September 28, 2022

PEARL Online Calculator User Manual

In this post, we will review the PEARL online calculator features, discuss the mechanisms behind them, and their applications.

Regular Eyes

This page allows the user to perform a calculation for 'standard eyes', i.e. eyes which have not undergone refractive surgery, have no corneal scar, and for which information about the first operated eye is not available.

The "keratometric index" field allows selection of the index used by the user's biometer. In most cases, it is 1.3375 ; however, some biometers use a different value. The PEARL methodology is based on thick lens equations and does not use the K value delivered by the biometer in its equations, but rather the anterior corneal radius of curvature measurement. It is then fundamental to select the right keratometric index value, because it is used by the calculator to determine this anterior corneal radius of curvature.

The "Biometer" field is used to apply slight changes to the AL measurement. If "Argos" or "US (immersion)" are selected, the approximation of the Sum-of-Segments AL utilised in the PEARL formula is not used. If US (applanation) is selected, a correction factor of 0.15 mm (1-4) is added to the AL measurement to compensate for the underestimation due to inadvertent compression.

The "IOL Model" field allows the calculator to determine which correction factor should be applied to modify the AL value used in the Theoretical Internal Lens Position (TILP) prediction algorithm. It also allows to perform a calculation for a meniscus IOL (e.g. Alcon MA60). In this case, the A constant can't be modified, which is normal : the geometrical particularities of this meniscus lens is integrated into the formula and customized for each lens power along the -5 to +5 range.

You may notice that the "LT", "CCT" and "WTW" fields are optional, this is to allow for calculation even with biometers which do not perform those measurements. However, it is strongly recommended to fill them in if they are available, especially the LT and CCT.

Complex Eyes

The "Complex Eyes" platform allows the user to perform a calculation for eyes which have undergone previous corneal refractive surgery, have corneal scarring, or an Implantable Collamer Lens (ICL) in-situ.

If the "Corneal Refractive Surgery" field is activated, it is mandatory to determine which LVC procedure (myopic or hyperopic) was performed, as the corneal power adjustment will not be the same. Selecting this option also automatically removes the ARC and CCT from the lens position predictors.

If available, the "Pre-LVC anterior corneal radius" or the "Corrected ametropia" fields will allow to respectively obtain or approximate the pre-LVC anterior corneal radius, which will then be used to predict the posterior corneal radius more accurately.

If the "Radial Keratotomy" option is activated, a different method is used to predict the corneal power. ARC and CCT are also removed from the lens position predictors in this scenario. This option can be combined with the Corneal Refractive Surgery fields in order to perform a calculation for eyes with a history of RK and subsequent PRK.

The "Scarred cornea" option allows removal of the anterior corneal radius from the lens position predictors. It is wise to use it in those cases, to avoid adding a lens position prediction error to the corneal power prediction error which will probably occur.

The "ICL" option is used to apply a small correction to the AL measurement, due to the AL measurement error induced by those lenses, as described in the literature (5,6).

Second Eyes

In this section, it is possible for the user to fill in the information allowing to back-calculate the lens position of the first operated eye, which will then be used as a predictor to enhance the lens position prediction of the second eye.

The options here are the same as the options available for the 'Complex Eyes' section.

Note that the Second Eye function is not available for eyes with a discordant corneal surgery history, neither for MA60 IOLs.

References :

1. Rose LT, Moshegov CN. Comparison of the Zeiss IOLMaster and applanation A-scan ultrasound: biometry for intraocular lens calculation. Clin Exp Ophthalmol. 2003 Apr;31(2):121-4.

2. Bjeloš Rončević M, Bušić M, Cima I, Kuzmanović Elabjer B, Bosnar D, Miletić D. Comparison of optical low-coherence reflectometry and applanation ultrasound biometry on intraocular lens power calculation. Graefes Arch Clin Exp Ophthalmol. 2011 Jan;249(1):69-75.

3. Rajan MS, Keilhorn I, Bell JA. Partial coherence laser interferometry vs conventional ultrasound biometry in intraocular lens power calculations. Eye (Lond). 2002 Sep;16(5):552-6.

4. Pereira A, Popovic M, Lloyd JC, El-Defrawy S, Schlenker MB. Preoperative measurements for cataract surgery: a comparison of ultrasound and optical biometric devices. Int Ophthalmol. 2021 Apr;41(4):1521-1530.

5. Chen X, Zhang D, Liu Z, Liu Y, Cai H, Wu Q, Zhang Y. Effect of Implantable Collamer Lens on Anterior Segment Measurement and Intraocular Lens Power Calculation Based on IOLMaster 700 and Sirius. J Ophthalmol. 2021 Dec

6. Pitault G, Leboeuf C, Leroux les Jardins S, Auclin F, Chong-Sit D, Baudouin C. Biométrie optique des yeux avec implants phaques [Optical biometry of eyes corrected by phakic intraocular lenses]. J Fr Ophtalmol. 2005 Dec;28(10):1052-7.