Ⅰ. INTRODUCTION
Compared to the relative commonness of TMJ disorders such as degenerative osteoarthritis, temporomandibular myofascial dysfunction, and internal derangement, synovial chondromatosis of the TMJ is a rare condition1. Clinically, its initial symptoms often do not differ significantly from those of more prevalent TMJ disorders, typically presenting with preauricular pain, joint noises, and restricted mandibular movement1, 2. As a result, in the early stages, especially during Milgram’s stage 1 or when distinct radiographic findings are absent, synovial chondromatosis may easily be misdiagnosed as a common TMJ disorder3, 4. Diagnostic confusion is further compounded by the fact that, even as the disease progresses and patients develop grating sounds or crepitus due to intra-articular loose bodies, similar noises can be observed in other TMJ conditions, such as osteoarthritis or disc displacement with degenerative changes5. Because of this clinical and symptomatic overlap, a thorough understanding of the characteristic clinical, radiographic, and pathologic features of synovial chondromatosis is critical for accurate and timely diagnosis6. Key distinguishing features may include the identification of multiple calcified loose bodies in the joint space on imaging, progressive and persistent joint crepitus that does not entirely resolve with standard conservative therapy, and histopathologic findings of nodules composed of chondrocytes within a chondroid matrix4, 7. Early recognition and differentiation from more common TMJ disorders enable prompt surgical intervention, which is crucial for preventing joint damage and improving patient outcomes6, 8, 9.
In this report, we present a case of synovial chondromatosis of the TMJ and review its distinguishing characteristics in comparison to previous literature, aiming to highlight important considerations for diagnosis and management. By sharing our clinical observations and a review of diagnostic pitfalls, we emphasize the necessity for clinicians to maintain a high index of suspicion for rare entities such as synovial chondromatosis when evaluating patients with persistent TMJ symptoms that do not respond to conventional modalities. In addition, we discuss its pathogenesis and treatment strategy.
Ⅱ. CASE PRESENTATION
A 73-year-old female presented with bilateral preauricular pain that occurred spontaneously and during mastication, with greater severity on the left side. In addition, a grating sound was noted in the left TMJ during mastication. The pain and discomfort started 3 years ago when she had been wearing a complete denture only on the left side, for which periodic follow-up examination had been performed in a local clinic. Initially, the pain was localized to the left TMJ, but it subsequently involved both TMJs and the mandibular angle area. Two weeks ago, multiple calcified nodules was found in the joint space at the local clinic, and she was transferred to our hospital for further evaluation. Physical examination revealed tenderness over both TMJs and their surrounding areas, without swelling. Evaluation of occlusion was difficult because she was wearing complete dentures. There was no limitation of mouth opening. The patient reported bruxism, and other dental findings were nonspecific. Her medical history included hypertension, diabetes mellitus, and previous knee chondroplasty.
Radiologic features showed multiple radiopaque mass within the left articular space, and the left condylar surface was flattened with irregularity, suggestive of synovial chondromatosis (Fig. 1A,B). Otherwise, findings were nonspecific. The mass containing calcified nodules was surgically removed via preauricular excision exposing the left TMJ and submitted for histologic evaluation. The submitted specimen measured 1.8X1.0X0.7cm in diameter and consisted of multiple fragments of yellow-white bony tissue with surrounding soft tissue. Histopathologic examination revealed multiple discrete nodules composed of clusters of chondrocytes separated by active cartilage matrix, embedded within fibrous tissue (Fig. 2A,B). The chondrocytes showed hyperchromasia and nuclear enlargement (Fig. 2C). However, there was no prominent nuclear atypia or mitotic activity. Endochondral calcification and bony tissue formation were observed in the peripheral region of chondroid mass (Fig. 2A). The final diagnosis was synovial chondromatosis.
After the operation, the patients complained of facial nerve weakness and headache. However, facial nerve weakness improved after one month. At two months postoperatively, the patient reported noise in the TMJ when dentures were removed, with no noise noted while wearing dentures. At four months, facial nerve function had fully recovered and no discomfort was present in the TMJ area. Ten days after last follow-up, the patient presented with headache and pain in the right TMJ. The issue was determined to be unrelated to the previous lesion, and the patient was referred to the Department of Oral Medicine.
Ⅲ. DISCUSSION
Synovial chondromatosis most commonly affects large synovial joints such as the knee, hip, and elbow, with the TMJ accounting for only approximately 3% of all cases10, 11. While the prevalent age for this disease is generally considered to be between 30 and 50 years, regardless of the affected site, there are case reports documenting occurrence in both younger and older individuals12, 13. In our report, we observed a case in a patient in her 70s, demonstrating that synovial chondromatosis of the TMJ can also occur in elderly individuals and that age above 70 should not be excluded from the differential diagnosis. It has been reported that TMJ synovial chondromatosis occurs more frequently in females14, whereas, in large joints, the condition is more common in males13. Our case also involved a female patient. Although the underlying reasons for this gender difference remain unclear, some have suggested a possible influence of estrogen15. However, given the occurrence of synovial chondromatosis in the elderly, as shown in this and other reports, the relevance of hormonal factors such as estrogen may be limited.
Similar to synovial chondromatosis in large synovial joint, where joint pain, swelling, and limited range of motion are the main symptoms, patients with TMJ synovial chondromatosis commonly present with analogous features7. Additionally synovial chondromatosis may be associated with joint effusion and crepitus during movement, and the patients sometimes report tenderness around the affected joint9. However, not all patient display the full spectrum of symptoms. Previous studies indicate that pain is the most prominent and consistent finding7, 9, a pattern also observed in our case. Nonethelss, the nature and characteristics of the pain are typically not specific enough to distinguish synovial chondromatosis from other TMJ disorders. Limited mouth opening is considered the second most common presenting symptom of TMJ synovial chondromatosis, with an incidence of reported in the literature of approximately 54 to 57%9. In our case, however, this finding was not observed, suggesting that limited mouth opening is not an essential criterion for diagnosis. While the presence of calcified nodules can produce noise within the joint, the incidence of clicking and crepitus in TMJ synovial chondromatosis is relatively low, with rates of approximately 9.94 to 14.27% and 19.34 to 28.57%, respectively9. In our case, a grating sound rather than a distinct click was noted, which aligns with the histologic finding of multiple calcified nodules. This observation suggests that a grating or crepitus-type noise may be more indicative of synovial chondromatosis than a simple click, although crepitus is not a consistent feature of the disease. In summary, our patient did not present with limited mouth opening or swelling. Taking into account the presence of pain, tenderness, and intermittent crepitus, it is important for clinicians to consider synovial chondromatosis as a possible diagnosis in cases of persistent TMJ pain and crepitus, particularly when these symptoms are unresponsive to prolonged and appropriated for TMJ disorders, even though this condition is rare.
To establish effective therapy and achieve better prognosis, a clear understanding of the pathogenesis of synovial chondromatosis is essential. Historically, synovial chondromatosis was regarded as a metaplastic process, wherein synovial pluripotent stem cells undergo abnormal differentiation into cartilage3. However, recent studies increasingly support a neoplastic origin, proposing that genetic alterations within synovial stem cells drive clonal proliferation and cartilaginous transformation. For instance, overexpression of genes associated with the Hedgehog signaling pathway, such as GLI1 and PTCH1, has been identified in affected tissues16. Furthermore, recurrent gene rearrangements involving FN1 and/or ACV2A have been reported in several cases17. Evidence of locally invasive behavior in some patients also strengthens the neoplastic hypothesis18. Despite these findings, it should be noted that metaplastic changes may still occur in synovial stem cells, potentially triggered by altered signaling pathways due to genetic or environmental factors. Thus, the pathogenesis of synovial chondromatosis remains a subject of debate, and a definitive classification as either metaplastic or neoplastic cannot be made at this time. It is possible that the concept of metaplasia, as it relates to this disease, may evolve further as our understanding of its molecular characteristics improves.
Most cases of synovial chondromatosis are completely benign and do not recur after complete removal. However, malignant transformation of synovial chondromatosis into synovial chondrosarcoma has been reported, although the incidence is low at approximately 1 to 5%19. Malignant transformation typically occurs after repeated recurrences and over a prolonged period, with an average latency of 11.2 years19, 20. Patients who experience malignant transformation often present with more severe pain and rapid clinical deterioration compared to the original benign lesion, but the clinical and radiologic features may remain similar to those of benign synovial chondromatosis20. Therefore, periodic follow-up and careful management are essential to ensure a favorable prognosis.
