This study analyzed the effects of injury and removal of the articular disc on maxillomandibular growth in young rats.
Materials and Methods
Thirty 1-month-old male Wistar rats were divided into 3 groups: injury, removal, and sham operated. Unilateral injury of the articular disc, removal of the articular disc, or only surgical access was performed. The animals were sacrificed at 3 months of age. Specimens were submitted to radiographic incidences and cephalometric mensurations were performed using a computer system. Data were subjected to statistical analyses among groups and between sides in each group.
There were degenerative changes of the condylar process in the injury and removal groups. Significant differences between sides were observed for length of the premaxilla, height of the mandibular body, and length of the mandible. Concomitant group comparisons showed significant differences in the height of the mandibular body ( P = .003) and the length of the mandible ( P = .001). There were important decreases to the height of the mandibular body and mandibular length in the injury group, whereas there was an important decrease only for the length of the mandible in the removal group. Specific measurements of mandibular ramus structures exhibited differences between the injury and sham-operated groups and between the removal and sham-operated groups.
Experimental injury and removal of the articular disc during the growth period in rats had deleterious effects on ramus structures and induced asymmetry of the mandible, with worse outcomes resulting from injury to the articular disc.
Mandibular deformities, especially mandibular asymmetries, have been associated with previous trauma to the temporomandibular joint (TMJ). Clinical studies have implicated prepubertal trauma as a cause of asymmetric growth of the mandible. This phenomenon can be explained by the presence of early condylar fractures that lead to disturbed growth of the fractured side, resulting in mandibular deviation. The mandibular condyle is considered the cornerstone of mandibular form and function and damage to this structure in growing children can adversely affect the growth of the jaw and the occlusion. Such changes would be a direct consequence of inflammatory or mechanical damage to the condylar cartilage.
In contrast, an association between internal derangement of the TMJ and the development of mandibular asymmetry has been suggested. In addition, children with temporomandibular disorders have exhibited a shorter mandibular corpus and vertical height of the ramus. Experimental studies have shown that surgical disc displacement can alter the growth of the mandible. This outcome would result in a retrognathic growth pattern.
The possibility of soft tissue changes in patients who have sustained injuries to the TMJ has been reported. An experimental model of indirect trauma to the TMJ has shown isolated injuries to structures such as the articular disc, condyle fibrocartilage, or temporal fossa. Soft tissue injuries of the TMJ can potentially lead to internal derangement, osteoarthrosis, and possibly fibrous ankylosis owing to disc displacement and condylar cartilage damage. Experimental disc perforation is initially followed by hypertrophy of the condylar cartilage and later by degeneration of the condylar surface. In addition, in growing rats, disc displacement or removal leads to an unfavorable outcome in the repair of condylar fractures, inducing condylar growth disorders. However, the role of trauma to the articular disc in maxillomandibular bone growth is not well understood.
The purpose of this study was to analyze the effects of unilateral injury and removal of the articular disc on maxillomandibular growth in young rats.
Materials and Methods
Thirty 1-month-old male Wistar rats were used for this study. All animals were fed with rodent feed (Labina, Agribands Purina, Paulínia, SP, Brazil) and water. They were divided into the following 3 groups: injury (n = 10), removal (n = 10), and sham operated (n = 10). The study was approved by the local subcommittee on ethics in animal experimentation (process 13/2009).
General anesthesia was induced with xylazine hydrochloride 10 mg/kg (Rompum, Bayer, Porto Alegre, RS, Brazil) and ketamine hydrochloride 25 mg/kg (Dopalen, Vetbrands, Paulínia, SP, Brazil). A single dose of benzylpenicillin 16,000 IU (Benzetacil, Fontoura-Wieth, Itapevi, SP, Brazil) was given, and the right side was shaved and cleansed with a povidone-iodine solution. A 1-cm preauricular incision was made, and blunt dissection was performed through the masseter muscle just below the zygomatic arch, with exposure of the lateral surface of the mandibular ramus. Injury to the articular disc was caused by using mosquito (Halstead) forceps, resulting in a crush in the anteroposterior direction. In the removal (discectomy) group, the articular disc and associated attachments were removed using a scalpel and forceps. The sham-operated group was subjected to exposure of the TMJ. In all groups, care was taken to prevent damage to the articular surfaces. The procedures were concluded by suturing in layers. Postoperative analgesia was provided with tramadol 0.3 mg/kg by injection. The animals were sacrificed at 3 months of age, and their heads and mandibles were carefully removed. After formalin fixation, radiographs with axial projections of the skull and lateral projections of the hemimandibles were obtained. These images were taken with a dental machine (Spectro II, Dabi-Atlante, Ribeirão Preto, SP, Brazil) at 56 kV and 10 mA, with an exposure time of 0.4 second for the skulls and an exposure time of 0.3 second for the hemimandibles. Periapical films were used (Dental Intraoral E-speed Film, Carestream Health, Rochester, NY).
The radiographs were subjected to a computerized cephalometric evaluation and digitized using an optical reader (Fotovix II, Tamron Co, Saitama, Japan). Measurements were obtained with Imagelab software (Softium Informática, São Paulo, SP, Brazil). Using skull radiographs, the following distances relative to the maxilla were measured bilaterally: the tympanic bulla (TB; the most anterior portion of this round structure of the skull base) to the mesial root of the first molar (the apex of this root) relative to posterior maxillary length; the TB to the infraorbital foramen (IF; the vertex of the image of this foramen) relative to maxillary length; and the IF to the incisal point (IP; the intersection of the lingual face of upper incisors with the midline) relative to the length of the premaxilla ( Fig 1 ). On hemimandible radiographs, the following distances relative to the mandible were measured bilaterally: the condylar process (CP; the highest point of this structure) to the angular process (AP; the apex of this structure peculiar to the rodent mandible) relative to mandibular ramus height; the distal face of the third molar (TM; intersection with the mandibular ramus) to the antegonial notch (AN; located on the mandibular base anterior to the mandibular angle) relative to mandibular body height; and the lower insertion of the incisor (II; the most anterior limit of the lower bone insertion of this tooth) to the CP relative to mandibular length ( Fig 2 ). Also, on the hemimandible radiographs, specific measurements relative to ramus structures were made. Initially, a line (A) tangent to the sigmoid notch and the most anterior point on the posterior aspect of the ramus was drawn. A perpendicular line (B) through the highest point on the condyle and then 2 additional perpendicular lines (C and D) were drawn tangent to the superior aspect of the coronoid process and the inferior border of the mandible. Using the horizontal line (A) and a perpendicular line to the top of the condyle, the height of the condyle (B) was recorded. The vertical height of the coronoid process (C) also was measured. The distance between perpendicular lines C and D was measured to determine the transverse width of the mandible (C-D). A line parallel to line A and bisecting line B measured the transverse width of the condyle (E; Fig 3 ).
To evaluate differences between mean values for the right and left sides in each group, the Wilcoxon signed posts test was used. The Kruskal-Wallis test was used to verify possible differences among the 3 groups when concomitantly compared and the Mann-Whitney test adjusted by Bonferroni correction was used to identify which groups differed from one another. SPSS 21.0 software (IBM Software Group, Chicago, IL) was used to conduct the analyses. The level of significance was set at 5% ( P < .050).