Commentary: Reliability of different biometric measurements in implantable collamer lense procedure.

Phakic intraocular lens (IOL) is an appropriate option for correction of refractive errors, especially when the refractive error is high and ablative procedures such as corneal refractive surgeries are contraindicated. However, for the placement of a phakic IOL, it is critical to select patients with sufficient anterior chamber depth (ACD) and anterior chamber angle diameter.[1]

The implantable collamer lens (ICL), a posterior chamber phakic IOL, effectively corrects moderate to high myopia. It is made from collamer, a biocompatible material.

Postoperative complications have been reported, however, with low risk–benefit ratio. Most of these complications are associated with vaulting of the lens (i.e., distance between the posterior surface of the ICL and the anterior surface of the crystalline lens). High vaulting conditions lead to increased intraocular pressure and inflammation by causing mechanical contact between the ICL and iris. Pigment dispersion, iris atrophy, secondary glaucoma, and formation of metabolic cataracts have also been associated with high vaulting conditions.[2] Low vaulting conditions have been reported to induce mechanical contact between the ICL[3] and crystalline lens and cause inadequate aqueous circulation in the perilenticular space. Mechanical contact between the ICL (posterior surface) and crystalline lens (anterior surface), as well as impaired circulation of aqueous humor are considered to play a crucial role in the development of anterior subcapsular cataracts.[4]

ICL sizing based on sulcus-to-sulcus (STS) diameter, white-to-white (WTW) diameter, and ACD has been established as the gold standard for achieving optimal vaulting.

Different devices used to measure the parameters are IOLMaster 700 biometer (Carl Zeiss Meditec, Jena, Germany), which is based on swept-source optical coherence tomography (OCT); Cirrus OCT (Carl Zeiss Meditec), which uses low-coherence interferometry to produce high-resolution images of the anterior segment; Visante OCT, which is an anterior segment OCT (Carl Zeiss Meditec) that uses low-coherence interferometry with a light source wavelength of 1310 nm; and Pentacam rotating Scheimpflug camera (Oculus, Wetzlar, Germany) imaging system, which is a rapid noncontact modality that enables the study of the eye’s anterior segment using a blue light-emitting diode and a rotating Scheimpflug camera. However, different modalities give slightly different readings, which adds to the perplexity of choosing accurate parameters like ACD and WTW diameter.[5]

Accuracy of measurements is of utmost importance for good outcome of ICL implantation. So, the above-mentioned study was conducted to evaluate the agreement between the biometric measurements used to calculate the size of the ICL with different technologies: swept-source OCT, spectral domain OCT, and Scheimpflug tomography. The authors concluded that for ACD measurements, there is a good agreement between the different devices under evaluation (Pentacam, IOLMaster, Cirrus OCT, and Visante OCT) and for WTW, the values measured with the different devices show large discrepancies with low correlation levels, especially when comparing the tomographs with the other devices under evaluation.

Most of the studies were conducted with a single parameter to compare a pair of devices. The present study included all parameters with the most commonly used devices in clinical practice.[6]

The concept of creating a different nomogram for each device seems to be a good approach, like NK nomogram by Nakamura et al., so as to avoid undesired postimplantation vaults.[7]

The review article concludes that there is a good agreement between the different devices under evaluation for ACD measurements, similar to Shajari et al.’s[5] study.

Contrary to what was observed for ACD, this excellent agreement across devices does not exist for WTW measurements. The correlation is better between Pentacam and IOLMaster, but it is considerably lower when comparing these two devices with the two OCTs. This may be because of their measurements with different landmarks,[8] for example, Pentacam platforms do measure WTW distances, while the Visante OCT relies on angle-to-angle (ATA) distance and the Cirrus OCT on scleral spur to scleral spur distance.

As there is less agreement between measurements by different equipment, further studies comparing each equipment with ultrasonic biometry will give an idea as to which instrument to rely upon for WTW accurate measurement of ICL diameter calculation.

Measurements from which instrument were used finally to calculate IOL diameter is not clearly mentioned.

To conclude, the review article is a good approach to find the agreement between different devices based on different technologies for preoperative measurements for ICL size calculation.[9]