Different Automated Instruments Produce Distinct White to White Measurements

Corneal topography
Corneal topography. Ophthalmologist scanning a patient’s eye to obtain a three-dimensional image of the cornea.
Researchers compare measurements of 3 different technologies.

Measuring mean white to white (WTW) distances with different automated instruments will result in significantly different lengths, according to a report published in Eye & Contact Lens.

Improperly sized implantable collamer lens (ICL) posterior chamber phakic intraocular lenses (pIOL) may lead to pigment dispersion or lens dislocation. The FDA recommends lens sizing with WTW distance, which can be measured in multiple manners. Researchers sought to determine whether the data automated instruments obtain are interchangeable.

They reviewed data from 65 eyes of 38 patients who underwent measurements for refractive surgery with Orbscan IIz® (Oculus),, OPD Scan III® (Oculus), or IOL Master 700® (Carl Zeiss Meditec). The investigators excluded patients with prior ocular surgery, trauma, or known limbal pathologies.

Differences in mean WTW distance measurement were statistically significant. The mean measurement was consistently greatest with IOLMaster 700 (12.1±0.5 mm), which was 0.33 mm greater (95% CI, 0.28–0.38; P <.001) than Orbscan IIz (11.8±0.4 mm). Mean difference between OPD Scan III (mean 12.0±0.4 mm) and Orbscan IIz was 0.24 mm (95% CI 0.21-0.28 P <.001).

Roughly a third of cases had longer lens length recommendations in OPD Scan III or Orbscan IIz, compared with IOLMaster 700 recommendations. After adjusting IOLMaster 700 and OPD Scan III WTW measurements with the aforementioned mean differences, one-fifth of lens lengths variations with change in instrument remained.

“WTW distance was on average shorter when measured with Orbscan IIz, followed by OPD Scan III and finally IOLMaster 700,” the researchers explain. “However, our data do not support that raw or adjusted WTW measurements can be used interchangeably for pIOL ICL size calculation. To the best of our knowledge, the Orbscan IIz will not be manufactured in the future, and its information may not be clinically useful anymore. More studies are needed for evaluating accuracy and reliability of the new nomograms models. Future refinement of these models may allow improvement in vault outcomes, thereby decreasing the adverse event rate in these patients.”

Limitations of the study involve generalization, its retrospective nature, and possible intervariability or intravariability.


Cruz S, Valenzuela F, Stoppel J, et al. Comparison of horizontal corneal diameter measurements using Orbscan IIz, OPD Scan III, and IOLMaster 700. Eye & Contact Lens. 2021;47(10):533-538. doi:10.1097/ICL.0000000000000786