± 2.63; range 3.1–18.5 mm), with lowest values present in mandibular 06s (7.7 ± 2.85; range 1.8–19.7 mm) and mandibular 11s (9.3 ± 2.94; range 2.6–18.0 mm).
OS-PH - individual pulp horns: ANOVA, with or without the youngest age group showed OS-PH to also be significantly (P<0.0001) correlated to individual pulp horn positions. The thinnest subocclusal secondary dentine was found over the caudally located pulp horns, i.e. 2nd pulp horn (8.42 ± 0.16; range 1.6–20.0 mm) and the most caudo-medially situated pulp horn (i.e. 4th maxillary or 5th mandibular pulp horn – see Fig 1) (combined values of 8.57 ± 0.17 mm; range 3.1–22.1 mm). The thickest secondary dentine was above the 6th pulp horn. (10.79 ± 0.40; range 1.8–19.7 mm) – although the Triadan 06s overall had the thinnest subocclusal secondary dentine (see above). The remaining pulp horns that were assessed had mean OS-PH values of between 9.06–9.28 mm.
OS-PH - age group: Subocclusal secondary dentine thickness consistently decreased with age in all age groups (Table 2). However, no statistically significant correlation was found between OS-PH and age group (P = 0.24) when the analysis included all age groups, When the youngest age group was excluded, a highly significant negative correlation (P<0.001) was found between these 2 variables, but only starting from the 16–20 year old age groups onwards, showing that subocclusal dentinal thickness significantly decreases with age in older horses.
Interproximal space - pulp horn distance (IPS-PH)
IPS-PH - Triadan positions: IPS-PH was highly significantly (P<0.0001) correlated to individual Triadan positions (when ANOVA was performed, both with and without the youngest age group) with Triadan 09s (5.35 ± 1.35 mm), and 06s (5.26 ± 1.55 mm) having significantly lower values (i.e. the occlusal aspects of their pulp horns were closer to the interproximal space) than those in the other Triadan positions.
IPS-PH - individual pulp horns: IPS-PH was highly significantly (P<0.0001) correlated to individual pulp horns (when analysed with and without the youngest age group). The most caudo-medial pulp horns (i.e. 4th maxillary or 5th mandibular pulp horns) consistently had significantly lower values (combined mean values of 4.89 ± 0.10; range 1.8–8.4 mm), i.e. their pulp horns were closer to the interproximal space than the other examined pulp horns that had mean IPS-PH values (mm) of 5.86 in pulp horn 1; 5.92 in pulp horn 2; 5.64 in pulp horn 3 and 5.91 in pulp horn 6.
IPS-PH - age group: When the analysis included all age groups, no significant correlation was found between IPS-PH and age group (P = 0.43) but when the youngest group was excluded, a highly significant negative correlation was found between these 2 variables (P<0.001), starting from the 11–15 year group onwards, showing that the occlusal aspects of the pulps become closer to the interproximal space in older horses.
Interproximal space - pulp horn distance15 (IPS-PH15)
IPS-PH15 - Triadan positions: IPS-PH15 was highly significantly (P<0.0001) correlated to individual Triadan positions (when ANOVA was performed with and without the youngest age group) with the Triadan 07 (5.54 ± 1.55; range 2.4–9.9 mm) and 08 (5.64 ± 1.52; range 2.3–10.1 mm) positions having significantly higher values (i.e. pulp horns further from the interproximal space) than the 4 other Triadan positions.
IPS-PH15 - individual pulp horns: IPS-PH15 was highly significantly (P<0.0001) correlated to individual pulp horns (when analysed with and without the youngest age group) with the 4th maxillary and 5th mandibular pulp horns (mean 5.19. s.d. 0.10; range 2.2–9 mm) consistently having significantly lower values (i.e. pulp horns closer to the interproximal space) at a level of 15 mm below occlusal surface than the other examined pulp horns that had mean IPS-PH15 values (mm) of 5.23 in pulp horn 1; 5.19 in pulp horn 2; 5.34 in pulp horn 3 and 5.87 in pulp horn 6.
IPS-PH15 - age group: When the analysis included all age groups, no significant correlation was found between IPS-PH15 and age group (P = 0.26), but when the youngest group was excluded, a highly significant negative correlation (P<0.001) was found between these 2 variables, but just starting from the 21–25 year group onwards, showing that IPS-PH15 decreases with increasing age, i.e. the pulps at 15 mm below the occlusal surface become closer to the interproximal space in aged horses.
Considering that the mean distance found between the interproximal space and pulp horn (IPS-PH) was 5.75 mm and that many currently-used diastema burrs4,5 are 4.5–6.5 mm in diameter, the diastema burring procedure is theoretically safe and should not cause pulpar exposure. For example, if the widening procedure begins exactly in the middle of the interproximal space utilising the above sized burrs, a maximum of 2.3–3.3 mm of dental tissue would be removed from the tooth on each side of a diastema (depending on burr size) and therefore would not expose a pulp horn in the majority of CT. Additionally, as this procedure is only performed on abnormally wide interproximal spaces (i.e. clinical diastemata) that have a median width of 2 mm on the occlusal surface (du Toit et al. 2009), even less dental tissue (approximately 1.3–2.3 mm) would be removed from each tooth. Many clinically significant diastemata are valve diastema that are 2.4 times as wide at their gingival than occlusal aspect (du Toit et al. 2009), and the abnormal angulation of the 2 CT beside a valve diastemata would further reduce the risk of pulpar exposure.
However, whilst the above generalisation is true, individual pulp horns can be just 1.3 mm from the interproximal space and 5.3% of pulps are less than 3.5 mm from the interproximal space. Consequently, a real risk of pulpar exposure or thermal damage exists if a CT with a narrow layer of peripheral calcified tissue on its rostral or caudal aspect is burred, and even more so, if dental tissue is not removed evenly from both adjacent CT. Additionally, some pulp horns were just 1.6 mm beneath the occlusal surface (OS-PH = 1.6 mm) and therefore pulp exposure could occur on the occlusal, rather than the interproximal aspect of the pulp horn.
In particular, the risk of pulpar damage increases if most of the dental tissue is removed from the caudal aspect of the tooth, where the most caudo-medial (4th maxillary or 5th mandibular) pulp horns lie closest to the interproximal space (both IPS-PH and IPS-PH15) and to the occlusal surface (OS-PH). The former feature is even visible on gross examination of most CT (Figs 2, 3). White and Dixon (2010) found lower, but not statistically significant different values, between subocclusal secondary dentine thickness in the combined values of the 2 caudal as compared to the combined values of 2 rostral pulp horns. It is possible that if these horns were analysed separately, as in the present study, a significant difference would have been found between individual pulp horns. White and Dixon (2010) found high variability in subocclusal secondary dentine thickness within and between teeth, very low values (e.g. 2 mm) in some horses and a tendency for subocclusal secondary dentine thickness to decrease with age, similar to the current findings.
The current findings suggest that it would be prudent to use as narrow a diastema burr as possible (e.g. 4–5 mm diameter) when treating CT diastemata, although the narrow diameter (<3.5 mm) solid carbide diastema burrs are prone to break during diastema widening, if the horse moves its head suddenly during the procedure (P.M. Dixon, unpublished observations). Additionally, the application of a slight caudal pressure during diastema widening (Dixon et al. 2008) should reduce the risk of pulpar damage, by removing additional dental material from the rostral aspect of the CT on the caudal aspect of the diastema, i.e. adjacent to pulp horns 1 and 3, which are the furthest from the interproximal space.
This study showed that the occlusal aspect of the pulp horn often lies just beneath the gingival margin, a feature also noted in all pulps in the CT of 5 out of 13 horses examined by Rucker (2006). However, this generalisation hides that fact that in some CT, viable pulp lies just 1.6 mm beneath the occlusal surface (Table 1). Mandibular CT clinical crowns are taller on their medial (lingual) aspects and maxillary CT clinical crowns are taller on their lateral (buccal) aspects and so separate analyses of mandibular and maxillary clinical crown height in the current study would have been preferable. Additionally, severe gingival recession occurs around some chronic diastemata, and consequently diastema widening is performed beneath the normal gingival margin in these cases. Also of clinical relevance was the negative correlation between clinical crown height (MCHL, MCHM) and IPS-PH, indicating that the taller a clinical crown is physiologically, the closer its pulps will be to the interproximal surface, and consequently the higher the risk of pulpar injury during diastema widening. Overgrown CT were not examined in this study and the possible effects of an increased rate of eruption and reduced occlusal contact on secondary dentine deposition on teeth that overgrow due to lack of effective occlusal contact remains unclear.
Whilst the above discussion has been on the risk of actually exposing viable pulp, pulp can also be severely thermally injured without being directly exposed (Wilson and Walsh 2005). Consequently, even if operators stick to the anatomical guidelines suggested in this paper, irreversible pulpar damage and possible loss of tooth can occur if the diastema burr is allowed to overheat significantly.
The positive correlation between OS-PH and IPS-PH can be clinically useful if creating ‘bit seats’, because it is possible to indirectly assess the thickness of subocclusal secondary dentine by examining the occlusal surface to evaluate the distance of the secondary dentine over the 6th pulp horn from the rostral edge of the 06s. The greater this distance, the greater also will be the thickness of subocclusal secondary dentine and consequently, the lower the risk of pulpar exposure of the 6th pulp horn; the converse is also true.
When the whole population, including the 0–5 year age group were considered, there was no statistical difference in the means of the 3 variables (OS-PH, IPS-PH and IPS-PH15) between the different age groups. However, when the youngest group (0–5 years) was excluded a statistical differences was found for all 3 variables, indicating that the data from the youngest group greatly increased the variability of data. Exclusion of the youngest group from the analyses showed a significant decrease in IPS-PH values, starting from the 11–15 year group onwards, and a decrease of IPS-PH15 starting from the 21–25 year group. These results show that diastema burring (and creating bit seats) actually carries a slightly higher risk of pulpar exposure in older horses, which is in contrast with current beliefs (Collins and Dixon 2005), where it is stated that younger horses are at higher risk of pulpar damage because their pulp horns are wider. Whilst the volume of the pulp horn and also pulp horn width is maximal in the recently erupted tooth and progressively reduces with the deposition of secondary dentine (Shaw et al. 2008), the most critical anatomical aspect of diastema widening is the distance of the pulp horn from the interproximal space (IPS-PH) and occlusal surface.
This study found that IPS-PH decreases with age, from 5 years onwards (Table 2); however, this decrease was only statistically significant from the 11–15 year old group onwards. In older horses, this finding may be explained by the normal tapering in of the CT in an apical direction. However, it is more difficult to explain why in younger and middle-aged horses, where such tapering in of the CT is not grossly apparent, the distances of the pulp horns to the rostral and caudal interproximal margins of the teeth slightly decreases, at the time that secondary dentine deposition around the pulp chambers has been shown to occur (Show et al. 2008). However, Shaw et al. (2008) microscopically measured the thickness of secondary dentine on the medial and lateral aspects of CT, whilst the current study grossly measured overall thickness of calcified tissue on the rostral and caudal margins. Equine CT commonly have minimal peripheral cementum on the rostral and caudal aspects of their CT, presumably due to slight, physiological movement of teeth that wears it away. It is possible that there is more tooth movement and thus more wear of peripheral cemental (and possibly even some peripheral enamel) in older horses and therefore, although secondary dentine may be thicker in older CT, the combination of less peripheral cementum in addition to the tapering in of CT results in the pulp becoming closer to the interproximal space in older teeth.
In contrast to expectations, this study also found subocclusal dentine thickness (OS-PH) to decrease with age from a mean of 11.2 mm in horses aged 0–5 years to 5.4 mm in horses aged >25 years. The findings of White and Dixon (2010) support the current findings, with these authors reporting CT subocclusal secondary dentine thickness to vary from a mean thickness (above all pulp horns) of 12.8 mm (range 5–33 mm) in a 4-year-old horse to 7.5 mm (range 2–24 mm) in a 16-year-old. The values of both above studies appear higher than those of Dacre et al. (2008) who reported that 22% of pulp chambers were exposed when young CT (median dental age of 4 years) were sectioned 2–6 mm (variation due to occlusal transverse ridges) below the occlusal surface. The differences between these findings may be explained by differences in the ages and breeds of horses used in the different studies. Du Toit et al. (2008) found that donkey CT subocclusal secondary dentine increased with age (e.g. mean mandibular CT values increased from 11.4 mm in donkeys <15 years to 14.4 mm in donkeys >15 years).
The mandibular Triadan 09s had lower IPS-PH values than the other Triadan positions. This may partially be an age-related effect, because the 09s are the first permanent CT to erupt. It thus appears that the Triadan 09s are at a slightly higher risk, while 07s and 08s that have thicker calcified dental tissue occlusally and interproximally to their pulp horns are at a slightly lower risk of pulpar exposure during CT diastema widening than the remaining Triadan positions.
In conclusion, this study has found that diastema widening would be theoretically safe and not cause pulpar exposure in the majority of CT. However, the pulp horns in some normal CT are just 1.3 mm from the caudal interproximal space, and are within 3.5 mm of an interproximal space in 5.3% of pulps in horses of all ages and consequently, widening of diastemata should be cautiously performed in all horses, using as narrow a burr as is feasible. The caudo-medial (4th maxillary, 5th mandibular) pulp horns are closest to the interproximal space and therefore the operator should try and remove slightly more dental tissue from the rostral aspect of the cheek tooth lying caudal to the diastema. Cheek teeth with physiologically tall clinical crowns (especially on their buccal aspect) tend to have pulp horns close to the interproximal space and so are at increased risk of pulp exposure as compared to teeth with shorter clinical crowns. In contrast to expectations, the distance between pulp horns and the interproximal space and occlusal surface slightly decreases with advanced age and thus puts aged horses at slightly increased risk of pulpar exposure during diastema widening or ‘bit seating’, as compared to younger horses.
Thanks are due to Professor Paolo Carnier, University of Padua, for help with the statistical analyses, Drs C. Stasyzk and N. du Toit for reading the manuscript and to Dr Ilaria Iacopetti.
1 Buehler, Coventry, UK.
2 Malvern Lapidary, Malvern, Worcestershire UK.
3 Knighton Tool Supplies, Leicester, UK.
4 Powerfloat, D & B Enterprises, Calgary, Alberta, Canada.