Measurements of masticatory forces in the horse

Gross Anatomy of the Equine Masseter Muscle: Lamination and Intramuscular Course of the N. Massetericus

The masticatory muscles of the horse are arranged in an asymmetrical pattern. Four individual muscles on the medial side of the mandible are opposed by one muscle (M. masseter) on the lateral side. However, recent studies on various herbivorous mammals indicate that the masseter muscle features a complex stratigraphic structure that might account for a functional diversity resembling an arrangement of several individual muscles. The functional consideration of the multidirectional equine masticatory movements leads to a similar hypothesis. In order to elucidate the detailed anatomy of the equine masseter muscle, eight cadaveric equine heads were dissected. Additional 29 skull specimens were assessed with regard to the masseteric attachment within the Fossa masseterica. A constant arrangement of nine individual muscular layers within the masseteric muscle was determined. The individual layers were clearly separated by tendon plates and their attachment areas at the masseteric fossa of the mandible were arranged in a constant pattern of bony ridges. With similar consistency, the main trunk of the masseteric nerve was found to run from dorsocaudal to ventrorostral: On that course, the nerve penetrated between the muscular layers from medial to lateral at constant positions. The findings of this study serve as a basis for further studies with the aim of developing biomechanical concepts of equine masticatory movement.

Equestrian-associated injuries of the hand: a retrospective analysis of injury mechanisms and patterns

INTRODUCTION

Hand injuries are frequently caused by sports and are associated with long periods of inability to work and high economic health costs. After ball sports and cycling, the most common cause of hand injuries is horseback riding. Therefore, measures should be taken to prevent these risks and increase safety in sports, however data characterizing equestrian sports-associated injuries are limited.

MATERIALS AND METHODS

A retrospective monocentric cohort study was performed including 39 patients (mean age: 35 ± 2.6 (range 9-65) years, female 89.7% who sustained an injury to the hand while practicing equestrian sports during 2016-2021 and presented to our hand surgery center. Data analysis was performed to characterize the trauma mechanism and injury patterns by evaluating the clinical information system and conducting telephone interviews.

RESULTS

Overall, 53.8% of the injuries occurred while leading the horse owing to traction by bridles or a lead rope on the fingers, whereas only 33.3% were caused by a fall and 12.8% by a bite injury. The majority (87.2%) of cases were injuries to the phalanges (metacarpus: 7.7%; carpus: 5.1%). Fractures were present in 51.3% of cases. The most serious injuries included avulsion amputations in 23.1% of patients (10.3% subtotal; 12.8% total amputation).

CONCLUSIONS

Equestrian-associated injuries occur more frequently during horse handling than riding, resulting in severe avulsion amputations due to traction of the lunge or bridle, requiring complex microsurgical treatment. We recommend that appropriate protective gloves are worn for prevention of hand injuries. Additionally, the use of self-opening panic hooks with overload protection can prevent excessive traction.

LEVEL OF EVIDENCE

III.

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.

Changes in eating time, chewing activity and dust concentration in horses fed either alfalfa cubes or long-stem hay

BACKGROUND

Chewing is an essential physiological process in horses; yet, the physical form of feeds may affect their chewing and feeding behaviour with consequences for equine digestion and health.

OBJECTIVES AND METHODS

The present study evaluated the potential of a commercial forage cubes made from alfalfa and mixed meadow grasses to maintain chewing activity when compared with a traditional long and fibre-rich hay. An additional aim was to measure the dust formation during feeding. The experiment was a crossover design with six horses (11 ± 4 years old, mean ± SD), fed with 5 kg (as-fed basis) of their feed (long hay or alfalfa cubes) overnight. Eating and chewing activity was measured using the EquiWatch system with a sensor-based halter.

RESULTS

Data showed that by feeding the same amount of feed overnight, cube feeding led to 24% shorter eating time (on average 67 min less) and 26% fewer total chews compared to the long hay. On an hourly basis, horses also spent more time eating and chewing the long hay compared to the cubes. The cube feeding led to a higher dust concentration of the inhalable fraction (<100 μm), but not of the thoracic (<10 μm) fraction. Nevertheless, average dust concentrations were generally low for both cubes and hay, whereby both were of sound hygienic status.

CONCLUSION AND RECOMMENDATION

Our data suggest that feeding alfalfa-based cubes overnight generated shorter eating time and less chews than the long hay without major differences in the thoracic dust. Therefore, because of the decreased eating time and chews, alfalfa-based cubes should not be fed a sole forage source, especially when it is to be fed without restriction.

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.

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.

Comparison of computed tomography and high-field (3.0 T) magnetic resonance imaging of age-related variances in selected equine maxillary cheek teeth and adjacent tissues

Background

Modern imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) have the advantage of producing images without superimposition. Whilst CT is a well-established technique for dental diagnostics, MRI examinations are rarely used for the evaluation of dental diseases in horses. Regarding equine endodontic therapies which are increasingly implemented, MRI could help to portray changes of the periodontal ligament and display gross pulpar anatomy. Knowledge of age-related changes is essential for diagnosis, as cheek teeth and surrounding structures alter with increasing age. The aim of the present study was to highlight the advantages of CT and MRI regarding age-related changes in selected equine cheek teeth and their adjacent structures.

Results

The CT and MRI appearances of the maxillary 08 s and 09 s and adjacent structures were described by evaluation of post-mortem examinations of nine horses of different ages (Group A: <6 years, B: 6–15 years, C: ≥16 years). Most of the tissues selected were imaged accurately with MRI and CT. Magnetic resonance imaging gives an excellent depiction of soft endo- and periodontal units, and CT of hard dental and bony tissues. Negative correlation between dental age and pulpar sizes was found: 71.3% of the changes in pulp dimensions can be explained by teeth aging. Pulpar sizes ranged from 14.3 to 1.3 mm and were significantly smaller in older horses (p < 0.05). A common pulp chamber was present in 33% of the teeth with a mean dental age of 2.25 years. Ninety-four percent of the 08 and 09 alveoli of all groups were in direct contact with the maxillary sinus. An age-related regression was found (R² = 0.88) for the distance between alveoli and the infraorbital canal.

Conclusions

The present study provides information about the dental and periodontal age-related morphology and its visibility using different imaging techniques. These results aid in evaluating diagnostic images and in deciding which is the superior imaging modality for clinical cases.

Functional anatomy of the equine temporomandibular joint: Collagen fiber texture of the articular surfaces

In the last decade, the equine masticatory apparatus has received much attention. Numerous studies have emphasized the importance of the temporomandibular joint (TMJ) in the functional process of mastication. However, ultrastructural and histological data providing a basis for biomechanical and histopathological considerations are not available. The aim of the present study was to analyze the architecture of the collagen fiber apparatus in the articular surfaces of the equine TMJ to reveal typical morphological features indicating biomechanical adaptions. Therefore, the collagen fiber alignment was visualized using the split-line technique in 16 adult warmblood horses without any history of TMJ disorders. Within the central two-thirds of the articular surfaces of the articular tubercle, the articular disc and the mandibular head, split-lines ran in a correspondent rostrocaudal direction. In the lateral and medial aspects of these articular surfaces, the split-line pattern varied, displaying curved arrangements in the articular disc and punctual split-lines in the bony components. Mediolateral orientated split-lines were found in the rostral and caudal border of the articular disc and in the mandibular fossa. The complex movements during the equine chewing cycle are likely assigned to different areas of the TMJ. The split-line pattern of the equine TMJ is indicative of a relative movement of the joint components in a preferential rostrocaudal direction which is consigned to the central aspects of the TMJ. The lateral and medial aspects of the articular surfaces provide split-line patterns that indicate movements particularly around a dorsoventral axis.

The effect of acute unilateral inflammation of the equine temporomandibular joint on the kinematics of mastication

REASONS FOR PERFORMING STUDY

Diseases of the temporomandibular joint (TMJ) are well reported in man and some domestic animals other than the horse. The pathophysiology of equine TMJ disease and the effects of disease on the kinematics of mastication are unknown.

OBJECTIVES

To determine whether transient unilateral inflammation of the equine TMJ results in alterations in the masticatory cycle.

STUDY DESIGN

An experimental controlled study utilising 6 horses of various ages with normal dentition.

METHODS

Each horse was equipped with an optical motion tracking (kinematic) system. Horses were observed chewing grass hay over 3 min intervals. Regardless of the initial side of the power stroke in the masticatory cycle, lipopolysaccharide (LPS) was injected in the left TMJ in each horse and the horses were reassessed after 6 h.

RESULTS

Four horses developed effusion of the injected TMJs; 2 of these also began quidding. All horses injected on the original side of the power stroke switched sides while the 2 injected on the contralateral side did not. All horses developed reduced vertical pitch (vertical opening) of the mandible. Overall, rostrocaudal movement of the mandible did not change; however, the timing of this movement relative to the phase of the masticatory cycle did. Injection with LPS did not affect the amount of lateral movement of the mandible.

CONCLUSIONS

Injection of LPS into the TMJ significantly altered the masticatory cycle compared with baseline values representing avoidance behaviour due to inflammation of the joint, despite which the horses continued to eat using the contralateral mandible. Lipopolysaccharide administration also led to quidding and a loss of feed efficiency (in some individuals).

Bilateral iatrogenic [corrected] maxillary fractures after dental treatment in two aged horses

This clinical report describes two horses with bilateral maxillary fractures following dental treatment. The fractures occurred during dental treatment by a veterinarian, and both had rostral, transverse, and complete bilateral maxillary fractures with instability and minimal displacement. The fractures were repaired using bilateral intraoral wiring with the patients under general anesthesia. The postoperative period was without complications and the fractures healed as expected. Maxillary fractures during or after routine dental treatment are rare, but can occur, especially in older horses. Appropriate sedation and, if necessary, regional nerve blocks might reduce the risk of iatrogenic fractures of the maxilla. General anesthesia must be considered for tooth extraction in horses that are difficult to restrain.

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.

Equine idiopathic cheek teeth fractures. Part 1: Pathological studies on 35 fractured cheek teeth

REASONS FOR PERFORMING STUDY

There is little published information on the pathology of idiopathic fractures of cheek teeth (CT).

OBJECTIVES

To perform pathological examinations on equine CT with idiopathic fractures in order to establish fracture patterns and to gain information on their aetiopathogenesis.

HYPOTHESIS

Gross and histological examination of CT with idiopathic fractures, including measurements of dentinal thickness, will provide information on fracture patterns, and on the duration and aetiopathogenesis of these fractures.

METHODS

Of 35 CT with idiopathic fractures that were examined to determine their fracture patterns, 20 underwent gross, histological and ultrastructural examinations, including dentinal thickness measurements, with the latter compared to dentinal measurements of dental age and Triadan position matched control CT, to help determine the duration of any pre-existing endodontic disease.

RESULTS

The fracture planes involved the pulp chambers in 30 out of 35 CT examined, and ran through coalesced, carious infundibula in the other 5 (maxillary) CT. The maxillary CT, particularly the Triadan 09 position were preferentially affected. The most common fracture plane, which was termed a maxillary buccal slab fracture, occurred through the 2 lateral (1st and 2nd) pulp chambers of maxillary CT and usually involved only the clinical crown. Buccal slab fractures of mandibular CT (through 4th and 5th pulp chambers) and midline sagittal fractures through the infundibula (of maxillary CT) were the next most common fracture patterns. Reduced dentinal thickness (probably indicating prior pathological changes in the fractured CT) was present in 25% of fractured CT.

CONCLUSIONS

Most idiopathic equine CT fractures involve the pulp chambers, especially those on the buccal aspect of both upper and lower CT. No predisposition to fracture was found in the majority of affected CT that appeared to have a normal endodontic appearance prior to development of fractures through their pulp chambers. The remaining idiopathic CT fractures were caused by advanced infundibular caries.

POTENTIAL RELEVANCE

All CT with idiopathic fractures are all at risk of pulpar or even apical infection.

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

The forces generated during equine mastication influence the teeth, as well as the periodontium, the jaw bones, the temporomandibular joints and materials used in dental therapy. Due to the limited accessibility of the cheek teeth located farther caudal in the equine oral cavity the measurement of the masticatory forces on these teeth is virtually impossible. The calculation of such forces is an appropriate, indirect alternative to direct measuring. Hence, a mathematical model, which respects several morphological features of the equine dentition, e.g. the Curve of Spee, is presented. The relevant parameters were measured on laterolateral radiographs of 24 cadaver heads and 6 skulls. Our calculations yielded two major results. (1) The masticatory forces increased from rostral to caudal. This increase was due to distinct anatomical features of the equine dentition, i.e. the Curve of Spee, the position of the teeth, and the length of the mandible. (2) The masticatory forces decreased with age. This decrease was due to the reduced height of the Curve of Spee in older horses. These results are discussed in view of veterinary dental treatment, with special reference to the geriatric horse.

The blood vessel system in the periodontal ligament of the equine cheek teeth – Part II: The micro-architecture and its functional implications in a constantly remodelling system

The micro-vasculature of the equine periodontal ligament (PDL) was investigated using corrosion casts for scanning electron microscopy. Specimens from eight healthy warm-blooded horses were examined. Specific vascular features such as large ampullae with a diameter of up to 300 microm and blind vascular casts were found in the specimens of the equine PDL. The ampullae-shaped venules occurred only in the peripheral layer, where they were formed at the confluence of several vessels. Two types of blind stumps were identified. The first was developed in all three layers, while the second was found only in the peripheral layer. Blood-flow-regulating structures were not present in the equine PDL. As vascular valves are absent, the blood can flow in all directions. This polydirectional shift exists in all vascular layers. The vascular ampullae, present exclusively in the peripheral layer, are discussed here as specific features of a mechanism that can receive masticatory forces. Apart from its mechanical role, the vasculature must constantly adapt to the spatial changes resulting from the life-long eruption of the equine cheek tooth. This is facilitated by vascular sprouting and intussusceptive vascular remodelling. The blind ending stumps of the corrosion casts are typical equivalents of vascular sprouting. The large ampullae are interpreted as the first step in an intussusceptive process.

Collagen fiber architecture of the periodontal ligament in equine cheek teeth

The objective of this study was to examine the spatial arrangement of the fiber apparatus of the equine periodontium which is supposed to meet two contrary requirements: (1) to attach the tooth firmly and elastically to the alveolar bone; and, to be appropriately remodeled and reconstructed in order to facilitate the prolonged eruption of the tooth. Specimens of periodontal ligament were obtained from the buccal and lingual/palatal aspects of the first molars from the maxilla and mandible of 12 horses. The animals were assigned to three age groups. Histological sections were prepared from three specific horizontal levels of the periodontal ligament and examined with conventional and polarized-light microscopy. At the gingival level, collagen fascicles (diameter > 200 microm) were densely packed. Their spatial alignment was the same in all age groups. The architecture of the collagen fiber apparatus differed at the middle and apical levels in the three age groups. There was a clear distinction between fibers, bundles, and fascicles. Bundling of collagen fibers, density of the fiber arrangement, and collagen fascicles with an alveolo-cemental orientation increased with age. The collagen fiber apparatus of the equine periodontal ligament is highly adaptive, responding continuously to the dynamic changes in the periodontal environment. Site-specific arrangements and age-dependent structural variations are assumed to maintain tooth support as the reserve crown gradually decreases in length with progressive dental attrition. Most of the age-dependent changes to the periodontal ligament in teeth examined in this study occurred at the apical level. The apical region of young teeth had no periodontal attachment, while the roots of older teeth were firmly attached to the alveolus. When evaluating periodontal ligament development, the individual tooth's 'dental age' should be considered rather than the animal's age to account for individual tooth eruption times.