Optimizing Biometry for Best Outcomes in Cataract Surgery
Optimizing Biometry for Best Outcomes in Cataract Surgery
The formulas in widest use utilise two biometric measurements (axial length and keratometry) and a single IOL constant (Hoffer Q, Holladay 1, and SRK/T). The Haigis formula uses three measurements (axial length, keratometry, and pre-operative anterior chamber depth) and three IOL constants, Olsen's formula five uses measurements (those used for the Haigis, plus pre-operative refraction and lens thickness) and one IOL constant, and the Holladay 2 formula uses seven measurements (those used by Olsen, plus patient age, and the horizontal white-to-white measurement) and one IOL constant.
There is presently no single formula that is suitable for all eyes. There are many studies comparing the performance of various formulas in different patient groups, but often the number of eyes analysed is small. The most authoritative comparison to date is that of Aristodemou et al, which evaluated the performance of the Hoffer Q, Holladay 1, and SRK/T formulas in 8108 eyes. The authors concluded that the Hoffer Q was best for axial lengths below 21.5 mm and the SRK/T for those above 26.0 mm. For the intervening range, there was no statistically significant difference between the formulas, although the Holladay 1 showed a modest performance advantage over the others.
The T2 formula is a modification of SRK/T, which corrects for systematic non-physiological behaviour in the latter, and which results in a 10% improvement in PE. The Haigis formula is reported to give accurate predictions for both short and long eyes, but Olsen's formula may produce results slightly better than Haigis'. The Holladay 2 formula is suggested to be 'one of the more accurate formulas available', but there is little systematic evidence for this assertion. In the absence of compelling comparative evidence of the superiority of these formulas, it is justifiable to continue using an appropriate combination (according to axial length) of the two variable single constant formulas.
Formula Choice
The formulas in widest use utilise two biometric measurements (axial length and keratometry) and a single IOL constant (Hoffer Q, Holladay 1, and SRK/T). The Haigis formula uses three measurements (axial length, keratometry, and pre-operative anterior chamber depth) and three IOL constants, Olsen's formula five uses measurements (those used for the Haigis, plus pre-operative refraction and lens thickness) and one IOL constant, and the Holladay 2 formula uses seven measurements (those used by Olsen, plus patient age, and the horizontal white-to-white measurement) and one IOL constant.
There is presently no single formula that is suitable for all eyes. There are many studies comparing the performance of various formulas in different patient groups, but often the number of eyes analysed is small. The most authoritative comparison to date is that of Aristodemou et al, which evaluated the performance of the Hoffer Q, Holladay 1, and SRK/T formulas in 8108 eyes. The authors concluded that the Hoffer Q was best for axial lengths below 21.5 mm and the SRK/T for those above 26.0 mm. For the intervening range, there was no statistically significant difference between the formulas, although the Holladay 1 showed a modest performance advantage over the others.
The T2 formula is a modification of SRK/T, which corrects for systematic non-physiological behaviour in the latter, and which results in a 10% improvement in PE. The Haigis formula is reported to give accurate predictions for both short and long eyes, but Olsen's formula may produce results slightly better than Haigis'. The Holladay 2 formula is suggested to be 'one of the more accurate formulas available', but there is little systematic evidence for this assertion. In the absence of compelling comparative evidence of the superiority of these formulas, it is justifiable to continue using an appropriate combination (according to axial length) of the two variable single constant formulas.