Comparison of white-to-white measurements using four devices and their determination of ICL sizing

Patients and methods

This study is a single-center, single-surgeon, retrospective, cross-sectional review of medical records of healthy patients who underwent refractive surgery using ICL at an ambulatory surgicenter. It was approved by the ethics review board of the institution and adhered to the tenets of the Declaration of Helsinki.

Data collection

A database search was performed of consecutive patients with moderate to high myopia and myopic astigmatism who underwent ICL surgery from March 1, 2018 to July 31, 2021. Patients aged 18–50 years old who underwent preoperative refractive screening for ICL surgery, and had complete preoperative caliper, Orbscan II, IOLMaster 700, and Pentacam AXL measurements were included. Data completeness was confirmed by obtaining a printout of the measurements or accessing the files saved on the respective machines. Patients who did not have a one-month postoperative follow-up with an anterior segment OCT measurement were excluded from this study. The following data were gathered from the medical records: sex, laterality of the eye, preoperative and postoperative uncorrected visual acuity at distance, manifest refraction. The WTW measured from 0° to 180° by a single examiner using the caliper was also noted. Biometric parameters including WTW, internal ACD, keratometry, and central corneal thickness were measured using the following machines: Orbscan II, IOLMaster 700, and Pentacam AXL.

The Orbscan (Bausch and Lomb, Germany) topographer was introduced in 1995. It was based on the innovative principle of measuring the dimensions of a slit-scanning beam projected on the cornea. Later, with the advent of computerized topography (1999), the Orbscan II (Orbtek, Inc.) evolved. One of its clinical applications is to use digital image processing for WTW measurements. Information about corneal shape, corneal thickness, and ACD is acquired by scanning the anterior eye segment with a slit beam. The computer automatically detects the corneal limbus (the border between the white sclera and darker iris image) by comparing gray-scale steps and calculates the corneal diameter. [12]

The IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) is the latest generation biometer in the IOLMaster series and one of the most popular biometers used in the world today. It uses swept-source optical coherence tomography capable of generating b-scans using lasers with variable wavelengths (high-frequency 1,055 nm tunable laser source) to produce biometric data such as axial length (AL), lens thickness, central corneal thickness, keratometry, pupil size, ACD, and WTW. [13]

The Pentacam AXL (Oculus Optikgeräte GmbH, software 1.25r15), introduced in 2015, consists of a Scheimpflug camera with partial coherence interferometry optical biometer. It can measure anterior segment tomography, ACD, WTW corneal diameter measurements, central corneal thickness, anterior and posterior corneal surface and aberrations, AL, and corneal topography. [13]

Ramon Castroviejo invented a graduated compass-like caliper which measured from 0 to 20 mm in 1 mm increments [14, 15]. The Castroviejo caliper is still often used in ophthalmology today and has applications in other medical branches [15]. However, its use in comparative studies of ocular dimensions has not been very encouraging, possibly because of its low resolution [16]. WTW measurements in ICL sizing calculations require sub-1 mm readings. Since our caliper's graduation markings are in 1.0 mm increments, readings were estimated to the nearest 0.1 mm reading.

Intraoperative and postoperative data obtained include the actual ICL size implanted during surgery, and the one-month postoperative vault measured using an anterior segment OCT (Visante OCT, Carl Zeiss, Germany). The vault measurements obtained were categorized into groups with a specified vault size range (< 0.1 mm, 0.1–0.24 mm, 0.25–0.50 mm, 0.51–0.75 mm, 0.76–1.00 mm, 1.00–1.25 mm and > 1.25 mm). The number and corresponding percentages of eyes falling into each group and those within the recommended vault range of 0.25–0.75 mm and 0.25–1.00 mm were also determined.

ICL size calculation

The Visian ICL is a plate-haptic design lens made of proprietary collamer material with a central convex/concave optical zone. This design features a forward vault intended to minimize ICL contact with the anterior capsule of the natural lens. The more recent models, VICMO (EVO), VTICMO (EVO Toric), VICM5 (EVO +), or VTICM5 (EVO + Toric), have a central port or hole measuring 0.36 mm (KS-Aquaport). The central hole was meant to obviate the need for a Nd:YAG iridotomy previously required with the older V4 model to allow physiologic aqueous humor circulation. Long-term studies investigated the safety profile of the ICL, and reports have shown generally low rates of adverse events. [17]

The Visian ICL has four different manufactured ICL sizes to accommodate normal variations of intraocular anatomy, namely 12.1, 12.6, 13.2, and 13.7 mm. [11] The EVO + and EVO + Toric models have a larger optic size (6.1 mm) than the EVO and EVO Toric, but all models have the same general design and overall diameter across the manufacturer's size variations. The ICL sizing calculation is the same for these ICL models. The online ICL size calculator determines the ICL size based on the Orbscan WTW and ACD data entered into the calculator. The surgeon receives the recommendation and makes the final decision in terms of lens size to be implanted.

ICL sizing was determined using the STAAR Surgical Online Calculation and Ordering System (OCOS™, Staar Surgical, USA). For this study, we performed six OCOS calculations using the WTW and ACD obtained from different devices: Orbscan WTW with Orbscan ACD, caliper WTW with Orbscan ACD, IOLMaster WTW with IOLMaster ACD, caliper WTW with IOLMaster ACD, Pentacam WTW with Pentacam ACD, and caliper WTW with Pentacam ACD. The ICL size obtained using the WTW and ACD measurements from the Orbscan was arbitrarily designated as the benchmark to which all other calculations were compared because this was our standard point of reference and was the basis for final selection of ICL sizing in our practice. Results obtained were validated by our supervising technician who checked for artifacts and invalid measurements. The ICL sizing determined from these different combinations of raw data were compared to the ICL size implanted per eye.

Surgical technique

On the day of surgery, patients were administered with Sanmyd-P (Tropicamide + Phenylephrine Hydrochloride, Santen Pharmaceutical Company, Shiga, Japan) as dilating agent and Alcaine (Proparacaine Hydrochloride, Alcon, Fort Worth, Texas, USA) as anesthetic agent to the operative eye. A Visian ICL [VICMO (EVO), VTICMO (EVO Toric), VICM5 (EVO +), or VTICM5 (EVO + Toric) model] was inserted through a small, 3.2 mm, clear corneal incision. The lens was injected through a clear corneal incision at 220° if it involved the right eye and 30° if it involved the left eye using a Staar MicroSTAAR injector (STAAR Surgical Co., Monrovia, CA, USA) and allowed to unfold slowly. The distal and proximal footplates were tucked under the iris with a modified intraocular spatula. Correct positioning of the ICL in the center of the pupillary zone was verified. Any remaining viscoelastic was meticulously irrigated out of the anterior chamber with balanced salt solution.

Statistical analysis

Demographic data were analyzed using SPSS Statistics for Windows, version 21 (IBM Corp., Armonk, NY, USA). Mean and standard deviation were used for continuous variables, while frequency and percentage were used for categorical variables. To determine if there were significant differences between mean diameters obtained with devices, paired t-test and Bland-Altman analyses were performed. The limits of agreement (LOA) were defined as mean ± 1.96 standard deviations of the differences between two measuring devices. P values of less than 0.05 were considered significant.

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