The purpose of this study was to verify the role of the lateral pterygoid muscle in the reconstruction of the condyle shape during the sagittal fracture healing process by histological methods. Twenty-four adult sheep underwent an osteotomy to create a sagittal fracture of the left condyle; the sheep were then divided randomly into two groups. The lateral pterygoid muscles of the sheep in the experimental group were maintained on the internal poles of the condyles, and their functions remained stable. The lateral pterygoid muscles of the sheep in the control group were cut, and their functions were blocked. The shape, erosion, and calcification of the condyles were observed and measured after 4, 12, and 24 weeks of healing ( n = 4 from each group). The condyles were then submitted to haematoxylin and eosin, Ponceau S, and Sirius red studies. The results of the histology studies showed increased bone formation in the experimental group in which the functions of the lateral pterygoid muscle remained the same. The results of this study suggest that the lateral pterygoid muscle affects the reconstruction of the condylar shape during the healing process of a sagittal fracture of the mandibular condyle, and may even be involved in the formation of ankylosis.
Ankylosis of the temporomandibular joint (TMJ) is a serious and disabling disease that severely restricts the movements of the mandible. It can cause problems with mastication, swallowing, digestion, speech, and breathing, and is even known to contribute to psychological disorders. When ankylosis of the TMJ occurs during childhood, it can retard mandibular development and lead to more serious maxillofacial deformities and dysfunction, such as facial asymmetry, micrognathia, and malocclusion. Ankylosis of the TMJ is difficult to treat and its cause is quite complex. It can result from physical trauma, or from local or systemic infections. Reankylosis is the most common complication after treatment, and the recurrence rate is 4–31%. An exploration of the pathogenesis of this disease might provide new avenues for the prevention and treatment of TMJ ankylosis.
In previous reports, trauma has been considered the main cause of ankylosis. Both animal experiments and clinical observations have indicated that sagittal fracture of the mandibular condyle is more likely to lead to ankylosis than other types of condylar fracture. However, the specific mechanism by which ankylosis develops during the wound healing process remains the subject of controversy. Some researchers believe that the pathogenesis of traumatic TMJ ankylosis is the organization and ossification of an intracapsular haematoma after injury. Animal experiments, though, have shown that intracapsular haematoma alone does not cause ankylosis. Recently, some researchers have proposed that ankylosis of the TMJ is a progression, which like hypertrophic non-union, develops in long bone. Because of the way the mouth moves, these injuries cannot completely recover and eventually form bone adhesions. We hypothesized that distraction osteogenesis of the lateral pterygoid muscle may be an important factor in the genesis of traumatic TMJ ankylosis. Recent animal studies by our group further confirmed that the lateral pterygoid muscle plays an important role in the reconstruction of the condyle shape during the sagittal fracture healing process. Based on the research performed, we postulate that pathological osteogenesis may be caused by the traction of the lateral pterygoid muscle, which is crucial in the formation of traumatic ankylosis in the TMJ.
The primary goal of this study was to substantiate the role of the lateral pterygoid muscle in the sagittal fracture healing process of the mandibular condyle, through histological methods.
Materials and methods
The study protocol was approved by the ethics committee of the military medical university. All sheep were cared for in accordance with the guidelines for animal research set by the animal research centre laboratory of the military medical university. Twenty-four 1-year-old, healthy sheep were divided randomly into two groups of 12. All operations were done under satisfactory anaesthesia. After creating the fracture, the function of the lateral pterygoid muscle was cut off in the control group, while the muscle was left unaltered in the experimental group.
The sheep were anaesthetized with xylazine hydrochloride (0.1 ml/kg) and their temporal regions were shaved and sterilized. We exposed the zygomatic arch and panniculus carnosus muscles at the surface of the capsule of the TMJ using a curved pre-auricular skin incision. A horizontal incision was then made through the capsule at the condylar neck in order to open the inferior joint space. The condylar head was then isolated and the superior joint space exposed. After pushing the disc inward, an oblique vertical osteotomy was made from the lateral pole of the condyle to the medial side of the condylar neck, using an ultrasound osteotome. The lateral pterygoid muscles of the sheep in the control group were separated and completely cut off, and the muscles of the sheep in the experimental group were maintained. The wound was closed in layers without suture of the capsule. After the operation, penicillin (20 mg/kg, twice a day) was given to each sheep to prevent infection, and this was continued for 3 days. Four sheep in each group were sacrificed at intervals of 4 weeks, 12 weeks, and 24 weeks after surgery. The TMJ was isolated, observed, and measured. Specimens were then examined histologically under a microscope.
After the TMJ of the study animals had been isolated, we observed the shape, erosion, and calcification of the joint. The sizes of the condyles in both groups were measured using a Vernier caliper.
Specimens were fixed in a 4% formaldehyde solution for 2 weeks, and then decalcified in ethylenediaminetetraacetic acid (EDTA) for 3 months. Semi-serial sections measuring 5 μm in thickness were cut in the sagittal plane. The sections were stained with haematoxylin and eosin (H&E), Ponceau S, and Sirius red, as described previously. The sections were then subjected to histological analysis.
SPSS version 16.0 software (SPSS Inc., Chicago, IL, USA) was used for all statistical analyses. The significance of the variations between the two groups was assessed using the Student’s t -test; P < 0.05 was accepted as significant.
Healing was uneventful following sagittal fracture in all 24 sheep. None of the sheep exhibited a clinical infection. Their skin exhibited only minor signs of inflammation during the first few weeks of healing. The animals were observed carefully and histological analyses were performed at 4 weeks, 12 weeks, and 24 weeks after the operation.
In the biopsies obtained after 4 weeks, the surface roughness of the condyle was increased in both groups. The change was more apparent, with observable protuberances, in the experimental group ( Fig. 1 A and D ). The size of the condyle was measured with a Vernier caliper. As shown in Table 1 , the mediolateral size of the condyle in the experimental group was significantly larger than that of the control group ( P < 0.05), while the anteroposterior size of the condyle showed no significant difference ( P > 0.05).
|Muscle cut (Control)||14.5 (2.1)||15.4 (3.1)||15.6 (3.1)|
|Muscle not cut||15.1 (2.6)||16.8 (3.9) a||17.5 (4.3) a|
|Muscle cut (Control)||22.6 (3.0)||24.6 (4.0)||24.8 (4.2)|
|Muscle not cut||23.3 (3.9) a||28.3 (5.4) a||29.6 (5.6) a|
H&E examination showed the osteoblasts and chondrocytes to be actively growing in the fracture zone of both groups, but this was more distinct in the experimental group. Blood vessels were found in the fracture callus. Fresh bone formation was more vigorous in the experimental group than in the control group ( Fig. 2 A and D ). Ponceau S examination showed a large amount of new bone in the fracture zone and the quantity of this was greater in the experimental group than in the control group ( Fig. 3 A and D ). Sirius red examination showed the presence of collagen type I and III in both groups, while some thin collagen type II was found in the control group. Collagen type III appeared in larger amounts in the experimental group than in the control group ( Fig. 4 A and D ).
After 12 weeks of healing, the volume of the condyle had increased in both groups. However, it was better defined in the experimental group, with larger and more obvious protuberances ( Fig. 1 B and E). Both the anteroposterior size and the mediolateral size of the condyle in the experimental group were significantly larger than those of the control group ( P < 0.05; Table 1 ).
On H&E examination, osteoblasts around the mature trabecular bone, plus further deposition of new bone was seen in the experimental group, while bone structure close to maturity with osteoblasts was hardly seen at all in the control group ( Fig. 2 B and E). Ponceau S examination showed a mass of mature bone matrix to have appeared in the fracture zone. New bone was still forming slowly in the experimental group when compared to the control group ( Fig. 3 B and E). Sirius red examination showed a large quantity of collagen type I and less collagen type III in the experimental group. Collagen type II had disappeared completely in the control group, and the size of collagen type I was smaller than that of the experimental group ( Fig. 4 B and E).
At 24 weeks after the operation, the volume of the condyle had increased slightly in both groups, and protuberances had morphed into apophysis in the experimental group ( Fig. 1 C and F). Both the anteroposterior size and the mediolateral size of the condyle in the experimental group were significantly larger than those of the control group ( P < 0.05; Table 1 ).
On H&E examination, osteoblasts existed only in the experimental group; mature trabecular bone was found in the experimental group compared with lamellar bone in control group ( Fig. 2 C and F). Ponceau S examination showed that new bone was mature in both groups. A small amount of the newly generated bone could be seen in the experimental group as compared to the control group ( Fig. 3 C and F). Sirius red examination showed that collagen type I was most common, whereas collagen type III was rarer in both groups ( Fig. 4 C and F).