**5. Ocular biometry and IOL power calculation**

Accurate measurements are critical for determining the correct power of a premium IOL before it is implanted during cataract surgery. The emmetropia is key factor of a successful refractive lens exchange to gain spectacle independence. Attaining this goal requires eliminating astigmatism and achieving a precise postoperative plano refraction within ±0.25 D.

Ocular biometry involves anatomical measurements of the eye, including the axial length (AL), keratometry, anterior chamber depth (ACD), lens thickness (LT), horizonal white to white (HWTW) which are the parameters for IOL power calculation [52].

Even the ultrasound biometry is still used in some difficult cases such as brunescent cataract, white cataract and severe subcapsular cataract. A hyperopic surprise often appeared in high myopic patients by using ultrasound biometry, because A-scan measured the deepest part of the staphyloma while macula was on the edge of the staphyloma which led to false longer axial length.

With IOLMaster (Zeiss) introduced in 1999, optical biometry technique provide a directly measurement from the macula to the corneal vertex. It becomes golden standard as it is highly accurate, easy to perform, non-invasive and comfortable for the patient. The accuracy of optical biometry, and in particular the IOLMaster 500 (Zeiss) and Lenstar 900 (Haag-Streit), have been extensively confirmed across a wide range of scientific studies [53, 54]. New generational optical biometry IOLMaster 700 (Zeiss) has integrate swept source optical coherence tomography to measure axial length. It allows for penetration of dense cataracts, determination of lens thickness (not available on the prior generations of IOLmaster), and visualization of the foveal pit to both ensure alignment of the image and possibly detect

pathology like epiretinal membrane or cystoid macular edema which is influenced the premium IOLs power calculation [55].

Besides the accuracy biometry, the IOL power calculation formula choice also is critical for premium IOLs surgery. Though the third and newer generation formula can get accuracy refractive result in normal axis length and keratometry eyes, attention must be paid to the long axial length eye as well as the abnormal corneal power cases [56]. New IOL power calculation formula like Barrett, Hill-RBF and Olsen will achieve more precision and accuracy in longer and short axial length eyes [57].

The IOL power calculation in post corneal refractive surgery eyes always is a challenge issue. Whether corneal radical keratotomy or PRK/LASIK always change the corneal shape of in different ways. Errors in evaluation of the correct corneal power and errors in estimating the effective lens position with the classical thin-lens formulas lead to underestimate the IOL power and hyperopic postoperative refractive surprise. Many adjustment methods had been developed to estimate the true corneal keratometric data such as Haigis-L formula, Shammas no-history formula [58]. The new device like schiempflug or swept source OCT which can directly measure the anterior/posterior/total corneal power to obtain more accuracy results [59]. Modern IOL formulas, such as the Barrett True-K and ascrs.org web-based IOL power calculator can provide greater refractive predictability [60].

Cataract surgeon must personalize his IOL constants for premium lenses. Although the design of the IOL is the primary factor in the constant, variations in surgical technique such as the placement of the IOL, the location and design of the incision, and differences in biometry and technicians also affect the personalized lens constant. Preoperative biometric data and post-operative refractive error of 20 to 40 cases should be collected in order to personalize lens constant [52]. This process is the only way to achieve superior results with these IOLs and accuracy to within ±0.25 D for 95 percent of patients. Personalizing the lens constant is critical to eliminating the systematic variations that make excellent results and happy patients the rule with multifocal lenses.
