EDV Research Papers

As published in Journal of Veterinary Dentistry, Vol 32 No. 3 Fall 2015

Summary:

Cephalometric studies are important to quantify abnormalities of jaw length and positioning. In this study, 4 to 7-month-old Quarter horse foals (n = 51) were examined to determine overjet (horizontal overlap) prevalence and measure the size of the physiological diastemata. Results were analyzed in relation to age, sex, and lineage. Another aim of this study was to develop a simple field technique for measuring incisor malocclusion and physiological diastemata dimensions that could be used to monitor the growth of the rostral components of maxilla, incisive bone, and mandible. The overall prevalence of overjet lesions in these foals was 51%. Females were overrepresented (61.5%). Overjet occurred more commonly in show foals (50% prevalence) than other working (7.7%) and race (42.3%) lineage foals. Significant differences were found between maxillary and mandibular physiological diastemata lengths in foals of all ages and, as expected, there was a positive statistical correlation between age and maxillary and mandibular physiological diastemata measurements. Incisor overjet was present in 44.4% of 4-month-old foals, 45.5% of 5-month-old foals, 58.3% of 6-month-old foals, and 60% of 7-month-old foals. There was a weak positive correlation between age and the presence of incisor overjet. It was concluded that incisor overjet was common among Quarter horse foals, especially those from show and race lineages. The field technique for physiological diastema measurements was considered effective. J Vet Dent 32(3) 173-175, 2015

Introduction

Equine dentistry has attracted increased attention from veterinarians, dental technicians, owners, and trainers over the past two decades.¹ It had previously been reported that dental related work was a vital part of equine practice, representing 10% of a UK equine veterinarian’s work.²

Some congenital dental disorders such as incisor overjet may show no obvious external clinical signs and are only detected by oral examination. The importance of a routine oral examination in young foals has been emphasized since severe malocclusions may be corrected if diagnosed early.³ Radiography can be used to quantify the extent of such disorders, but is time consuming, requires sedation, and usually not essential.⁴ It has been recommended that craniofacial alterations can be more accurately assessed by means of cephalometric studies.⁵

Occlusion has been described as normal contact between maxillary and mandibular teeth and malocclusion is any deviation of normal contact.⁶ Overjet is described as protrusion of the maxillary incisor teeth rostral and horizontally beyond the limits of the mandibular incisor teeth in centric occlusion, and overbite refers to a greater extent of horizontal, rostral, and vertical overlap of maxillary incisor teeth, also in centric occlusion.⁷

Mandibular brachygnathism, a class 2 malocclusion commonly known as “parrot mouth”, is a condition in which maxillary incisor teeth protrude more rostrally than mandibular incisor teeth resulting in absence of occlusal contact between maxillary and mandibular central incisor teeth.⁹ Mandibular brachygnathia leads to an overbite or overjet and may be caused by maxillary prognathism.^8-10 Whether the class 2 malocclusion is related to mandibular shortening or maxillary elongation, the reduced skull measurements is termed brachycephalia.¹¹ Mandibular brachygnathism is the most common malocclusion in horses with a prevalence reported as 2 to 5% in the equine population. ^12,13 A greater prevalence has been reported in males and Quarter horses are commonly affected by overjet.^4,14

Diastema is defined as a space between 2 adjacent teeth in the same dental arcade and is considered a pathological condition.^15,16 In herbivores, this should not to be confused with the physiological diastema between incisor and cheek teeth, also known as the interdental space or bars of the mouth.

The objectives of this study were to evaluate the prevalence of incisor malocclusions in Quarter horse foals of different ages and lineages. Additionally, we sought to test and validate a simple field technique to quantify incisor malocclusions and measure the dimensions of the physiological diastemata in foals.

Materials and Methods

In this study, 51 Quarter horse foals (23 males, 28 females) from Tatuí, São Paulo state, Brazil, were used. Foals were grouped by age: I = 4-months-old (n = 18), II = 5-months-old (n = 11), III = 6-months-old (n = 12), IV = 7-months-old (n = 10). These foals and their dams had constant access to pasture ad libitum and received commercial feed twice daily in their stable. Four of the 4-month-old and all older foals were weaned. Twenty- five of the 51 foals were from racing lineages, 18 were from show lineages (animals with an excellent conformation but unsuitable for racing or non-racing work) and 8 were from non-racing work lineages (animals used for routine work such as ranch horses).

Measurements of the physiological diastemata between the maxillary and mandibular second incisor teeth (502, 602, 702, 802) and the first cheek teeth (506, 606, 706, 806)¹⁷ were obtained in unsedated foals using a metric tape with an attached marker (paperclip) [Fig. 1 ]. The 02 was chosen because it was erupted in each foal. The front of the tape was placed on the gingival margin at the mesial aspect of 06 and the marker was dragged to the gingival margin of the distal aspect of 02 in each quadrant. The tape with attached marker was then placed on a flat surface and measured with digital calipers from the beginning of the tape to the marker. Measurement of the distance between rostral (labial) surfaces of the maxillary and mandibular incisor teeth was done interproximally between the first incisor teeth (01s) using a caliper ruler (Fig. 2). The head was in a natural (neutral) position during measurement to avoid erroneous data related to caudal mandibular movement when the head is elevated and rostral movement when the poll is flexed.^18,19 However, there was some difficulty in keeping the head and neck of some unsedated foals in a neutral position when obtaining physiological diastema and overjet malocclusion measurements since some foals resented the measurement procedures.

Statistical analyses of measurements were performed using statistics software. Variables were analyzed for normality using Kolmogorov-Smirnov test. Comparisons between mean maxillary and mandibular physiological diastemata measurements were made using Wilcoxon test. A Spearman test was performed to evaluate possible correlations between age and diastemata values, and age and the presence of overjet. P values < 0.05 were considered significant.

Results

There were significant differences in length between maxillary and mandibular physiological diastemata in all age groups (Table 1). There was a positive correlation (P< 0.0001) between age and both maxillary and mandibular physiological diastemata lengths. Measurements from different groups indicated that maximum uniform growth of rostral components of the maxilla and mandible occurs in foals between 4 and 5-months of age. Less mandibular growth occurs between 5 and 6-months, and even less between 6 and 7-months of age (Table 1).

Table 1

Overjet and diastemata measurements (mm)* in Quarter horse foals.

Group	n	Maxillary Diastema	Mandibular Diastema	P†	Overjet
I	18	82.36 ± 0.36	79.46 ± 0.23	0.001	0.58
II	11	89.17 ± 0.15	85.61 ± 0.12	0.01	1.12
III	12	92.99 ± 0.15	88.77 ± 0.06	0.002	1.27
IV	10	96.37 ± 0.26*	90.68 ± 0.37	0.005	1.74

* mean + SD; †P < 0.05 considered significant

Of the 51 foals examined, 26 (51%) had overjet present in a neutral head position including 8 (15.7%) in group I, 5 (9.8%) in group II, 7 (13.7%) in group III, and 6 (11.8%) in group IV. Of these 26 affected foals, 16 (61.5%) were female. They included 13 (50%) show lineage foals, 11 (42.3%) race lineage foals, and 2 (7.7%) work lineage foals. Of the 19 foals with overjet greater than 1-mm, 10 (52.6%) were show foals, 7 (36.8%) race foals, and 2 (10.5%) work foals. None of the examined foals were affected with overbite.

The prevalence of overjet in group I was 44.4% (8/18); 45.5% (5/11) in group II, 58.3% (7/12) in group III, and 60% (6/10) in group IV. There was no correlation between age and overjet values (P = 0.074). The prevalence of overjet was 72.2% (13/18) in show lineage, 44% (11/25) in race lineage, and 25% (2/8) in work lineage foals (Fig. 3).

It was observed that the head shape of show and race lineage foals were grossly different. Race foals demonstrated a much more convex face profile and generally had a bigger head than show foals.

Discussion

There is limited knowledge about normal mandibular and maxillary growth parameters, and physiological diastema lengths in equids. Some foals develop overjet between 1 and 6-months of age.^4,14 These observations reflect the theory that few foals are born with maximal expression of the overjet syndrome, with worsening clinical signs that might progress beyond 7-months of age.²⁰

It has been assumed that the rostral component of the mandible was affected in brachygnathism, when the mandibular physiological diastema was shorter.¹⁰ The results reported here show that the maxillary and mandibular physiological diastemata lengths were different in all foals examined, with 98% having a longer maxillary diastema regardless of the presence of malocclusion. These results suggest that a difference in maxillary and mandibular physiological diastemata lengths is normal for foals in the age range reported here. It has been suggested that overjet may be hereditary in Quarter horse foals.^4,21 We found that overjet was most common in show lineage Quarter horse foals. Show horses are usually selected based on appearance while race or working horses are selected based on performance. Show horse breeders use selective breeding to produce individuals of a certain head type (shorter and with a straight or lightly concave facial profile). This conformation preference may be a factor in the greater prevalence of malocclusions in show foal lineages.

In this study, although results showed a positive correlation between age and both maxillary and mandibular diastemata lengths as would be expected, there was no correlation between age and severity of overjet, as described previously.¹³ The results of this study suggest that there is an age-related tendency towards development of overjet lesions and this could occur because of different growth patterns of the maxilla (endochondral ossification) and mandible (endochondral ossification and epiphyseal growth) that occur independently.^2,13

The results of this study show that overjet is common in female Quarter horse foals of show lineage. The methods used to measure diastemata lengths and incisor overjet were efficient and repeatable. There were differences in maxillary and mandibular physiological diastemata lengths in all foals. There appears to be a peak of uniform growth of rostral maxillary and mandibular components between 4 and 5-months and less mandibular growth between 5 and 7-months of age. Further cephalometric studies are necessary to further understand craniofacial abnormalities in different horse breeds.

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