Kinematics of the equine temporomandibular joint

Exploring the Use of Animal Models in Craniofacial Regenerative Medicine: A Narrative Review

The craniofacial region contains skin, bones, cartilage, the temporomandibular joint (TMJ), teeth, periodontal tissues, mucosa, salivary glands, muscles, nerves, and blood vessels. Applying tissue engineering therapeutically helps replace lost tissues after trauma or cancer. Despite recent advances, it remains essential to standardize and validate the most appropriate animal models to effectively translate preclinical data to clinical situations. Therefore, this review focused on applying various animal models in craniofacial tissue engineering and regeneration. This research was based on PubMed, Scopus, and Google Scholar data available until January 2023. This study included only English-language publications describing animal models' application in craniofacial tissue engineering (in vivo and review studies). Study selection was based on evaluating titles, abstracts, and full texts. The total number of initial studies was 6454. Following the screening process, 295 articles remained on the final list. Numerous in vivo studies have shown that small and large animal models can benefit clinical conditions by assessing the efficacy and safety of new therapeutic interventions, devices, and biomaterials in animals with similar diseases/defects to humans. Different species' anatomical, physiologic, and biological features must be considered in developing innovative, reproducible, and discriminative experimental models to select an appropriate animal model for a specific tissue defect. As a result, understanding the parallels between human and veterinary medicine can benefit both fields.

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.

Functional anatomy of the equine temporomandibular joint: Histological characteristics of the articular surfaces and underlining tissues

It has been assumed that dental conditions cause disorders of the equine temporomandibular joint (TMJ), due to biomechanical overload or aberrant loading. However, the incidence of published TMJ disorders in horses is low and this leads to the question whether the equine TMJ is adapted well to its biomechanical requirements or is able to remodel its articular surfaces in response to modified loading conditions. The aim of this study was to determine the histological characteristics of healthy equine TMJs. The tissue components of the articular surfaces of 10 TMJs obtained from horses without any clinical history of dental or TMJ disorders were analysed. Apart from the mandibular fossa of the temporal bone, the osseous aspects of the TMJ exhibited a uniform zoning pattern. The articular surfaces were composed of three tissue layers: (1) a superficial cell-rich dense connective tissue layer; (2) a middle fibrocartilage layer; and (3) a deep hyaline-like cartilage layer. The articular disc was composed of an inner core of fibrocartilage and hyaline-like cartilage meshwork covered with both cell-rich dense connective tissue and fibrocartilage at its dorsal and ventral aspects. In contrast, the mandibular fossa was only covered by a dense connective tissue, frequently supplemented by a synovial membrane, suggesting low biomechanical stress. Glycosaminoglycans, which are indicative of compressive loads, were predominantly present within the rostral part of the articular tubercle and the retroarticular process, the dorsal part of articular disc and the entire mandibular head, but were absent within the mandibular fossa. The results of this study suggest the presence of different biomechanical demands in the dorsal and ventral compartment of the equine TMJ.

Preclinical Animal Models for Temporomandibular Joint Tissue Engineering

There is a paucity of in vivo studies that investigate the safety and efficacy of temporomandibular joint (TMJ) tissue regeneration approaches, in part due to the lack of established animal models. Review of disease models for study of TMJ is presented herein with an attempt to identify relevant preclinical animal models for TMJ tissue engineering, with emphasis on the disc and condyle. Although degenerative joint disease models have been mainly performed on mice, rats, and rabbits, preclinical regeneration approaches must employ larger animal species. There remains controversy regarding the preferred choice of larger animal models between the farm pig, minipig, goat, sheep, and dog. The advantages of the pig and minipig include their well characterized anatomy, physiology, and tissue properties. The advantages of the sheep and goat are their easier surgical access, low cost per animal, and its high tissue availability. The advantage of the dog is that the joint space is confined, so migration of interpositional devices should be less likely. However, each species has limitations as well. For example, the farm pig has continuous growth until about 18 months of age, and difficult surgical access due to the zygomatic arch covering the lateral aspect of joint. The minipig is not widely available and somewhat costly. The sheep and the goat are herbivores, and their TMJs mainly function in translation. The dog is a carnivore, and the TMJ is a hinge joint that can only rotate. Although no species provides the gold standard for all preclinical TMJ tissue engineering approaches, the goat and sheep have emerged as the leading options, with the minipig as the choice when cost is less of a limitation; and with the dog and farm pig serving as acceptable alternatives. Finally, naturally occurring TMJ disorders in domestic species may be harnessed on a preclinical trial basis as a clinically relevant platform for translation.

Biomechanical analysis of the masticatory movement before and after adjusting dental occlusion in equine

ABSTRACT The aim of this study was to evaluate through three-dimensional kinematic analysis the influence of occlusal adjustment by tooth wear in masticatory biomechanics of horses. Seven clinically healthy thoroughbred Arabian horses with strong occlusal irregularities were used, of which seven castrated males and one female of between seven and nineteen years of age. Three digital video cameras and seven spherical reflective markers placed on the horses’ face were employed. The animals were filmed twice in succession: while chewing hay before and after the occlusal adjustment by tooth wear. Following that, kinematic analysis was made of the movements obtained by means of the images which were synchronized, segmented and reconstructed three-dimensionally with the help of Dvideow program. Mathematical functions were applied in Matlab environment for obtaining the values of the amplitudes of movements. The results of the biomechanical analysis showed that the occlusal adjustment increases the range of mandibular movements in horses.

Occlusal Angles of Equine Incisors

The angulation of equine incisors is frequently used as a parameter for dental corrections. However, the term incisor angle is only vaguely defined, and no studies exist presenting a series of reliable measurements in individual incisors of multiple horses. The aim of this study was to establish an exact method to determine incisor angles and to test whether clinically accessible landmarks (facial crest and bars) are suitable to estimate incisor angles. Eighteen horses were used to create 3-dimensional (3D) reconstructions of the skulls from computed tomography (CT) data sets. Reference planes (median and transverse plane) were calculated using defined anatomical landmarks. Subsequently, occlusal planes for incisors and for incisor quadrants were calculated. Occlusal table angles were measured in relation to the reference planes. For each incisor, sagittal and transverse angles were measured. Mean values of individual incisor angles ranged from 3.5° to 6.8° (transverse angle) and from 32.6° to 44.9° (sagittal angle). No significant differences were detected in mean between the left and the right side when teeth in same Triadan positions were compared. However, in individual horses, marked differences between the left and the right side of the jaws occurred. Mandibular incisors showed significantly steeper sagittal angles than maxillary incisors. Furthermore, angles of opposing incisors were correlated with each other. The facial crest and the upper jaw bars featured a curved shape and were therefore of limited use to estimate the angulation of the upper incisors. In contrast, the lower jaw bars were suitable to determine the angulations of lower incisors.

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).

Analysis of Temporalis and Masseter adaptation after routine dental treatment in the horse via surface electromyography

Limited knowledge of how routine dental treatment (rasping) alters the mastication cycle exists. To our knowledge, Masseter and Temporalis muscle activity after rasping has not been previously evaluated. A descriptive, experimental study compared muscle activity pre- and post-routine dental treatment using surface electromyography (sEMG) to investigate the hypothesis that Masseter activity would increase and Temporalis activity decrease, over a 6 week time period after routine dentistry. Motor unit action potential amplitude (MUAP) and peak amplitude contraction (PAC), for 5 chewing cycles, were measured using sEMG in the right and left Masseter and Temporalis muscles of 10 horses, selected opportunistically due to their dental pathology that required routine rasping (week 0). Routine dental treatment was undertaken and sEMG measurements repeated at 1, 3 and 6 weeks post treatment. Mean MUAP and PAC were calculated for each week and compared both across the cohort and for each horse. For the cohort: Temporalis MUAP fluctuated after rasping, but was only significant in the left muscle between weeks 0 and 6, and weeks 1 and 6 (P<0.02). PAC did not differ significantly, with the exception of a decrease occurring on the left from week 0 to 1 (P<0.01). Masseter activity varied throughout the investigation, but few significant differences were found. A non-significant but consistent reduction in magnitude of PAC was found. For the individuals: MUAP and PAC in the Masseter and Temporalis muscles varied (increased / decreased) on an individual basis throughout the six weeks post rasping, although only MUAP fluctuations between weeks were significant (P<0.01). Adaptation occurs in the Masseter and Temporalis of individual horses after routine rasping; this appears to be associated with kinematic changes within the chewing cycle and is still occurring 6 weeks post-treatment.

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.

Computed tomography and magnetic resonance imaging features of the temporomandibular joint in two normal camels

Computed tomography (CT) and magnetic resonance (MR) image features of the temporomandibular joint (TMJ) and associated structures in two mature dromedary camels were obtained with a third-generation equipment CT and a superconducting magnet RM at 1.5 Tesla. Images were acquired in sagittal and transverse planes. Medical imaging processing with imaging software was applied to obtain postprocessing CT and MR images. Relevant anatomic structures were identified and labelled. The resulting images provided excellent anatomic detail of the TMJ and associated structures. Annotated CT and MR images from this study are intended as an anatomical reference useful in the interpretation for clinical CT and MR imaging studies of the TMJ of the dromedary camels.

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.

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.

Comparison of mandibular motion in horses chewing hay and pellets

REASONS FOR PERFORMING STUDY

Previous studies have suggested that temporomandibular joint (TMJ) kinematics depend on the type of food being masticated, but accurate measurements of TMJ motion in horses chewing different feeds have not been published.

HYPOTHESIS

The temporomandibular joint has a larger range of motion when horses chew hay compared to pellets.

METHODS

An optical motion capture system was used to track skin markers on the skull and mandible of 7 horses as they chewed hay and pellets. A virtual marker was created on the midline between the mandibles at the level of the 4th premolar teeth to represent the overall motion of the mandible relative to the skull during the chewing cycle.

RESULTS

Frequency of the chewing cycles was lower for hay than for pellets. Excursions of the virtual mandibular marker were significantly larger in all 3 directions when chewing hay compared to pellets. The mean velocity of the virtual mandibular marker during the chewing cycle was the same when chewing the 2 feeds.

CONCLUSIONS

The range of mediolateral displacement of the mandible was sufficient to give full occlusal contact of the upper and lower dental arcades when chewing hay but not when chewing pellets.

POTENTIAL RELEVANCE

These findings support the suggestion that horses receiving a diet high in concentrate feeds may require more frequent dental prophylactic examinations and treatments to avoid the development of dental irregularities associated with smaller mandibular excursions during chewing.

Effect of rostrocaudal mobility of the mandible on feed digestibility and fecal particle size in horses

OBJECTIVE

To evaluate the effect of rostrocaudal mobility (RCM) of the mandible during extension and flexion of the atlanto-occipital joint on weight gain, feed digestibility, and fecal particle size in horses.

DESIGN

Randomized controlled trial.

ANIMALS

34 pregnant mares housed in 1 barn.

PROCEDURES

Horses were randomized into a treatment (n = 17) or control (17) group. All horses were sedated, and the distance between the rostral portions of the upper and lower incisor arcades was determined during extension and flexion of the atlanto-occipital joint; mandibular RCM was calculated as the difference between these values. In the treatment group, measurements were made after dental floating. Body weight was recorded 1 day before dental floating and at intervals after mandibular RCM determination for a period of 24 weeks. Feces were collected from each horse during two 5-day periods. Samples of feed and feces were analyzed to determine feed digestibility; particle size analysis was performed on dried fecal samples.

RESULTS

Data obtained from each group of horses revealed that RCM of the mandible did not affect weight gain, feed digestibility, or particle size in feces; there were no differences among breeds. Controlling for age and number of dental lesions did not significantly affect results.

CONCLUSIONS AND CLINICAL RELEVANCE

In horses, RCM of the mandible did not appear to affect weight gain, feed digestibility, or fecal particle size. On the basis of these and other published data, the assumption that dental abnormalities affect digestion detrimentally in horses needs to be reevaluated.