The Lens Haptic Plane (LHP) a Fixed Reference for IOL Implant Power Calculation

OBJECTIVE: To establish an unambiguous method to assign A-constants and similar constants required by the various formulas in use for the calculation of intraocular lens (IOL) implant power, without recourse to clinical investigations. METHOD: Retrospective analysis of clinical data involving widely different IOL designs and several surgeons was performed to find the position of the lens haptic plane (LHP). The clinical data had been generated previously for entirely different purposes. Inclusion criteria were visual acuity of 0.5 (20/40) or better and availability of a postoperatively (3-12 months) measured ACD (distance from anterior cornea to anterior IOL). Ray-tracing calculations were performed to find the ACD compatible with the optical data (preoperative L and K, and IOL power). The exact design (front and back radii, and central thickness) of the IOL was used in these calculations. The calculated ACD was verified against the postoperatively measured ACD. Lens haptic plane was calculated from ACD using the geometry of each particular design. RESULTS: Data from 688 cases, 8 different clinics and 4 IOL models were analysed. On average the calculated ACD coincided with the postoperatively measured ACD when applying paraxial ray-tracing and thick lens theory (Gaussian optics). An average eye with K = 43.63 D, L = 23.24 mm and LHP = 4.02 mm was established. For any new IOL design, the power to make this average eye emmetropic can be determined by applying paraxial ray-tracing. Subsequently the A-constant or any other formula constant can be determined. LEMMA: There is no need to utilize constants inherent to various formulas. The calculation model used to find the constants could just as well be used directly to find the exact implant power, provided that the position of the LHP is known and that the manufacturer supplies the necessary design information. Because the LHP concept is independent of IOL model, the advantage would be that the surgeon could concentrate on refining his technique to estimate LHP preoperatively, rather than personalizing constants for every new IOL model.