In our study, we examined exposure rates and risk factors for exposure after BGI implantation. The rate of exposure for BGI implanted at Hiroshima University Hospital between April 1, 2012 and October 31, 2016 was 15.0%. This rate is higher than the 5–6% rate that has been reported previous studies [8, 19, 20]. We consider that the causes of the high exposure rates in our study are related to following three things.
First, we consider that racial differences may have affected. Whereas previous reports have mainly focused on Causasians, our reports have focused on Asians. Asians have more friction of the GDD against the ocular tissues than do Caucasians. Friction between the ocular tissue and the GDD may increase the risk for GDD exposure. The average palpebral-fissure width among Asians is narrower than that in Caucasians [23]. Muir et al. also pointed out small orbits as the reason for the high exposure risk in children and women [19]. We speculate that Asians with tight orbits may have more friction with the implant than Caucasians, leading to increased exposure. Moreover, the prevalence of dry eye disease is higher in Asians than in Caucasians, which may increase the friction on the ocular surface and lead to GDD exposure [24].
Second, most of the patients in our study had had prior ocular surgeries before GDD implantation. In some studies, prior surgeries have been reported to be a risk factor for exposure [16, 21]. In Byun et al.'s study, the majority of patients had no history of prior ocular surgeries (153 out of 256, 57.5%) [16]. In Levinson et al.'s study, 18.2% of included cases had not had previous ocular surgery [20]. However, in our study, only 6.1% of patients had not.
Third, the use of different patch-graft materials to cover the tubes may have affected the rate of implant exposure. In our facility, due to the limited number of human donor scleral grafts, we covered the tubes with a self-scleral flap in most cases. Human donor scleral grafts were used only in the eight most-severe cases, in which the conjunctival sac had been shortened. However, previous studies about implant exposure in Westerners have mostly used donor sclera, cornea or pericardium [19, 20]. A previous report showed that the tube-exposure rate was 30% when the surgeons initially covered the tube with a self-scleral graft, but that the exposure rate was lowered to less than 5% with the advent of donor or autologous tissue for covering the tube [25]. The BGI exposure rates in Asians only reported from Tojo et al [26]. They counted only tube exposure in their report, and the tube exposure rate was 3.8%. In their study, they used human donor scleral grafts in all cases, and nearly half of the cases were robustly covered with donor scleral graft and self-scleral flap. Perhaps, Asians’ eyes need to be covered with more robust patch graft to prevent exposure than those of Caucasians.
We observed that use of the BG 102–350 in diabetic patients was associated with 15-times greater odds of BGI exposure, compared with the use of BG 101–350 and BG 103–250. Of the four cases in which the BG 102–350 was exposed, the Hoffman elbow exposure occurred in three cases, except for one case in which plate was exposed after pars plana vitrectomy for diabetic retinopathy. Unlike the BG 101–350 and BG 103–250, the BG 102–350 has a Hoffman elbow, which is a 5.3-mm2 plastic plate. The Hoffman elbow is larger and harder than the tube. The exposure of a Hoffman elbow always started from the tip of the wing. Because a Hoffman elbow moves up and down, with the scleral insertion part acting as a fulcrum, the force to lift the tip of the Hoffman elbow's wing might easily be applied by eye movements and blinking. It is conceivable that friction between the Hoffman elbow and ocular tissue, including the conjunctiva, is likely to occur. Moreover, it is well known that diabetes delays wound healing [27]. Owen et al. reported that there is poor conjunctival perfusion in diabetic patients and that it may result in poor tissue strength [28]. Because diabetic patients may have weaker tissue strength due to poorer conjunctival blood flow, in diabetic patients implant exposure might occur more easily with the BG 102–350 with its Hoffman elbow.
We consider that implantation of BG101-350 or BG103-250 would be a better way to prevent exposure in diabetic patients. When anterior type of implant such as BG101-350 or BG 103–250 is inserted into the posterior chamber or vitreous cavity, the tube may touch the lens or the backside of the iris [29]. Depending on the patient's condition, if BG 102–350 implantation is required, it will need to be covered with a more robust patch graft.
Our study has some limitations. First, it is limited by the retrospective analysis. Because the information was not uniformly recorded for each patient, there were several factors including the history of ocular surface disease and systemic disease other than diabetes that we could not examine, which might have had an association with implant exposure. Second, it included a small sample size, particularly in terms of examining the risk factors for implant exposure. The small sample size may have made it difficult to detect significant differences as a risk factor for exposure. We found that BG 102–350 had a higher risk of exposure than other BGI types in patients with diabetes. However, it is unclear that BG 102–350 is also risk of exposure in non-diabetes patients. In our study, we were not able to analyze the relationship between BG102-350 and exposure non-diabetic patients since BG102-350 was implanted in only a few non-diabetic patients. Therefore, we believe that further investigation is needed with larger sample sizes.