Multicentre study: reliability and repeatability of Scheimpflug system measurement in keratoconus

Abstract

PurposeTo assess the repeatability and reliability of the most important tomographic parameters for characterising keratoconus measured with a Pentacam HR (high resolution).MethodsOverall, 230 eyes in 158 patients with keratoconus were analysed. We performed five consecutive corneal tomography examinations for each eye with a Pentacam HR in patients with keratoconus. Study eyes were classified into three groups depending on the maximum posterior elevation (max_BFS_post): grade 1 for cases of keratoconus with a max_BFS_post of 40 µm; grade 2 for those with a max_BFS_post of between 41 and 75 µm and grade 3 for those with a max_BFS_post of over 75 µm. We calculated the intraclass correlation coefficients (ICCs) and repeatability limits of parameters from tomography and aberrometry.ResultsAll the parameters were found to have excellent ICCs (0.9). The repeatability limits for the key parameters were higher than 0.5D for the power parameters, 20° for the axis of corneal astigmatism and 10 µm for the thinnest corneal thickness. Further, we obtained repeatability limits of above 0.1 µm for the aberrometry values and overall greater than 15° for the coma axis. All the values increase with the severity of keratoconus, except for that of the coma axis which falls with keratoconus grade.ConclusionsThe reliability indicated by ICCs supports the view that the Pentacam HR is useful for the diagnosis of keratoconus. The repeatability limits suggest that new criteria should be established for monitoring progression taking into account the real measurements that can be made using this system.

Evaluation of a new nomogram for Ferrara ring segment implantation in keratoconus

“…Several factors may account for the pachymetric increase found in our series including a potential redistribution of corneal tissue due to the ring implantation or the level of intrasession repeatability of Pentacam pachymetric measurements. Indeed, de Luis Eguileor et al [51] have recently confirmed that the repeatability limits for the thinnest corneal thickness measurements obtained with the Pentacam system is 10 µm. Finally, we have analyzed the potential effect on the outcomes of the use of the mechanical or femtosecond-laser assisted technique for the implantation of the ring segments as a more precise dissection plane depth has been demonstrated to be achieved using the femtosecond technology [52] .…”

“…In any case, some discussions comparing with the results of previous nomograms have been included in the current article but taking care of extracting definitive conclusions as a direct clinical comparison (with or without randomized assignment) has not been done. Another limitation was not including the analysis of ocular wavefront aberrations as with this information a more exhaustive analysis of astigmatism could have been performed, including the analysis of discrepant astigmatism (DA), which is the discrepancy between refractive astigmatism and the sum of total corneal astigmatism and lenticular astigmatism (derived from ocular wavefront aberrometric analysis) [51] . It should be considered that DA is normally negligible in eyes with normal optics but can become significant when coma-like high order aberrations refract as astigmatism [54] .…”

Current Developments in Corneal Topography and Tomography

“…Distinguishing topography and tomography is critical, as each of these examinations have their own characteristics. For corneal topography, we can expect excellent agreement in corneal power between measurements; for tomography the agreement limits are wider [85]. On the other hand, tomography allows critical stereometric data to be obtained, which are not available in corneal topography.…”

Comparison of corneal tomography using a novel swept-source optical coherence tomographer and rotating Scheimpflug system in normal and keratoconus eyes: repeatability and agreement analysis

“…Therefore, measurement devices must provide a fast and reliable single measurement with high precision. RSC is known to reliably measure corneal tomographic parameters; however, there is a discrepancy in repeatability of this technology in the measurement of moderate or advanced KC cases, where corneal scars may be present [ 11 – 14 ]. Swept-source optical coherence tomography (SS-OCT) is a novel technology to measure corneal tomography characterized by a longer wavelength (845 nm [ 15 ] or 1300 nm [ 16 ]), instead of blue light, as it is used in Scheimpflug technology (475 nm).…”