This article is more than 5 years old. Information may no longer be current.
Determining effective lens position
Click Here to Manage Email Alerts
 The Krukenberg spindle of pigment dispersion syndrome is seen in this eye, which has also had prior radial keratotomy. |
One of the challenges of IOL calculations is determining exactly where in the eye the IOL will end up — the effective lens position. This is particularly complicated in this eye with radial keratotomy (RK) incisions and pigment dispersion syndrome.
The two-variable IOL calculation formulae (Holladay 1, Hoffer Q and SRK/T) use the keratometry value and axial length to determine the effective lens position. But these formulae can be fooled in unusual eyes. This patient has a measured average keratometry of 33 D, so the assumption would be that with such a flat cornea, the anterior chamber must be relatively shallow. However, that is not the case: The anterior chamber is deep, and the only reason the Ks are flat is that the patient has had prior RK.
The Krukenberg spindle on the corneal endothelium points to pigment dispersion syndrome. In this eye, there appears to be an unusual iris configuration and an anterior chamber that is unusually deep, which may have induced the physical loss and dispersion of iris pigment. This further complicates the IOL calculations because the anterior chamber depth is at the extreme.
To help make the IOL estimations (probably a better word than calculations) more accurate using the two-variable formulae, we can use the Aramberri double-K method. This involves using different keratometry values for the actual power calculation vs. the effective lens position determination. In this case, the estimation of true central keratometry (33 D measured, which probably means 30 D true) for the power estimation and then the preop Ks or even an average K (43 or 44 D) for the determination of effective lens position.
Finally, there can be a benefit to using newer formulae such as the Holladay 2 and Haigis. The Holladay 2 uses seven input variables, including anterior chamber depth and lens thickness, and allows for the double-K method to arrive at a more accurate result. Whatever method is chosen, it would be wise to aim for a somewhat myopic postop result, because the postoperative surprise is usually hyperopic in these eyes and residual myopia is useful, particularly in eyes that have undergone RK that may drift toward hyperopia over time.
See Dr. Devgan share more expert insight live at OSN New York 2010, to be held November 19-21, 2010 at the Sheraton New York Hotel & Towers. Learn more at OSNNY.com.