Most of the reasons for the failure of endoscopic DCR can be attributed to a poor understanding of endonasal anatomy and the lacrimal sac position on the lateral nasal wall, which can lead to a wrong location of the rhinostomy5. Different from the procedure of external approach, the first step of endonasal DCR is to determine the projection position of the lacrimal sac on the lateral nasal wall. In order to locate the lacrimal sac accurately, it is necessary to find a stable surgical reference mark and determine its relative position with lacrimal sac on the lateral nasal wall.
The axilla of the middle turbinate is a constant endonasal anatomical landmark, which can be utilized to localize the lacrimal sac in DCR surgery. Wormald et al. ’s study on CT-DCG showed that the sac is located 8–10 mm above the axilla of the middle turbinate6. In other studies, the fundus of the sac is an average of 4.73mm ± 2.86mm, 6.6mm ± 1.3mm above the axilla of the middle turbinate, respectively8–9. Apart from the ethnic origin, were the differences in these findings influenced by other factors, such as size? Previous studies have rarely addressed the above issues further.
Through the measurement of CT-DCG images, SSF of all cases was located above the MTA in this study. And the average distance from the axilla to the sac fundus was 7.52mm ± 3.23mm. Our result is very close to the results of Wormald et al. However, this study grouped according to the transverse diameter of the lacrimal sac showed that the average distance from the axilla to the sac fundus in the large, medium, and small lacrimal sac groups was 8.63mm ± 3.05mm, 7.59mm ± 2.92mm, 5.96mm ± 2.56mm, respectively. With the increase of the transverse diameter of the sac, the distance from the the axilla to the sac fundus gradually increased. Our results show that the size of the lacrimal sac is an important factor affecting the relative position of the superior fundus of the sac. The relative position of the lacrimal sac fundus is not constant.
In our study, the CC of most patients is above the MTA, and the average distance from the CC to the axilla is 3.95mm ± 2.49mm. Moreover, there was no statistically significant difference between groups. These results indicate that the position of the CC relative to the axilla is relatively stable and is less affected by the size of the lacrimal sac. Because the CC is interwoven and surrounded by hard and thick tissues such as muscle fibers and medial canthal ligaments, it is speculated that the relatively stable position of the CC may be related to these anatomical factors10. One of the main reasons given for the failure of external DCR surgery is inadequate bone removal in the sac projection area, which may also be an important factor for the failure of endoscopic DCR surgery5. The CC provides a valuable landmark for endoscopic surgeons. If the CC is visible through the open sac, the surgeon can be reassured that the bone removal are sufficiently high and most of the sac is exposed. In ideal DCR procedure, the area of bony resection around the CC should be at least 3–5 mm in diameter6, 11. If the level of the CC is determined on the lateral nasal wall during the DCR operation, the ideal position of nasal mucosal incision and bone resection can be obtained by referring to it. Our study shows that It is feasible to determine the level of the CC on the lateral nasal wall through the MTA, which is of great significance for accurate localization of the lacrimal sac and full opening of the lacrimal sac cavity in DCR surgery. There are few reports about the location of the CC relative to the MTA. The reason may be related to the difficulty in displaying the CC and the MTA on a coronal CT-DCG image at the same time, and the difficulty in directly measuring their distance. In this study, the distance from the axilla to the CC was obtained by calculating the difference between the distance from the SSF to the axilla and the distance from SSF to the CC. It is expected that there will be better measurement methods to further verify the positioning of the CC in the future.
Wormald et al.5used CT-DCG to show that the fundus of the sac lies about 5mm above the common canalicular opening. In our study, the average distance from the fundus of the sac to the CC was 3.41mm ± 1.31mm. With the increase of the transverse diameter of the sac, the distance from the fundus of the sac to the CC gradually increases. The reason for the difference between the studies may be related to the ethnic origin of the research subjects6. In addition, since the previous studies seldom grouped according to the size of the sac, the composition ratio of the size of the sac may also be a factor leading to the differences.
In view of the importance of fully exposing the common canalicular opening in the sac during endoscopic DCR surgery, the MFP at the level of the CC must be removed during the operation. We measured the thickness of the MFP at the level of the CC, and found that the bone thickness was on average 3.50mm ± 1.24mm, which had no significant correlation with the size of the sac. This is very close to the results of previous studies in which the bone thickness of the MFP was 3-6mm above the maxillary line6, 9. In the process of powered endoscopic DCR, the height of mucosal incision was weakened by the drill grinding the thick bone of MFP as one progresses posteriorly6. Therefore, in order to fully open the lacrimal sac and reach the target height, the nasal mucosa incision should be designed with an additional 2-3mm incision height.
In the DCR procedure, for patients in the large and medium lacrimal sac groups, there is a large space around the CC in the lacrimal sac. We believed that the design of the nasal mucosal incision need not aim at opening the fundus of the sac. And it should refer to the level of CC, increase the exposed height above the common canalicular opening by 2mm, and add an additional 2-3mm to compensate for the loss of the incision height caused by the bone thickness of the MFP. According to the average distance from the axilla to the CC is 4mm in our study, it is estimated that the position 8-9mm above the axilla can be designed as the upper boundary of the endoscopic DCR nasal mucosa incision. This is consistent with Wormald et al. 's theory that the first mucosal incision should be made at 8 to 10 mm above the axilla12. Due to the abundance of nerves and vessels in the nasal roof, determining the location of mucosal incision can ensure the effective opening of the sac and CC, and reduce the risk of bleeding and injury of olfactory nerve fibers caused by excessively high incision design.
The particularity of the small lacrimal sac DCR operation lies in a thick sac wall, a narrow cavity, and less normal mucosal tissue around the CC, difficulty in exposing sac, and the small DCR ostium is easily blocked by granulation and cicatrix. Small lacrimal sac DCR surgery is difficult, and the success rate of surgery is not ideal13–14, which has always been a thorny problem in lacrimal drainage reconstruction surgery. The key to small lacrimal sac DCR surgery is to create a sufficient sac opening on lateral nasal wall with exposure of the CC, increase the area of normal mucosa around the CC, and reduce nasal mucosa damage and bone exposure over the location of the ostium13, 15–16. Therefore, we convinced that the preoperative positioning of the lacrimal sac should be aimed at exposing the sac fundus. Thus can open the narrow diverticulum-like space between the sac fundus and the CC, increase the area of the normal mucosa around the CC. Furthermore, in the small sac DCR operation, due to the small sac cavity and the small sac mucosal flap, the large mucosal incision made in conventional DCR surgery may not be suitable. The larger the mucosal excision, the more mucosal damage and the bare bone, which increases the risk of ostium granulomas proliferation and cicatricial closure17. Individual design of mucosal incision is of great significance to improve the success rate of small lacrimal sac DCR surgery.
In conclusion, our study shows that it is feasible to locate the sac through the relative position of the CC and the MTA. The distance of the SSF to the MTA was influenced by the lacrimal sac size. Accurately positioning the lacrimal sac and upper nasal mucosal incision is beneficial to fully open the lacrimal sac and expose the CC. At the same time, it can avoids much more mucosal incisions than necessary during the operation, which can shorten the operation time, reduce the surgical trauma, and make the endoscopic DCR operation more safer, more efficient, and less invasive. The above is of great significance for improving the success rate of endoscopic DCR surgery. Since this study only discussed the availability of locating the lacrimal sac by CT-DCG, surgical data was limited. In the future, more locating information of lacrimal sac and nasal mucosa incision in endoscope DCR surgery should be collected to further supplement and verify the clinical application of the above results.