TMJ is a bilateral joint structure where both rotational and translational movement occurs. As mentioned previously, MRI can image the nature of condylar structure, a component of TMJ, more effectively than the plain X-ray or arthrogram does [15, 16]. Also, MRI is currently the most representative diagnostic method for the assessment of the soft tissue component of TMJ [17]. So, for the imaging TMJ structures, whether it is bony or soft tissue, MRI will be the best structure we can rely on today.
On the condylar fractures, the positional changes of the condylar head occur, in terms of the displacement or dislocation, and they in turn accompany inevitably the deformation or destruction of TMJ structure. And, this suggests the possible problems of disc position or structure and also destructional changes to joint capsule and/or surrounding ligaments. Christiansen et al. [18] investigated 43 patients with TMJ trauma using the CT and reported that the disc displacement was observed in 16 patients (37 %) while 15 patients (34 %) had the mandibular fracture. Raustia et al. [19] also examined 46 post-traumatic condylar fractures of 40 patients using the CT and plain X-rays after an average of 47 months and reported that the anterior disc displacement was found in six joints among 21 joints, which was far more than expected.
But, it would not be easy to expect their results to be reliable, since it was hard to gain an accurate understanding of structural changes for TMJ by CT imagings. Therefore, Jones et al. [20] and Goss et al. [21] evaluated, respectively, 14 and 20 patients of condylar fracture with arthroscope, and reported that discs were conserved intact in all cases, even though the considerable synovial ecchymosis was observed. These reports were, however, argued by subsequent studies using MRI. Sullivan et al. [22] and Takaku et al. [23] have reported that all the discs were displaced in the anterior/inner directions along with the fractured condyle, which was contrary to the findings from arthroscope-based studies.
This study aimed at presenting the possible disc displacement with accurate MRI imaging. We could find differences between the displacement degree of fractured fragment (on plain X-rays) and displacement of fractured proximal segment of condyle (on MRI) (Table 2). This probably came from the lack of information obtained from plain X-rays with the limited disclosure of the degree of fractured fragment displacement. It also raised the possibility of more distant segmental displacement and also the need of more careful diagnosis and treatment planning when we rely on the plain X-rays. The displacement pattern of the TM disc seemed to be similar situation as that of condylar segment (Table 3). And, our observation about the positional relationship between the condyle and disc indicated that the general position of the disc was near the condylar segment, especially when we disregard the minor difference of anterior or posterior discal position (Table 4). Dwivedi et al. [17] reported a significant relationship between the extent of condylar fracture and the degree of impairment to the disc. But, it also indicated that the final positional relationship between the displaced condylar and discal structure were not always identical. The number of condylar position “near eminence” from the dislocation and displacement group consisted of 23 cases out of 28 cases (Table 2). And, the number of disc positions near the eminence was 22 cases (Table 3). In addition, the condylar segments from the non-displacement group had six cases of posterior position within the glenoid fossa, while none from the same group had the posterior disc positions. All these findings indicated that the position of the disc went along with the condylar fracture segment, but with more anterior positional tendency.
Changes of the disc shape were found in seven cases on the fracture side and one case on the contralateral side (Table 5). In most cases, disc shapes were changed to have the anterior bending, which is typically different in shape from that of TMD with disc displacement with/without reduction. This morphological change seemed to be related to the traumatic injury, but hard to explain due to the lack of information. According to Dwivedi et al. [17], the higher condylar fracture tends to cause the greater injury to retrodiscal tissue, and to cause the lower capsular tears with the minimally displaced condylar fracture.
The morphological changes of the retrodiscal tissue were also thought to be caused by the traumatic injury, to indicate the tearing by the anterior dislocating condylar segment along with the disc. We could find this kind of injury to the retrodiscal tissue in six cases (13 %) at the condylar fractured sides (Table 5), while 39 to 50 % by Sullivan et al. [22] and Takaku et al. [23]. They regarded the retrodiscal tissue as highly susceptible to damage associated with condylar fractures.
However, the signal changes of retrodiscal tissue were found in seven cases from dislocation group, two cases from displacement and non-displacement group, and even two cases from contralateral side (Table 6). Thus, it seemed not to be highly related with the degree of displacement. And, this signal intensity change was found to be localized at the area immediately posterior to the disc, without any low signal intensity. Thus, this signal changes seemed to be correlated with the high chance of strong impact onto the retrodiscal tissue by direct trauma or indirect to the displaced condylar segment. In the same context, Sano et al. [24] reported that the high signal intensity of the retrodiscal tissue on T2-weighted images could be related closely to the severe joint pain. And, Takaku et al. [23] has reported that the existence of abnormal signal intensity of retrodiscal tissue after trauma might suggest the presence of trauma-induced inflammation.
The occurrence of high signals at both the condylar segment and retrodiscal tissue were found in three cases, and they were all minimal or undisplaced fractures. This finding shows the possible great traumatic injury to the condylar head and its adjacent tissues even if they are not accompanied by the fracture. And, it also raises the possibility of the higher traumatic injury onto the non-fracture side or minimal displaced fracture. Thus, we need to pay the same attention to the non-fracture side as to the fractured condyle.
Regarding the signal intensity (on T2-weighted images) of the joint space, they were found in 91 % (43 joints) of fracture side and 38 % (eight joints) of contralateral side for the superior joint cavity, and 72 % (34 joints) of fracture side and 43 % (nine joints) of contralateral side for the inferior space (Table 7). It could be diagnosed as the hemarthrosis, which might be mixed with bleeding and synovial fluid within the joint cavity.
In arthroscope-based studies, Jones et al. [20] found the hemarthrosis joints with condylar fractures in 13 joints out of 15 cases, and Goss et al. [21] observed it from 82 % cases when observed within 5 days. And, Takaku et al. [23] observed the high signal intensity from all of the 12 joints on MR images and found hemarthrosis on six joints during the surgery. Gerhard et al. [25] also found the significant relationship between the degree of condylar injury and hemarthrosis findings on MRI. These authors all proposed the hemarthrosis when the capsule or retrodiscal tissue was damaged. And, their significances are definitely related to the possible adhesion or ankylosis of the joints in case we fail to manage them properly.