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Corneal ring segments decrease corneal curvature but do not affect biomechanics
J Cataract Refract Surg. 2009;35(10):1761-1767.
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Intrastromal corneal ring implantation in keratoconic corneas markedly reduced corneal curvature but did not influence corneal biomechanical characteristics, a study showed.
The authors assessed biomechanical and morphological changes in keratoconic corneas implanted with Intacs intrastromal corneal ring segments (Addition Technology).
"To our knowledge, this is the first study that shows that Intacs placement changes the morphological characteristics of the cornea but not the biomechanical viscoelastic response parameters, such as [corneal hysteresis] and [corneal resistance factor]," the authors said.
The retrospective study included 18 eyes of 18 patients with a mean age of 31.3 years (range: 13 to 50 years). Patients underwent a complete ocular examination before surgery and 6 months after surgery. Investigators performed corneal topography with the Orbscan II (Bausch & Lomb) and assessed biomechanical factors with the Reichert Ocular Response Analyzer.
Mean preoperative corneal hysteresis was 7.7 mm Hg and mean postoperative corneal hysteresis was 7.4 mm Hg. Mean preoperative corneal resistance factor was 6.6 mm Hg and mean postoperative corneal resistance factor was 6.1 mm Hg. Neither reduction was statistically significant.
"In our study, both IOP and central pachymetry remained stable postoperatively and thus played a role in the lack of a significant change in [corneal hysteresis] and [corneal resistance factor] values," the authors said. "We hypothesize that because of the lamellar structure of the cornea, the [intrastromal corneal ring] segments might reshape the center of the cornea without changing the fundamental biomechanical properties of the corneal tissue, at least in the short-term postoperative period."
However, mean minimum and maximum central keratometry values decreased more than 5 D; the differences were statistically significant (P < .0001).
Further study with a larger patient group and longer follow-up interval is needed to determine the effect of Intacs implantation on corneal hysteresis and corneal resistance factor, the authors said.
Intracorneal ring segments have been well documented to decrease central keratometry and cylinder in patients with keratoconus. Dauwe et al confirm this finding by demonstrating a mean reduction in central K of almost 6 D. The novel information provided in this study is that corneal hysteresis measurements do not change significantly following the placement of intracorneal ring segments for keratoconus. Corneal hysteresis is a measurement of the biomechanical properties of the cornea. These measurements suggest that the intracorneal ring segments change the curvature of the cornea but not the underlying pathological condition.
– Eric D. Donnenfeld, MD
OSN
Cornea/External Disease Board Member