Interpreting corneal changes after PRK: Integrating biomechanics with tomographic remodeling

Abstract

Photorefractive keratectomy (PRK) alters corneal biomechanical and tomographic measurements, but postoperative change, an abnormal index classification, and progressive ectasia are distinct. Saif et al. reported biomechanical changes at 6 months; a separate Pentacam cohort showed tomographic changes at 1 month. Clinically and topographically normal eyes assessed approximately 25 years after PRK also had frequent abnormal classifications. These different cohorts and observation periods cannot establish a common postoperative trajectory or show that an abnormal result is benign in an individual eye. Prospective paired Pentacam and Corvis ST measurements in the same eyes, followed by independent validation, are needed to develop postoperative references. Until then, serial clinical and tomographic findings should inform interpretation of isolated index abnormalities.

Dear Editor,

We read with interest Saif et al.’s assessment of corneal biomechanics following LASIK, PRK, and cross-linking using Corvis ST [1]. At 6 months, LASIK and PRK eyes showed increased deformation amplitude, deformation amplitude ratio, corneal biomechanical index (CBI), and tomographic biomechanical index (TBI), with reduced stiffness parameter at first applanation and Ambrósio relational thickness [1]. These measurements characterize postoperative biomechanical behavior; they do not by themselves establish progressive ectasia in an individual eye.

Postoperative measurement change, a postoperative value classified as abnormal by an ectasia-related index, and serial evidence of progressive ectatic disease require different interpretations. The first may reflect expected surgical alteration; the second is a classification under existing rules; the third concerns change in an individual eye. Tissue ablation can alter measurements, while existing ectasia-related classifications may not retain their diagnostic meaning after surgery. An abnormal value is not a diagnosis of ectasia, but it also cannot be dismissed as benign remodeling without appropriate follow-up. Neither finding alone establishes whether progression is present. Post-PRK diagnostic thresholds for CBI, TBI, and BAD-D remain insufficiently validated for determining early ectasia after surgery.

In a separate Pentacam study of 102 PRK eyes, posterior keratometric parameters and elevation changed at 1 month; BAD components Dp, Dt, and Da and total BAD-D increased, whereas Df did not change significantly [2]. These early group findings demonstrate that uncomplicated PRK can alter posterior corneal and Belin/Ambrósio variables commonly used in ectasia assessment, but cannot distinguish remodeling from early ectasia in an individual eye.

The studies involve different cohorts and intervals: 1-month tomography, 6-month biomechanics, and assessment approximately 25 years after PRK [1–3]. Because the same eyes were not followed across these intervals with both instruments, these findings do not establish a continuous trajectory. Among clinically and topographically normal eyes assessed approximately 25 years after PRK, 71.9% had both high-risk CBI and diseased BAD classifications [3]. This cross-sectional observation illustrates discordance between index classification and clinical assessment; it neither documents serial stability over 25 years nor proves that a similar classification is harmless in another eye.

A prospective PRK cohort could undergo paired Pentacam and Corvis ST measurements before surgery and repeatedly through early, intermediate, and longer follow-up. A reference trajectory would describe the expected distribution and variability of serial within-eye changes in posterior elevation, BAD-D components, CBI, TBI, and stiffness, accounting for baseline thickness, ablation depth, correction, and measurement variability. The resulting candidate postoperative ranges and changes over time should then be validated in an independent cohort against progression defined by serial clinical and tomographic evidence without using the index under evaluation. Because post-PRK ectasia is uncommon, multicenter follow-up with adequate outcome events would be needed to assess diagnostic performance. This approach would directly test whether observed changes differ between uncomplicated eyes and those with documented progression. Such procedure-specific postoperative reference trajectories and thresholds remain a research objective rather than an established clinical standard.

Duch et al. examined the influence of tomographic and biomechanical indices on refractive-surgery indications [4]. TBI was developed by combining Scheimpflug tomography and biomechanics to enhance ectasia detection [5]. Neither establishes validated post-PRK criteria that distinguish remodeling from early progression. Saif et al.’s findings therefore motivate postoperative validation, not a new diagnostic threshold [1].

In an individual post-PRK eye, an isolated abnormal index warrants interpretation alongside symptoms, examination, preoperative and surgical data, and repeat tomographic and biomechanical measurements. Documented progression deserves particular attention; the sensitivity and specificity of individual postoperative indices for early ectasia remain uncertain.

Author contributions

M.K. conceived the correspondence and drafted the manuscript. G.A. contributed to the literature review and critically revised the manuscript. K.Y. supervised the work and critically revised the manuscript. All authors read and approved of the final manuscript.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Ethics approval and informed consent were not required because this Letter to the Editor does not report original research involving human participants, animals, or plants.

Competing interests

The authors declare no competing interests.