Comparative Medicine Vol 61, No 6

Copyright 2011 December 2011

by the American Association for Laboratory Animal Science Pages 510–513

Case Report

Bilateral Traumatic Temporomandibular

Joint Luxation in a Rat

Lisa J Shientag,1,* Karen L Rosenthal,2 Heidi K Chandler,3 and Suzanne M Wheeler1

Bilateral temporomandibular joint (TMJ) luxation was diagnosed postmortem in a female, 6-mo-old CD rat (Rattus norvegicus)
after probable head entrapment and subsequent disentanglement from a protective jacket. Clinical antemortem signs included
inability to close her mouth, prehend food and drink water, anxiety, and linear skin erosions extending down the right and left
commissures of the mouth. Radiography revealed rostral displacement of the mandible with concomitant malocclusion. The com-
bination of clinical signs, acute nature of the presentation, and torn appearance of the protective jacket were strongly indicative of
a traumatic etiology. To our knowledge, this is the first reported case of TMJ luxation in a rat.

The temporomandibular joint (TMJ) is a defining feature of of bilateral TMJ luxation in a rat (Rattus norvegicus) that resulted
mammals and separates them from other vertebrates.8 It is a con- from a presumed traumatic incident.
dylar, synovial joint that shares many anatomic features among
mammals but has also evolved specialized characteristics in the
various mammalian species in response to biomechanical feeding
Case Report
A female, 6-mo-old CD rat (Crl:CD/SD) acutely presented
forces.4,8,14 The TMJ functions to maintain alignment of the maxilla
unable to close her mouth, as noted by a veterinary technician.
and mandible and allow movement of the jaws.6
While in her cage, the rat repeatedly wiped her open mouth with
The basic structure of the TMJ includes the condylar process of
her front paws while standing on her hindlimbs. Ten minutes ear-
the head of the mandible, which articulates with the mandibular
lier, the veterinary technician had removed a protective canvass
fossa in the squamous part of the temporal bone in most mam-
jacket from this animal and noticed no abnormalities of anatomy,
mals.8,14 Both the condylar process and mandibular fossa are
posture, or behavior. The rat had been wearing the jacket for 2 wk
covered in hyaline cartilage and are separated by a biconcave,
to discourage ongoing self-trauma to the skin on her dorsum and
fibrocartilagenous disc that divides the joint into dorsal and ven-
flanks. During that time, no adverse effects had been noted.
tral components.6,12,14 The joint is surrounded by a fibrous capsule
The rat was individually housed in a polysulfone cage with
with a synovial lining that is reinforced in some veterinary spe-
corncob bedding in a conventional facility in accordance with the
cies with ligamentous attachments to the mandibular and tempo-
animal care policies and procedures of the Department of Animal
ral bones.6,8,13,14
Medicine (University of Massachusetts Medical School, Worces-
Luxation of the TMJ occurs when the components of the joint
ter, Massachusetts). She was provided with ad libitum, bottled,
disarticulate and move in an aberrant direction, usually as a re-
acidified water and standard rodent chow (Lab Diet, 5POO Pro-
sult of some kind of head trauma.5,7,9 Temporomandibular joint
lab, RHI 3000, PMI Nutrition International, Brentwood, MO)
luxation can be bilateral or unilateral. Sometimes mandibular or
consumed from a hopper. The housing room was on a 12:12-h
other cranial fractures will be present.5 The condylar process al-
light:dark cycle. Nestlets (Ancare, Bellmore, NY) and Shepherd
most always luxates in a rostrodorsal direction with accompany-
Rat Shacks (Shepherd Paper Products, Watertown, TN) were pro-
ing dental malocclusion.5-7,14, Luxations also can occur as a result
vided for enrichment.
of TMJ dysplasia.1,5,6,13
The rat was part of an IACUC-approved study that involved
Several case reports in the veterinary literature describe trau-
bilateral resection of the lattisimus dorsi muscles 2.5 mo prior to
matic or pathologic TMJ luxations and ensuing treatments in cats,
this clinical presentation. At the time of surgery, the rat weighed
dogs, horses, and guinea pigs.5-7,9,10 Here, we describe the first case
334 g. Sterile procedures were followed, in accordance with the
guidelines for survival rodent surgery at the facility. She was not
intubated or masked, eliminating the possibility of iatrogenic
Received: 02 May 2011. Revision requested: 10 Jun 2011. Accepted: 10 Aug 2011. trauma from either of those manipulations. During surgery, she
1
Department of Animal Medicine, University of Massachusetts Medical School, Worcester,
was monitored with a veterinary pulse oximeter and kept warm
Massachusetts; 2Department of Clinical Studies, Matthew J Ryan Veterinary Hospital,
School of Veterinary Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; with a recirculating water heating pad placed on top of the sur-
3
Olympus Biotech, Hopkinton, Massachusetts. gery table. The surgery was uneventful, and the rat recovered
*
Corresponding author. Email: lisa.shientag@umassmed.edu

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 TMJ luxation in a CD rat

normally after the procedure. Postoperative pain management

consisted of 2 doses of ketoprofen (5 mg/kg SC daily) and 2.5 d of
buprenorphine (0.08 mg/kg SC) administered every 8 h.
Twelve days after the surgical procedure, the rat was seen
scratching at the skin on her dorsum, initially near the incision site
and then, in coming days, along her dorsum and flanks, leading
to superficial, moist, inflamed, discrete lesions of various sizes.
The treatment plan consisted of twice-a-week nail trimmings and
topical antibiotics when necessary, but the lesions kept recurring,
and the rat was seen licking at the wounds on several occasions.
The cause of the self-trauma was not determined but included
possible nerve damage from the surgery.
To protect the skin from further trauma and allow the current
lesions the opportunity to heal, the rat was dressed, about 1 mo
after presenting with the excoriations, in a protective canvass
jacket designed specifically for rats by a commercial vendor (Lo-
mir Biomedical, Notre-Dame-de-I’lle Perrot, Canada). The jacket
was 4 in. at its longest and 3 in. at its widest points, with 2 holes
for the forearms and a hook-and-loop closure supplemented with
eyehooks for ties, although ties were not used in this instance.
Two weeks after the rat was dressed in the jacket, the skin lesions
were almost healed. For the entire 2 wk, the rat was noted to be
in good health, eating and drinking normally with good body
condition. It was decided that the jacket would be removed.
At the time of jacket removal, the rat presented with an open
mouth and appeared to be in distress, intermittently pawing at Figure 1. Dorsoventral plain-film radiographic view of rat skull. Thin
her mouth and running back and forth between the food hopper arrows indicate the rostral shifting of the right and left coranoid proc-
and water bottle. Her hydration status, mucous membrane color, esses of the mandible toward the upper third of the zygomatic bone
and body condition were normal. Differential diagnoses in mam- region. Thick arrows show that the right and left condylar processes of
the mandible are rostrally shifted toward the caudal area of the right
malian species for an inability to close the mouth include oral for-
and left zygomatic bones. Triangles indicate that the right and left an-
eign body, TMJ luxation or subluxation, mandibular or zygomatic gular processes of the mandible are visible just beyond the ends of the
fractures, TMJ dysplasias with open-mouth locking, mandibular zygomatic bones. These radiographic signs indicate a rostral shifting of
neoplasia, and trigeminal neurapraxia.1,5,6,9 the mandible after bilateral luxation of the TMJ.
The rat was lightly anesthetized with isoflurane gas. Examina-
tion revealed 2 fairly deep, linear, erosive skin lesions running The lateral radiographic view of the head revealed rostral pro-
down the rat’s mandible from the right and left commissures of trusion of the mandibular incisors that incorrectly overlapped the
the mouth. A cursory oral examination did not reveal any laxity upper incisors (Figure 2). In addition, the upper and lower molar
of the mandible or other cranial or jaw bones, and there were no arches did not articulate appropriately, with the mandibular third
other obvious abnormalities. Rostral luxation of the mandible molar nearly apposed to the first maxillary molar (Figure 2). In
was a differential diagnosis, but an attempt to reduce the man- normal, resting rodent occlusion, the upper incisors overlap the
dible under the light plane of anesthesia was unsuccessful. lower incisors, and the molar arcades are aligned, with the third
The principle investigator of the study was contacted and de- maxillary molar meeting the third mandibular molar, and so on.16
cided with the veterinarian to euthanize the rat with CO2 gas Additional details of the TMJ anatomy could not be discerned
because she appeared to be in distress that could not easily be with the imaging technique used. However, the radiographic
relieved and was near the endpoint of the study. Radiographs of anomalies indicate bilateral rostrodorsal dislocation of the TMJ
the carcass were obtained with a mounted, portable radiograph with concomitant malocclusion. These radiographic abnormali-
machine (HF80+, MinXray, Northbrook, IL). The dorsoventral ties combined with the linear skin erosions along the right and
view of the head showed a significant rostral shifting of the man- left sides of the chin suggest a traumatic etiology, with string en-
dible, with the right and left coronoid processes of the mandible trapment from the jacket being the most likely scenario. The rat
visible toward the rostral third of the zygomatic bone (Figure 1). may have tried to remove the jacket or was biting at the jacket at
The condylar processes of the right and left mandibles were vis- some point over the weekend, with her lower jaw becoming en-
ible as radioopaque densities toward the caudal one-third of the snared in the fabric. She then could have struggled to break free,
zygomatic bone, and the right and left angular processes of the luxating her mandible in the process. The removed jacket had
mandible could be observed jutting just past the bottom edge of several frayed areas in the fabric (Figure 3), although when these
the zygomatic bone (Figure 1). Skull radiographs from normal torn areas first appeared is unknown.
rats show the coronoid processes toward the caudal third of the
zygomatic bones, the condylar processes superimposed with the
caudal end of the zygomatic bone, and the angular processes jut- Discussion
ting just below the level of the tympanic cavity.15 To our knowledge, this is the first report of a bilateral TMJ luxa-
tion in a rat (Rattus norvegicus). The injury most likely occurred as

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Comparative Medicine
December 2011

Figure 3. Protective canvass jacket that was removed from the rat before
clinical signs were observed. The black arrow indicates the frayed areas
of fabric where the rat’s head could have been ensnared. The rat may
have struggled to break free, potentially bilaterally luxating the TMJ.

and often a tape muzzle or interarcade wiring is applied for 1 to

Figure 2. Lateral plain-film radiographic view of rat skull. The thin ar- 2 wk after the reduction to allow time for healing.5,6 Although the
row shows that the mandibular incisors are malpositioned under the horse case cited7 involved unilateral luxation, the scenario of head
maxillary incisors. Triangles show that the upper and lower molar ar- entrapment and subsequent escape, along with the traumatic and
cades are misaligned, with the maxillary first molar nearly apposed to acute nature of the incident, shares many details with that of the
the mandibular third molar. These radiographic signs reveal the dental rat in the current report.
malocclusion after bilateral luxation of the temporomandibular joint TMJ luxations are rare in horses due to the inherent stability
due to presumed traumatic injury.
of the joint, which is strengthened with an overlay of muscula-
ture and strong attachments of the joint capsule and mandibular
a result of a traumatic incident involving head entrapment and ligaments.9 These abnormalities are also relatively rare in dogs
subsequent disentanglement from a protective canvass jacket. and cats for similar reasons,5,14 although TMJ dysplasia with as-
The rat had been dressed in this jacket to discourage self-trauma sociated open-jaw locking or subluxation occurs more frequently
after repeated incidents of self-excoriation on her dorsum and in brachycephalic and chondrodyplastic breeds of dogs and cats
flanks after experimental surgery was performed on the back than in other breeds.1,6 One report describes the use of computed
muscles several weeks prior to the appearance of the skin lesions. tomography for diagnosis of and treatment planning for open
Few references in the veterinary literature describe cases of in- mouth locking due to TMJ dysplasia in a 2.5-y-old, female Persian
juries involving protective jackets or other coverings designed cat.1 The cat presented pawing at her open mouth, as did the rat
to promote healing in animals. These include anecdotal reports in our report. The use of a tomographic scanner and 3D recon-
of injuries to animals dressed in Elizabethan collars, 2,3 and one struction of the joint images eliminated many of the problems,
report of 2 dogs accidentally asphyxiated when their Elizabethan such as the superimposition of the complex structures of the joint,
collars became ensnared in plastic bags.17 One experimental study associated with conventional plain-film radiographic imaging of
described depression of feed intake and weight loss in rats wear- the TMJ.1,6,14
ing neck collars.11 Caudal luxation of the condylar process from the mandibular
Case reports and references to cases discuss unilateral or bilat- fossa is rare in cats and dogs because of the bony tubercle called
eral TMJ luxations in cats, dogs, horses, and guinea pigs, usually the retroarticular process, which is a caudoventral extension of
as a result of head trauma but sometimes due to TMJ dyspla- the mandibular fossa.6,14 Rostral luxation of the condylar process
sia.1,5-7,9,10 One publication describes unilateral, right TMJ luxation in those species is limited by the articular tubercle in the rostral
in a 20-y-old American Saddlebred gelding that presented un- mandibular fossa.6 This structure is relatively small in dogs,
able to close his mouth after his head had been caught in a fence.7 slightly more pronounced in cats, and also exists in herbivore and
Radiographs revealed rostral displacement of the right condylar omnivore species.14 However, the mandibular fossa in male, adult
process and lower molar arcade with no associated fractures. The Wistar rats is reported in one study12 to be flat with no bony tu-
TMJ luxation in this horse was repaired by closed reduction under bercles. The authors12 speculated that the absence of articular tu-
anesthesia.7 Closed reduction is recommended as a conservative bercles in rats may make the movement of their mandibles highly
treatment for dogs, cats, and horses, but would be impractical for specialized for extensive protrusive movements. 12 The lack of
most laboratory rodents involved in studies. For cats and dogs, articular tubercles may have contributed to the forward bilateral
one recommended treatment involves placing a dowel transverse- TMJ luxation in the rat in our report. The previously cited report12
ly between the maxillary and mandibular molars to distract the did not remark on the presence of ligamentous structures in the
condyle distally while the rostral mandible and maxillary bones rat TMJ, but absence of these structures might increase the risk of
are squeezed together.5 This method of reduction can be unstable,

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traumatic TMJ luxation in rats. Extensive rostral-caudal gliding The rat discussed in this case report probably was injured after
movements have been noted in the temporomandibular articula- becoming entangled in a protective jacket. This explanation is the
tions of rats;4 in addition, the temporomandibular articulations most plausible, given the evidence of torn fabric on the jacket and
subluxate during normal gnawing motions in rats, resulting in the animal’s clinical signs, which included linear skin erosions
transient mandibular prograthism while feeding.4 All of these extending down the sides of her mouth, normal body condition,
features of the rat TMJ apparatus may predispose rats to rostral and acute presentation. In this instance, the benefit of the jack-
temporomandibular luxations in certain circumstances. et was overcome by the devastating, but unusual, injury that it
Although the current report is the first of a rat with traumatic, caused. More frequent monitoring of the rat may have resulted in
bilateral TMJ luxation, chronic molar malocclusion in guinea pigs the detection of any entrapment, although the time frame of this
due to molar overgrowth may lead to overextension of the mas- incident could not be determined.
ticatory muscles, concomitant jaw laxity, and possible TMJ lux- TMJ luxations are rare in veterinary species, but clinical signs
ation.10 These animals may present with anorexia, weight loss, can be identified, and diagnostic and treatment options are avail-
inability to prehend food or masticate, drooling, facial swelling, able. Similar to the rat reported in our case, diagnostic and treat-
and sometimes exopthalmos.4,10 ment options can be extrapolated across species.
Clinical signs of unilateral or bilateral TMJ luxation include
inability to close the mouth or prehend and chew food or drink
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