Effects of injury or removal of the articular disc on maxillomandibular growth in young rats

nkx3.2 mutant zebrafish accommodate jaw joint loss through a phenocopy of the head shapes of Paleozoic jawless fish

The vertebrate jaw is a versatile feeding apparatus. To function, it requires a joint between the upper and lower jaws, so jaw joint defects are often highly disruptive and difficult to study. To describe the consequences of jaw joint dysfunction, we engineered two independent null alleles of a single jaw joint marker gene, nkx3.2, in zebrafish. These mutations caused zebrafish to become functionally jawless via fusion of the upper and lower jaw cartilages (ankylosis). Despite lacking jaw joints, nkx3.2 mutants survived to adulthood and accommodated this defect by: (a) having a remodeled skull with a fixed open gape, reduced snout and enlarged branchial region; and (b) performing ram feeding in the absence of jaw-generated suction. The late onset and broad extent of phenotypic changes in the mutants suggest that modifications to the skull are induced by functional agnathia, secondarily to nkx3.2 loss of function. Interestingly, nkx3.2 mutants superficially resemble ancient jawless vertebrates (anaspids and furcacaudiid thelodonts) in overall head shape. Because no homology exists in individual skull elements between these taxa, the adult nkx3.2 phenotype is not a reversal but rather a convergence due to similar functional requirements of feeding without moveable jaws. This remarkable analogy strongly suggests that jaw movements themselves dramatically influence the development of jawed vertebrate skulls. Thus, these mutants provide a unique model with which to: (a) investigate adaptive responses to perturbation in skeletal development; (b) re-evaluate evolutionarily inspired interpretations of phenocopies generated by gene knockdowns and knockouts; and (c) gain insight into feeding mechanics of the extinct agnathans.

The effectiveness of photobiomodulation in the management of temporomandibular pain sensitivity in rats: behavioral and neurochemical effects

This study analyzed the effects of photobiomodulation (PBM) with low-level laser therapy on nociceptive behavior and neuronal activity in the trigeminal nucleus after experimental unilateral temporomandibular joint (TMJ) disc injury. The animals were divided into 4 groups (n = 10 each): group 1, surgical injury of the articular disc and PBM; group 2, sham-operated subjected to PBM; group 3, surgical injury of the articular disc; and group 4, control (Naïve). Ten sessions of PBM were performed using GaAs laser with a wavelength of 904 nm, power of 75 W pico, average power of 0.043 W, area of the beam of 0.13 cm², duration of the pulses of 60 nseg (in the frequency of 9500 Hz), energy density of 5.95 J/cm², energy per point of 0.7 J, and power density of 333.8 mW/cm², and the irradiation was done for 18 s per point. Neuropathic symptoms were evaluated using the von Frey test. Trigeminal ganglion samples underwent immunoblotting to examine the expression of substance P, vanilloid transient potential receptor of subtype-1 (TRPV-1), and peptide related to the calcitonin gene (CGRP). There was a total decrease in pain sensitivity after the second session of PBM in operated animals, and this decrease remains until the last session. There was a significant decrease in the expression of SP, TRPV-1, and CGRP after PBM. Photobiomodulation therapy was effective in reducing nociceptive behavior and trigeminal nucleus neuronal activity after TMJ disc injury.

Effects of articular disc or condylar cartilage resection on mandibular growth in young rats

OBJECTIVE

This study was aimed to compare the effects of articular disc and condylar cartilage resection on mandibular growth in Sprague Dawley rats.

DESIGN

Eighty-four male Sprague Dawley rats (age = 4 weeks) were grouped according to the following procedures: group A (n = 21), exclusive surgical exposure of articular disc and condylar cartilage; group B (n = 21), exclusive surgical resection of articular disc; group C (n = 21), exclusive surgical resection of condylar cartilage; group D (n = 21), surgical resection of both articular disc and condylar cartilage. All surgery was performed in unilateral. One rat was killed in each group immediately after the surgery. Hematoxylin and eosin (H&E) staining was used to confirm the completely removal of the disc or cartilage. Five rats in the four groups were sacrificed in 1, 3, 6, and 9 weeks post-operation. The heights and lengths of the mandibles were measured and analyzed statistically.

RESULTS

The mandibular height of group D (5.01 ± 0.25 mm) was statistically lower than group A (5.59 ± 0.17 mm) at 1 week post-operation. The height of group C (5.62 ± 0.26 mm) was significantly lower than group A (6.27 ± 0.31 mm) 3 weeks after surgery. The height of group B (6.38 ± 0.36 mm) was significantly lower than group A (6.95 ± 0.10 mm) 6 weeks after surgery. At 9 weeks post-operation, the mandibular heights in groups B, C, and D were lower than group A, group D was lower than group C, and group C was lower than group B. The lengths of the mandibles were not significantly decreased until 9 weeks post-operation in group D.

CONCLUSIONS

The increase in mandibular height was interfered after either articular disc or condylar cartilage was resected, and mandibular height deficiency likely occurred earlier and more severely when cartilage was resected. However, the increase in mandibular length was barely interfered when either articular disc or condylar cartilage was resected.

Impact of Temporomandibular Joint Discectomy on Condyle Morphology: An Animal Study

PURPOSE

Temporomandibular disorders lead to parafunctional activity that may alter bone remodeling of mandibular components. This animal study aimed to assess the impact of temporomandibular joint discectomy on condylar bone microarchitecture.

MATERIALS AND METHODS

A total of 30 one-month-old Wistar rats were assessed and divided into 3 equal groups (2 test groups and 1 control group) of 10. The first test group underwent disc removal, the second test group underwent disc and condylar cartilage removal, and the 10 remaining rats were analyzed as sham-operated controls, following a split-mouth design. The rats were killed humanely 2 months after surgery, and the respective mandibles were scanned with micro-computed tomography for quantitative morphometric analysis.

RESULTS

There were significant differences among the 3 groups analyzed (disc removal, disc and condylar cartilage removal, and sham-operated control) for bone volume fraction (ratio of bone volume to total volume, P = .044), structure model index (P < .001), fractal dimension (P = .024), and porosity (P = .023). In addition, operated and contralateral nonoperated sides significantly differed for all variables in at least 1 of the test groups (P < .05) but not in the control group (P > .05).

CONCLUSIONS

Within the limitations of this study, our results suggest that discectomy may lead to alterations of the mandibular condylar morphology.

An Animal Model for Inducing Deviation of the Mandible

PURPOSE

Altering the occlusal surface is still a common choice for inducing a deviated mandible in an animal model. Botulinum neurotoxin type A (BoTx/A) can block the action potential transmission in neuromuscular junctions by inhibiting acetylcholine release without damaging the nerves and muscle structures. Our present study was aimed at developing an easy-to-reproduce animal model of asymmetric mandibles in which injection of BoTx/A was applied.

MATERIALS AND METHODS

A total of 96 healthy 4-week-old male Sprague-Dawley rats were divided into 2 groups: an experimental group (n = 48) with BoTx/A injection and a control group (n = 48) with sterile saline injection at 4 sites of the right masseter muscle. Twelve rats from each group were humanely euthanized at weeks 1, 2, 3, and 4 for morphometric analysis using the micro-computed tomography (CT) findings.

RESULTS

The micro-CT scans revealed facial asymmetry in the experimental group, with no facial asymmetry in the control group after injection. Significant differences were found between the experimental and control groups regarding the indexes containing the mandibular length (length from condyle to menton, length from coronoid to menton, and length of mandibular corpus from gonion to menton) and ramus height (posterior border and middle region near coronoid, and height of anterior mandible at vertical distance from menton).

CONCLUSION

Our data have indicated that this deviated mandible animal model induced by injection of BoTx/A is highly reproducible and might be proved suitable for future studies of the asymmetric mandible.