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Obstruction of the Lacrimal System: Treatment with a Covered, Retrievable, Expandable Nitinol Stent versus a Lacrimal Polyurethane Stent¹

¹ From the Departments of Radiology (G.Y.K., H.Y.S., T.S.S., K.B.S., H.K.Y.) and Biomedical Engineering (T.H.K.), Asan Medical Center, University of Ulsan College of Medicine, 388-1 Poongnap-Dong, Songpa-Ku, Seoul 138-736, Korea. Received October 12, 2001; revision requested January 8, 2002; final revision received July 1; accepted August 19. Supported by grant HMP-98-G-2-043 from the Highly Advanced National (HAN) Project, Ministry of Health and Welfare, Republic of Korea. Address correspondence to H.Y.S. (e-mail: hysong@amc.seoul.kr).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To compare the clinical effectiveness of a covered nitinol stent with that of a polyurethane stent for treatment of lacrimal system obstructions.

MATERIALS AND METHODS: A nitinol stent was knit from a single thread of 0.1-mm nitinol wire in a tubular configuration and was covered by dipping the stent into a polyurethane solution. The stent was 4 mm in diameter and 30 or 35 mm long. With fluoroscopic guidance, a covered nitinol stent (n = 33, group A) or a polyurethane stent (n = 35, group B) was placed in 68 patients. The following items were evaluated retrospectively: technical success, procedure time, cumulative patency rate, and complications. An unpaired Student t test was used to analyze the difference between the procedure times. Kaplan-Meier survival curves and a log-rank test were used to compare the cumulative patency rates.

RESULTS: Stent placement was technically successful in 31 (94%) of 33 patients in group A and in all 35 (100%) patients in group B. After stent placement, all patients showed resolution of epiphora. Average procedure time was 400 seconds (range, 270–900 seconds) in group A and 260 seconds (range, 150–900 seconds) in group B. The difference between the procedure times was statistically significant (P = .0003). During the mean follow-up period of 40 months, there was recurrence of epiphora in 30 of 31 patients in group A and 26 of 35 patients in group B. The difference of the cumulative patency rates was statistically insignificant (P = .2).

CONCLUSION: Although the polyurethane stent used for treatment seemed to be more effective than the nitinol stent, selection of these stents for placement should be made with caution, because the long-term patency rates are not encouraging.

© RSNA, 2003

Index terms: Lacrimal gland and duct, 223.24 • Lacrimal gland and duct, interventional procedures, 223.126 • Stents and prostheses

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Epiphora, the imperfect drainage of tears through the lacrimal passages so that they fall over the margin of the lid onto the cheek, is a common ophthalmic problem, composing 3% of ophthalmologic clinic visits (1,2). Fluoroscopic placement of a polyurethane stent is a relatively new method for treating epiphora. Published data suggest that the procedure is a safe, easy, and effective way to treat epiphora (3–5). In addition, the procedure is believed to cause less bleeding than surgery, and it leaves no facial scar. Despite the encouraging results of fluoroscopic placement of a lacrimal polyurethane stent, the procedure has been limited by obstruction of the stent, either by mucoid material or by granulation tissue (6,7).

To overcome the limitations of the lacrimal polyurethane stent, a covered, retrievable, expandable nitinol stent was devised. The purpose of this study was to investigate the clinical effectiveness of a covered nitinol stent and to compare it with a polyurethane stent for treatment of lacrimal system obstructions.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Stent Construction
The covered, retrievable, expandable nitinol stent was constructed in our research laboratory. The stent was knit from a single thread of 0.1-mm nitinol wire in a tubular configuration in an interlocking, diamond-shaped pattern (Fig 1). The stent was 4 mm in diameter and 30 or 35 mm long when fully expanded. To prevent tissue from growing into the stent lumen, the stent was covered by being dipped in a 12% polyurethane solution (Biospan; Polymer Technology Group, Emeryville, Calif). To provide a firmer covering for both ends of the stent, these areas were covered with 100% nylon mesh and then coated by dipping them into the polyurethane solution. Two gold rings were attached at both ends of the stent so that it could be easily detected with fluoroscopy.

View larger version (197K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Covered, retrievable, expandable nitinol stent (left) and lacrimal polyurethane stent (right).

The clinical characteristics of groups A and B are shown in Table 1. In group A, the obstruction was at the junction between the lacrimal sac and the nasolacrimal duct in 28 lacrimal systems and at the lacrimal sac in the remaining five systems. Seven of the 33 systems also had stricture at the common canaliculus. The causes of obstruction were idiopathic in 30 systems and traumatic in three. In group B, the obstruction was at the junction between the lacrimal sac and the nasolacrimal duct in 30 systems and at the lacrimal sac in the remaining five systems. Four systems also had stricture at the common canaliculus. The causes of obstruction were idiopathic in 33 systems and congenital in two.

In group A, the stent was placed by using a 6.3-F lacrimal stent introducer set (Song Nasolacrimal Duct Stent Set; Cook, Queensland, Australia). The introducer set consisted of a dilator, sheath, stent loader, and pusher catheter. A sheath with a dilator from the set was passed retrograde over the guide wire and advanced across the obstruction until the proximal tip of the dilator was inside the dilated lacrimal sac. To place the tip of the sheath more properly in the lacrimal sac, the sheath was advanced approximately 3 mm into the lacrimal sac while the dilator was withdrawn from the sheath. After the dilator was removed from the sheath, a stent was introduced over the guide wire into the sheath with the aid of a stent loader and advanced by means of a pusher catheter until the radiopaque tip of the stent was located at the sheath tip. The pusher catheter was then held in place while the sheath was withdrawn. This freed the stent and allowed it to expand and to rest within the lacrimal system. After the stent was released from the sheath, the sheath containing a pusher catheter was pulled out through the inferior nasal meatus and the guide wire through the superior punctum. Dacryocystography was performed immediately after the procedure to verify the position and patency of the stent. Subsequently, irrigation of the stent was performed by passing saline solution through the needle. In group B, placement of the lacrimal polyurethane stent (Song Nasolacrimal Duct Stent; Cook) was performed in the same way as described previously (8).

Patients were given oral prophylactic ampicillin for 24 hours before the procedure. Ampicillin and topical antibiotic-steroid eye drops were continued for 7 days after the procedure.

We removed the stent from a patient if a stent obstruction occurred. The stent was removed through the nose with use of a stent retrieval hook (Cook) with fluoroscopic guidance or with use of a hemostat with nasal endoscopic guidance. Immediately after removal of the stent, dacryocystography was performed through the inferior lacrimal punctum with a 24-gauge needle (Cook) to verify the patency of the lacrimal system. The lacrimal system was then irrigated with saline solution through the inferior punctum to wash out possible blood clots or mucoid material.

Follow-up
Clinical examinations and dacryocystography were performed at 1 week, 6 months, 1 year, and 2 years after the stent placement to assess the stent patency or migration. The patients were also advised to visit the outpatient clinic if they had recurrence of symptoms. In patients who underwent stent removal because of occlusion, clinical examinations and dacryocystography were performed 3 months following stent removal. Thereafter, all patients were contacted by telephone every 6 months to obtain information concerning epiphora.

The following items were evaluated retrospectively in each group: technical success and limitations, required time for stent placement, cumulative patency rate, and complications. In patients with recurrence, we also evaluated the causes of recurrence on dacryocystographic findings, ease of stent removal, stent-occluding materials, and clinical effectiveness after stent removal.

Initial patency of the lacrimal system was defined as the flow of contrast medium through the stents into the inferior meatus of the nasal cavity as seen on dacryocystograms obtained immediately after stent placement. Maintained patency of the lacrimal system was defined as improvement in epiphora and patency of the lacrimal system on follow-up dacryocystograms. To assess clinical improvement, epiphora was graded in six levels from grade 0 (no epiphora) to grade 5 (constant tearing) according to the scale of Munk et al (9).

Statistical Analysis
The Student t test was used to analyze the differences in age, symptom duration, and average time required for stent placement between group A and group B. The ² test was used to analyze the differences in sex, causes of obstruction, obstruction level, and past treatment between the two groups. Kaplan-Meier survival curves and a log-rank test were used to compare the cumulative patency rate between the two groups. Multivariate analysis (variables: stent type, age, sex, causes of obstruction, obstruction level, symptom duration, and past treatment) was used to analyze prognostic factors for the patency period. Analysis was conducted with use of SPSS software (version 10.0.7; SPSS, Chicago, Ill), and P values less than .05 were considered to indicate a significant difference.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Stent placement was technically successful and was well tolerated in 31 (94%) of the 33 patients in group A and in all 35 (100%) of the patients in group B (Figs 2, 3). After successful stent placement, all patients showed complete or partial resolution of epiphora. However, insertion of the polyurethane stent in group B was more rapid with fewer failures than insertion of the nitinol stent in group A. Stent placement failed in two patients in group A because of acute angulation of the sheath at the nostril during retrograde insertion of a covered, retrievable, expandable nitinol stent. These patients underwent successful placement of a polyurethane stent immediately after failure of the nitinol stent placement. The average time required for the procedure from the beginning of the local anesthesia of the infratrochlear nerve to completion of the postprocedural dacryocystography after stent placement was 400 seconds (range, 270–900 seconds) in group A and 260 seconds (range, 150–900 seconds) in group B. There was a statistically significant difference in the procedure time between group A and group B (P = .0003). No major complications occurred in either patient group during stent placement.

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 2a. Covered, retrievable, expandable nitinol stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Lateral fluoroscopic image obtained during retrograde stent insertion. Note the compressed stent (arrowheads) in a sheath and four radiopaque markers (two at each end [arrows] of the stent). (c) Posteroanterior fluoroscopic image obtained after sheath removal shows the stent (arrowheads) in the tract from the top of the lacrimal sac to the nasal floor. Note radiopaque markers (arrows) at ends of the stent. (d) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after placement. (e) Six months after stent placement, posteroanterior subtraction dacryocystogram shows complete obstruction of the stent with a large filling defect (arrowheads) in the lacrimal sac.

View larger version (125K): [in this window] [in a new window] [Download PPT slide]   Figure 2b. Covered, retrievable, expandable nitinol stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Lateral fluoroscopic image obtained during retrograde stent insertion. Note the compressed stent (arrowheads) in a sheath and four radiopaque markers (two at each end [arrows] of the stent). (c) Posteroanterior fluoroscopic image obtained after sheath removal shows the stent (arrowheads) in the tract from the top of the lacrimal sac to the nasal floor. Note radiopaque markers (arrows) at ends of the stent. (d) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after placement. (e) Six months after stent placement, posteroanterior subtraction dacryocystogram shows complete obstruction of the stent with a large filling defect (arrowheads) in the lacrimal sac.

View larger version (138K): [in this window] [in a new window] [Download PPT slide]   Figure 2c. Covered, retrievable, expandable nitinol stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Lateral fluoroscopic image obtained during retrograde stent insertion. Note the compressed stent (arrowheads) in a sheath and four radiopaque markers (two at each end [arrows] of the stent). (c) Posteroanterior fluoroscopic image obtained after sheath removal shows the stent (arrowheads) in the tract from the top of the lacrimal sac to the nasal floor. Note radiopaque markers (arrows) at ends of the stent. (d) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after placement. (e) Six months after stent placement, posteroanterior subtraction dacryocystogram shows complete obstruction of the stent with a large filling defect (arrowheads) in the lacrimal sac.

View larger version (99K): [in this window] [in a new window] [Download PPT slide]   Figure 2d. Covered, retrievable, expandable nitinol stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Lateral fluoroscopic image obtained during retrograde stent insertion. Note the compressed stent (arrowheads) in a sheath and four radiopaque markers (two at each end [arrows] of the stent). (c) Posteroanterior fluoroscopic image obtained after sheath removal shows the stent (arrowheads) in the tract from the top of the lacrimal sac to the nasal floor. Note radiopaque markers (arrows) at ends of the stent. (d) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after placement. (e) Six months after stent placement, posteroanterior subtraction dacryocystogram shows complete obstruction of the stent with a large filling defect (arrowheads) in the lacrimal sac.

View larger version (82K): [in this window] [in a new window] [Download PPT slide]   Figure 2e. Covered, retrievable, expandable nitinol stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Lateral fluoroscopic image obtained during retrograde stent insertion. Note the compressed stent (arrowheads) in a sheath and four radiopaque markers (two at each end [arrows] of the stent). (c) Posteroanterior fluoroscopic image obtained after sheath removal shows the stent (arrowheads) in the tract from the top of the lacrimal sac to the nasal floor. Note radiopaque markers (arrows) at ends of the stent. (d) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after placement. (e) Six months after stent placement, posteroanterior subtraction dacryocystogram shows complete obstruction of the stent with a large filling defect (arrowheads) in the lacrimal sac.

View larger version (117K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. Polyurethane stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction of the lacrimal system at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after stent placement. (c) Eighteen months after stent placement, posteroanterior subtraction dacryocystogram shows near complete obstruction of the stent with a filling defect (arrowheads) at the "mushroom" portion of the stent.

View larger version (114K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. Polyurethane stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction of the lacrimal system at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after stent placement. (c) Eighteen months after stent placement, posteroanterior subtraction dacryocystogram shows near complete obstruction of the stent with a filling defect (arrowheads) at the "mushroom" portion of the stent.

View larger version (123K): [in this window] [in a new window] [Download PPT slide]   Figure 3c. Polyurethane stent placement. (a) Posteroanterior subtraction dacryocystogram shows complete obstruction of the lacrimal system at the junction (arrow) between the lacrimal sac and the nasolacrimal duct. (b) Posteroanterior subtraction dacryocystogram shows good flow of contrast medium through the stent immediately after stent placement. (c) Eighteen months after stent placement, posteroanterior subtraction dacryocystogram shows near complete obstruction of the stent with a filling defect (arrowheads) at the "mushroom" portion of the stent.

View larger version (21K): [in this window] [in a new window] [Download PPT slide]   Figure 4. Graph of overall cumulative patency after stent placement. The curves show the difference in cumulative patency rates between the covered, retrievable, expandable nitinol stent and the lacrimal polyurethane stent.

In group B, there was recurrence of epiphora in 26 (74%) of the 35 patients during the 40-month mean follow-up period (range, 35–48 months) following stent placement. In these patients, occlusion occurred in the lacrimal sac in 18 patients and in the common canaliculus in eight patients (Fig 3). The stent was reopened in two patients with use of forceful irrigation with saline solution through the inferior punctum. Stents were removed from the remaining 24 patients with recurrence by using a stent retrieval hook 1–42 months (mean, 18 months) after placement. Stents were occluded by mucopurulent material in four patients and by granulation tissue in the other 20. During the 21-month mean follow-up period (range, 7–33 months) following stent removal, six (25%) of 24 patients did not require further treatment because their obstructions resolved. The remaining 18 patients underwent either placement of a second stent or a dacryocystorhinostomy.

Stent removal was successful and well tolerated in all but one patient in group A. In this patient, the distal tip of the stent was successfully grasped with a retrieval hook, but the stent could not be pulled out of the nose because the grasped stent became disengaged from the retrieval hook. The stent was removed by an ophthalmologist during a dacryocystorhinostomy, and granulation tissue in the proximal end of the stent was revealed. The majority of these patients expressed feeling mild pain during the stent removal and had slightly blood-stained nasal discharge, which stopped spontaneously 1–12 hours after stent removal.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Recently, two kinds of interventional procedures, balloon dilation and expandable metallic stent or plastic stent placement, have been advocated for the treatment of epiphora (3–6,8–14). These procedures offer advantages over invasive surgical procedures, including the following: general anesthesia is not needed, the procedures are simple and safe, no facial scar is produced, bleeding is less of a problem, substantial anatomic alteration is minimized, and the procedures are more easily tolerated by patients. Technical and clinical success rates of balloon dilation between 56% and 95% have been described in the literature (9,11–18); however, long-term effectiveness of balloon dacryocystoplasty is not encouraging in cases of complete obstruction of the lacrimal system (11,12,15–21).

To overcome the limitations of balloon dacryocystoplasty, a lacrimal polyurethane stent and an uncovered, expandable, metallic stent were designed (6,8,10,22,23). In a series of 283 systems in 236 patients with complete obstruction of the lacrimal system, 262 (97%) of 270 systems in which the polyurethane stent was successfully placed showed complete or partial resolution of epiphora at the time of 7-day follow-up dacryocystography (6). Of the 262 systems, 181 systems (69%) maintained patency of the lacrimal system and improvement of epiphora during a mean follow-up period of 95 weeks. Pabon et al (23) placed the polyurethane stent in 70 obstructed lacrimal systems in 62 patients and reported an 87% initial technical success rate and a 73% 1-year patency rate. When Ilgit et al (10) and Song et al (22) placed an uncovered, expandable metallic stent in patients with severe epiphora after failed balloon dacryocystoplasty, they obtained favorable short-term results. Despite the encouraging results of fluoroscopic placement of the lacrimal polyurethane stent and the uncovered, expandable metallic stent, a limitation of the procedure has been stent obstruction by either mucoid material or granulation tissue (6,10,22). In addition, an uncovered, expandable metallic stent lacks longitudinal flexibility, and it cannot be removed nonsurgically.

To overcome the limitations of a lacrimal polyurethane stent and an uncovered, expandable metallic stent, we devised a covered, retrievable, expandable nitinol stent. The lacrimal polyurethane stent was 2 mm in outer diameter and 1.5 mm in inner diameter. The covered, retrievable, expandable nitinol stent we used was 4 mm in outer diameter, and the internal diameter was almost the same as that of the external diameter, because the thickness of the covering membrane was negligible. Singh and Garg (24) used a lacrimal polyethylene tube of different outer diameters. They emphasized that a wider tube has several advantages over a narrow one, including a firm fit in the nasolacrimal duct, less chance for blockage by blood or exudate, less chance of a kink blocking the tube, and greater drainage capacity. In our series, all patients with a covered, retrievable, expandable nitinol stent showed resolution of epiphora. No filling defects, such as impacted mucus or blood clots, were seen on the follow-up dacryocystograms obtained 1 week after stent placement. In 13 of the 31 patients with the covered, retrievable, expandable nitinol stent, a polyurethane stent was placed after removal of the nitinol stent due to recurrence of epiphora. In these patients, there was no outstanding difference in the tear drainage; in two patients however, filling defects in the lacrimal sac were seen on the follow-up dacryocystograms obtained 1 week after stent placement. In addition, in two patients from group B, some filling defects in the lacrimal sac were seen on the follow-up dacryocystograms obtained 1 week after stent placement. Thus, we assume that the covered, retrievable, expandable nitinol stent is superior to the polyurethane stent for tear drainage and for drainage of impacted mucus, dacryoliths, or blood clots. Regarding the diameter of the lacrimal stent, lacrimal polyurethane stents have limitations because they cannot be compressed, so they should be introduced with a large introducing system. On the other hand, the covered, retrievable, expandable nitinol stents can be compressed and introduced with a much lower profile introducing system; however, the polyurethane stent also has some advantages such as rapid placement of the stent and a long-term patency rate that is superior to that of the nitinol stent.

We anticipated a more favorable long-term patency rate for the patients in group A than for those in group B because a nitinol stent has a wider inner diameter than a polyurethane stent. The patency rate was higher in group A than in group B until approximately 1 year after stent placement; however, the long-term patency rate after 1 year was lower in group A than in group B. In both groups, most patients with recurrence of epiphora revealed complete obstruction at the lacrimal sac with a filling defect. In addition, removed stents revealed that in most cases the filling defect consisted of granulation tissue. Thus, we assume that an expandable nitinol stent induces granulation tissue formation more chronically and actively than does a polyurethane stent because the nitinol stent can constantly irritate the endothelium of the lacrimal sac due to its expansile force and large diameter.

In our study, the difference of the cumulative patency rates in group A and group B was statistically insignificant; however, in the multivariate analysis to analyze prognostic factors for patency period, three variables (stent type, sex, and obstruction level) were significant for patency period, and the most significant variable was stent type. Thus, the covered, retrievable, expandable nitinol stent warrants further investigation in the treatment of epiphora.

Removal of an occluded covered, retrievable, expandable nitinol stent or of an occluded lacrimal polyurethane stent was relatively easy and safe in most patients. Song et al (7) reported that the patency rate after a lacrimal polyurethane stent removal was 33% during the mean 22-month follow-up period. Although six (25%) of our 24 patients in group B experienced improved epiphora after stent removal, only two (8%) of 25 patients in group A did not require further treatment after stent removal due to epiphora improvement. Some investigators have reported that newly developed granulation tissue in the esophagus and trachea appearing after stent placement can disappear after stent removal (25,26). Although granulation tissue that develops after stent placement can disappear or diminish after stent removal, the size of the filling defects was larger in group A than in group B because of the larger internal diameter of the stent. This discrepancy may contribute to the different patency rates of the lacrimal system following stent removal. Thus, we surmise that the placed stent should be removed after an appropriate period of time for treating the obstruction, even though the appropriate period is not optimized. Janssen et al (27) reported favorable results after temporary placement of polyurethane stents in patients with a chronic or subacute lacrimal abscess who had not responded to conventional antibiotic therapy. In addition, periodic lavage of a placed stent may improve patency of the stent. Lee et al (28) reported excellent long-term clinical results of 8% occlusion after a mean follow-up of 22 months with lavage at 1, 3, and 6 months and thereafter every 6 months after placement.

In summary, we achieved excellent initial clinical results with placement of a covered, retrievable, expandable nitinol stent and a lacrimal polyurethane stent in patients with complete obstruction of the lacrimal system. In addition, these stents can be removed with a retrieval hook without major complications in most cases if there is recurrence of epiphora. Although the polyurethane stent used for treatment seemed to be more effective than the nitinol stent, selection of these stents should be made with caution as a first-line therapeutic option, because the long-term patency rates are not encouraging.

	FOOTNOTES

 
Author contributions: Guarantors of integrity of entire study, G.Y.K., H.Y.S.; study concepts, H.Y.S.; study design, G.Y.K., H.Y.S.; literature research, G.Y.K., T.S.S.; clinical studies, G.Y.K., H.Y.S., T.S.S.; data acquisition, G.Y.K., T.H.K.; data analysis/interpretation, G.Y.K., H.Y.S.; statistical analysis, G.Y.K., H.K.Y.; manuscript preparation, T.H.K., G.Y.K.; manuscript definition of intellectual content, G.Y.K.; manuscript editing, K.B.S., H.Y.S.; manuscript revision/review, H.K.Y.; manuscript final version approval, H.Y.S., G.Y.K.

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