Linear regression software for intraocular lens implant power calculation

Regression equations for the calculation of intraocular lens power are popular. Because some of the variables in cataract surgery are surgeon-specific, every surgeon should ideally have a personalized formula. We have developed software enabling those with access to IBM-compatible microcomputers to develop equations based on their own data.     

Development of the SRK/T intraocular lens implant power calculation formula

A new implant power calculation formula (SRK/T) was developed using the nonlinear terms of the theoretical formulas as its foundation but empirical regression methodology for optimization. Postoperative anterior chamber depth prediction, retinal thickness axial length correction, and corneal refractive index were systematically and interactively optimized using an iterative process on five data sets consisting of 1,677 posterior chamber lens cases. The new SRK/T formula performed slightly better than the Holladay, SRK II, Binkhorst, and Hoffer formulas, which was the expected result as any formula performs superiorly with the data from which it was derived. Comparative accuracy of this formula upon independent data sets is addressed in a follow-up report. The formula derived provides a primarily theoretical approach under the SRK umbrella of formulas and has the added advantage of being calculable using either SRK A-constants that have been empirically derived over the last nine years or using anterior chamber depth estimates.     

Intraocular lens power calculation for lens exchange

PURPOSE: To examine patients who had intraocular lens (IOL) exchange for large postoperative refractive errors and determine the factors that contributed to the error in IOL power calculation. SETTING: Thirteen affiliated hospitals in Japan. METHODS: This study comprised 34 cases that required IOL exchange because of large refractive errors after primary lens implantation. Patients with intraoperative complications were excluded from the study. The potential contribution of axial length, corneal refractive power, IOL manufacturer, and IOL fixation to errors in the predicted power was examined retrospectively. Axial length was calculated by the SRK/T and Holladay formulas using refraction after primary IOL implantation. RESULTS: There was no statistical difference between the corneal refractive power before and after cataract surgery. The axial lengths calculated using the SRK/T and Holladay formulas were longer than the ultrasonic axial lengths in 24 and 23 cases, respectively. Using IOLs from the same manufacturer for both primary implantation and exchange reduced the error in predicted refraction. CONCLUSION: Axial length and IOL manufacturer were important factors in predicting refraction power in eyes requiring IOL exchange.     

Value of a new non-contact biometer for intraocular crystalline lens power calculation

PURPOSE: One of cataract surgery's current imperatives involves refraction: the power of the lens implant must be calculated as accurately as possible. Here we present a new method of biometric ocular measurement using the partial optical consistency interferometer. MATERIAL AND METHODS: This investigation studied the axial length measurement of 100 eyes. Five measurements were taken with a classic echobiometric contact technique using the ultrasonic mode; 5 others were taken with the infrared noncontact technique (IOL Master, Zeiss Humphrey). The latter technique is based on interferometric biometry with optical consistency and measurements were taken with an infrared luminous ray. With extreme rapidity and no contact, the device provides a complete biometry, including axial length, keratometry, and anterior chamber depth. It includes a built-in computer. RESULTS: Comparing the ultrasonic and infrared measurements emphasizes the precision and particularly the high reproducibility of the infrared method. The standard deviations of the samples were significantly lower for the 100 measurements. Its limitations depends on the type of cataract since success was not obtained for certain posterior subcapsular opacities. DISCUSSION: This new method of performing a biometry with a partial consistency interferometer contributes a number of advantages: speed, its noninvasive nature with no contact, the high reproducibility of the exam, as well as precise measurements as shown by the difference in the standard deviations of the two methods. CONCLUSION: Biometry using the optical consistency interferometer seems to be a reliable, reproducible, and precise technique that brings great precision for the calculation of the power of the intraocular implant in cataract surgery.     

儿童人工晶状体屈光力的计算

选择准确合适的人工晶状体是儿童白内障手术的关键,儿童人工晶状体屈光力的计算存在较大的预测误差,这是由测量误差和人工晶状体计算公式误差造成的,所以恰当地选择各类测量仪器和人工晶状体计算公式很重要.Holladay 2公式对短眼轴患儿计算人工晶状体数值更准确.应用该公式时,需要测量7个参数,即角膜白到白直径、晶状体厚度、眼轴长、角膜屈光力、术前前房深度、术前屈光状态及年龄.该公式一定程度上实现了人工晶状体屈光度数计算的个性化,但所需参数多,对患儿配合要求高.另外,随着年龄的增长以及眼球的发育成熟儿童眼睛屈光度仍会改变,所以我们还应选择正确的术后屈光目标.     

Intraocular lens power calculation in children

With improving surgical technique and equipment, the acceptable age for placing an intraocular lens in infants and children is becoming younger. The tools for predicting intraocular lens power have not necessarily kept up, as current theoretical and regression intraocular lens power prediction formulas are largely based on adult eyes at axial lengths, anterior chamber depth, and keratometric values much different than those seen in infants. In addition, the adult eye has matured and is no longer growing, whereas the eyes of infants and children may continue to note changes in axial length, keratometric values, and possibly optical characteristics. Another source of error in intraocular lens power selection that is more likely to occur in pediatric patients than in adult patients is inaccuracy in measurement of axial length or keratometric power. A review of current tools and considerations for intraocular lens power prediction in infants and children is presented.     

The emmetropic and the iseikonic implant lens: computer calculation of the refractive power and its accuracy.

The preoperative calculation of the power of a lens prosthesis and the resulting aniseikonia is given, as well as diagrams for reading the emmetropizing and the iseikonizing power of the lens. The consequences of an inaccuracy in the measurement of the dimensional and refractive properties of the eyes are calculated. It can be concluded that the main determinant for an accurate prediction of the power of the intraocular lens prosthesis is the axial length measurement. Since this measurement is carried out by means of ultrasound biometry, it is necessary to emphasize the adequate application of this technique. The problem as to whether an emmetropizing, or an iseikonizing lens should be implanted may be solved with the data supplied by the computer program. A flow chart for this program is given in an appendix.     

Biometry and intraocular lens power calculation

PURPOSE OF REVIEW: Heightened patient expectations for precise postoperative refractive results have spurred the continued improvements in biometry and intraocular lens calculations. In order to meet these expectations, attention to proper patient selection, accurate keratometry and biometry, and appropriate intraocular lens power formula selection with optimized lens constants are required. The article reviews recent studies and advances in the field of biometry and intraocular lens power calculations. RECENT FINDINGS: Several noncontact optical-based devices compare favorably, if not superiorly, to older ultrasonic biometric and keratometric techniques. With additional improvements in the internal acquisition algorithm, the new IOL Master software version 5 upgrade should lessen operator variability and further enhance signal acquisition. The modern Haigis-L and Holladay 2 formulas more accurately determine the position and the shape of the intraocular lens power prediction curve. SUMMARY: Postoperative refractive results depend on the precision of multiple factors and measurements. The element with the highest variability and inaccuracy is, ultimately, going to determine the outcome. By understanding the advantages and limitations of the current technology, it is possible to consistently achieve highly accurate results.     

Biometry and intraocular lens power calculation

This article surveys the literature of 1 year, between July 2003 and August 2004, on the topic of biometry and intraocular lens power calculation for cataract surgery. There is an increasing demand for low postoperative refractive error with rising patient expectations, especially with patients who have already undergone refractive surgery, and with developing intraocular lens technologies such as multifocal, accommodating, or toric intraocular lenses. Optical biometry has become an invaluable tool for axial length measurement, especially for a setting with a less experienced biometrist. Introduction of ray tracing for power calculation and new methods of dealing with power calculation in eyes that have undergone previous refractive surgery seem promising. New intraocular lens designs that allow adjusting the axial optic position and therefore the effective refractive power of the intraocular lens have been evaluated in animal studies.     

Significance of intraocular lens power calculation.

A total of 94 patients underwent extracapsular cataract extraction and insertion of Sinsky style two-loop posterior chamber intraocular lenses. Forty-six eyes received a standard power IOL and 48 eyes were given a preoperatively calculated IOL. A significant difference was found in the two groups with regard to the postoperative refractive error and uncorrected visual acuity.     

人工晶状体计算公式比较

目的 以临床资料验证比较SRK－Ⅱ公式、SRK／T公式和SCDK公式的准确性。方法 选择380只老年性白内障眼,术后3个月以上测量眼轴长度、角膜屈光度、眼屈光度,将测量值分别代入3个公式计算平均绝对屈光误差值,比较3个公式的准确性。结果 在眼轴长度小于26mm时,3个公式计算的平均绝对屈光误差值相差不大。眼轴长度大于26mm时,SRK－II公式的平均绝对屈光误差值为1．42D,SRK／T公式误差值为0．84D,SCDK公式误差值为0．89D;屈光误差值大于2D者,SRK－II公式为15．53％,SRK／T公式为4．74％,SCDK公式为5．26％。结论 在眼轴长度大于26mm时,SCDK和SRK／T公式准确性明显高于SRK－Ⅱ公式,SCDK和SRK／T公式的准确性相差不大,均高于SRK－Ⅱ公式。     

Intraocular lens implant power calculations: investigations controlling for lens type

Many second generation intraocular lens power calculation formulas have recently been introduced. This study explores the performance of these formulas while controlling for a potential source of variation--the lens type. For this study, all 1,157 cases studied used the Cilco CPLU posterior chamber lens. All surgeries were performed using similar phacoemulsification techniques by only two physicians (R.M.C. and S.C.G.). The SRK, SRK II, Holladay, and Binkhorst formulas were compared among themselves and also with a piece-wise nonlinear regression formula ("best fit") developed specifically from these data by the authors. Performance of the SRK II, Holladay, and best fit were better than the older SRK and Binkhorst for most axial length ranges. For these data, the Holladay and best fit formulas performed marginally better overall than the other formulas. It was also found that manipulation of specific surgeon constants significantly affected the performance of the Binkhorst formula, but had little effect on the other formulas.     

Intraocular lens power calculation in short eyes

PURPOSE: To compare the accuracy of the Hoffer Q and SRK-T formulae in eyes below 22 mm in axial length, using biometry measured with partial coherence inferometry (PCI), without a customised ACD constant. METHODS: Data were retrospectively and prospectively collected by identifying eyes of axial length below 22 mm in the records of the intraocular lens (IOL) master and in preoperative notes. Biometry was performed using PCI and IOL power was calculated using both SRK-T and Hoffer Q formulae. Refractive outcome was measured and the accuracy of the two formulae compared. RESULTS: Forty-one eyes of 41 patients were identified with an axial length <22 mm. Axial lengths ranged from 21.96 to 20.29 with a mean of 21.51 mm, and IOL power ranging from 23 dioptres (D) to 29 D. The Hoffer Q formula showed a mean prediction error of 0.61 D (SD 0.80) compared with the SRK-T, which showed a mean prediction error of 0.87 D (SD 0.829). A paired t-test found that the Hoffer Q was significantly more accurate than the SRK-T formula (P<0.001). CONCLUSIONS: Hoffer Q was found to be more accurate than the SRK-T formula in this series of eyes <22 mm axial length when customised ACD constants are not used. Royal College of Ophthalmologists guidelines may need to be adjusted in accordance with these findings. This study underlines the importance of monitoring outcomes, and suggests different customisations are needed for different formulae, with a higher correction if the SRK-T formula is used for short eyes.