The values for the Axial length and Keratometry obtained through the various methods as mentioned above should be consistent so as to rule out sources of error.
The need to repeat these measurements can be obtained based upon the HOLLADAY CRITERIA which states:
Repeat Km if:
Km47D for either eye
Difference between the two eyes >1 D
Corneal cylinder doesnot correlate well with the refractive cylinder
Repeat AL if:
AL< 22mm
AL> 25mm
Difference between the two eyes > 0.3 mm
Doesnot correlate well with refraction
Repeat all measurements if difference between both the eyes >1D
Repeat all measurements if >3d variation from a specific lens style
FORMULAE FOR IOL POWER CALCULATION
Theoretical formulas are the ones based upon the optics of …show more content…
For an eye with shorter axial length ELP decreases and increases for an eye with longer axial length. ELP also varies with type of IOL and the position of its optical centre.
Third generation formulas:
These formulas are a combination of empirical and theoretical formulas.
They include:
• Holladay-I formula: o It utilizes a constant known as the ‘Surgeon Factor (SF)’.The surgeon factor is distance between anterior plane of iris and Intraocular lens optic plane.
o The ‘SF’ depends upon lens style, lens manufacturer, surgeon technique’s etc.
ELP = anatomical ACD + SF
• SRK/T formula: o It is a theoretical formula o Accurate for eyes with long Axial length o It estimates using optimized postoperative ACD, corneal refractive index, optical axial length
Optical AL = Retinal thickness + AL
• Hoffer-Q formula: o It is a theoretical formula o Used for eyes with short Axial length
• Haigis Formula: o It utilizes three constants: a0, a1,
The need to repeat these measurements can be obtained based upon the HOLLADAY CRITERIA which states:
Repeat Km if:
Km47D for either eye
Difference between the two eyes >1 D
Corneal cylinder doesnot correlate well with the refractive cylinder
Repeat AL if:
AL< 22mm
AL> 25mm
Difference between the two eyes > 0.3 mm
Doesnot correlate well with refraction
Repeat all measurements if difference between both the eyes >1D
Repeat all measurements if >3d variation from a specific lens style
FORMULAE FOR IOL POWER CALCULATION
Theoretical formulas are the ones based upon the optics of …show more content…
For an eye with shorter axial length ELP decreases and increases for an eye with longer axial length. ELP also varies with type of IOL and the position of its optical centre.
Third generation formulas:
These formulas are a combination of empirical and theoretical formulas.
They include:
• Holladay-I formula: o It utilizes a constant known as the ‘Surgeon Factor (SF)’.The surgeon factor is distance between anterior plane of iris and Intraocular lens optic plane.
o The ‘SF’ depends upon lens style, lens manufacturer, surgeon technique’s etc.
ELP = anatomical ACD + SF
• SRK/T formula: o It is a theoretical formula o Accurate for eyes with long Axial length o It estimates using optimized postoperative ACD, corneal refractive index, optical axial length
Optical AL = Retinal thickness + AL
• Hoffer-Q formula: o It is a theoretical formula o Used for eyes with short Axial length
• Haigis Formula: o It utilizes three constants: a0, a1,