Rotation of toric implant
O-rings implants are placed in the cataract surgery in patients who present a astigmatism corneal, and wish to not wear glasses in vision by far (or close) after surgery. Some Diffractive multifocal implants are available in ring version (ex: implant AT Lisa toric, Zeiss). The power of the implant is calculated through biometric examination: in the case of an implant, o-ring, the power of the implant contains a spherical component and cylindrical (implant generates astigmatism). The implant should be aligned according to an axis, which depends on the corneal astigmatism, and the incision made during the cataract surgery. The presence of marks on the implant (next to the less powerful meridian of the implant) allows to identify the orientation of the implant during surgery.
The implant manufacturers offer calculators 'online', where the surgeon between useful data in the calculation of the implant before cataract intervention (axial length, value of powers and extreme IOL axes, etc.). Generated by the calculated implant astigmatism "cancels" one that is generated by the operated eye cornea.
One axis error in the final orientation of the toric implant results in a correction of astigmatism correction. An error of 15 ° results in an under 50% of astigmatism correction: a 30 ° axis error results in the absence of reduction in the magnitude of total ocular astigmatism. A 90 ° error logically results in a doubling of the value of the operating cylinder in post.
Axis errors observed in clinical practice are generally less; However, an error of 10 ° can induce the persistence of an astigmatism can reduce significantly the acuity uncorrected in post operative.
It is important to identify and quantify the error of axis. THEOPD SCAN III is a topographer, aberrometer, which includes a software module designed to help the surgeon to manage the correction of corneal astigmatism in cataract surgery with o-rings implants.
Here is the map OPD SCAN obtained after cataract surgery and an o-ring implant (Zeiss, AT Torbi): there is a residual astigmatism eye whose magnitude is-2.25 x 95 °.
Biometrics in preoperative suggested a toric implant of a spherical power of 20 d and a cylindrical power of + 4.50 D facing 54 ° (in this case figure, the implant has a higher vergence 4.50 diopter according to the axis perpendicular to 54 °, or 144 °). Post operative target refraction was-2.50 D (without astigmatism).
The calculation of internal astigmatism reveals that astigmatism (related to the o-ring placed implant) is oriented to 65 ° (lower vergence axis) / 155 ° (largest vergence axis): it is a mistake to close axis of 11 ° (65 ° - 54 °).
The use of the "Toric IOL Summary" module allows to precisely measure the error of axis. Located on the implant brands are visible (mode retro illumination). By aligning a crosshair on the axis of these brands, the software automatically calculates the value of the angle with the axis of the arched corneal Meridian. In the clinical example, this value is estimated at 12 °.
A vector calculation (see the page on) astigmatism and axis error) made for a cylindrical correction of 4.50 D 'shifted' to 12 ° predicted postoperative astigmatism persistence close to that which is reached the operated eye. This confirms the interest of a rotation of the implant to 12 ° in the opposite direction to position it according to the desired axis.
Using Callisto (Zeiss) system allows the identification of the axis in real time during surgery thanks to a digital camera built into the operating microscope Zeiss Lumera. In this specific case, ink brands were affixed next to the axis of the implant, and act as 'reference' for the Callisto system axis. The features "superimposed" on the screen are "added" by the system (augmented reality) based on the data entered by the user and materialize the axis that will have to be realigned the implant (this axis is located at 12 ° in a clockwise direction with that of the implant).
Inserting a surgical manipulator microphone allows you to easily turn the implant into the capsular bag, under protection visco elastic, so as to bring the implant in the desired axis.
In post operative, the OPD SCAN III (module "Toric IOL summary") used to determine the positioning of the implant.
Optical measures objectify total corneal astigmatism correction, attesting to the interest of the position of the o-ring implant re.