The lengths of the LS and the NLD and the inclination of the LS–NLD vary among individuals and between ethnic groups.19–23 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 confirmed 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 flat 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 flowed 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 8.8 ± 14.1°; that of the posterior bending type (66.7% of cases) was −13.8 ± 13.3°. In an anatomical survey of Japanese cadavers, Narioka et al. reported that the anterior bending type accounted for 80% of cases, with an average angle of +8.9 ± 5.0° (range, 0°–19°), and the posteriorly-bending type accounted for 20% of cases, with an average angle of −12.3 ± 9.0° (range, −2° to −26°).25 By contrast, Park et al. reported that the mean LS–NLD angle was −10.3° and that about 90% of cases were posterior bending.26 Our data was provided by a larger number of cases and was normally distributed. For more accurate values, it is necessary to increase the number of measurements and to accumulate data on the LS–NLD angle and the frequencies of anterior and posterior bending variations.
There are several limitations to this study. First, we did not evaluate normal lacrimal duct morphology; we evaluated the contralateral lacrimal duct of patients diagnosed with unilateral PANDO. We cannot exclude the possibility that a unilateral PANDO case in this study might develop into a bilateral PANDO case. 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.