Most of the reasons for 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 rhinostomy [8]. 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 utilize a stable surgical reference mark and determine its relative position with the lacrimal sac on the lateral nasal wall.
The axilla of the middle turbinate and the maxillary line are the major landmarks utilized by endonasal surgeons to localize the lacrimal sac. In addition,the lacrimal sac is adjacent to the maxillary frontal process(FPM), the lacrimal bone, agger nasi air cell and the uncinate process [9–13].The axilla of the middle turbinate tends to be a constant endonasal anatomical landmark, which can be utilized to localize the lacrimal sac in DCR surgery [6]. Wormald et al. ’s study on CT-DCG showed that the sac is located 8-10mm above the axilla of the middle turbinate [5]. 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, respectively [9, 14]. Apart from the ethnic origin, were the differences in these findings influenced by other factors, such as the lacrimal sac size? Previous studies have rarely addressed the above issues further.
Through the measurement of CT-DCG images, the 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’s. However, the grouped study 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 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 factors [15]. 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 surgery [16]. The CC provides a valuable landmark for endoscopic surgeons [17]. If the CC is visible through the open sac, the surgeon can be reassured that the bone removal is 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 3–5mm in diameter [18]. 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 of different sizes 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 the SSF to the CC. To decrease the measuring error, all the measurements of CTDCG images were taken three times by two radiologists respectively and then averaged. It is expected that there will be better measurement methods to further verify the positioning of the CC in the future.
Wormald et al.used 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 subjects [14, 19]. In addition, since previous studies seldom grouped according to the size of the sac, the composition ratio of the sac size 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 should 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 line [6, 14]. In the process of powered endoscopic DCR, the height of the mucosal incision was weakened by the drill grinding the thick bone of MFP as one progresses posteriorly. Therefore, in order to fully open the lacrimal sac and reach the target height, the nasal mucosa incision should be designed to add an additional 1-2mm incision height.
In the routine DCR procedure, the design of the nasal mucosal incision should aim at fully exposing the CC opening rather than the SSF. In our study, the position of the CC relative to the axilla is relatively stable and is less affected by the size of the lacrimal sac. The average distance from the axilla to the CC is about 4mm. Refer to the level of the CC, increase the exposed height above the common canalicular opening by 2mm, and add an additional 1-2mm to compensate for the loss of the incision height caused by the bone thickness of the MFP. It is estimated that the position 7-8mm above the axilla can be designed as the first nasal mucosa incision of the endoscopic DCR for patients with large and medium lacrimal sac. In general, this method can simplify the process of CTDCG images analysis and the lacrimal sac localization on the lateral nasal wall in each case.
Small lacrimal sac DCR surgery has its uniqueness, and the success rate of surgery is not ideal [20, 21]. The particularity of the small lacrimal sac DCR operation lies in a narrow cavity, and less normal mucosal tissue around the CC, difficulty in exposing the sac, and the small DCR ostium is easily blocked by granulation and cicatrix. In small lacrimal sac DCR surgery, it is important to create a sufficient sac opening on the lateral nasal wall with exposure of the CC, increase the area of normal mucosa around the CC, reduce nasal mucosa damage and bone exposure over the location of the ostium [22, 23]. In our study, the distance of the SSF to the CC varies from a minimum of 0.6mm to a maximum of 4.2mm in the small sac group. Therefore, we are convinced that the preoperative positioning of the small lacrimal sac should be aimed at exposing the CC, also 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, the large mucosal incision made in conventional DCR surgery may not be suitable due to the small sac cavity and the small sac mucosal flap. The larger the mucosal excision, the more mucosal damage and the bare bone, which increases the risk of ostium granulomas proliferation and cicatricial closure [24]. Referring to the level of the CC and the distance from the SSF to the CC, personalized nasal mucosal incision can be designed for patients with small lacrimal sac. And 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 on CT-DCG images,with reference to the size of the lacrimal sac. 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 in 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 were limited. More locating information of the lacrimal sac and the nasal mucosa incision in endoscope DCR surgery is needed to further supplement and verify the clinical application of the above results.