Ⅰ. INTRODUCTION

Synovial cysts are true cysts that commonly occur in the wrist, knee, and feet. In the field of oral and maxillofacial surgery, it rarely occurs in the temporomandibular joint (TMJ). 1) Although it usually occurs unilaterally, it can be also bilateral.2-4) The most common symptoms are pain and swelling in the preauricular area. In rare cases, symptoms may include tenderness, neuralgia, pain aggravated by TMJ movements, numbness, paresthesia, limited mouth opening, and clocking. 5) Several studies revealed that synovial cysts tend to affect women, with an average age of 46 years.3, 5) Herniation and displacement of the synovial lining are the main causes. Trauma and inflammation can also cause synovial cyst. In addition to synovial cysts, various diseases can cause pain and swelling in the preauricular area. Essentially, ganglion cysts must be differentiated from synovial cysts. Although ganglion and synovial cysts are clinically and radiologically indistinguishable, they can be distinguished histopathologically. Other lesions that can cause pain and swelling in the preauricular area should be differentiated. Sebaceous cysts and retention cysts of the parotid, synovial chondromatosis, osteochondroma, osteoma, osteoblastoma, aneurysmal bone cyst, hemangioma, sarcoma, benign cervical lymphoepithelial cyst, and benign vascular or neural mass should be also considered.5, 6) Synovial cysts are usually diagnosed by computed tomography (CT) or magnetic resonance imaging (MRI). A histopathological examination is required to confirm the defini- tive diagnosis.

In this report, we present a rare case of a synovial cyst in the TMJ and conduct a literature review.

Ⅱ. CASE REPORT

A 54-year-old woman visited the Department of Oral and Maxillofacial Surgery of the Kyungpook National University Dental Hospital, with chief complaints of right TMJ swelling. She had no specific medical history. Right TMJ swelling was palpable for 10 days and appeared to be getting bigger. No pain and limited mouth opening were noted. On physical examination, the right TMJ was slightly swollen, and a round, movable mass was felt on palpation. Panorama X-ray images taken at the time of the visit did not show any special findings in the right TMJ. The patient had undergone enhanced CT from another hospital before the present visit. CT revealed a 1.1 × 0.8 × 1 cm well-defined rounded cystic lesion on the lateral area of the right TMJ (Fig. 1A, B). Based on the clinical and radiological findings, a synovial or ganglion cyst in the right TMJ was suspected. After aspiration attempts, surgical removal under general anesthesia was planned as necessary. At the next visit, ultrasound-guided aspiration was attempted in the outpatient department. No cystic fluid was found. After explaining to the patient the possibility of postoperative facial nerve damage, surgery under general anesthesia was decided. The preauricular approach was used. The cystic lesion was exposed by blunt dissection, avoiding the facial nerve and parotid gland. The lesion was well-circumscribed. When the cystic fluid was aspirated, a gelatinous fluid was noted. The entire cystic lesion was removed. Postoperatively, on the right TMJ, compression by gauze was done. In addition, the gauze on the TMJ was wrapped with a facial band. It can reduce postoperative edema and dead space. No postoperative facial nerve weakness was observed. The specimen was sent for histopathologic examination. Microscopic examination showed lumen surrounded by loose fibrous tissue, and the lining is in a folded form and is composed of synovial cells (Fig 2A, B). Eventually, it was diagnosed as a synovial cyst.

Ⅲ. DISCUSSION

Synovial cysts commonly occur in women in their 40s and rarely occur in the oral and maxillofacial area. Two types of cysts occur in the TMJ, namely, synovial cysts, and ganglion cysts. Although TMJ cysts are commonly found on the lateral side of the joint, they can also occur on the anterior and medial sides.1) In the TMJ, cysts are commonly unilateral and rarely bilateral. The present case can be considered a representative case because it occurred on the lateral side of the right TMJ and the patient was 54 years old.

Synovial and ganglion cysts are clinically and radiologically very similar. Clinically, on physical examination, they cause TMJ swelling and pain. Radiologically, on CT and MRI, well-defined cystic lesions can be found. In the previous literature, the terms “synovial cyst” and “ganglion cyst” are used interchangeably; however, they can be differentiated by microscopic examination.7) Synovial cysts are true cysts lined with synovial cells and filled with synovial fluid. It may or may not communicate with the joint cavity. Trauma or inflammation can increase joint pressure, causing herniation or displacement of the synovium, and these factors can cause synovial cysts. Ganglion cysts are pseudocysts lined with fibrous connective tissue and filled with gelatinous material. The exact cause is unknown; however, it is thought to be caused by myxoid degeneration of the connective tissue of the joint capsule. Whether trauma can cause ganglion cysts is also unclear. In this case, the patient had no history of inflammation or trauma.8-10)

When diagnosing synovial cysts, CT and MRI are commonly used and effective. It is usually not detected on dental panorama; however, in rare cases, it can be seen as a radiolucency around the TMJ area or as a contour defect of the condyle.5) Ultrasonography is favorable for identifying parotid involvement and can be a good alternative in pregnant women.11) CT and MRI are effective for identifying the lesion and their relationship to the joint. Specifically, MRI has excellent soft tissue contrast, which is helpful for diagnosis in the TMJ area with varying soft tissues. Moreover, if the lesion is not observed on CT because of its small size, it may be observed on MRI.4) Cystic lesions appear as hypointense on T1-weighted images and hyperintense on T2-weighted images.1) In the present case, ultrasound- guided aspiration did not obtain synovial fluid. The lesion was observed clearly on CT; thus, MRI was not necessary. The definitive diagnosis of synovial cysts can be confirmed by histopathologic examination.

The gold standard of treatment of synovial cysts is complete removal via surgical excision. In most cases, a preauricular incision is made under general anesthesia. During surgery, the facial nerve and parotid gland must be preserved. Conservative treatments include careful observation and fine-needle aspiration. In some cases, spontaneous regression occurred by observation and aspiration alone. With these conservative treatments, cysts can recur more often.4, 12) Recurrence is rare if it is removed completely.

In conclusion, various diseases can cause swelling or pain in the TMJ; however, if cystic lesions are found on CT or MRI, they can be diagnosed as synovial or ganglion cysts. These can be removed surgically, and the definitive diagnosis can be made histopathologically. Synovial cysts are uncommon diseases. Oral and maxillofacial surgeons should be familiar with the clinical, radiologic, and pathologic characteristics of synovial cysts and be able to manage them.