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JUST as pseudophakic IOL power calculation is problematic in eyes that have undergone laser vision correction surgery, so too might implantation of a phakic IOL introduce the potential for IOL power calculation errors if cataract surgery becomes necessary, said Kenneth J. Hoffer MD at the XXI Congress of the ESCRS. He has coined the terms "biphakia" and "biphakic" to describe those latter eyes, and explained that the source for error is inaccurate ultrasound-measured axial length due to the effect of the phakic implant on ultrasound velocity through the eye. "When performing an A-scan, the assumption made by the instrument is that the ultrasound is travelling through the eye at an average speed of 1555 m/sec. However, that value can be altered considerably when a phakic IOL is present, depending on the material of the implant and its power (central thickness). Accounting for this alteration may be important for accurate determination of axial length and pseudophakic IOL power," Dr. Hoffer explained.
The speed of ultrasound through the various materials used in phakic IOL construction varies widely. Relative to its average speed of travel through the normal phakic eye, ultrasound goes faster through PMMA, acrylic, and collamer at rates of 2660, 2026, and 1740 m/sec, respectively, while it is slowed down to 980 m/sec by silicone, Dr. Hoffer said. However, the impact of the implant on ultrasound-determined axial length depends further on its power. Overall, it is greater for hyperopic lenses that have a thick centre than for the relatively thin myopic phakic IOLs. Dr. Hoffer has developed a simple model to allow correction of the axial length, taking into account IOL material and power, and he has described it in a recently published article (Hoffer KJ, J Cataract Refract Surg 2003;29:961-5). He explained that correction of the ultrasound-measured axial length involves subtracting the erroneous distance measured through the phakic implant and adding back the actual thickness of the IOL (TA).
The unknown erroneous distance (i.e., the erroneous lens thickness [TE) can be solved for by knowing that the ratio of the implant's actual thickness (TA) to the actual velocity of ultrasound through the phakic IOL material (VA) is directly proportional to the ratio of the erroneous thickness (TE) to the erroneous velocity used by the ultrasound instrument (TE): TA/VA = TE/VE. Thus, TE = TA x (VE/VA). By substituting for TE in the formula for calculating the corrected axial length ( AL ) and combining terms, a final formula for corrected AL is derived: AL (corrected) = AL(measured) + TA(1- VE/VA).
In an appendix to his article, Dr. Hoffer lists the TA values for all phakic IOLs across all materials and their entire range of available powers. Knowing VA for each implant material and using the formula for corrected AL , he calculated that in eyes implanted with a PMMA phakic IOL, it is necessary to add 42% of the implant thickness to the measured axial length to arrive at the corrected value. For acrylic, the adjustment factor is +23% of the phakic IOL thickness, while it is only 11% for the collamer ICL. Since ultrasound velocity is slowed by silicone, in eyes implanted with the PRL, it is necessary to subtract 59% of the implant thickness to calculate the correct axial length. The accuracy of the formula will be tested in the real world by evaluation of the refractive outcomes achieved with its use, he added.
Kenneth J. Hoffer MD, FACS Clinical Professor of Ophthalmology, Jules Stein, Los Angeles
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