In patients following Rx Sx the assumtions of the keratometry are incorrect
This results in a hyperopic shift in myopic correction and myopic shift in hyperopic correction.
Error due to different refraction index, different ACD/AL, different corneal radius (K).
Hyperopic surprises after myopic correction greater than myopic surprises after hyperopic correction dt > central changes in myopic correction.
Historical method: subtract postop keratometry (after stabilization, before nuclear sclerotic myopic shifts) from preop keratometry at corneal plane - does not take into consideration cataractous myopic shift
Hard Contact lens method: subtract the change in refraction (SEQ) following HCL wear
Use another formula which does not take into account ELP for calculating IOL power Haigis
Use online calcs ESCRS ASCRS DocHolladay etc.
Use topography (EKR) taking into account both anterior and posterior corneal surfaces and measuring instead of assuming the central cornea