Ocular Biometry and Calculation of IntraOcular Lens Power

This chapter first details the physical principle of ocular biometry, then presents results, with the distances, in millimeters, depending on the celerity in the different eye structures (cornea, anterior chamber and vitreous, lens) and finally the quality criteria of a “good” biometry: as high as possible peaks, on equal height, with repetition echoes and results consistent with refraction. Considering the technique, A-mode biometry (contact and immersion) is first presented, the manual acquisition mode being much more preferable. B-mode guided biometry is then presented and discussed; it has several advantages, particularly better intra- and inter-examiner reproducibility. Special cases are also discussed, according to age (neonates, infants and children), the refraction (myopia or hyperopia) or the pathology (aphakia, pseudophakia, phakic eye with a refractive phakik intraocular lens [pIOL], intravitreal silicone, melanoma and age-related macular degeneration, risk of closed angle glaucoma, hypotonic eye, and eye trauma). Then, pachymetry allowing the measurement of corneal thickness, and optical biometry using interferometry, are briefly presented. The second part of the chapter deals with the different formulas for IOL power calculation and how to use them. We successively present the historical method, also known as the 1.25 dpt correction; the first-generation theoretical formulas; and the regression formulas SRK and SRK II, to concentrate on the latest generation of theoretical formulas: SRK/T, Holladay, Hoffer-Q, Haigis, and Barrett’s Universal II. The importance of the keratometry value on the result is then analyzed, and we discuss the biometry for toric IOLs, the special case of contact lens wearers, special cases when keratometry is not possible, IOL design for children and infants (congenital cataracts), and special cases of corneas modified by refractive surgery. We present tables for the advantages and disadvantages of ultrasound and optical axial length measurement techniques and the proposed choices of theoretical formulas to be used as a axial length function taking into account the optimal efficiency of each formula. These tables help readers master this subject of ocular biometry and IOL power calculation.