Actual position of the implant
The actual position of the implant is the position the implant in the eye after cataract surgery. It affects the value of the final refraction, because more implant is "far" from the cornea, and less effective the couple horny + crystalline power is important. From this position,. that must be anticipated (and is then based on an "assumption") when calculatingdepends on the power calculated for the implant (biometrics). Modern biometric calculation formulas differ mainly in the way in which she "predict" the actual position of the implant.
This implant power, expressed in diopters (unit of the vergence: 22D for example) is intended to give the eye the desired refraction. For a same eye (axial length and keratometry fixed) and a same target refraction, more the implant in a close position of the cornea, and less power is high. Conversely, if the position of the implant is later (further), then its power must be higher. A difference of one to two diopters in the plan of the implant (0.5 to 1 5 D in the plan of a bezel correction) can be obtained depending on the position of the implant.
The following illustration were achieved through the biometer IOL Master 700 (Zeiss):
If an eye is Emmetropic perfectly (no glasses for distance vision) after the cataract surgery, is that the calculation of implant was made by predicting a position of the implant very close to that obtained in fine, after the intervention. If the implant has a different position, a myopia (implant located a bit further than expected) or a farsightedness (implant located a bit further back) will be observed.
Here is a list of the reasons that explain the inability to predict with certainty the position of the implant in the eye after the surgery:
-the lens has a large volume: when it is removed, the volume of the casepulaire bag is by definition reduced:
-the volume and the implant geometry differs from that of the lens: the implant consists of an optical (6 mm diameter), supported by the 'legs' (haptic), which may be of the same material as Optics (one-piece said implant) or different composition (implant "3 pieces": a perspective and two haptic).
-implants differ from share their geometry and their final position in the capsular bag (size of the implant, angulation of the optics live in the haptics of the implant screw, etc.). The focal power of the implant should be measured from the master plan image of it: the position of this master plan differs depending on the refractive index, the thickness and the curvature of the surfaces of the implant.
-Preoperative anatomic depth of the anterior Chamber is of course correlated with depth anatomical post operative: but this correlation is weak: for example, in case cataract evolved, the previous 'protrusion' of the reduced lens depth of the anterior Chamber: it greatly deepens after withdrawal of the cristallinien material. Pre operative of the lens thickness also plays a role in the final position of the implant.
Here is an illustration of two cuts of the House prior to the eye, made by Scheimpflug camera, before and after cataract surgery. There is a deepening of the room earlier of the eye, related in part to the withdrawal of the volume of the lens which tends to protrude "" in front of the pupil (the volume of the lens increases with time).
The following figures also show these data.
The first is an image cut OCT of the anterior segment of the eye of a patient with bilateral cataracts. The eye is a hymermetropie, the lens is large and slightly push the iris plan. The preoperative anterior chamber depth is reduced.
After removing the lens (cataract surgery by phaco emulsification) and asked for the implant, the OCT shows a deepening of the anterior Chamber.
Therefore, it is important to understand this point: ocular biometry with calculation of implant contains a bit of extrapolation: the final (or actual) position of the implant in the eye.
To address this uncertainty, manufacturers have initially proposed to use biometric calculation 'average' value, called "A constant", which allows to perform the biometric calculation based on a position 'average' of the implant in the eye. This value was proposed after completing studies on eyes operated on cataract and who received the implant concerned. Based on the gap between desired and obtained refractions (biometric error) it is possible statistically to find for what average value of position of the implant the error is minimal. This method requires that the other sources of error (measure of the axial length, corneal power rating) are the lowest possible.
Average effective location of the implant
The average position of the implant is provided by the manufacturer, not directly, but in the form of a constant expressed in diopters: this "convention" is related to the first generation formulas, including the so-called "empirical formula" or "regression" called SRK (from the initials of its authors). This formula is abandoned, but the SRK - T (T for theoretical) formula is still used for most of the eyes (as long as they are not too 'short' - the axial length must be greater than 22 mm). SRK - T form part of the "theoretical" formulas, because it relies on a physical-mathematical model of the eye. This constant is related to the distance between the corneal vertex and the plan view (master plans) of the implant: should not be confused with the anterior chamber depth, which is a pre-operative anatomical data (distance between the top of the cornea and the front part of the lens). After the intervention, the depth of the anterior Chamber may be estimated based on the distance between the top of the cornea and the iris plan. The optics of the implant position is different from the plan of the iris. It depends on various factors, such as the geometry of the implant, the thickness of his view, the geometry of the capsular bag (and its insertion on the ciliary trial of the eye), etc. In addition, the surgical technique used is also able to influence this average position slightly. Thus, the variable "average position of the implant" saw substitute that of 'Sucker factor (SF)' or 'Factor related to the surgeon' (formula of Holladay).
The constant A is expressed in diopters: it is possible to connect it to the value of the average actual position (PEM) of the implant through the following formula:
EMP = ((A x 0.5663 constant) - 65.6 + 3.595) / 0.9704)
The constant A is also convertible into "Sucker Factor" (SF):
SF = (constant a. x 0.5663) - 65.6
The value of the constant A is usually close to 120 D. In the first formula, aimed to convert to "average position" (mm), observed that the introduction of such value (120) module about the value of the PEM around 5 mm.
In the second formula, the resulting value is lower: this value is a 'fix' to apply to a constant representing the average position of the implant.
Actual position individualized implant
The individualization of the actual position predicted the implant using biometric data for a given eye is at the heart of the challenges of modern biometric calculation formulas. The first author to have intentionally changed the value of the constant was Binkhorst; He proposed to increase the value of the actual position of the implant in proportion to axial length measured. The accuracy of the prediction of the actual position for a particular eye determines the quality of the refractive result. In my eyes 'atypical', or operated in the past of refractive surgery, the assumptions of the 1st, 2nd and 3rd generation formulas (formulas of implant) proved wrong. 4th and 5th generation formulas are more able to make a more accurate calculation.
The influence of the actual position of the implant is illustrated in the calculation of the power of the implant after surgery (refractive)LASIK for myopia), in a cataract patient once it had been LASIK surgery for myopia ten years previously.
The biometric calculation, designed to provide the power of the iplant, can be accomplished using two different formulas. These calculations are based on the same elements (keratometry, axial length, and depth of the anterior Chamber to Haigis formula). However, they provide slightly different results, because formulas are predicting a different effective position of the implant.