The incidence of TAC was 7.5% in our study. After TAC was described by Mafee et al. 6,
several studies were published about this topic. The calcification incidence was reported at varying rates (Table-2
). The lowest ratio of TAC was 3% in 100 CT scans which was reported by Murray et al. 1
. Ko and Kim 3
reported the highest incidence of 16% in 216 patients. In our study, incidence of TAC is correlated with the literature.
The trochlea is a cartilaginous saddle attached to periorbita of the frontal bone in the superior nasal orbit. This cartilaginous saddle is separated from a fibrovascular sheath by a bursa-like space. The tendon of the superior oblique runs within this sheath 7. Although the exact location of calcification at the trochlear apparatus is not clear, cartilage, the synovial sheath, and the tendon are possible sites. The TAC etiology is not certain, however metabolic, inflammatory and degenerative diseases considered to be potential causative factors 8. TAC thought to be a marker for diabetes mellitus (DM) for patients younger than 40 years old 8,10. However in a recently study performed by Buch et al. 9 did not find an association between TAC and DM. The authors declared that they had not enough data to reveal the relationship among them. Buch et al. 9 also reported a significant correlation with autoimmune diseases, elevated serum alkaline phosphate levels and TAC.
The shape of TAC was not reported in earlier mentioned reports because CT technology was not good enough to obtain thin slices. With the advances in CT technology, it is possible to acquire thin slices less than 1mm and multiplanar reformatted images. Xiao et al. 5 reported the first study evaluating the TAC morphology with MDCT. They assessed the orbital CT scans obtained in 3mm slice-thickness. The authors described three types of trochlear calcification as “comma, dot and inverted U” types. Most of the TAC in their study was “comma” type (66.7%), followed by “dot” (22.2%) and “inverted U” (11.1%) types, respectively. We realized an additional type of TAC which had a “linear” shape. The most common type was “dot-like” similar to Xiao et al. study 5. However the ratio was lower and the second type “inverted U” had close ratio in our study. The shape of TAC may be related to the exact localization at the trochlear apparatus. Classification of TAC according to morphologic appearance can be helpful in differentiating from intraorbital foreign body especially in trauma patients 5.
An interesting finding in our study is the attachment of TAC to the orbital wall although TAC was previously described as a discrete calcification 8. We realized that in 8 (21.05%) subjects TAC was attaching with the orbital wall. As our best knowledge our report is the first one describing this finding. Slice thickness can be less than 1 mm and reformatted images are available in different planes with MDCT. Finding may not be detected previous reports because most of them were performed with thicker slices than our study. Most of TAC attaching to orbital wall was “comma” type. Attachment of TAC to orbital wall might be an additional finding in differentiating TAC from the foreign bodies.
Gender difference was investigated in previous reports. Xiao et al. 5 reported TAC in males two times more than females. The number of male subjects was approximately four times more than female subjects. This may explain the male dominance in their study. The other reports had lack of significant difference between genders like our study 3,8,9. The ratio of male and female subjects was nearly equal in our study and TAC was detected with same ratio in both genders. In an early study about TAC, an incidence increasement with aging reported 8. For this reason the authors suggested TAC could be a degenerative process. However there was no significant correlation found between TAC incidences and aging in the latter reports, including our study 1,3,9
It is important to differentiate TAC from a metallic foreign body in patients whom require magnetic resonance imaging (MRI) 11. Metallic foreign bodies can move during MRI and may cause severe complications like blindness 12. Both TAC and metallic foreign body may have averaged attenuation in CT images.
This study has some limitations because of its retrospective nature. We couldn’t investigate the association of TAC with systemic pathologies because we didn’t have enough data. Our study was based on the imaging findings and we were not able to correlate the calcification types with histopathologic findings.