Spectral domain OCT used for assessment of corneal allogenic intrastromal ring segments
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Keratoconus is a corneal ectatic disease that affects young individuals with significant visual symptoms.
There have been various treatment options in the last 2 decades, including contact lenses, corneal cross-linking and intrastromal corneal ring segments. In 2017, Jacob and colleagues introduced the first corneal allogenic intrastromal ring segments (CAIRS) as an alternative treatment option. In this column, we present the optical coherence characteristics of CAIRS using spectral domain OCT.
Source: Dhivya Ashok Kumar, MD, FRCS, FICO, FAICO, Soosan Jacob, MS, FRCS, DNB, and Amar Agarwal, MS, FRCS, FRCOphth
CAIRS technique
The donor cornea was mounted on a Barron artificial anterior chamber, and the donor epithelium was completely removed from limbus to limbus. The donor cornea was then dismounted and placed upside down in a Lieberman Teflon block to strip the donor Descemet’s membrane. A specially designed double-bladed circular trephine (Jacob CAIRS trephine, Madhu Instruments) with an inner blade diameter of 6.5 mm and an outer blade diameter of 8 mm was centered on the donor corneal button and used to punch the tissue. Two concentric cuts were thus simultaneously created on the rim to yield a CAIRS ring within the two blades.
The Bowman’s layer side of the punched-out donor corneal stroma was marked while still on the trephine. The trephined ring was then extracted from the trephine and placed over the Teflon block, followed by cutting it at one site to yield a long segment. This segment was flattened out using the blunt sides of non-toothed forceps. The linear segment was then further cut into two segments with a 15° blade. One of the cut segments was carefully placed on the 4.6-mm optic zone on the Jacob CAIRS customizer (Epsilon Instruments), and a fine-tipped marker was used to make the required markings for planned arc length as per the preoperative topography plan of the patient. The segment was then cut along the inked marks, and the final CAIRS segment was ready for insertion. Central marking of the host cornea was done on the corneal light reflex, visualized coaxially. Then the Jacob clock gauge marker (Epsilon Instruments) was placed over the host cornea, and using a Jacob single-blade radial marker (Epsilon Instruments), segment insertion was done.
The VisuMax 500 kHz femtosecond laser platform (Zeiss) was used to create a circular channel in the host cornea. The parameters used were inner diameter of 4.8 mm and outer diameter of 7.2 mm at 50% depth of the minimum stromal thickness in the 5-mm optical zone, with VisuMax laser default energy setting of 300 nJ for intracorneal ring incision treatment method. Two entry incisions into the channel were created 180° opposite each other to allow optimal placement of the CAIRS. The segment was pushed through using a Jacob curved Y-rod (Epsilon Instruments) into the channel and then carefully pulled through the other end using a Jacob curved reverse Sinskey hook (Epsilon Instruments). The CAIRS was laid out straight before being pulled in to prevent twisting of the segment.
OCT analysis of CAIRS segments in vivo
Spectral domain OCT (MS-39, CSO) with 3.9 µm resolution was performed in the postoperative period in the operated eyes with CAIRS. Width (microns), length (mm) of the segments, corneal thickness near the channel and central corneal thickness were measured using the caliper tool option. The CAIRS width was determined by measuring two points from the hyperreflective rim of the CAIRS segment on SD-OCT image on the vertical axis. The length was measured in the same method on the horizontal axis. The chord length of the segment was determined by drawing a perpendicular line down from the outer edges on each end into the anterior chamber and measuring the distance between them in the eye. True fractional depth, fractional shallowing and fractional compression were determined from the measurements from the anterior cornea (corneal thickness anterior to CAIRS) and total thickness of the cornea at the level of CAIRS.
Inferences of OCT examinations
Depth of placement of the segment is important in the management of keratoconus and its functional outcome. True fractional depth helps determine the percentage of depth obtained postoperatively. Thus, one can use SD-OCT as a tool to assess surgical positioning of the segments. The analysis can help determine whether the desired depth was attained in placement as predicted. One can also compare and correlate the functional outcome with anatomical positioning of the segments. When anterior corneal thickness is thinner (in front of CAIRS), shallowing is more, and when anterior corneal thickness is thicker, depth is more.
Comparison with intrastromal corneal ring segments
CAIRS have an advantage over synthetic counterparts, with less risk for corneal melt or extrusion. The incidence rate of postoperative complications caused by intrastromal corneal ring segments has been noted to be between 1.1% and 2%. Shallow segment depth has been associated with several complications, including ring superficialization, stromal thinning and epithelial breakdown. However, in CAIRS, although the segment occupied anterior to mid-stroma, no such complications have been noted. Corneal tissue biocompatibility is the key in CAIRS as opposed to intrastromal corneal ring segments. This makes them suitable for implanting in thinner corneas, which may not be possible for synthetic segments. Another physical feature is the flexibility obtained with CAIRS as compared with synthetic segments. The OCT evaluation has shown good contour formation adapted by the CAIRS segments in situ and no extrusion or melting. The flexible nature of the corneal segment makes it easy to manipulate in the channel intraoperatively and can be aided by viscoelastics as well. There are no real-time studies on the analysis of CAIRS on anterior segment OCT. We recommend future studies to further analyze the functional and anatomical correlation in CAIRS in the short and long term.
- References:
- Alió JL, et al. J Cataract Refract Surg. 2006;doi:10.1016/j.jcrs.2006.02.044.
- Hersh PS, et al. J Cataract Refract Surg. 2011;doi:10.1016/j.jcrs.2010.07.030.
- Jacob S, et al. J Refract Surg. 2018;doi:10.3928/1081597X-20180223-01.
- Lai MM, et al. J Cataract Refract Surg. 2006;doi:10.1016/j.jcrs.2006.05.030.
- Rabinowitz YS. Surv Ophthalmol. 1998;doi:10.1016/s0039-6257(97)00119-7.
- For more information:
- Amar Agarwal, MS, FRCS, FRCOphth, director of Dr. Agarwal’s Eye Hospital and Eye Research Centre, is the author of several books published by SLACK Books, sister company of Healio publisher Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at aehl19c@gmail.com; website: www.dragarwal.com.
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