The challenge of

The challenge of abnormal corneas

An abnormal cornea may fool the chosen IOL power calculation formula, thus careful consideration to what formula is best in these eyes, as well as using all possible useful data, is essential, Thomas Olsen MD, Denmark, told delegates attending the 37th Congress of the ESCRS in Paris, France. Dr Olsen, the creator of the Olsen formula and software using the C-constant approach for the prediction of the IOL position, discussed the challenges of IOL calculations in the pathological cornea during the ESCRS/EuCornea Symposium: Cataract Surgery in Eyes with Diseased Corneas.

Since the advent of the original theoretical IOL power calculation formulas in the late 1960s there have been significant advances, Dr Olsen pointed out, referencing the development of improved thin lens formulas, improved biometry, estimated lens plane (ELP) and ray tracing methodology, leading to the newer statistical or hybrid fourthgeneration formulas.

However, Dr Olsen warned that pathological corneas can “fool” even the best formulas and be very challenging to choose the correct lens position.

Some IOL formulas assume a normal eye model for the K-reading for the estimation of the ELP (Barrett II, Hill-RBF, etc), while others do not depend on the K-reading for the estimation of the ELP (Olsen, Haigis, etc).

Dr Olsen focused on cases of post-LASIK cornea, keratoconus, megalocornea to highlight his points.

He quoted a post-LASIK study of 74 eyes with previous Rx correction ranging from -6D to -14D. All cases were measured pre-op with Lenstar biometry and Pentacam Scheimpflug photography. Even the best IOL calculations can be ‘fooled’ by the pathological cornea. Priscilla Lynch reports Thomas Olsen MD

PREDICTION ERROR Comparing the prediction error with four different calculation methods, the Olsen formula, using ray tracing to calculate lens power and the concept of the C-constant to predict implant location, had the best result (60.8% with a prediction error within 0.5D), with the poorest result from the Haigis formula (37.8%).

Dr Olsen also discussed a keratoconus case where ray tracing of Pentacam data was used in the calculation, and IOL implantation was performed as a staged procedure; postop unaided visual acuity was 20/40, while in a megalocornea case the prediction of the IOL position was calculated using the C-constant and there was also good visual results.

Concluding, Dr Olsen said there is still no perfect IOL power calculation formula for all eyes and better methods to obtain the true corneal power and to predict the best lens position are still needed, as are formulas that use all the information that can now be obtained from patients.