The role of the disc in the healing of displaced subcondylar fracture in the growing period: an experimental study in rats

Abstract

The purpose of this study was to evaluate the role of the disc in the healing of condylar fractures in the growing period. 40 growing and adult male Wistar rats were randomized into 4 equal groups: growing rats suffering condylar fracture with disc damage; growing rats suffering condylar fracture without disc damage; adult rats suffering condylar fracture with disc damage; adult rats suffering condylar fracture without disc damage. 3 months after the operation all animals were killed and the outcomes were compared using mandible deviation measure, body weight evaluation and histological observation. In growing rats, when the disc was undamaged maintaining its relation to the condyle head, even though the condylar fractured fragment was displaced, the condyle healed and recovered almost to normality in 3 months. In the same condyle fracture form, once the disc was damaged and lost the normal relation with the condyle head, the outcome was unfavourable. The occurrence of damage to the disc and its attachment to the condyle head are important factors in the healing of condyle fractures in the growing period.

The incidence of condylar fractures among all mandibular fractures is high, and in children the proportion is higher than in adults . Conservative treatment is advocated for condylar fractures in children . Most cases of condylar fracture in children are treated by conservative procedures and have a favourable prognosis .

Most paediatric condylar fractures heal with a favourable outcome, but in some patients sequelae subsequent to condylar fractures can result in mandibular asymmetric deformities, functional disturbances and temporomandibular joint (TMJ) ankylosis . A series of studies showed that the favourable results of condylar fractures in children managed by conservative procedures are associated with condylar growth and bone remodelling . The status of the disc during condylar fracture healing is seldom mentioned. The damaged or displaced disc is considered a key factor in the aetiology of ankylosis . The factors resulting in the sequelae subsequent to condylar fractures in children are complicated, and it is unclear whether disc damage is responsible for these complications. To the authors’ knowledge, few studies have focused on the relationship between the status of the disc and the prognosis of condyle fractures in children. The purpose of the present study was to evaluate the role of the disc in the healing of condylar fractures in the growing period.

Materials and methods

Growing, male Wistar rats with an average body weight of 70 g (1 month old, n = 20) and adult, male Wistar rats with an average body weight of 270 g (3 months old, n = 20) were used in this experiment. Rats were used because the growing condyle is similar in structure to that of humans and most other mammals . The growing and adult rats were segregated at random into 4 equal groups of 10 animals: growing rats suffering condylar fracture with disc damage (group A); growing rats suffering condylar fracture without disc damage (group B); adult rats suffering condylar fracture with disc damage (group C); adult rats suffering condylar fracture without disc damage (group D). The adult rats were the control groups.

Anaesthesia was achieved by intramuscular injection of ketamine hydrochloride. Each right preauricular area was shaved and prepared with antiseptic solution. An approximate 10 mm preauricular skin incision was made directly below the zygomatic arch over the right side. Blunt dissection was performed through the masseter muscle, and the condylar process was exposed. In groups B and D, a condylar fracture at a point about 1.5 mm beneath the top of the condyle was made using mosquito forceps, ensuring that the soft tissue over the condylar head was intact. Completeness of the fracture was confirmed by mobility of the condyle fragment, and the condyle fragment was displaced in the medial direction before the wound was closed ( Fig. 1 ). In groups A and C, the disc and associated attachments over the condylar head were removed using eye scissors and then the condylar fracture was made as in groups B and D.

3 months after the operation, the rats were killed. Their body weights were recorded for statistical evaluation using Student’s t -test to evaluate the significance of the difference between the mean values of both groups in growing and adult rats. A P value of <0.05 was considered to be statistically significant.

When the rats were killed, the whole skulls were obtained and fixed in 4% paraformaldehyde for 48 h. The skulls were radiographed to evaluate the deviation degree of the mandible, referring to the methods described by Luz and de Araujo . Radiograms were taken from directly above (focus film distance 40 cm, 25 kV voltage, and 40 s exposure). Angular measurements of mandible deviation were made on the radiograms, with reference to a line between both tympanic bullae (angle α , Fig. 2 ). To evaluate the significance of the difference between the mean vales of four groups, Student’s t -test was used and the level of significance set at P < 0.05.

The entire TMJ region of the right side was removed, decalcified, dehydrated and embedded in paraffin for histological observation by standard methods including haematoxylin eosin (HE) stain. All specimens were cut in the coronal plane.

Results

No animal died accidentally in the postoperative period. Healing after the operation progressed uneventfully. The mean values of body weights of the rats from the four groups were 220.79 ± 18.68 (group A), 295.74 ± 22.33 (group B), 272.64 ± 8.25 (group C), and 289.35 ± 10.81 (group D). The statistical difference was significant between the body weight mean values of the two groups of growing rats. There was no significant difference between the two groups of adult rats.

The deviation of the mandible to the right side was observed in all groups. The mean values for angle α from the four groups gave the sequence of the severity of the deviation ( Fig. 3 ) as: group A (85.74 ± 0.38) > group C (88.27 ± 0.31) > group B (88.66 ± 0.44) > group D (88.68 ± 0.25). Statistical evaluation suggested that there was significant difference between group A and group B, group A and group C, group C and group D. There was no significant difference between groups B and D.

Histological observation revealed the prognosis in the four groups. In group A ( Fig. 4 ), abnormal union of the condyle fracture resulted in a crooked condyle. The normal structure of the disc had disappeared, but the joint space still existed. The disorganized arrangement in cartilage layers of the anamorphic condyle was obvious, showing an increase in the total thickness of the condylar cartilage with chaotic proliferation. The prognosis in group B was favourable ( Fig. 5 ); the fractured condyle healed with an almost normal structure, and the fracture area was indistinct. The normal structure of the disc and the organized arrangement of the cartilage layers were visible. The appearance of the TMJ in groups C and D is shown in Figs. 6 and 7 , respectively. In group C, the condyle fracture showed malunion with irregular callus in the fracture area. The pathological changes included an anamorphic condyle and disappearance of the disc structure. In group D, the prognosis was almost favourable. The fractured condyle was united, but the callus in the fracture area was undergoing remodelling. The normal structure of disc and the organized arrangement of the cartilage were visible.