An experimental model of osteoarthrosis of the temporomandibularjoint in monkeys

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British Journal of Oral and Maxillofacial Surgery (2002) 40, 232–237 © 2002 The British Association of Oral and Maxillofacial Surgeons doi: 10.1054/bjom.2001.0746, available online at http://www.idealibrary.com on

BRITISH

Journal of Oral and Maxillofacial Surgery

An experimental model of osteoarthrosis of the temporomandibular joint in monkeys K. Tominaga, S. Hirashima, J. Fukuda First Department of Oral and Maxillofacial Surgery, Kyushu Dental College, Kitakyushu, Japan SUMMARY. The purpose of this study was to develop a model of osteoarthrosis of the temporomandibular joint in monkeys, which is remarkably similar in structure and function to that of humans. Nine juvenile monkeys, two as controls and seven as an experimental group, were used in this study. In the experimental group, the articular eminence on both sides was surgically made steeper. Two animals were killed at 1 week, four at 6 months, and one at 1 year postoperatively and the temporomandibular joints were examined macroscopically and microscopically. Typical changes of osteoarthrosis were observed in the 6-month and 1-year specimens. These comprised clustering of chondrocytes which resulted in vertical and horizontal splitting in the articular cartilage, and fibrillation of the articular surface resulting in fibrous union in the joint cavity. These degenerative changes advanced progressively over time. Slight anterior displacement and degenerative changes in the articular disc were also seen. © 2002 The British Association of Oral and Maxillofacial Surgeons

Seven animals (14 TMJs) were used as the experimental group. The control group comprised one monkey that was killed immediately and another that was killed after 1 year for observation of age-related changes. In the experimental group, two animals were killed 1 week after operation, four at 6 months, and one at 1 year postoperatively. All experiments were done according to the guidelines and with the permission of the local Ethics Committee for Animal Research, Kyushu Dental College.

INTRODUCTION Experimental models of osteoarthrosis of the temporomandibular joint (TMJ) have been induced by artificial disc perforation and/or condylar surface injury in rabbits or sheep.1,2 The sheep model of TMJ osteoarthrosis has been used not only for research into the development of osteoarthrosis3 but also for studies of diagnostic techniques4 and management methods.5 However, induction of osteoarthrosis by direct intervention in the joint cavity, particularly on the articular surface, does not seem ideal because it is difficult to distinguish repair processes of the direct injury from degenerative changes. In addition, the function and structure of the TMJs of sheep and rabbits are not similar to those of the human. It is therefore better to induce osteoarthrosis without intervention into the joint cavity and to use primates that have a joint structure and masticatory function similar to those of humans. We have developed a surgically induced model of osteoarthrosis of the TMJ in juvenile monkeys that does not involve opening the joint.

Operation Animals were anaesthetized with an intramuscular injection of a mixture of 5% ketamine (20 mg/kg, Ketalar®, Sankyo, Tokyo, Japan) and 2% xyladine (1 mg/kg, Seractal®, Bayer, Germany). The zygomatic process of the squamosal bone was exposed by preauricular incisions extended to the temporal region. Osteotomy of the whole root of the zygomatic process including the articular eminence and retroarticular process was done with a thin dental fissure bar and a thin osteotome, following the line shown in Fig. 1. An oblique osteotomy of the zygomatic arch in the anterior region of the articular eminence was achieved with a small diamond disc. The whole bony component that covered the disc–condylar complex was mobilized to the squamosal bone and rotated anteriorly. The oblique cut upper edge of the mobilized zygomatic arch was seated at the lower edge of the remnant of the zygomatic arch and rigidly fixed

ANIMALS AND METHODS Experimental animals and experimental groups Nine juvenile Japanese monkeys (Macaca fuscata) (six female and three male), aged 2–3 years, weighing 7–9 kg, were used in this experimental study. 232

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B

Fig. 1 (A) Osteotomy line on a dry skull. (B) Close-up of osteotomy line.

The specimens were then fixed in 10% neutral buffered formalin in a centric occlusion position. They were decalcified in 20% formic acid and embedded in paraffin in a conventional manner. Sagittal sections 6 ␮ thick were cut and stained with haematoxylin and eosin. Degenerative changes in the articular eminence, disc and condyle were sought in the central one-third of each section. These changes included hyalinization, disorganization of the collagen network, fibrillation, clustering of chondrocytes, thinning of cartilage, denudation of the subchondral bone, fibrosis of bone marrow and perforation of the disc. Fig. 2 Photograph taken during operation. The mobilized fossa– eminence component is rotated anteriorly and fixed with a titanium mini-plate.

with a titanium mini-plate (Fig. 2). The inside of the TMJ was not touched during this procedure. The wound was thoroughly irrigated and closed in layers. The operation was performed bilaterally in all experimental animals. The animals were given antibiotics intramuscularly for 3 days after the operation. They were fed with conventional pellets for monkeys. Observation protocol Food intake was monitored and behaviour at eating was observed. The animals were killed with an overdose of pentobarbital (Nembutal®, Dainippon, Osaka, Japan) after each follow-up period. The fresh TMJs were cut sagitally in the centre with a band-saw (Maruto, Tokyo, Japan) and examined macroscopically.

RESULTS General observation There were no notable differences between experimental and control animals in food intake and behaviour at eating during the follow-up period. Control group: macroscopic and microscopic observations The mandibular condyle and the eminence faced vertically in the centre of the articular disc (Figs 3 and 4). There were no macroscopic or microscopic differences between the younger and older animal in the control group. One week after operation Macroscopic observations The fossa–eminence component including the retroarticular process was rotated anteriorly. The articular eminence

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Fig. 3 Macroscopic view of the control joint.

Fig. 4 Photomicrograph of the specimen shown in Fig. 3 (magnification
2).

was anterosuperior to the condyle. The disc was rotated slightly anteriorly as a result of the anterior translocation of the upper part of the joint, but there was no change in the attached position of the disc to the condyle or glenoid fossa (Fig. 5). Microscopic observations

Fig. 5 Macroscopic view of the specimen at 1 week after operation. The fossa–eminence component is rotated anteriorly.

Fig. 6 Photomicrograph of the specimen shown in Fig. 5. There is no histological change in any joint component (magnification
2).

Table 1 Histological changes in the temporomandibular joint: figures indicate number of the joints which have degenerative changes Status Control (n:4) 1 week after operation (n:4) 6 months after operation (n:8) 1 year after operation (n:2)

Eminence

Disc

Condyle

0 0 7 2

0 0 4 2

0 0 5 2

There were no histological changes in any joint component (Fig. 6; Table 1). Microscopic observations Six months after operation Macroscopic observations In many specimens there was slight anterior displacement of the disc proper. Deformity of the disc and fibrous adhesion in the upper joint cavity were also seen (Fig. 7).

In most of the specimens, clustering of chondrocytes and consequent vertical and horizontal splitting of the articular cartilage in the eminence were observed. Fibrillation of the articular surface in the upper joint cavity and fibrous adhesion were seen (Figs 8–10). These degenerative changes were more severe in the anterior part of the joint. In half the specimens, there was loosening of the

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Fig. 7 Macroscopic view of the specimen 6 months after operation. There is slight deformity of the eminence and the disc, and fibrous adhesion in the upper joint cavity (arrow). The disc is slightly displaced anteriorly.

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Fig. 9 Higher-powered photomicrograph of the area inside the rectangle in Fig. 8. There is fibrous adhesion in the upper joint cavity as well as vertical and horizontal splitting of the articular cartilage (magnification
8).

Fig. 8 Photomicrograph of the specimen shown in Fig. 7. There are degenerative changes in the eminence and the disc (magnification
2).

Fig. 10 Photomicrograph of the articular eminence in another specimen from the 6-month group. Clustering of chondrocytes has resulted in vertical splitting. Note the intact articular fibrous layer (magnification
13).

collagen network in the disc proper and hyalinization in the centre of the disc. In the condyle there was proliferation of chondrocytes and formation of clusters (Table 1).

cartilage layer of the articular eminence resulted in extensive fibrous adhesions in the upper joint cavity, and total splitting of the cartilage to the subchondral bone could be seen (Figs 12 and 13). There was also considerable thinning or loss of condylar cartilage. The bone marrow of the eminence and the condyle looked fibrosed. The centre of the disc was thinner than in the 6-month specimens and one of them was perforated with severe fibrillation and adhesion.

One year after operation Macroscopic observations The articular eminence and the condyle were deformed as a result of severe thinning of the cartilage layer (Fig. 11). Microscopic observations

DISCUSSION

There was progressive degeneration in each joint component (Table 1). Severe degenerative changes in the

The anatomy and the function of the TMJ are remarkably similar in monkeys and humans, so monkeys are an

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Fig. 11 Macroscopic view of the specimen at 1 year after operation. There is severe deformity of the eminence, the disc and the condyle, and severe thinning of the cartilage layer of the condyle.

Fig. 12 Photomicrograph of the specimen shown in Fig. 11. There are severe degenerative changes in all components of the temporomandibular joint. Extensive fibrous adhesion of the upper joint cavity has resulted in total splitting between the cartilage and the subchondral bone (arrows) as well as perforation of the disc (magnification
2).

excellent model for osteoarthrosis. We used mixed sexes in the experimental animals. Adult male monkeys have long canines while female monkeys do not; however, in juvenile monkeys aged 2–3 years old there is no sex difference in occulusion.6 Dijkgraaf et al. in their review divided osteoarthrosis of the human TMJ into four stages: initial stage, early stage, intermediate stage and late stage.7 The histopathological change in the initial stage is proliferation of chondrocytes. In the early stage there is focal swelling of the articular cartilage and proliferation of chondrocytes, resulting in formation of clusters. In the intermediate stage, fibrillation of articular cartilage causes vertical and horizontal splitting, resulting in detachment of articular cartilage. Thinning of the cartilage from mechanical wear and severe cluster formation, and

Fig. 13 Higher-powered photomicrograph of the area inside the rectangle in Fig. 12 (magnification
5).

advanced disorganization of the collagen network are also seen. In the late stage of osteoarthrosis, there is extensive fibrillation and detachment of articular cartilage, and eventually denudation of the subchondral bone. These degenerative changes are seen more often in the eminence than in the condyle.8 Our model resembled osteoarthrosis of the human TMJ. The 6-month specimens showed the intermediate stage and 1-year specimens the late stage. Mechanical loading exceeding the capacity for adaptation may cause chondrocyte injury and destruction of the remodelling system. An imbalance between anabolism and catabolism of the extracellular matrix accelerates degradation changes in cartilage.9,10 Overloading of the TMJ causes increased friction and bigger shearing force to the articular surfaces with translatory movements. Subsequently horizontal splitting or adhesive wear of the cartilage results in flattening of the eminence and the condyle.7,10 In our model, surgical manipulation of the articular eminence and glenoid fossa caused mechanical overloading. That overloading led to horizontal splitting in the eminence, thinning of the cartilage in the condyle, and perforation of the disc. We used juvenile monkeys for this experiment to eliminate age-related changes. In our model, the articular disc proper was slightly displaced anteriorly. We could find no report of disc displacement without direct intervention to the disc. The disc displacements in experimental animals have been induced by surgical anterior traction of the disc itself.11,12 Stegenga et al.10 and recently Nitzan13 proposed an explanation of disc displacement. They suggested that cartilage breakdown, loss of lubrication and increased friction as a result of overloading impair disc movement. This induces repetitive stretching of the disc attachment and finally the stretched, elongated

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attachment permits displacement of the disc. This implies that osteoarthrosis of the TMJ may be followed by internal derangement. This new model of osteoarthrosis of the TMJ in monkeys will be useful not only to investigate mechanisms of osteoarthrosis of the TMJ but also to study therapeutic measures such as bite plane treatment.

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The Authors Kasuhiro Tominaga DDS, PhD Associate Professor Soichi Hirashama DDS, PhD Assistant Professor Jinichi Fukuda DDS, PhD Professor First Department of Oral and Maxillofacial Surgery Kyushu Dental College Kitakyushu, Japan Correspondence and requests for offprints to: Mr K. Tominaga, First Department of Oral and Maxillofacial Surgery, 2-6-1 Manazuru, Kokurakita, Kitakyushu 803-8580, Japan. Tel: +81 93 582 1131; Fax: +81 93 582 6000; E-mail: [email protected] Accepted 19 September 2001