Biomechanical evaluation of the equine masticatory action: calculation of the masticatory forces occurring on the cheek tooth battery

Temporomandibular joint biomechanics and equine incisor occlusal plane maintenance

In equine dentistry, the physiological incisor occlusal surface is visually perceived as a plane with a distinct inclination to the head's coronal plane, extending rostro-ventrally to caudo-dorsally. To better understand the formation of this inclined plane and its connection to dental wear, we investigated the hypothesis that it arises from masticatory movements and the considerable distance between mandibular articular heads and the incisor occlusal surfaces, acting as the three points of support for the mandibles. Leveraging data from a large-scale clinical study involving static and dynamic orthodontic measurements in horses, we approximated the mandibular movement range where incisor occlusion and dental wear occur. By introducing and testing a segment coordinate system, we explored possible angular deviations from the occlusal plane caused by mandibular roll and pitch rotations during two lateral mandibular movement patterns, protrusion and retrusion. Theoretical biomechanical calculations and simulations confirmed the visual perception of the incisor occlusal surface as a plane. To further examine our assumptions, we employed a simple mechanical simulator to assess incisor normal occlusion and provoked malocclusions (diagonal, smile, and frown bite) by modifying temporomandibular joint (TMJ) movement patterns. The results from clinical investigations were corroborated by both the theoretical analysis and mechanical simulations, strengthening our understanding of the biomechanical basis behind the physiological incisor occlusal plane maintenance in horses. These findings have significant implications for equine dental health and contribute to a thorough understanding of TMJ dynamics.

A Retrospective Observational Cohort Study on the Expiration of Maxillary Cheek Teeth Infundibula in 217 Horses

The determination of a horse's age by its dentition is a technique that has been employed by horse owners and professionals since at least 600 BC. While aging teeth based on incisor wear patterns is common, wear patterns in the cheek teeth may also be useful. As horses continue to live longer, we routinely examine horses into their 20's and 30's, and it is becoming more common to observe aging changes in the cheek teeth. To our knowledge, this study is the first to use easily obtained clinical observation of the expiration of cheek teeth infundibula to age horses of advanced ages (16 to 35 years). The results of this study indicate that the attrition of infundibula is likely due to the combined effects of eruption time, length of the tooth or infundibulum, forces placed on the tooth or infundibulum, and the presence of enamel or cemental hypoplasia. With additional prospective studies, the use of cheek tooth infundibular expiration may become a useful tool to determine the age of horses in the field.

Treatment of a Mandibular Diastemal Fracture Using Locking Compression Plate and Cerclage Wire in a Mare

This report describes the combination of two surgical fracture repair techniques and the postoperative management of a mandibular diastemal fracture in a two-year-old mare. The mare was referred to a veterinary hospital with a laceration over the body of the right mandible. Radiographic assessment revealed two mesial fracture lines involving the second premolar tooth and a ventrally displaced bone fragment. The mare was treated under general anesthesia and the fracture was corrected using open reduction and plate fixation. A 3.5 mm narrow 15-hole locking compression plate with seven locking screws were used in a bridge form. Cerclage wire was also used to anchor the incisor teeth to the second and third premolar teeth. The cerclage wire and incisor teeth were covered with polymethylmethacrylate to prevent implant failure and avoid injury to the oral mucosa. Implants were removed 55 days after surgery and the mare was discharged from hospital five days later. The mare returned for cerclage wire removal after 90 days and was allowed to resume exercise thereafter. The combination of two surgical techniques, proper implant choice and appropriate postoperative management, including use of pelleted feed, contributed to successful bone healing and return to function.

A Computerized Simulation of the Occlusal Surface in Equine Cheek Teeth: A Simplified Model

Equine mastication, as well as dental wear patterns, is highly important for the development of treatments in equine dentistry. During the last decades, the stress and strain distributions of equine teeth have been successfully simulated using finite element analysis. Yet, to date, there is no simulation available for dental tooth wear in equines. In this study, we developed a simplified two-dimensional computer simulation of dental wear. It provides a first tentative explanation for the development of the marked physiological inclination of the occlusal surface and for pathological conditions such as sharp enamel points in equine cheek teeth. The mechanical properties of the dental structures as well as the movement of the mandible during the equine chewing cycle were simulated according to previously published data. The simulation setup was optimized in preliminary test runs. Further simulations were conducted varying the lateral excursion of the mandible and the presence or absence of incisor contact during the chewing cycle. The results of simulations showed clear analogies to tooth wear patterns in living equids, including the formation of wear abnormalities. Our analysis indicates that small variations in the pattern of movement during the masticatory cycle, as well as incisor contacts, are leading to marked changes in the occlusal tooth wear patterns. This opens new research avenues to better understand the development of dental wear abnormalities in equines and might have serious implications on captive animal health, welfare, and longevity.

Tooth Elongation of Maxillary Second Premolars and Mandibular Third Molars and Associated Periodontal Disease in Horses: A Retrospective Study

Periodontal disease has been identified in horses and donkeys through the millennia at a prevalence of up to 75% and is a leading cause of tooth loss in horses. We hypothesize that there is a correlation between tooth elongations and periodontal disease. Dental records of all client owned horses examined by a single AVDC/equine board certified practitioner in 2016-2018 were analyzed. Inclusion criteria consisted of the examination finding of tooth elongation of one of the following teeth pairs 106/107, 206/207, 310/311, or 410-411. The included cases were then examined for the finding of periodontal disease in Stages 1-4 (PD 1-4). Bayesian correlation analysis revealed significant positive correlations of elongations and periodontal disease existed between PD1 and proximal tooth pairs 106-107 and 206-207, PD2 and PD4 with all four tooth pairs, and PD3 with 106-107. Clinically, early diagnosis and treatment of newly formed elongations, or prevention of their formation altogether through diligent dental prophylaxis could prove key in preservation of teeth through prevention of the associated development of periodontal disease in the horse.

Fracture Resistance of Equine Cheek Teeth With and Without Occlusal Fissures: A Standardized ex vivo Model

Background:Ex vivo fracture models are frequently used in human dentistry to provide insights in the fracture mechanisms of teeth. Equine cheek teeth fractures are an important dental pathology, but there has been no research performed to examine the fracture resistance ex vivo.

Objective: To evaluate the fracture resistance of equine cheek teeth and identify anatomical predictors that might influence fracture resistance in healthy teeth. It was further evaluated if the presence of a fissure caused a decrease in fracture resistance.

Study design:Ex vivo experimental design.

Methods: Individual cheek teeth were subjected to a compression load in a universal testing machine until fracture occurred. Testing was performed in two study groups. A first group of healthy cheek teeth was tested to examine anatomical predictors on fracture resistance. A second group comprised cheek teeth with occlusal fissures and an equal number of age- and size-matched fissure-free teeth as controls. The effect of possible predictors on fracture resistance was investigated by regression analysis.

Results: In the first group, fracture resistance was significantly influenced by the location on the tooth where testing was performed in both maxillary (p < 0.001) and mandibular teeth (p < 0.001). Additional significantly associated factors were Triadan number in mandibular (p = 0.009) and the mesiodistal length of the occlusal surface of maxillary teeth (p = 0.01). Experimentally induced crown fractures that extended below the simulated bone level were more frequently associated with pulp horn exposure (p < 0.001). In the second group, significant lower fracture loads were recorded in teeth with fissures (mandibular p = 0.006; maxillary p < 0.001), compared to fissure-free teeth.

Main limitations: This ex vivo model does not imitate the in vivo masticatory forces and lacks the shock-absorbing properties of the periodontal ligament.

Conclusions: The methodology used in this study provides an ex vivo experimental set-up to test fracture resistance of equine cheek teeth enabling evidence-based research to examine the potentially weakening effects of tooth pathology and its treatments. Crown resistance to fracture differed along the occlusal surface of healthy equine cheek teeth, and the presence of fissures further decreased fracture resistance.

Equine "Idiopathic" and Infundibular Caries-Related Cheek Teeth Fractures: A Long-Term Study of 486 Fractured Teeth in 300 Horses

Background: Limited objective information is available on the prevalence of non-traumatic equine cheek teeth fractures, the signalment of affected horses, and the clinical features and treatment of these fractures.

Objectives: This study aims to document patterns of idiopathic and infundibular caries-related cheek teeth fractures in a referral population and evaluate associations between fracture patterns and horse age, Triadan position of affected teeth, clinical signs, and deemed necessity for treatment.

Study Design: A retrospective case review.

Methods: The clinical records at Edinburgh University Veterinary School (2010–2018) were examined for the presence of non-traumatic equine cheek teeth fractures. Variations in the frequencies of different fracture patterns were compared between horse ages, Triadan tooth positions, clinical signs, and deemed necessity for treatment.

Results: Records of 300 horses with 486 non-traumatic cheek teeth fractures including 77% maxillary and 23% mandibular teeth with a mean of 1.6 (range 1–10) fractured teeth/horse were available. Fracture patterns included maxillary first and second pulp horn (“slab”) cheek teeth fractures (n = 171), caries-related infundibular fractures (n = 88), other maxillary teeth fracture patterns (n = 92), mandibular first and second pulp horn (“slab”) fractures (n = 44), other mandibular fracture patterns (n = 62), and complete clinical crown loss (n = 29; including 23 maxillary and 6 mandibular teeth). The median age of affected horses ranged from 11 years with maxillary “slab” fractures to 15 years with infundibular caries-related fractures. Triadan 08–10s were the most commonly (86%) fractured maxillary teeth. The Triadan 08 and 09 positions were the most commonly (64%) fractured mandibular teeth. No clinical signs were noted in horses with 48% of the fractured teeth; oral pain/quidding was recorded with 26%, clinical apical infection with 23%, and bitting/headshaking problems with 6%. Treatments included extraction of 40% fractured teeth, extraction of small/loose fragments (10%), and odontoplasty. Stable remnants of 60% of fractured teeth were left in horses without clinical signs.

Main Limitations: Long-term follow-up information was not available for all cases.

Conclusions: There is increasing recognition of equine non-traumatic cheek teeth fractures, with about half not causing clinical signs. Teeth with apical infection, multiple fractures, or advanced caries require extraction. Other fractured teeth with subclinical endodontic disease may not need exodontia unless they later cause clinical signs.

Biomechanical comparison of locking compression plate fixation and a novel pedicle screw external fixation to repair equine mandibular fractures

OBJECTIVE

To determine the biomechanical properties of pedicle screw external fixation (PDW) for equine mandibular fracture repair and compare PDW to locking compression plates (LCP).

STUDY DESIGN

Cadaveric study.

SAMPLE POPULATION

Sixteen adult equine mandibles.

METHODS

Four mandibles were kept intact, while 12 were osteotomized and stabilized with the LCP or PDW construct (6 mandibles/group). Failure, stiffness, and yield were calculated from quasi-static ramp to failure and compared with previous analysis of mandibular fracture constructs. Tooth root involvement and method of failure were determined from radiographs and videos.

RESULTS

Locking compression plate constructs achieved greater stiffness and load at failure (4656 ± 577 N-m/radian, 558 ± 27 N-m P < .05) compared with PDW constructs (2626 ± 127 N-m/radian, 315 ± 48 N-m). Yield did not differ between types of fixation (369 ± 57 N-m, 193 ± 35 N-m, P = .145). Tooth involvement was noted in two LCP constructs with failure via bone fracture. Pedicle screw external fixation constructs failed via wire unraveling and screw bending.

CONCLUSION

Locking compression plate fixation increased stiffness and failure of constructs but did not influence yield. It also increased the risk to tooth root involvement relative to fixation with PDW. Compared with another study, PDW offered stiffness and failure similar to an intraoral splint with interdental wires, external fixator (EF), and external fixator with wires (EFW) and yield similar to an EF, an EFW, and a dynamic compression plate.

CLINICAL RELEVANCE

Pedicle screw external fixation offers biomechanical stability comparable to other relevant mandibular fixation techniques and reduces the risk of tooth root damage compared with LCP fixation.

The Equine Gingiva: A Gross Anatomical Evaluation

Equine periodontal disease (ePD) usually starts with food impaction, formation of diastemata, gingival inflammation and formation of periodontal pockets. This process proceeds toward the dentoalveolar space, causing detachment of tooth supporting periodontal fibers. Although several therapeutical procedures have been proposed, ePD is often only diagnosed in advanced stages, requiring dental extraction. A similar dilemma has been observed in small animal medicine, but has been overcome by the introduction of reliable examination protocols for the early diagnosis of periodontal diseases (PD). These protocols are based on detailed anatomical descriptions of healthy gingiva, allowing for the determination of the pathognomonic signs of the onset of PD and providing a basis for grading systems and treatment plans. Consequently, proposals have also been made for periodontal examination protocols in horses. However, these protocols were widely adopted from small animal medicine assuming a similar anatomy of the equine and canine gingiva. To provide a solid anatomical basis for equine specific periodontal examinations, 20 equine heads were examined macroscopically, with special attention to the gingival sulcus, the gingival margin and the interdental papillae. Constant morphological patterns of the gingival margin and the interdental papillae were found for the vestibular and lingual/palatal aspects of the upper and lower cheek teeth arcades, as well as for the incisor arcades. A gingival sulcus measuring greater than 1 mm was present in only 6% of the investigated specimens. The inspection of the gingival margin and the interdental papillae, as well as the recognition of a gingival sulcus, may serve as criteria to establish equine specific periodontal investigation protocols.

Scintigraphic evaluation of cheek teeth in clinically sound horses

Dental disorders are one of the top-ranking clinical domains in equine practice. Scintigraphy has emerged as a useful diagnostic imaging modality for equine dental diseases. There is a paucity of the normal scintigraphic uptake and its correlation with age. This prospective, cross-sectional, descriptive and pilot-designed study aimed to describe the radioisotope uptake (RU) patterns of the reserved crown and periodontal bone of the maxillary and mandibular cheek teeth (CT) in clinically sound horses and to evaluate the age effect on RU. For this purpose, 60 horses that underwent a bone scintigraphy for reason unrelated to head were included and divided equally into four age groups. Regions of interests (ROIs) were positioned around alveolar and periodontal bone of each maxillary and mandibular CT including their reserve crown, and a reference ROI positioned at the mandibular ramus. The count per pixel of each ROI was measured using a dedicated software and the RU ratio relative to the reference region was calculated. The results showed that the maxillary and mandibular CT had a standard RU pattern, where it increased from rostral to caudal, and peaked in the middle of dental arcades and decreased slightly towards the last CT. The maxillary CT had a higher RU compared with the mandibular CT, and there was no significant difference in the maxillary and mandibular CT uptake between age groups. This information may aid veterinarians evaluating potentially abnormal dental scintigraphy.

The Frequency of Errors in Determining Age Based on Selected Features of the Incisors of Icelandic Horses

The structure and changes occurring to horses' teeth during ontogeny are not only used to assess the degree of somatic maturity but also the development of universal patterns and is therefore used to determine the age of horses. Research shows that methods of assessing the age of horses based on the appearance of teeth tend to suffer from relatively large errors. This is probably influenced by the results of intensive selection and being kept in living conditions that differ substantially from their natural environment. This study aimed to assess the suitability of selected features of the incisors to determine the age of Icelandic horses. One hundred and twenty-six Icelandic horses (78 mares and 48 stallions) of different ages (range: 0-24 years; groups: 0-2 years, >2-5 years, >5-11 years, and >11 years) were examined by an experienced horse person who was blinded to the actual age of the horse and did not know which age group horses were in. Age was determined by the inspection of each horse's teeth and was compared to the actual age of the horse recorded in the breeding documentation, and the percentage of mistakes made regarding the age group was calculated. The estimated age did not match the real age in 36.5% of the horses. The age was more often underestimated (19.0%) by, on average, 0.9 ± 1.0 years than overestimated (17.5%) by, on average, 1.3 ± 1.4 years. Within age groups, the least number of errors in determining age were made in young horses aged 0-2 years, when the eruption and growing of the deciduous incisors and the disappearance of the cups was considered. The average percentage of errors in this group (2.1%) was significantly lower (p < 0.01) than for older horses, whose age was estimated based on the exchange of deciduous to permanent teeth (55.8%), disappearance of the cups (68.0%), and shape changes on the grinding surface (40.0%). Significantly more frequent underestimation of age based on replacing deciduous for permanent incisors and significantly more frequent overestimation of age on the basis of the disappearance of the cup may indicate that Icelandic horses up to 5 years of age are characterized by a slower rate of growth than horses of other breeds, especially warmblood horses. These results suggest that patterns used to determine the real age of horses based on changes occurring on the incisors should be modified in order to consider the specificity of the course of growth and maturation processes of horses of various types and breeds.

A Computed Tomographic and Pathological Study of Equine Cheek Teeth Infundibulae Extracted From Asymptomatic Horses. Part 2: MicroCT, Gross, and Histological Findings

Background: Equine maxillary cheek teeth infundibulae are frequently affected by developmental and acquired disorders, but the imaging, gross, and histological features of normal and abnormal infundibulae remain incompletely understood. Objective: To perform MicroCT, gross examination, and histology on sectioned teeth and compare the imaging and anatomical findings. Study design: Ex vivo original study. Methods: Eight maxillary cheek teeth of different ages and with varying grades of unilateral (n = 5) or bilateral (n = 3) occlusal infundibular caries were extracted from equine heads obtained from an abattoir. The teeth were imaged by MicroCT, then transversely sectioned and grossly and histologically examined, with the imaging and gross and histological anatomical findings examined and compared. Results: Fifteen infundibulae, including two without occlusal caries had subocclusal cemental hypoplasia or caries. One infundibulum without occlusal caries had no subocclusal cemental defects. Histologically, hypoplastic cemental areas consisted of irregularly-shaped, wide central channels, with multiple, large, cylindrical side-branches that extended peripherally to a variable extent. Cementum with extensive, wide, empty channels, and cementum with a more irregular moth-eaten appearance had dark or eroded gross appearance and a low HU on CT. Some infundibulae had cement-free areas that only contained fragments of collagen-like material, especially at their apical aspects (apical cemental hypoplasia). Carious subocclusal areas had connections with the occlusal surface and had disrupted cemental architecture, including of their central vascular channel that, along with their side branches, contained degraded food, and cemental debris. Main Limitations: No clinical histories or accurate ages were available for these eight teeth. Conclusions: Hypoplastic cemental lesions, including at central linear and apical sites, histologically contain areas with multiple wide-branched, cylindrical channels or even areas of total cement hypoplasia visible on gross sections When such cemental defects contact the occlusal surface due to normal wear, food impaction, and caries can ensue.

Equine cheek teeth occlusal fissures: Prevalence, association with dental wear abnormalities and occlusal angles

BACKGROUND

Fissures of the occlusal surface of equine cheek teeth are commonly encountered during oral examination but their aetiology is unknown.

OBJECTIVES

To examine the prevalence and characteristics of occlusal fissures in cadaver teeth. It is hypothesised that their prevalence is influenced by masticatory forces. Consequently, their possible association with wear disorders and occlusal angles were examined.

STUDY DESIGN

Cross-sectional survey.

METHODS

The dental abnormalities and occlusal fissure findings in the cheek teeth of 143 cadaver heads were recorded. The cheek teeth occlusal angles were measured using the stiff-hinge technique. Multiple regression analyses were performed to establish possible relationships between age, sex, dental wear, occlusal angle and fissure prevalence.

RESULTS

Occlusal fissures were found in 103/143 (72%) heads. Sex and age were determining factors in the prevalence of fissures. A similar prevalence was found in mandibular (54.1%) and maxillary teeth (45.9%, OR = 1.10; 95% CI = 0.95-1.29, P = 0.2). Mandibular fissures were more commonly located on the buccal aspect (OR = 1.42; 95% CI = 1.16-1.65, P < 0.001), whereas for maxillary fissures there was no difference in prevalence between palatal and buccal aspects (OR = 1.19; 95% CI = 0.97-1.46, P = 0.1). Two main fissure types were identified. Type 1a fissures were the most prevalent type (39.5%). No significant correlation was found between the presence of wear abnormalities or the occlusal angle of cheek teeth, and the prevalence of fissures.

MAIN LIMITATIONS

No dental histories were available.

CONCLUSION

Equine cheek teeth show a high prevalence of occlusal fissures. Despite some evidence of predilection sites on the tooth surface that might indicate a mechanical aetiology for these lesions, no associations were found with wear abnormalities or occlusal angles of affected cheek teeth. Further histological and ultrastructural studies are warranted to elucidate their aetiology and possible role in other dental diseases.

A new understanding of oral and dental pathology of the equine cheek teeth

Equine dental disease has a high prevalence. Because of developmental, functional, and anatomic differences, limited inference can be made from brachydont dental pathology to that of equine cheek teeth. This article reviews the pathology of equine cheek teeth and their associated oral tissues, with specific information on periodontitis, pulpitis, maxillary infundibular changes, dental fractures, dental overgrowths, mucosal ulceration, and the regenerative capacity of equine teeth.

Periodontal biomechanics: finite element simulations of closing stroke and power stroke in equine cheek teeth

Background

In equine dentistry periodontal diseases, especially periapical inflammation, are frequently occurring problems. Anachoresis is believed to be the most common cause for the development of such disorders. Nevertheless, there is still no substantiated explanation why settlement of pathogen microorganisms occurs in equine periodontal tissues. It is expected that excessive strains and stresses occurring in the periodontal ligament (PDL) during the horse’s chewing cycle might be a predisposing factor. In this study this assumption was examined by finite element (FE) analyses on virtual 3-D models of equine maxillary and mandibular cheek teeth, established on the basis of μCT datasets. Calculations were conducted both under conditions of closing and power stroke.

Results

Results showed a uniform distribution of low stresses and strain energy density (SED) during closing stroke, whereas during power stroke an occurrence of high stresses and SED could be observed in the PDL near the alveolar crest and in periapical regions.

Conclusion

The concentration of forces during power stroke in these specific areas of the PDL may cause local tissue necrosis and inflammation and thus establish a suitable environment for the settlement of microorganisms.

Kinematic analysis of equine masticatory movements: comparison before and after routine dental treatment

The objective of this study was to compare masticatory movements before and after dental treatment using kinematic analysis. The masticatory movements of 15 adult Warmblood horses with mild dental pathology chewing standardised hay were recorded on three consecutive days before and three times after (days 7, 21, 28) dental correction. The results of the leading mandibular tracking marker, located at the caudal edge of the inter-mandibular suture, were compared statistically. Reproducibility of measurements prior to dental treatment was excellent (P<0.05). Rostrocaudal mandibular motion was significantly reduced after dental correction (9±2mm vs. 8±2mm; P=0.046). Oscillations during the power stroke were significantly reduced after dental correction (R(2)=98.3%±0.3 vs. 98.8%±0.3; P=0.050). Although significant changes were observed in individual horses, the overall results of lateral and dorsoventral mandibular motion did not reveal significant differences. In conclusion, kinematic analysis of masticatory movements provided reproducible results and may be useful to evaluate changed movement patterns following dental correction.

Measurement of the Curve of Spee in horses

The Curve of Spee is well known, yet hardly understood in the horse, as there is a striking lack of biometrically related data which could help to explain its biomechanical significance. Older horses display drastic alterations of the Curve of Spee in routine clinical examinations. Since the Curve of Spee is known to be biomechanically related to the masticatory forces in humans, the same may apply to the equine, and masticatory deficits can be expected in the geriatric horse. Quantitative data of the Curve of Spee cannot be obtained directly by means of measurements in the horse's long and narrow oral cavity. Instead, the radiographs of 24 heads and 6 skulls of horses of different age were used to develop a geometrical procedure facilitating a quantitative analysis of the height of the Curve of Spee. The geometrical values gave clear evidence that the height of the Curve of Spee was smaller in groups of older horses compared with those of younger horses. This finding was in accordance with biomechanical studies which reported decreased masticatory forces in older horses.