CALCULATING IOL POWER

Patients have long come to expect to dispose of their glasses after cataract surgery. Having good uncorrected distance visual acuity is sometimes even valued more than the improvement in vision caused by the removal of the cataract itself. So, accurately calculating the correct IOL power is a crucial step in the patient’s perception of overall success of cataract surgery. The techniques currently used to calculate the IOL power have been highly refined during the past 30 years. A significant amount of this work was done by Dr Kenneth J Hoffer, who has consolidated all the relevant information into one book, IOL Power, published by Slack Incorporated. Formatted along the lines of the IOL power courses that Dr Hoffer has taught at AAO and ASCRS meetings over the past 36 years, this book is a crucial read for every cataract surgeon – and an excellent way to help avoid unpleasant post-op refractive surprises.

Most surgeons are familiar with the frequently used formulas by Hoffer, Holladay and Haigis. These allow for rapid and accurate calculations for the vast majority of eyes. But how did these formulas come to be? And, more importantly in clinical practice, how can they best be used for “non-standard†eyes? Developing a thorough understanding of the background and significance of each measurement helps the clinician avoid the pitfalls that could ultimately lead to a very unhappy patient. IOL Power is divided into two sections. Section I, “Basics and Accurate Biometry,†begins with an admission by the author that “this science is rather dry.†It does indeed seem quite technical to a beginner, although each component is interesting and very useful to the surgeon looking to perfect his or her outcomes.

By breaking the process down into manageable components, the author makes it all more accessible. The ultrasound, A-scan biometry, laser interferometry and the IOLMaster, corneal power, automated keratometry, the pentacam and corneal power measurements and IOL position are each discussed in their own chapter(s). For example, following an introduction to axial length measurements with the ultrasound, the important differences between immersion and applanation techniques are discussed. A-scan biometry is explained in detail. This allows the reader to select those aspects of the procedure of greatest interest, or those in which (s)he needs some fine tuning.

Section II, “Formulas and Special Circumstances,†goes further to discuss the calculations in non-standard situations. “It is important to understand the history of IOL power formulas so that one gains an understanding of the vagaries of today’s modern formulas,†begins his section. Since the theoretical beginnings in the late 1960s, five generations of formulas have been introduced, with the later formulas allowing optimisation for both IOL style and surgeon. Further, the use of different formulas is encouraged, depending on the eye’s axial length: for eyes smaller than 24.5mm, use the Hoffer Q; for those 24.5 – 26.0, use the Holladay 1; and for those >26.0mm, the SRK/T should result in the best outcomes. Complicating factors such as the postoperative effective lens position are also discussed. This section continues with discussions of axial length measurement in atypical eyes. The authors are very straightforward about the difficulties that can be encountered. Staphyloma eyes “may be the most frequent condition in which a precise AL measurement may not be obtained.†This book is recommended for residents during their cataract surgery rotation; fellows and young ophthalmologists who are still learning the complexities of the calculations; and more experienced surgeons looking to fine-tune their results.