Precis: A larger proportion of iStent inject microstents than suggested by gonioscopic examination could be entirely burrowed within the trabeculum (45.7%). The length of device protrusion within the anterior chamber is linked to postoperative intraocular pressure (IOP). Introduction: The iStent inject is a relatively new device designed to be implanted ab-interno through the trabecular meshwork. Although some preliminary studies have shown IOP-lowering potential, the full extent of the mechanisms through which IOP is reduced remains unconfirmed. The aim of this study was to use in vivo optical coherence tomography analysis of the anatomic and physiological effects of iStent inject device positioning on the structures of the iridocorneal angle and IOP. Methods: In total, 25 eyes of 19 subjects (mean age, 75.5 +/- 10.5 y) with mild to moderate open-angle glaucoma were enrolled in this prospective study. All patients underwent implantation of 2 iStent inject devices combined with cataract surgery. In the postoperative period, after IOP stabilized, anterior segment optical coherence tomography (AS-OCT) was performed and several measurements were made of the Schlemm canal (SC) and the devices' positions. Thirteen unoperated fellow eyes served as control eyes. Results: Overall, 92% of devices were visible on AS-OCT versus 88% visible on gonioscopic examination. Of all visible devices, 54.3% had a protruding portion into the anterior chamber with a mean protruding length of 88.5 mu m, and 72% of the devices' heads were not positioned within the SC, with a mean shortest distance of 109.4 mu m. In operated eyes, the average major diameter of SC was 308.7 +/- 197.4 mu m (median, 303.8 mu m) versus 126.9 +/- 60.3 mu m in control eyes. Device protrusion and larger SC diameters were associated with lower postoperative IOP (r(s)=-0.54; P=0.005 and r(s)=-0.43; P=0.04, respectively), whereas the distance between the devices' heads and SC was inversely associated with SC dilatation (r(s)=-0.41; P=0.04). Conclusions: This study highlights the value of AS-OCT in exploring the positioning of trabecular bypass devices such as the iStent inject. Our study shows that more microstents than suggested by gonioscopy could be completely burrowed within the trabeculum. It also suggests that the surgical outcomes of cataract surgery combined with iStent inject implantation may be connected to the amplitude of device protrusion within the anterior chamber as well as the postoperative dimensions of the SC.