Fast OBL (opaque bubble layer)
Normal bubble pattern
6.3 Patient Positioning
The VisuMax® laser has a curved interface which applanates the patient’s cornea with a low corneal suction. The advantage of this system is that patient fixation is maintained even after suction is applied and incidence of subconjunctival hemorrhage is negligible, but the downside is higher incidence of suction loss. Any obstruction to the movement of the gantry will result in a higher incidence of suction loss during the procedure, and this can be avoided by proper patient positioning before applanation and suction application.
Proper exposure can be attained by elevating the chin and turning the face in the opposite direction. The head rest of the VisuMax® bed can be moved up and down to obtain an optimal chin elevation, without making the patient uncomfortable.
This maneuver is particularly useful in patients having a prominent nose, and it is prudent to ensure that there is a clear gap from the patient’s nose and laser gantry before corneal applanation and suction. It is also important to orient the suction tube temporally.
6.4 Nomogram Adjustment
Results vary with every laser and every surgeon, and it is imperative that all surgeons develop a nomogram adjustment to refine their results. I have found that there is a tendency for undercorrection with my initial results of SMILE. The refraction is also pupil dependent , and I like to add 10 % to myopic astigmatic treatments in patients who are between 20 and 30 years and if scotopic pupil size is larger than 6 mm.
6.5 Astigmatism Correction
The VisuMax® does not have cyclotorsion compensation, but it is important to correct for cyclotorsion in higher cylinders. Cyclotorsion can occur for various reasons : (1) cyclotorsion occurring naturally from sitting to supine position, (2) positioning of head and turning the face, (3) speculum and patient’s resistance to it and Bell’s phenomenon, and (4) applanation and suction can induce cyclotorsion due to difference in the corneal curvature and curvature of the interface.
Manual compensation for cyclotorsion: The 0–180° axis is marked on the limbus extending about 2 mm on either side onto the cornea with the patient sitting upright. Take care so that the marking does not extend into the optical zone of the laser as this may interfere with the laser delivery and make dissection tougher. Following applanation and suction, check for the alignment of the 0–180 marks with respect to the horizontal line on the reticule of the microscope ocular or the VisuMax screen, then rotate the patient interface (suction cup) so that it is aligned before proceeding with the laser delivery (Video 6.1).
I prefer to correct for cyclotorsion in cases where the cylindrical power is greater than 0.5 D. I also like to overcorrect astigmatism by 10 % as per my nomogram correction and use an optical zone of more than 6.5 mm. This gives me a good postoperative outcome even in high cylinders and pure astigmatism.
6.6 Applanation and Suction
Angle kappa and centration: VisuMax does not have pupil centration and tracking. The centration is along the patient’s line of sight and may be away from the pupil center if the patient has a large angle kappa. Asking the patient to look at the microscope light gives an idea about the angle kappa apart from the preoperative topography, which will also give this information. This is important in patients who have a large angle kappa as after applanation and suction you may find the centration away from the pupil center and you may unnecessarily try to undock and correct for this. Patient fixation and centration along the line of sight gives a better centration of the corneal flattening and visual outcomes than pupil centration .