EQUINE VETERINARY JOURNALEquine vet. J. (2006) 38 (2) 143-147

143

Summary

Reasons for performing study: The minute anatomy of the temporomandibular joint (TMJ) in horses is poorlydocumented in the literature.

Objectives: To examine in detail the anatomy of the normal equine TMJ and the relationship of the joint toadjacent structures.

Methods: The anatomy of the TMJ was examined in 12 equidae using dissections, synovial and vascular injectionsand frozen and plastinated sections. Relevant structureswere photographed.

Results: Rostral and caudal compartments were identified inthe dorsal and ventral synovial pouches of the TMJ. Lateraland caudal ligaments were identified and attachments of themasticatory muscles over the TMJ were documented. Directrelationships between the TMJ and components of the earwere found, and the vascular supply described.

Conclusions: This joint is related to some components of theear as in man, and has its own peripheral vascular supply.

Potential relevance: A knowledge of the detailed anatomy ofthe equine TMJ is a prerequisite to interpreting theimaging techniques, and will enable improved diagnosis ofTMJ pathologies.

Introduction

As opposed to in man, the equine temporomandibular joint (TMJ)has received little attention and its minute anatomy is poorlydocumented. The TMJ of the horse is a synovial condylar jointbetween the base of the zygomatic process of the temporal boneand the condylar process of the mandible (Barone 1980; Nickel etal. 1986; Sandoval and Agüera 1999; Getty 2001). Theretroarticular process is immediately rostral to the external auditorymeatus. The ventral articular surface corresponds to the condylarprocess of the mandible, which has a rostrocaudal (sagittal) axisthat is more convex than the lateromedial (transverse) axis. Thelateromedial axis is longer than the rostrocaudal axis (Barone1980; Nickel et al. 1986); furthermore, it forms an angle of 10° with the bicondylar line of the mandible (Barone 1980).

Due to the incongruent articular surfaces, an articularfibrocartilage disc is developed with a thick periphery and a thin

central part. A fibrous expansion arises from its caudomedialaspect and spreads between the retroarticular process and thecaudal aspect of the mandibular condyle (Barone 1980; Weller etal. 2002). The disc is biconcave on its lateromedial axis (Barone1980; May et al. 2001; Weller et al. 2002); its entire perimeter isfirmly attached to the capsule and its neurovascular bundle enterscaudally through the retrodiscal tissue, according to Barone(1980) and May et al. (2001).

A complete articular capsule has been described. It is thickerrostrolaterally and 2 capsular reinforcements, lateral and caudalligaments, are differentiated (Barone 1980; Nickel et al. 1986;Weller et al. 1999, 2002; Sandoval and Agüera 2000; Getty 2001;May et al. 2001). The articular disc is covered with a synovialmembrane dividing the articular space into 2 synovial pouches,dorsal and ventral (Barone 1980; Weller et al. 1999, 2002; May etal. 2001). There are discrepancies about the communication ofthese pouches and their number of compartments.

In anatomical textbooks, the masseter, temporalis and lateraland medial pterygoid muscles and occipitomandibular part of thecaudal belly of the digastric muscle are referred to as TMJstabiliser muscles, but the precise relationship is not specified(Barone 1980; Nickel et al. 1986; Getty 2001). Other importantstructures related to the equine TMJ have been documented,including the parotid gland, vascular and neural structures(transverse facial artery and vein, facial nerve, superficialtemporal vein) (Weller et al. 1999, 2002; May et al. 2001),temporohyoid joint (Rosenstein et al. 2001) and lateral aspect ofthe guttural pouch (Rosenstein et al. 2001; Weller et al. 2002).

The purpose of this report was to document the detailedanatomy of the normal equine TMJ and its relationship to adjacentstructures such as the ear.

Materials and methods

The heads of 12 pure Spanish breed horses, without a history ofany disease related to the articulation, were used in this study(mean age 7 years, mean bodyweight 380 kg).

Synovial and vascular injections

Temporomandibular joints were subjected to a dorsal synovialpouch injection into its caudal compartment (Rosenstein et al.

Anatomy of the equine temporomandibular joint: study bygross dissection, vascular injection and sectionM. J. RODRÍGUEZ*, A. AGUT, F. GIL† and R. LATORRE†

Departments of Animal Medicine and Surgery and †Anatomy and Comparative Anatomy, Faculty of Veterinary Medicine, Murcia University,Espinardo Campus, 30071 Murcia, Spain.

Keywords: horse; temporomandibular joint; anatomy; vascularisation; section; ear

*Author to whom correspondence should be addressed.[Paper received for publication 04.07.05; Accepted 15.08.05]

144 Anatomy of the equine temporomandibular joint

2001) with an 18 gauge, 40 mm needle aimed ventrorostrally. Theneedle was placed making an angle of 45° with the prominencesof the mandibular fossa and mandibular condyle, and directedcaudodorsolateral-rostroventromedially; 6–10 ml green-colouredlatex were then introduced. The ventral synovial pouch wasinjected through its rostrolateral compartment (Weller et al. 2002)with a 20 gauge, 25 mm needle directed rostrovetrolateral-caudodorsomedially, introducing 1 ml red-coloured latex.

For vascular injection, 70 ml red epoxy (Araldit CY 223,Aradur HY 2967)1 was introduced through the common carotidartery. The heads were then frozen at -30°C for 48 h to obtainblocks containing the TMJ. Two heads were subjected toenzymatic digestion by means of a Pancreatin technique(Tompsett 1970).

Gross dissection

Six TMJ were used. The lateral prominences of the zygomaticprocess of the temporal bone and the condylar process of themandible were landmarks to achieve the lateral dissection,which was started caudolaterally and proceeded fromsuperficial to deeper anatomical structures. For medialdissections, the heads were cut near the midline by cryosectionand the mandible and lateral pterygoid muscle were removed.The articular discs and the latex cast of the synovial poucheswere removed from 4 TMJ by dissection.

Sections

For sectioning, a marker system and a high-speed band saw wereused. The blocks containing TMJ were then frozen at -70°C for 1 week. Cryosections were obtained on transverse and obliqueplanes: 1) lateroventrocaudal-mediorostrodorsal, making anangle of 45° with the sagittal axis and 45° with the transverseaxis of the head; 2) rostrodorsomedial-caudoventrolateral,angulating the saw 15° with the horizontal plane and 20° withthe transverse plane of the head; and 3) caudodorsomedial-rostroventrolateral, making 10° with the horizontal plane and45° with the transverse plane of the head. Sections wereplastinated using the P-40 plastination method (von Hagens etal. 1987; Latorre et al. 2003).

Results

The superficial lateral dissection revealed the relationshipbetween the TMJ and the following structures: the parotid gland;parotidoauricular muscle; maxillary artery and vein and itsbranches; transverse facial vessels; transverse facial branch of theauriculotemporal nerve; facial nerve; and temporalis and massetermuscles. In a deeper dissection (Fig 1), the articular capsule wasfound to be complete, attaching from temporal bone to mandibularcondyle. The caudal aspect of the articular capsule was in closeproximity to the facial nerve and the superficial temporal arteryand vein. Lateral and caudal ligaments of the TMJ were identifiedand isolated. As the masseteric nerve crossed over the mandibularincisure, it was surrounded by fibres of the articular capsule anddisc. Medially, the TMJ was closely adjacent to the temporohyoid

Fig 1: Dorsocaudal view of the lateral dissection of lefttemporomandibular joint region. Maxillary vein and articular capsule wereremoved. 1 = Zygomatic process of the temporal bone; 2 = articulartubercle; 3 = mandibular fossa; 4 = retroarticular process; 5 = mandibularcondyle; 6 = mandibular neck; 7 = caudal aspect of mandibular branch; 8 = articular disc; 9 = caudal fibrous expansion of the disc; 10 = lateralligament; 11 = caudal ligament; 12 = facial nerve; 13 = parotid gland; 14 = guttural pouch; 15 = external auditory meatus; 16 = massetermuscle; 17 = extraperiorbital fat tissue.

Fig 2: Caudodorsomedial-rostroventrolateral section of the lefttemporomandibular joint (viewed from the rostral aspect). 1 = Mandibularcondyle; 2 = mandibular neck; 3 = mandibular branch; 4 = retroarticularprocess; 5 = zygomatic process of the temporal bone; 6 = articular disc;7 = caudal fibrous expansion of the disc; 8 = articular cartilage; 9 = intra-articular fat tissue; 10 = caudal compartment of the dorsalsynovial pouch; 11 = articular capsule; 12 = parotid gland; 13 = externalauditory meatus; 14 = retroarticular space; 15 = masseter muscle; 16 = medial pterygoid muscle.

Fig 3: Ventromedial view of the left temporomandibular joint;boney surfaces and articular disc position. 1 = Occipital condyle; 2 = paracondilar process; 3 = basilar portion of the occipital bone; 4 = foramen lacerum; 5 = temporal bone (tympanic and petrousportion); 6 = articular disc; 7 = dorsal synovial pouch; 8 = caudalcompartment of the dorsal synovial pouch; 9 = mandibular condyle; 10 = mandibular branch.

M. J. Rodriguez et al. 145

joint, dorsal part of the stylohyoid bone, guttural pouch, pterygoidvenous plexus, mandibular nerve and external acoustic meatus(Fig 2). The retroarticular space (tympanic incisure) was filledwith the caudal ligament which was surrounded by the facial nerveand attached from the capsule to the external acoustic meatus andpetrotympanic fissure.

Dorsal and ventral injected synovial pouches were examinedand no communication was found between them. The dorsalsynovial pouch was wider and had a large caudal and small rostralcompartments. The caudal compartment spread ventromedially tothe tympanic part of the temporal bone and the petrotympanicfissure (Fig 3). The ventral synovial pouch was smaller, but alsocontained rostral and caudal compartments, with the former a littlebigger than the latter.

The insertion of the articular disc to the capsule and the bonysurfaces of the temporal bone and mandibular condyle wasidentified. The removed articular disc had an oval shape with astronger concavity of the ventral side because of a caudal fibrousexpansion; this also caused the disc to be ‘L’-shaped. All discs

contained some fat tissue on the lateral half of the caudal edge,where the caudal compartment of the dorsal synovial pouch wassituated. The synovial membrane which covered the fat tissueadopted small irregular elevated shapes. Injected vesselssurrounded homogeneously the disc perimeter (Fig 4).

Enzymatic corrosion casts of the TMJ area revealed the bloodsupply arising from the transverse facial, superficial temporal,caudal deep temporal and tympanic arteries. From the caudal deeptemporal artery arose a branch which passed through the

Fig 5: Lateral view of the left temporomandibular joint (TMJ) arterialsupply in a foal. Tympanic part of the temporal bone was removed. 1 = Retrotympanic process; 2 = tympanic incisure; 3 = retroarticularprocess; 4 = mandibular fossa; 5 = articular tubercle; 6 = mandibularcondyle; 7 = mandibular incisure; 8 = coronoid process; 9 = massetericfossa; 10 = maxilar artery; 11 = caudal auricular artery; 12 = superficialtemporal artery; 13 = rostral auricular artery; 14 = transverse facialartery; 15 = TMJ branches of the transverse facial artery; 16 = TMJbranches of the caudal deep temporal artery.

Fig 4: Dorsal view of the right articular disc after vascular injection.Caudal is to the top and lateral to the left. 1 = Dorsal face; 2 = rostralborder; 3 = caudal border; 4 = lateral border; 5 = medial border; 6 = caudal fibrous expansion; 7 = intra-articular fat tissue.

Fig 6: Lateroventrocaudal-mediorostrodorsal section of the lefttemporomandibular joint (viewed from the rostral aspect). 1 = Mandibularcondyle; 2 = mandibular neck; 3 = caudal aspect of the mandibularbranch; 4 = mandibular branch; 5 = zygomatic process of the temporalbone; 6 = articular capsule; 7 = mandibular incisure; 8 = dorsal synovialpouch; 9 = caudal compartment of the dorsal synovial pouch; 10 = ventralsynovial pouch; 11 = extraperiorbital fat tissue; 12 = temporalis muscle;13 = parotidoauricular muscle; 14 = parotid gland, 15 = transverse facialvessels; 16 = facial nerve; 17 = masticatory nerve.

Fig 7: Rostrodorsomedial-caudoventrolateral section of the lefttemporomandibular joint (viewed from the rostral aspect). 1 = Mandibularcondyle; 2 = zygomatic process of the temporal bone (articular tubercle);3 = coronoid process; 4 = articular disc; 5 = rostral compartment of thedorsal synovial pouch; 6 = articular capsule; 7 = parotid gland; 8 = guttural pouch; 9 = stylohyoid bone; 10 = temporal muscle; 11 = lateral pterygoid muscle; 12 = extraperiorbital fat tissue; 13 = eye.

146 Anatomy of the equine temporomandibular joint

mandibular incisure to supply the articulation at this aspect;rostrally, this branch joined together with branches of thetransverse facial artery (Fig 5).

The attachments of the temporalis muscle onto the medial androstral sides of the capsule and disc and the masseter muscle ontothe lateral and rostral sides of the capsule and laterally to the discwere observed (Fig 6). Some fibres of the lateral pterygoid musclewere fixed onto the capsule and the disc rostromedially, while themedial pterygoid muscle had only a weak attachment onto thecapsule (Fig 7).

Discussion

Anatomical descriptions in the literature of the equine TMJ arebrief and there is a comparatively low level of knowledge of thisarticulation in the horse. Conversely, there is great interest in thehuman TMJ and the joint has been studied thoroughly, withdocumentation of the most frequent pathologies suffered(Emshoff et al. 1997, 2002; Motoyoshi et al. 1998; Uysal et al.2002). The degree of development in human and equine TMJ issimilar (May et al. 2001), and the present study reports therelationship of the masticatory muscles and middle ear. Thesefindings were possible because of the oblique sections thatallowed visualisation of other TMJ aspects not possible onclassic sections.

The articular disc was biconcave, the concavity being deeperin the lateromedial than the rostrocaudal axis. However, May etal. (2001) reported a strong concavity in rostrocaudal axis. Thelateral, medial and rostrolateral edges of the disc were thick andthe central part was thin; because of the caudal fibrousexpansion, the disc adopted an ‘L’ shape. This morphology wasalso described by Barone (1980) and Weller et al. (2002) inhorses and by Schmolke (1994) in man. The disc insertedlaterally onto the capsule and the mandibular condyle andmedially, onto the capsule and temporal bone. This finding wasnot reported by Rosenstein et al. (2001) or Weller et al. (2002)and was not in agreement with May et al. (2001), who describeda circumferential attachment of the disc to the mandibularcondyle and joint capsule. Furthermore, the disc also attachedcaudomedially in the retroarticular space and the tympanic partof the temporal bone. The masseteric nerve was entrapped byfibres of the articular capsule and articular disc as it passedthrough the mandibular incisure, similar to man (Schmolke1994; Lippert 2000), but until now had not been described inhorses. The peripheral vascular supply of the equine articulardisc arose from the transverse facial, caudal deep temporal andtympanic arteries, differing from the rostral and caudal nutritiondocumented in man (Schmolke 1994) and from the lone caudalsupply reported in the horse through the retrodiscal tissue byBarone (1980) and May et al. (2001).

In the present study, the isolated articular capsule wascomplete, differing from the descriptions reported in man byBermejo-Fenoll et al. (1992), who stated that a complete capsuledoes not exist but articular reinforcements, short and long fibresare present. The TMJ capsule is in close association with thefacial nerve, mandibular nerve, superficial temporal artery andvein, pterygoid venous plexus and transverse facial vessels inhorses (Barone 1980; May et al. 2001; Rosenstein et al. 2001;Weller et al. 2002) and man (Schmolke 1994; Lippert 2000).These features were identified in all specimens in our study.Indeed, all these authors concurred that the lateral and caudal

ligaments were solely capsular reinforcements but our resultsdiffered in that these ligaments were easily identified andisolated. Further, a third capsular reinforcement located medially,the pterygomandibular ligament, was described by Barone (1980)and Sandoval and Agüera (1999) but not observed in ourdissections or sections.

There exists an intimate relationship of the caudal ligament,capsule and disc with the external acoustic meatus, the tympanicpart of the temporal bone and the petrotympanic fissure that linksthe TMJ and middle ear in horses. This does not appear to havebeen documented previously in the horse, but has in man(Schmolke 1994; Cheynet et al. 2003). Cheynet et al. (2003) alsodescribed 2 ligaments, the discomallear and malleomandibular,from the TMJ to the malleous of the middle ear.

Rostral and caudal compartments of both noncommunicatingdorsal and ventral synovial pouches were identified in our study.The most recent report by Weller et al. (2002) did not mention arostral compartment of the dorsal synovial pouch. May et al.(2001) documented rostral and caudal compartments of the dorsalsynovial pouch, but no compartment for the ventral articularspace. Getty (2001) and Barone (1980) reported only the caudalcompartment of the dorsal synovial pouch. In man, the reports ofBermejo-Fenoll et al. (1992) and Lippert (2000) did not refer toany compartmentalisation of the articular cavities. In addition,Weller et al. (2002) stated that there were synovial villi in botharticular spaces, but only elevations of the membrane by the fattissue were observed in our study.

The lateral pterygoid muscle inserted into the medial androstromedial aspects of the capsule and into the rostromedialaspect of the disc, as previously reported (Nickel et al. 1986; Getty2001). Many fibres of the temporalis muscle attached to themedial and rostromedial aspect of the capsule, and the massetermuscle inserted directly onto the rostrolateral and lateral aspectsof the capsule. These findings contrasted with those reportedpreviously by Getty (2001) and Weller et al. (2002).

The attachment of the medial pterygoid muscle onto theventromedial side of the capsule, the rostromedial nexus betweenthe temporalis fibres and disc, and the attachment of the massetermuscle to the rostral and lateral fibres of the articular disc is newinformation regarding the horse, although these features have beenreported previously in man (Schmolke 1994).

Temporomandibular joint disorders reported in the horse arelimited. One reason is the nonspecific symptomatology that can bemistaken for behavioural or neurological diseases (e.g.headshaking, quidding) (May et al. 2001). Many of these clinicalsigns could be explained by the direct relationship between theTMJ and the ear. Therefore, a detailed anatomical knowledge ofthe TMJ in horses is a prerequisite for assisting diagnostictechniques such as ultrasonography, magnetic resonance imaging(MRI) or computed tomography (CT). Consequently, theinformation provided by our study should be a useful tool toimprove diagnosis and treatment of equine TMJ disorders.

Acknowledgement

We would like to acknowledge the critical revision of Dr RobertW. Henry from the University of Tennessee.

Manufacturer’s address

1Huntsman Advanced Materials (Europe) BVBA, Everberg, Belgium.

M. J. Rodriguez et al. 147

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