OPTICAL ZONES, THIN CORNEAS, HIGH MYOPIA AND LARGE PUPILS
The future just came to Alaska with the new Variable Spot Scanning (VSS) and large 8.00 mm. blend optical zones. Now people with larger pupils under going LASIK, PRK and LASEK have a greater probability that they will retain good night vision.The normal pupil size in daylight conditions is 4.0 mm. Some people’s pupils can dilate up to 7.5mm under low light conditions. This is why it is important to measure the pupil size preoperatively under low light levels. This is done with the infrared pupilometer.
People with pupils that dilate larger than the old treatment ablation zone (6.0 or 6.5) may experience halos, glare, starburst effect or ghosting of the image.
We now have the capability to make the effective optical zone larger to reduce the chances that people will have these unwanted optical aberrations.
The blend zone option uses the Variable Spot Scanning (VSS) technology to create a treatment for myopia and/or astigmatism which enlarges the outer diameter of 6.0 mm. (standard zone) and 6.5 mm. (larger zone) ablations to 8.00 mm. diameter. The shape of this ablation includes an inner optical zone and an outer transition zone. The outer transition zone provides a gradual change from the optical zone (6.0 or 6.5) out to the non-ablated cornea.
Ablation profile of the 6.5mm optical zone and blend zone
We can see the enlargement of the effective optical zone size on corneal topography (below). The transition zone extends out to 8.0mm. (extended ablation area). The pupil size is 5.6mm. The treatment optical zone option is 6.0mm.
The computer software takes 1.00 diopter of the proposed myopic correction off of the original myopia to be corrected. It holds this in reserve until the excimer laser ablates the major part of the myopic correction by the standard multizone approach. First, two separate zones utilizing the opening diaphragm of the myopic module of the excimer laser is ablated on to the corneal surface (A and B below).
The multizone approach
This is followed by the Variable Spot Scanning (VSS) technology creating a blended transition zone from the original 6.5mm. (large optical zone option) out to 8.0mm. The blend starts in the periphery and ends up in the center, adding the original 1.0 diopters of myopic correction back in.
The blend treatment starts out with the 1 mm spot size and proceeds around the edge of the multizone treatment. The 2,3,4,5,and 6mm. spot size is then applied in sequence. The scanning of the spot is achieved by the computer moving the off set lens. The different sizes of the spot are achieved by opening the diaphragm. The following histogram is the % of pulses that the computer uses to divide up the 1.0 diopters into the blend zone treatment. The total number of pulses the excimer laser uses for this is 56.
Distribution of blend zone pulses
The extra amount of corneal tissue removed to accomplish this added benefit is small compared to old standard 8.0 mm optical zones. You can see from the table below that it adds very little to the added ablation depth (4.0 to 11.0 microns).
|amount of myopia
< -7.00 diopters
< -7.00 diopters
-7.0 to -11.0 diopters
> -11.0 diopters
6.0 to 6.5 mm.
6.0 to 6.5 mm.
|increase in ablation depth (blend)
Extra amount of tissue removed by the blend zone compared to the 6.5mm optical zone
We have four treatment zone options available to choose from.
1. 6.0 mm. (standard zone)
2. 6.0 mm. (standard zone) + blend zone (8.0mm.)
3. 6.5 mm. (larger zone)
4. 6.5 mm. (larger zone) + blend zone (8.0mm.)
If an 8.00mm. multizone straight ablation (no blend) were to be performed on a person with 10 diopters of myopia it would remove a total of 213 microns of corneal tissue. If an 6.00mm. multizone straight ablation with a 8.0mm blend were to be performed on a person with 10 diopters of myopia it would remove a total of 128 microns of corneal tissue. This simple example demonstrates the tissue saving attribute of the new Variable Spot Scanning (VSS) and large 8.00 mm. blend optical zones.
This becomes increasingly important when considering doing LASIK on people that have high amounts of myopia, thin corneas and large pupils.
There are five critical factors or dimensions that must be considered when contemplating performing LASIK. They are the flap thickness, the amount of residual corneal stroma, the diameter of the excimer laser ablation, the depth of the excimer laser ablation and the pupil diameter under low light conditions. These values determine the key elements of LASIK – safety of the procedure, stability of the procedure, quality of the laser correction and quantity of the laser correction.
Any corneal ablation procedure, whether photo refractive keratectomy (PRK) or LASIK, when it attempts to sculpt a lens over the surface of the cornea, must remove some corneal tissue and must follow some rules related to ablation diameter and depth. The relationship between optical zone diameter desired and the central ablation depth is described by the Munnerlyn formula for myopic ablations. Simply stated it says that the central ablation depth is equal to the optical zone squared times the amount of myopia, divided by 3
The ablation has to be deeper for larger amounts of myopia and for larger optical zones.
Because the maximal amount of tissue will be photoablated from the central cornea and the cornea being thinnest at the center, the thickness of the central cornea becomes important when LASIK is performed for high amounts of myopia with its larger ablation depths. The possibility of progressive corneal ectasia after LASIK has been reported from high amounts of ablation on thin corneas. Ectasia is the bulging forward of the central cornea. The following corneal topography demonstrates corneal ectasia following high amounts of ablation on a thin cornea. The red areas show forward bowing of the cornea.
Orbscan on an ecstatic cornea
According to studies carried out long time ago by Dr. Jose Barraquer, later by Dr. Ruiz and more recent studies with a 50 Megahertz ultrasonic B scan it has been determined that a minimum of 250 microns of residual corneal bed tissue has to be maintained to prevent corneal ectasia.
The average corneal thickness is approximately 550 microns. Since the flap thickness during the LASIK procedure is usually chosen to be 160 microns, and assuming the person has 6.0 diopters of myopia with a pupil size of 6.5mm, this cornea will have a residual bed thickness of 298 microns. This assumes that we elect to use the 6.5mm optical zone with the 8.0mm blend. Residual bed thickness = 550-(14×6.0+8) +160
Explanation of the example follows:
Corneal thickness = 550 Ablation depth from 6.5 (large optical zone) (Munnerlyn formula) = (OZ x OZ x (amount of myopia)) / 3 = 14×6.0 = (6.5 x 6.5 x 6.0) / 3 = (42.25 /3 x 6.0) = 14x 6.0 = 84
Ablation depth from 6.5 (large optical zone) and 8.0mm blend = 84 +8 = 92
Total ablation depth + flap thickness = 92 + 160 = 252
Residual bed thickness = 550 – 252 = 298
The corneal thickness is 502 microns. Since the flap thickness during the LASIK procedure is usually chosen to be 160 microns, and assuming the person has 6.0 diopters of myopia with a pupil size of 6.5mm, this cornea will have a residual bed thickness of 250 microns. This assumes that we elect to use the 6.5mm optical zone with the 8.0mm blend. Residual bed thickness = 502 -(14×6.0+8) +160. In this case it would be prudent to choose a flap thickness of 130 microns. Residual bed thickness then becomes 280 microns. Residual bed thickness = 502 -(14×6.0+8) +130.
The corneal thickness is 489 microns. Since the flap thickness during the LASIK procedure is now chosen to be 130 microns, and assuming the person has 6.0 diopters of myopia with a pupil size of 6.5mm, this cornea will have a residual bed thickness of 276 microns. This assumes that we elect to use the 6.5mm optical zone with the 8.0mm blend. Residual bed thickness = 489 -(14×6.0+8) +130. In this case it would be prudent to choose PRK because we want to be more conservative. We cant choose corneal rings because currently they treat up to 3.00 diopters of myopia.
This person under went a previous LASIK procedure that resulted in 290 microns of residual bed tissue. The resulting corneal thickness after the procedure was 450 microns. The person was an under responder with a residual amount of refractive error of 1.75 diopters. Originally we elected to use the 6.5mm optical zone with the 8.0mm blend because the pupil was 7.0 in the dark. We would elect to do the same now. Since the flap thickness during the original LASIK procedure was chosen to be 160 microns. The residual bed thickness is calculated to be 258 microns after enhancement treatment. In this case it is prudent to do corneal rings as the enhancement procedure. We want to be more conservative.
At Grendahl Eye Associates we have two different microkeratomes. One is called the ACS and the other is called the Moria CB. The ACS is capable of producing any flap thickness 240 to 30 microns. The ACS microkeratome 130 plate produces a flap thickness of 94 + or – 20 microns. This is the microkeratome that we chose for thinner corneas. The ACS microkeratome can only be used on average curved corneas (42.00 to 45.00). The thinnest flap that the Moria CB is capable of producing is 110 microns. The Moria CB microkeratome is capable of being used on flat (39.00 to 42.00), average and steep (45.00 to 49.00) corneas. Thinner flaps are more difficult to handle and require more surgical skill to put back into place. Thick flaps are easier to handle and almost fall back into position.
These are some of the reasons that we at Grendahl Eye Associates do two independent and different corneal thickness measurements on all preoperative corneas. Two different and independent measurements are made of the central corneal thickness. The first method used is the ultrasonic pachymeter.
The second method used is the orbscann optical pachymeter. This unit gives the corneal thickness at all points on the cornea and reads thicker than the ultrasonic unit.
The Orbscan reading can be multiplied by .96 to obtain the ultrasonic reading. The average cornea has a thickness of 550 microns.
Preoperative information on corneal thickness measurements, papillary size measurements, amount of myopia, choice of different flap thickness, two different types of microkeratomes and the new Variable Spot Scanning (VSS) with large 8.00 mm. blend optical zones allow us to tailor the correct laser vision correction procedure for your eyes. Sometimes the corneas are to thin, myopia to large and pupil size to large for LASIK to be safely performed. This is one of the reasons why we would then recommend PRK or corneal rings. This is why we do all of the refractive surgical procedures
|< < < previous|