Analysis of lacrimal duct morphology from cone-beam CT dacryocystography in a Japanese population

Dacryocystorhinostomy (DCR) is the rst-line treatment for lacrimal duct stenosis and obstruction in western countries. Endoscopic-assisted nasolacrimal duct intubation (ENDI) is spreading steadily as a minimally invasive treatment in Northeast Asia. ENDI is prevalent in this area because Northeast Asians have relatively at facial features, with a less elevated superior orbital rim than other ethnic groups. This allows for relatively easy manipulation of a dacryoendoscope. Evidence has accumulated that the morphology and inclination of the lacrimal duct differ among individuals and ethnic groups. In this study, we collected anthropometric data from a Japanese population of 100 samples—the parameters vital for designing a dacryoendoscope probe. The data we provided was essential in designing the overall length, bending point, and curve-line of the dacryoendoscope probe. Although the Japanese data measured in this study would not be directly applicable to other ethnic groups, we hope that the parameters provided by this study will contribute to the accumulation of valuable anthropometric data for the design of endoscopic probe morphologies and the development of therapeutic devices for lacrimal tract diseases—in terms of designing optimal morphologies, specic to ethnic groups and populations.


Introduction
The lacrimal duct extends from the lacrimal punctum to the lower opening of the nasolacrimal duct (NLD) on the lateral wall of the inferior nasal meatus. It passes through upper and lower punctum, the superior and inferior canaliculi, and the common canaliculus to reach the internal common punctum (ICP) in the lacrimal sac. The pathway to this point passes through eyelid tissue that is mobile and elastic. The lacrimal sac (LS) is located in the lacrimal fossa. The interosseous and meatal parts of the NLD are xed tissues.
Primary acquired nasolacrimal duct obstruction (PANDO) is an organic obstruction of the lacrimal duct that can occur anywhere from the punctum to the NLD opening. 1 Cases with obstruction from the punctum to the ICP are classi ed as lacrimal canaliculus obstruction, while cases with obstruction from the LS to the NLD opening are classi ed as nasolacrimal duct obstruction. Dacryocystorhinostomy (DCR) is the rst-line treatment for PANDO. Endoscopic-assisted nasolacrimal duct intubation (ENDI) is widely used as a minimally invasive treatment for lacrimal duct stenosis and obstruction in Northeast Asia. [2][3][4][5] The ENDI procedure is performed while directly observing the obstructed area in the lacrimal duct with a dacryoendoscope and observing the nasal cavity with a nasal endoscope. This reduces complications from false passage formation. Because ENDI can usually be performed under local anesthesia, it has evolved into a less invasive and safer procedure, which is one of the main reasons for its increasingly widespread use in Northeast Asia. Another reason is that Northeast Asians have relatively at facial features, with a less elevated superior orbital rim (SOR) than other ethnic groups. This allows for relatively easy manipulation of a dacryoendoscope. 6 In general, the long-term therapeutic outcomes of ENDI are not equivalent to DCR. Nevertheless, evidence has accumulated that the outcomes of ENDI are almost as effectual as DCR for canaliculus obstruction and PANDO (in cases of non-in ammatory or partial obstruction). 2,4,5,7−9 Since ENDI is a minimally invasive procedure for the treatment of PANDO, which can be performed under local anesthesia, further studies are needed to compare the long-term treatment outcomes of DCR and ENDI in PANDO, in terms of pathological conditions (e.g., site of obstruction, cause of obstruction, and duration of obstruction).
Since it was rst reported in 1909, dacryocystography (DCG) has undergone improvements in contrast media, injection method (using a cannula), and imaging method. DCG is still an essential preoperative evaluation for PANDO. 10,11 Since its rst application in dentistry in 1998, clinical applications of cone-beam computed tomography (CBCT) have gradually increased in the head and neck regions; CBCT is now widely used in medical facilities for dentistry, oral surgery, and otorhinolaryngology. [12][13][14][15] There are few reports on CBCT in the eld of ophthalmology. Nonetheless, CBCT-DCG is a valuable test for evaluating PANDO. It has the advantage of much lower radiation exposure than conventional multi-slice CT-DCG. [16][17][18] The length and inclination of the LS and NLD differ among individuals; there are also differences between races and ethnic groups. This is the rst study to report measurements of various parameters of the lacrimal duct in a Japanese population, based on CBCT-DCG. We hope that the measurements provided by this study will contribute to the accumulation of valuable anthropometric data for the design of endoscopic probe morphologies and the development of therapeutic devices for lacrimal tract diseases (in terms of designing optimal morphologies, speci c to ethnic groups and populations).

Results
The mean age of the 102 cases was 71.3 ± 11.7 years. Among them, 74 cases were female and 28 cases were male. There were 51 cases of right side PANDO and 51 cases of left-side PANDO. The maximum, minimum, and average values of the measured parameters are shown in Table 1. The angle formed by SOR-ICP-NLD opening The maximum value of the angle was 27° and the minimum value was −11°. The mean value was 10.2 ± 7.8°. The angle was positive in 92% (93/101) of cases, while 8% (8/101) of the subjects had a negative angle. An example image of a case with a large SOR-ICP-NLD opening angle is shown in Fig. 3. The large angle was due to the elevation of the SOR and anterior inclination of the NLD. The Shapiro-Wilk test gave a value of 0.55, indicating a normal distribution (Fig. 4A). For females, the mean was 9.9 ± 8.2°; for males, it was 10.8 ± 6.7°. There was no signi cant difference between males and females (p = 0.67).
The length of SOR-ICP

The length of LS
The maximum value was 17.1 mm and the minimum value was 4.3 mm. The mean was 8.9 ± 2.3 mm.
The Shapiro-Wilk test gave a value of 0.0002, indicating a non-normal distribution (Fig. 4D). For females, the mean was 8.7 ± 2.1 mm; for males, it was 9.6 ± 2.6 mm. There was no signi cant difference between females and males (p = 0.079).

The length of NLD
The maximum value was 20.7 mm and the minimum value was 5.7 mm. The mean was 13.2 ± 2.7 mm.
The Shapiro-Wilk test gave a value of 0.39, showing a normal distribution (Fig. 4E). For females, the mean was 13.0 ± 2.4 mm; for males, it was 13.7 ± 3.3 mm. There was no signi cant difference between females and males (p = 0.17).
The LS-NLD angle The maximum angle was 40° and the minimum was −43°. The mean was −6.3 ± 14.1°. The Shapiro-Wilk test gave a value of 0.30, indicating a normal distribution (Fig. 4F). The anterior bending type represented 33.3% (31/93) of cases; 66.7% (62/93) were of the posterior bending type. Examples of cases with anterior and posterior bending are shown in Figs. 5 and 6. For females, the mean was −6.9 ± 14.5°; for males, it was −4.6 ± 12.9°. There was no signi cant difference between males and females (p = 0.29).

Discussion
The lengths of the LS and the NLD and the inclination of the LS-NLD vary among individuals and between ethnic groups. [19][20][21][22][23] In this study, we measured various parameters of the Japanese lacrimal duct using CBCT-DCG images.
The average angle formed by the SOR-ICP-NLD opening was 10.2 ± 7.8°. The line formed by the SOR-ICP is the anatomical limit where the tip of a straight probe can reach most anteriorly after entering the NLD through the ICP. We con rmed that, in 92% of subjects, the line formed by the ICP-NLD opening was anteriorly inclined to the line formed by the SOR-ICP. This suggests that blind probing with a straight bougie, or manipulating a dacryoendoscope with a straight probe, is more likely to form a false passage posterior to the original lacrimal duct. Therefore, a probe with a bent anterior tip, or a curved probe, is more appropriate. In 8% of the subjects, the SOR-ICP-NLD angle was zero or negative. In such cases, a straight probe is considered more suitable than a curved one.
The mean length of the SOR-ICP was 24.3 ± 3.2 mm. The mean length of the ICP-NLD opening was 21.8 ± 2.7 mm. These new parameters could be measured because CBCT-DCG depicts the ICP. Although these parameters did not follow a normal distribution, they may be helpful for optimizing the length and the curve line settings of the dacryoendoscope probe. Generally, Northeast Asians have a low development of the SOR and a relatively at facial appearance. The angles and lengths in other ethnic groups that have a well-developed SOR may be different from our study's results. By measuring the angles and lengths of several other races and ethnic groups, it will be possible to develop dacryoendoscope probes that are more suitable for the anthropometric structure of the target population.
Based on anatomical measurements in Japanese cadavers, the average length from the lacrimal punctum to the ICP is 11 mm. The average axial length of the LS is 12-15 mm, with the average diameter being 3 mm (the lumen was 1-2 mm). The average length of the NLD is 12-17 mm. 21,24 In our study, the length from the ICP to the LS-NLD transition was 8.9 ± 2.3 mm; the length from the LS-NLD transition to the NLD opening was 13.2 ± 2.7 mm. In fact, as the length from the ICP to the LS-NLD transition refers to the length of the LS body, the actual axial length of the LS can be assumed to be 2-3 mm longer-the length of the fundus of the LS. It was challenging to measure the total length of the LS because the contrast medium had already owed out of the LS at the time of imaging. Thus, the fundus of the LS was often poorly visualized. Therefore, we measured the distance between the ICP and the LS-NLD transition, which was clearly delineated. The distance from the LS-NLD transition to the NLD opening was, in fact, the length of the bony nasolacrimal canal. The interosseous part of the NLD does not have an entirely linear structure but sometimes has a complicated and diverse curvature. Therefore, our measurements do not represent the actual length of the NLD. We acknowledge the necessity of developing a more accurate method for evaluating the length of the NLD.
Several studies have investigated the LS-NLD angle. We found a mean LS-NLD angle of −6.3 ± 14.1°( range, −43° to +40°). The average angle of the anterior bending type (33.3% of cases) was 8. Second, our measurements were obtained from two-dimensional images; in essence, we need to obtain measurements in three dimensions. It has been reported that, in coronal section, the LS is inclined laterally to the midline and the NLD is inclined medially to the LS. Approximately one-third of the NLD is medially inclined, and two-thirds are laterally inclined, relative to the midline. 25 Furthermore, the NLD does have a linear structure but bends in complicated and diverse ways. Although we used planimetric data in sagittal section, the actual lengths of LS and NLD (and their constituent angles) should be represented in three dimensions. In future research to evaluate the parameters of the lacrimal duct, DCG images should be converted into three dimensions.

Patient selection
The subjects of this study were patients diagnosed with unilateral PANDO at Ehime University Hospital from December 2015 to April 2021. Diagnosis was obtained through the irrigation test, dacryoendoscopic examination, and CBCT-DCG. We retrospectively analyzed the CBCT-DCG images of the contralateral side of 102 cases diagnosed with unilateral PANDO. There were no abnormalities on the contralateral side in all the above tests. A typical example of a CBCT-DCG image, sectioning the lacrimal duct, is shown in Fig. 1.
The patient had been diagnosed with left-sided unilateral PANDO. Fig. 1 shows a DCG image of the right side, contralateral to the obstructed side.
In CBCT-DCG images of a sagittal section, the following parameters were evaluated: 1) the angle formed by SOR-ICP-NLD opening ( Fig. 2A); 2) The length of SOR-ICP (Fig. 2B); and 3) The length of ICP-NLD opening (Fig. 2B). To measure these parameters, the following method was applied. A straight line starting from the ICP was drawn in the direction of the SOR; the tangent point on the SOR was determined. The distal end of the interosseous NLD was de ned as the NLD opening. The angle formed by the line connecting the tangent point of SOR-ICP and the line connecting ICP-NLD opening was measured.

Figure 5
Example in which the NLD is inclined anteriorly to the LS (anterior bending type).On the left is the original image. On the right side, the long axis of the NLD was anteriorly inclined by +14° relative to that of the LS. Abbreviations: NLD, nasolacrimal duct; LS, lacrimal sac.

Figure 6
Example in which the NLD is inclined posteriorly to the LS (posterior bending type). On the left is the original image. On the right side, the long axis of the NLD was posteriorly inclined by −21° relative to the LS. Abbreviations: NLD, nasolacrimal duct; LS, lacrimal sac.