AUG 22, 2012
This study in the February issue of Cornea evaluated the use of optical coherence tomography (OCT) for the measurement of donor corneas for endothelial keratoplasty. While the use of ultrasound pachymetry (USP), the standard method for these measurements, requires contact with the tissue and thus may pose contamination risk, OCT is noncontact.
They found that donor corneal measurements by OCT were not as predictable as those by USP. Since predictability by OCT was better for corneas thinner than 600 µm compared with thicker ones, the authors suggest that the predictability of graft thickness could be optimized by using OCT to select corneas thinner than 600 µm and then using immediate precut USP to set the microkeratome depth.
The study included 154 donor corneas. They were imaged by OCT while immersed in preservation medium, with central corneal thickness (CCT) measured from the Bowman layer to the endothelium. The corneas were then mounted on an artificial anterior chamber and the epithelium removed. USP was used to measure CCT just before sectioning with a microkeratome, and central graft thickness (CGT) was measured by USP. The graft was then returned to the medium and imaged by OCT.
The average CCT measured by OCT (550 ± 63 mm) was thicker (P < 0.001) than that measured by USP (507 ± 54 mm). Similarly, CGT measured by OCT (158 ± 41 mm) was thicker (P = 0.0005) than that measured by USP (153 ± 38 mm). The predictability of cut depth, as assessed by pooled standard deviation (SD), was better (P = 0.023) for USP (41 µm) compared with OCT (48 µm). The graft was thicker (P < 0.001) peripherally than centrally in OCT images. The predictability of cut depth by OCT was better (P < 0.001) for corneas thinner than 600 µm (SD = 45.6 µm) compared with those thicker than 600 µm (SD = 86.9 µm).
The authors say it may be helpful to use OCT to screen out very thick corneas while they are still in vial before precutting them for endothelial keratoplasty. OCT also may be useful for imaging grafts to provide surgeons with an assessment of their average thickness and uniformity.
The authors note that OCT measurements were taken when the donor corneas were in a relaxed state in Optisol-filled vials. In Optisol, donor corneas swell with increased storage time because of the gradual decrease in the endothelial pump function. These swollen corneas could undergo the greatest deturgescence while mounted on the artificial anterior chamber. The authors say that an OCT system designed to scan corneas that have been conveniently mounted on the artificial anterior chamber might be a good option in the future.