The aim of this study was to evaluate the treatment success and safety of AbiC in combination with cataract surgery in phakic glaucoma patients. Furthermore, a significant IOP reduction was achieved at all postoperative follow-ups, the mean IOP reduction was 25.7% after 12 months. Comparable retrospective studies [9, 17, 20] observed an IOP reduction between 16.8 to 41.0% after AbiC. No significant increase in IOP one year after surgery was noted, however the number of anti-glaucomatous drugs did increase significantly between 2 months after surgery and 12 months. Therefore a stable IOP is likely the result of an increase in topical medication.

Interestingly, IOP is lowered significantly regardless of initial IOP. The subjects with an initial IOP ≥ 21 mmHg showed no difference in IOP at 12 months with the patients with an initial IOP < 21 mmHg (14.8 ± 4.4 mmHg vs. 14.2 ± 3.4 mmHg, p > 0.1). The early postoperative IOP 2 months after AbiC was significantly correlated to IOP prior to the surgery. The higher the IOP prior the higher the IOP 2 months after AbiC. The correlation was not significant, when looking only at the patients with an IOP > 21 mmHg prior to the surgery, however the smaller number of patients should be taken into consideration, when interpreting these results.

Some may see AbiC as an evolution of traditional CP. It is less invasive and can achieve the same IOP and glaucoma medication reduction as traditional CP, as shown by Gallardo et al. [21] in their paired eye study. Two further studies were published, presenting the 1-year and 3-year results in a small prospective study. 48 patients with an IOP < 30 mmHg were randomized into ABiC, ABeC (Ab externo Canaloplasty, i.e. traditional canaloplasty) and mini-ABeC (i.e. a modification of traditional canaloplasty) in a 1:1:1 ratio. No significant differences were in-between the 16 patients in each group were recorded, regarding IOP, at 12 months [22] as well as 3 years after surgery [23]. These results are promising, however the small number of patients included in this study should be taken into consideration. Further long-term Studies are needed if AbiC can really be as successful as AbeC over time. Finally. AbiC does not include the placement of a traction suture to leave the SC stretched long-term after surgery. The suture traction does not seem to have a significant effect on IOP during the first 12 months. Furthermore the suture can be used to perform a 360° trabeculotomy after failed canaloplasty, with promising results [24].

A significant decrease in topical medication was achieved after 2, 6 and 12 months. In our study the patients hat to take > 80% less medication at 2 months after surgery and the medication burden was reduced by more than 50% 12 months after AbiC. The increase in topical medication was significant between 2 and 12 months (Please Refer to Fig. 2 for visualization). This is in line with the findings of Ondrejka et al. [9] and Körber [17], which reported a significant decrease in topical medication after surgery. In contrast, Davids et al. [20] did not observe a difference of local topical glaucoma medication 12 months after surgery.

The complete success rate (without local therapy) was 31.6%, whereas the qualified success rate (with local therapy) was three times as high. Therefore, AbiC was able to successfully lower and control IOP, whereas most patients might still need topical medication to achieve target IOP.

All AbiCs conducted in this study, were performed in combination with cataract surgery. Cataract extraction as a standalone procedure can lead to an IOP reduction of 14% at 12 months after baseline [25].

Initially this might imply, that a combination with cataract surgery distorts the success rates. However, this seems rather unlikely, since in Ondrejka's study [9] AbiC as a standalone procedure resulted in higher IOP reduction than in combination with cataract surgery. In the study by Gallardo et al. [26] the IOP reduction rate after AbiC was approximately the same with and without cataract surgery. Further studies are needed to verify if AbiC should be performed as a standalone procedure rather than adjunct to a cataract surgery. Both approaches have their theoretical and practical merits.

Regarding the safety of AbiC, there has been no severe or long-lasting complications neither in our study nor in several other studies [8, 9, 17, 20, 26, 27]. The absence of serious complications is one of the key advantages of AbiC over traditional glaucoma surgery.

Gallardo et al. [26] points out that the minimally invasive approach without scleral and conjunctival manipulation is also a crucial advantage, since it is naive for future incisional glaucoma procedures if needed. Interestingly, the authors found comparable results concerning the IOP-lowering effect in ex-externo canalplasty compared to AbiC in a paired-eye comparison study after 1-year follow-up. Furthermore, in contrast to bleb-dependent filtration surgery such as trabeculectomy, the postsurgical nursing is less complex [8]. Zhang et al. mentions as a possible disadvantage that AbiC might not be applicable on advanced glaucoma due to its limited IOP reduction. Moreover, an extended learning curve and pricey surgical equipment is disadvantageous. Nevertheless, no permanent implant is inserted and unlike other MIGS several anatomic and physiological levels are targeted to increase aqueous humor flow [8, 18].

Of the initial 43 eyes 7 were lost to the final follow up. Therefore the number of included eyes was reduced to 36, which is substantial but still a relatively limited number. Furthermore only Caucasian patients happen to be included in our study, the treatment results might not be applicable to different ethnicities. We believe that the study represents a meaningful and valid contribution to the existing rather sparse literature regarding long-term results after AbiC. Furthermore the lack of a control group (patients with similar glaucoma stages just receiving cataract surgery) is a major limit factor, as we are thereby not able to quantify the sole effect of AbiC in our setting.

In conclusion, AbiC in combination with cataract surgery seems to be a safe as well as effective microinvasive surgical method for mild to moderate glaucoma. By catheterisation and viscodilatation of the Schlemm's canal using VISCO 360, a successful and lasting IOP reduction and decrease of the need for local glaucoma medication for 12 months could be achieved. However, only one third of the subjects did not need any local medication after 12 months, Gallardo et al. [26] described a medication-free rate of 40% after AbiC. Therefore a complete medication free controlled IOP might not be achievable, however these techniques are helpful in bridging the time until traditional filtering glaucoma surgeries might be necessary. Further studies need to show, if an additional trabeculotomy combined with the AbiC procedure (OMNI System, Sight Sciences, Inc. Menlo Park, CA, USA) might lead to higher success rates.

Relevant intra- or postoperative complications have not occurred in our study, highlighting the safety of this novel glaucoma treatment modality.