In IOL power formulas, lenses are represented by IOL constants, namely:
|
Formula |
IOL constant C |
|
SRK II |
A constant |
|
SRK/T |
A constant |
|
HofferQ |
pACD |
|
Holladay-1 |
sf |
|
Haigis |
a0 |
|
Every IOL constant C depends on axial length L, corneal radius R, IOL
power P, refraction X, and - as far as the Haigis a0 constant is concerned - on
the preop anterior chamber depth d. Any change in one of the variables L, R,
P, X or d will cause a corresponding change in the respective IOL constant C. If a new axial length measuring device is bought, it may measure e.g.
0.3 mm longer than the old one. This change DL=0.3 mm in
axial length determination will be responsible for a change DC in the respective IOL constant. Likewise, if a new keratometer
consistently gave 0.10 mm steeper radii than an old one, this shift of DR=0.10 mm will again be reflected in a change DCof the respective IOL constant. |
|
Formula |
IOL constant |
Change D in
IOL constant |
|
SRK II |
A |
DA = 2.50 DL –
5.12 DR |
|
SRK/T |
A |
DA = 2.49 DL – 4.69 DR |
|
HofferQ |
pACD |
DpACD = 1.74 DL – 3.74 DR |
|
Holladay-1 |
sf |
Dsf = 1.65 DL –
3.37 DR |
|
Haigis |
a0 |
Da0 = 1.88 DL –
3.97 DR – 0.40 Dd |
|
Example: (let the following
be averages for e.g. 20 patients, obtained with different measurement
instruments): |
|
Averages [mm] |
Ultrasound/keratometer |
IOLMaster |
Difference D |
|
Axial length L |
23.32 |
23.48 |
0.16 |
|
Corneal radius R |
7.73 |
7.63 |
-0.10 |
|
Anterior chamber depth d |
2.80 |
3.08 |
0.28 |
|
The new measurement setup causes the following
changes in IOL constants: |
|
Formula |
Change
D in IOL
constants |
||
|
SRK II |
DA =
2.50 DL –
5.12 DR |
= 2.50 x 0.16 + 5.12 x 0.10 |
= 0.91 D |
|
SRK/T |
DA =
2.49 DL –
4.69 DR |
= 2.49 x 0.16 + 4.69 x 0.10 |
= 0.87 D |
|
HofferQ |
DpACD =
1.74 DL –
3.74 DR |
= 1.74 x 0.16 + 3.74 x 0.10 |
= 0.65 mm |
|
Holladay-1 |
Dsf =
1.65 DL –
3.37 DR |
= 1.65 x 0.16 + 3.37 x 0.10 |
= 0.60 mm |
|
Haigis |
Da0 = 1.88 DL – 3.97 DR – 0.40 Dd |
= 1.88 x 0.16 + 3.97 x 0.10 – 0.40 x 0.28 |
= 0.59 mm |
|
Accordingly, the new constants are calculated from
the old constants as follows : |
|
Formula |
New (IOLMaster) constants |
|
SRK II |
IOLMaster A-constant = old A-constant + 0.91 D |
|
SRK/T |
IOLMaster A-constant = old A-constant + 0.87 D |
|
HofferQ |
IOLMaster pACD = old pACD + 0.65 mm |
|
Holladay-1 |
IOLMaster sf = old sf + 0.60 mm |
|
Haigis |
IOLMaster a0 = old a0 + 0.59 mm |
|
These results were in part presented at the 2001
ASCRS meeting in HAIGIS W, STROBEL J: Influence of measurement
conditions on IOL constants. |
Last revision: Jan 11, 2010