Aconstant conversion - Immersion Ultrasound Biometry to Optical Biometry
Situation 1 - Dr X has an optical biometry machine. However, the cataract is matured and the optical biometry machine fails to provide for the axial length. Dr X is forced to shift to immersion ultrasound biometry. What would be the aconstant for immersion ultrasound biometry for a lens which is otherwise quoted to be 119.0 with optical biometry.
Situation 2 - Dr Y is on immersion ultrasound biometry. His surgical (refractive ) outcomes are satisfactory. He intends to start a new IOL from a company which has quoted aconstat as 119.2 for optical biometry. What would the aconstant be for ultrasound biometry, that can be applied for the new lens with Dr Y's existing practice of immersion ultrasound biometry ?
Situation 3 - You have a personalized/optimized A constant for a particular IOL based on contact or applanation ultrasound biometry and you have recently purchased IOL Master. What would be the new A constant for the optical biometry of the same IOL.
Applying immersion biometry aconstant to optical biometry:
These are practical situations and questions I have come across often. The questions primarily stem from the belief that optical biometry measures from the Cornea Epithelium to the Retinal Pigment Epithelium ( Optical Path Length or OPL ) while ultrasound biometry measures till the Inner Limiting Membrane ( ILM ) which is often called Geometric Path Length ( GPL ). Therefore the general belief has been that the aconstant has to be changed to match the axial length calculated by two different technologies.
This is not entirely true. Yes, the optical biometry machines measure upto the RPE while the ultrasound biometry waves reach till the ILM. But this difference in path length is accounted for in optical biometry to match the readings from (immersion) ultrasound biometry. This was important to help optical biometry axial length to be applicable with current generation IOL calculation formulas like SRKT, etc, without having to change Aconstants. The conversion from OPL to GPL to match immersion based ultrasound biometry is applied by all optical biometry systems. Wolfgang Haigis calibrated the Zeiss IOL Master against immersion ultrasound biometery :
AL (Zeiss) = (OPL/1.3549-1.3033) / 0.9571 ; where AL(Zeiss) is the output Axial Length.
This formula made the reading output of Zeiss IOL Master based on PCI technology applicable to immersion based AL biometry.
So let us revisit the situations and see what is the solution
Situation 1 - Dr X has an optical biometry machine. However, the patient's cataract is matured and the optical biometry machine fails to provide for the axial length. Dr X is forced to shift to immersion ultrasound biometry. What would be the aconstant for immersion ultrasound biometry which is otherwise quoted to be 119.0 with optical biometry.
Possible solution : Do not change your (personalized/optimized ) optical a constant as you use the immersion ultrasound biometry for axial lengths less than 25mm. This is because Wolfgang Haigis had already calibrated to match Optical path Length ( distance from corneal epithelium to Retinal Pigment Epithelium) measured by IOL Master to match immersion ultrasound biometry ( Geometric Path Length - epithelium to Inner Limiting Membrane ).
However, Cooke et al in their publication in the JCRS, July 2022, found that for axial lengths greater than 25mm, their is a distinct advantage to reduce the optical Aconstant by .23 mm. This is to avoid hyperopic outcomes in long eyes.
Situation 2 - Dr Y is on immersion ultrasound biometry. His surgical (refractive ) outcomes are satisfactory. He intends to start a new IOL from a company which has only quoted optical biometry Aconstat as 119.2. What would be the aconstant for immersion ultrasound biometry, that can be applied for this new IOL with Dr Y's existing practice of immersion ultrasound biometry?
Possible soluition :
Use the Aconstant that is quoted by the company for optical biometry for less than 25 mm of AL. There should be no difference in Aconstant between immersion and optical biometry as the difference in axial length measurement has been calibrated in all optical biometry machines to match immersion ultrasound biometry. However, Cooke et al (JCRS July 2022) suggest that for eyes with AL greater than 25mm, .23 should be substracted from the optical biometry A constant. However, after atleast 30 cases do personalize the Aconstant based on refractive outcomes.
Situation 3 - You have a personalized A constant for a particular IOL based on contact or applanation ultrasound biometry (not immersion) and you have recently purchased IOL Master. What would be the new A constant for the optical biometry of the same IOL based on your existing applanation/contact ultrasound biometry .
Possible Solution A) Wolfgang Haigis has provided us a possible solution. This should work only when you have a personalized Aconstant for applanation ultrasound biometry and you are unsure of starting with optical biometry Aconstant provided by ULIB or IOLcon.org for the IOL.
Optical Biometry Aconst = Contact U/S Aconst +3* ( AL Optical Biometry - AL contact U/S Biometry ). You do not have to do the maths. Here is a quick converter for you. Use this calculator to get the optical biometry aconstant based on your personalized (or the aconstant that you have been using with good results) with contact or applanation based ultrasound biometry.
Possible Solution B): You may refer to IOLcon.org for an optimized optical aconstant of the same IOL as a start. Please do personalize the Aconstant based on atleast 30 cases with the IOL. For further personalization of Aconstant here is a simple spreadsheet I designed for you.
Zeiss IOL Master Manual; A Practical Operation Guide Choosing a Lens Modification Option.
Calculation of intraocular lens power: a review - Thomas Olsen; Acta Ophthalmol. Scand. 2007: 85: 472–474
Personal communications - Jack Holladay,MD.