Experimental evaluation of online optical coherence pachymetry for corneal refractive surgery.

Purpose: Online optical coherence pachymetry (OCP) allows to monitor central changes of the corneal cross section intraoperatively. In this experimental evaluation the validity of the optical measurements for corneal refractive surgery was assessed.

Methods: Online OCP based on low-coherence interferometry with a wavelength of 1310 nm and a measurement frequency of 74 Hz was directly integrated in a clinical excimer laser. In 16 patients the central corneal thickness was measured with online OCP and ultrasound pachymetry (US). Furthermore, the ablation characteristics were assessed in corneoscleral discs unsuitable for transplantation (n=12) and PMMA samples (n=18).

Results: Online OCP was possible in all patients and materials studied. The mean central corneal thickness was 537+/-31 microm (OCP) and 546+/-33 microm (US). The corneal reproducibility was +/-4.3 microm (coefficient of variation [CV] 0.8%) with online OCP and +/-3.7 microm (CV 0.68%) with US. The reproducibility in PMMA samples was +/-1.0 microm (CV 0.16%). There was a significant correlation between online OCP and US measurements (r=0.93, P<0.001). The mean difference was 9.1 microm or 1.69% (P=0.01), and the limits of agreement (95% CI) ranged from -15 microm to 33 microm. There was a significant linear relationship (r=0,95; P<0.001) between the calculated and the optically determined ablation depth with online OCP. Also ablation depth measurements in PMMA correlated positively with spectrophotometric values (r=0.98; P<0.001).

Conclusion: In this experimental evaluation, online OCP revealed to be a precise and reproducible method to assess the central corneal thickness and its changes intraoperatively. This could be important to monitor incisional and excimer laser-based corneal refractive procedures, such as PRK or LASIK.