This prospective study provided clinical evidence to support modified suture-assisted canaloplasty as an effective and safe alternative to canaloplasty using a flexible microcatheter in Asian POAG patients. This is the first study evaluating the efficacy and safety of modified suture-assisted canaloplasty for the treatment of Asian patients with POAG. Moreover, we demonstrated that the twisted 6/0 suture possessed similar physical and chemical properties to the microcatheter apart from the illuminated tip.

Besides the cost factor, the twisted 6/0 polypropylene suture was an ideal material for SC cannulation in terms of characterization. With the help of material specialists, three candidates for probing were compared. The twisted 6/0 suture was selected as a prober based on the material evidence. The microcatheter costs 170 times more than the polypropylene suture. The sharp margin in the cauterized tip of the 5/0 suture affected the circumferential cannulation. However, twisted 6/0 sutures possessed a smooth loop tip and similar body diameter to iTrack™. It also had a deformable loop tip helping to advance in the SC, which was better than iTrack™ and avoided false pathing[10]. Material characterizations indicate that the polypropylene suture has similar physical and chemical properties to iTrack™, although some differences exist such as without illuminated tip.

Three modifications were made to improve the efficacy of canaloplasty, including SC opening by viscocanalostomy, circumferential probing by a twisted 6/0 suture, and loose suture of the superior scleral flap. Attributed to our experience of viscocanalostomy for over twenty years [7, 14, 16], SC was accurately located during the dissection of the deep scleral flap. Twisted 6/0 suture was selected for its superior physical and chemical properties, which were demonstrated in the previous part. The superficial scleral flap was sutured in a loose way for external outflow of the aqueous humor. Canaloplasty is a new surgery aiming to restore the natural outflow way of aqueous humor by two mechanisms[17]. One is to extend SC and distal outflow by viscoelastic substance. The other is to persistently dilate the SC by placing a double-strand 10–0 polypropylene suture in the canal. Circumferential viscodilation of SC alone is also called canaloplasty or ab-interno canaloplasty[18, 19]. However, persistent suture tension was not included, which could be the key to sustained IOP reduction.

As is well known, the key of the modified procedure was successful circumferential cannulation of SC. In this study, the success rate of circumferential cannulation by twisted 6/0 sutures was 90%. Most of the trials studying canaloplasty were assisted by iTrack™. However, the success rate of 360°intubation varied from one to another. Previous studies by Lewis showed success rates of 360°cannulation were 78.7% [17] and 74% [20]. Hughes reported a probing rate of 80.9% by Visco360 or Omni System [21], Bull reported a successful probing of 89.9% [22], and Brusini reported a probing rate of 90.8% [23]. Furthermore, Xin reported a success rate of 89.2% [24]. Afterward, a few doctors applied sutures in canaloplasty instead of a dedicated microcatheter. Haus initiated the twisted 6/0 suture for circumferential probing and reported a probing rate of 71.6% [10]. Our previous study reported a cannulation rate of 88.7% by 5/0 suture in primary congenital glaucoma. In this study, the probing rate was higher than most of the studies of canaloplasty aided by either the illuminated microcatheter or suture. One possible reason is the surgeon’s experience of viscocanalostomy for over twenty years [14]. Viscocanalostomy helped to open SC accurately when making the deep scleral flap, which guaranteed the success of 360°cannulation. The other reasons are the smooth loop tip and flexibility of the twisted 6/0 polypropylene suture. In our study, circumferential cannulation failed in 4 patients (10%). Failed cannulation owed to adhesion, occlusion, and incorrect passage of probing material in SC [25].

Significant IOP reduction was achieved at each follow-up visit postoperatively. For an IOP ≤ 21, ≤ 18, and ≤ 15 mm Hg and ≥ 20% reduction, our result presented qualified success rates of 97.2%, 86.1%, and 66.7%, and complete success rates of 66.7%, 61.1%, and 52.8% at 12 months respectively. Viscocanalostomy distended only a third to half of SC and distal outflow way of aqueous humor, while canaloplasty could circumferentially expand SC. Thus, canaloplasty could achieve 2 mmHg of IOP reduction compared with viscocanalostomy[4]. Seuthe et al. reported complete success rates of 65.8% and 59.5% with an IOP ≤ 21 mmHg and 18 mmHg [26]. Vastardis reported that complete and qualified success rates were 74.31% and 90% with an IOP ≤ 21 mmHg [27]. Furthermore, Vastardis reported an absolute success rate of 20.19% in advanced POAG [28]. The reason for the low success rate could be the definition of the success rate (5 ≤ IOP ≤ 15 mmHg), which was lower than usual criteria (IOP ≤ 21 mmHg or IOP ≤ 18 mmHg). For IOP ≤ 21 mmHg, ≤ 18 mmHg and ≤ 16 mmHg, Brusini achieved a qualified success rate of 92.1%, 84.3%, and 68.5%, while a complete success rate of 70.8%, 67.4%, and 59.5% at 2 years postoperatively [23]. Our results presented superior IOP-lowering efficacy to most studies. This achievement may be due to combining canaloplasty and viscocanalostomy which supplied more drainage ways through TDM. Besides internal outflow through distended SC, the aqueous humor was drained out in several ways after TDM. Some went to the SC through both of the ostia, some was absorbed by the new aqueous humor veins on the sclera, some went through the uveoscleral outflow, and the remaining traveled by the subconjunctival path through loosely sutured scleral flap [1, 4].

Cox regression analysis showed that age, preoperative IOP, and SE negatively influenced the success rate significantly in this study. Grieshaber et al. reported younger age positively influenced IOP-lowering efficacy while preoperative IOP did not [29]. Thus, early surgical intervention was suggested because of a better prognosis. Hughes reported that high preoperative IOP positively influences the amount of IOP reduction [21], which was contrary to our study. Enrolled patients in Hughes’s study were diagnosed with mild to moderate glaucoma, while most of the patients were advanced glaucoma in our study. We supposed that the natural outflow pathway was more damaged and less likely to be restored in advanced POAG. Also, higher myopia predicted the failure of canaloplasty because myopia is associated significantly with POAG [30].

The number of IOP-lowering medications decreased from 3.2 ± 0.6 preoperatively to 0.5 ± 0.8 postoperatively significantly. The number of medications was similar to or less than previous studies [11, 31]. For high IOP after modified canaloplasty, laser goniopunture was adopted to make one or two tiny holes to increase queous humor from the anterior chamber to the scleral lake under the superficial scleral flap. Kodomskoi et al. applied laser goniopuncture (LGP) in 18% of the patients, and iris incarceration occurred in 4% [11]. LGP was performed in 9.9% of the patients in a study by Brusini et al. In this study, LGP was not adopted to avoid breaking the 10/0 polypropylene suture kept in the SC for dilating the canal.

In terms of complication, hyphaema was the most common and was cleared within 1 week postoperatively. Hyphaema was retrograde bleeding from episcleral venous because of dilated SC. It was deemed a predictor of surgical success because it indicated permeability of the distended trabecular meshwork and distal outflow of aqueous humor [32]. Another complication was transient high IOP. It stemmed from viscoelastic material [25]. In our study, the occurrence of IOP spike was 8.3% which is lower than other studies because viscocanalostomy supplied external outflow of aqueous humor at the early stage after surgery[10, 26, 32,33,34]. Only one patient had mild peripheral anterior synechia to the TDM. Neither Descemet’s membrane detachment nor cyclodialysis occurred in our study [35].

Potential limitations need to be considered in our study. Firstly, blind cannulation aided with the twisted 6/0 polypropylene suture could lead to complications due to a wrong turn. Secondly, the follow-up period was limited. Given the novel nature of this modified surgery, 12-month data was the best we could achievable. The long term follow-up is still on going. Thirdly, the sample size was small with a single-center study which may cause statistical bias. A multi-center study with a large sample size should be planned for rigorous analysis. Ideally, a head-to-head controlled study between suture-assisted canaloplasty and microcatheter-assisted canaloplasty will be needed to demonstrate their efficacy and safety in circumferential catheterization and the management of glaucoma.

In summary, we presented that the twisted 6/0 suture can be an ideal material for SC cannulation. Modified suture-assisted canaloplasty could achieve effective IOP reduction and decreased medication burden with few complications and low cost. Modified suture-assisted canaloplasty seems to be a promising, cost-efficient, and accessible alternative to microcatheter-assisted canaloplasty.

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