Department of Vision Sciences, Glasgow-Caledonian University, Scotland, United Kingdom.
Eye Contact Lens. 2010 Mar;36(2):106-15. doi: 10.1097/ICL.0b013e3181d0b604.
Orbscan scanning-slit optical pachymetry was introduced over a decade ago and yielded higher central corneal thickness (CCT) values to the "gold standard" of contact ultrasound pachymetry (U/S). An acoustic correction factor (AF) was introduced later to compensate for this difference. The goal of this review was to assess the magnitude and consistency of the difference, as well as to assess how useful the AF had been.
Using PubMed (Medline)-sourced citations, published articles were identified that included data on CCT from U/S and Orbscan, with the latter data checked to see whether an AF had been applied. Main comparisons were made between (1) Orbscan data without AF and U/S, and (2) Orbscan data with a 0.92 AF applied and the U/S data.
From 46 studies involving a total of 6136 eyes (average number per study of 133, range 6-1214), the average CCT values by U/S ranged from 0.520 to 0.580, for a group mean of 0.545 mm. For Orbscan without AF, the average CCT values ranged from 0.557 to 0.624 mm, for a mean of 0.582 mm, a net difference of 0.037 mm from U/S, with all Orbscan data (average values from any particular study) being higher than U/S. With a 0.92 AF applied, the net difference was -0.009 mm. The calculated limits of agreement between the two methods ranged from 0.004 to 0.073 without AF, but from -0.041 to + 0.023 mm with the AF. The overall outcome was essentially the same if weighted for cohort size and sample variability, or if only studies reporting on one eye were considered.
Orbscan pachymetry can be expected to yield CCT data that is approximately 7% higher than U/S. The global application of a 0.92 AF does not robustly align the Orbscan CCT data to that of U/S and, in fact, can easily result in the data being as much as 7% lower. Overall, the level of agreement between Orbscan and U/S is limited, and Orbscan data should simply be reported as measured without any adjustment.
角膜地形图扫描技术十多年前问世，与角膜超声测厚仪（U/S）的“金标准”相比，该技术测量出的中央角膜厚度（CCT）值更高。后来引入了一个声学校正系数（AF）来弥补这种差异。本研究的目的是评估这种差异的大小和一致性，以及评估 AF 的使用效果。
利用 PubMed（Medline）收录的文献，确定了包含 U/S 和 Orbscan 角膜厚度数据的已发表文章，并检查后者的数据是否应用了 AF。主要比较包括：（1）未应用 AF 的 Orbscan 数据与 U/S 之间的比较，（2）应用 0.92 AF 的 Orbscan 数据与 U/S 数据之间的比较。
46 项研究共涉及 6136 只眼（每项研究平均眼数为 133 只，范围为 6-1214 只），U/S 测量的 CCT 值范围为 0.520 至 0.580，平均值为 0.545mm。未应用 AF 的 Orbscan 平均 CCT 值范围为 0.557 至 0.624mm，平均值为 0.582mm，与 U/S 相比，净差值为 0.037mm，所有 Orbscan 数据（特定研究的平均值）均高于 U/S。应用 0.92 AF 后，净差值为-0.009mm。两种方法之间的一致性界限范围为 0.004 至 0.073 无 AF，而应用 AF 时为-0.041 至+0.023mm。如果根据队列规模和样本变异性进行加权，或者只考虑单眼报告的研究，总体结果基本相同。
角膜地形图仪测量的 CCT 值预计比 U/S 高约 7%。在全球范围内应用 0.92 AF 并不能使 Orbscan 的 CCT 值与 U/S 准确对齐，实际上，数据很容易低 7%。总体而言，Orbscan 与 U/S 之间的一致性水平有限，因此，Orbscan 数据应在未经任何调整的情况下直接报告测量值。
