Korean J Ophthalmol Vol.22, No.4, 2008 Authors Cornea Aqueous, vitreous Crystalline lens PMMA IOL Silicone IOL Acrylic IOL - - - 2718 m/sec 980 m/sec 1900 m/sec - 1532 m/sec 1641 m/sec 2718 m/sec 980 m/sec 2200 m/sec Hoffer KJ30 1641 m/sec 1532 m/sec 1641 m/sec 2690 m/sec 980 m/sec - 1640 m/sec 1532 m/sec - 2780 m/sec 980 m/sec 2180 m/sec 1532 VIOL AL = ALM1532 + LT(1- ) = ALM1532 + CALF where AL=actual pseudophakic AL, ALM1532=AL measured at 1532 m/sec, LT=center thickness of the IOL, VIOL=US velocity of the IOL material, and CALF=corrected AL factor. The center thickness of an IOL is variable according to the manufacturer's design and the diopteric powers, both of which are not always available to the surgeon performing the IOL exchange. Assuming an acrylic IOL with an US velocity of 2180 m/sec and a central thickness of 0.7-1.0 mm, a CALF of 0.207-0.297 mm is obtained. An US velocity of 2200 m/sec, as used by Lee and Choi,29 yields a CALF of 0.212-0.303 mm. This potential variability in the CALF may partly explain the mean shortening of the AL observed just before IOL exchange. A limitation of this approach is that the material and/or central thickness of an opacified IOL are not always available in clinical practice. There are also concerns regarding the extent to which the AL measurement may be affected by various degrees of opacification. The standard deviations (intercase variability) of the prediction errors were generally greater in the IOL exchange group compared with the initial IOL implantation group and were slightly larger for the in-the-sulcus group than for the in-the-bag group. Altered optical properties of the opacified IOLs and a US velocity that was not adequate for IOLs with various degrees of opacification may be related to the increase in the mean and standard deviation of the prediction errors. Operator-related errors could have also been responsible, because each pair of AL measurements was performed by different technicians. Increased variability in the actual postoperative positioning of the IOL with regard to the initial IOL implantation surgery may also have been responsible. Jin et al.9 reported their experience with IOL exchange due to incorrect lens power and suggested that the refractive surprise after cataract surgery could be attributed to inherent errors in the biometry and formula, which may not be appropriate for certain eyes. Their observation may also provide some explanation for the increased prediction errors related to IOL exchange in our study. However, our data suggest that if the biometric data obtained before IOL exchange are available and of good quality, IOL exchange with implantation of a new IOL in a more stable position (in-the-bag position) may be associated with a sufficiently 234 Table 3. US velocities used by various authors for various ocular tissues and IOL materials at body temperature (35-37°C) Holladay JT27 Lowery MD and et al.28 Lee and Choi29 small amount of prediction error of refraction. To our knowledge, this is the first clinical report from the Republic of Korea regarding the clinical efficacy and complications of the IOL exchange procedure for an opacified IOL. Our data showed excellent visual recovery results and good results with regard to complications. Although the predictability of the postoperative refraction of the IOL exchange was relatively good using biometric data obtained before the initial cataract surgery and employing a new IOL implantation in-the-bag, we faced difficulties with the refractive predictions calculated using biometric measurements obtained just before the IOL exchange. Difficulties in making the AL measurement in eyes with an opacified IOL was the most probable cause. Although our study was limited by a relatively small number of cases, our experience suggests that good quality initial biometric data and new IOL implantation in the bag can produce good postoperative refractive results. Use of an US velocity appropriate for the IOL is also likely to result in improved refractive prediction. We believe that an IOL exchange procedure should be performed only after comprehensive informed consent is given by the patient. Dagres et al.22 mentioned that various symptoms and complaints reported by patients with opacified IOLs did not correlate well with their VAs and that VA alone was not a good criterion for performing an IOL exchange. The surgeon must be deliberate in making decisions regarding whether or not an IOL exchange surgery should be performed for an eye with an opacified IOL. IOL exchange surgery is the only effective treatment for patients with an opacified IOL. However, indications for IOL exchange are no longer limited to complications following IOL implantation and they are becoming even larger due to the increasing refractive demands of patients previously treated with IOL implantation. It would be worthwhile for ophthalmologists to become familiar with the procedures, efficacies, limitations, and complications associated with IOL exchange. We hope that further studies with a larger group of patients and proper stratification may help provide solutions to the problems that we encountered with IOL exchange. References 1. Kim JC, Kim CS, Choi SH, et al. Clinical characteristics of