Questions from fitters regarding ROSE K2 XL and replies by Paul Rose
last updated 2019.12.12
Q: I often choose a semi-scleral lens for people who have dexterity issues – e.g. an 87-year-old registered blind first time user who had great success with Jack Allen’s S-Lim lens. I could not imagine this lady managing to bend over and balance a lens filled with saline on her fingers.
A: Lens handling will always be an issue for some patients although by far the majority overcome this with practice. Competence in insertion and removal must be demonstrated before the lenses can be dispensed. In the international trial on 120 fits, about 10% of patients reported handling issues, so often one teaching session is not sufficient. However, only 2% gave up because of handling issues.
Q: I almost never advocate the use of suckers – they are so easy to lose, but you never lose your fingers! You can also wash your hands easily but suckers get old and manky.
A: In my hands suckers are easier to use for insertion and removal of semi-scleral lenses and cause less trauma to the eye and to the patient psychologically than using compression and edge techniques using the fingers. However, if you can achieve competence without traumatizing the eye or the patient for insertion and removal without the use of a sucker, then this is ideal.
Q: What is the reason for the sealed system – does it give higher oxygen? I can’t see how, but that is the only reason I can think of. Is it possible to get the lenses with fenestrations for those patients who can’t manage the saline-filled insertion?
A: Because the lens rests mainly on the cornea and has a higher edge lift than most semi-scleral designs, when the lens is fitted correctly, tear exchange occurs with blinking. This can be seen. In the international trial with 117 fits on a range of conditions, no fitter requested fenestrations and there were no reports of corneal anoxia. However, if you wish to fenestrate, this can be done but should not be necessary in by far the majority of fits. Fenestrations are a great source of bacteria and protein build up so should be avoided unless absolutely required. Also, it is vital that any fenestration is placed on the lens in the correct place or studies show it can actually tighten the fit and or cause bubbling.
Q: Has removal with fingers been tried? Surely it should work if the lens is not fitted too tightly? If it is harder because of the lack of fenestrations that begs the question again – why are we going for a lens which sucks on more? What is the advantage?
A: To be clear, the XL design does not suck on more. The lens is designed like a regular GP lens to have some movement and tear exchange. Because the majority of support comes from the cornea which is much more rigid than the conjunctiva, seal off and subsequent binding of the lens are not issues if the lens is correctly fitted Yes, some fitters remove the lens using the finger technique. Correctly fitted however, the lens should be easy to remove and just as easy as a lens with a fenestration.
Q: How important is the position of the sucker on the lens for removal?
A: Extremely important. The sucker must never be placed centrally as this can cause considerable suction forces as the lens is pulled outwards off the eye, particularly if the lens is steep centrally. The sucker must always be placed off centre between the edge of the lens and the pupil margin ,and then on removal an outward and nasal rotation should be used to literally “peel” the lens off the eye.
Q: I have fitted a Pellucid Marginal Degeneration case and although the fit looks good after about an hour I am seeing bubbles accumulating under the lens over the pupil area.
A: Bubbles appearing over time can only enter from under the edge of the lens. In PMD this is usually around 6 o’clock where the cornea is steepest, causing the lens to lift off and allow small bubbles to enter at this point. There are several ways to resolve this problem:
1. Increase the diameter by at least 0.4mm.
2. Steepen the BC slightly to avoid any touch over the steep area just inside the limbus near 6 o’clock. An 0.1 mm steepening can be significant.
3. Decrease the edge lift. However this will decrease the edge lift at all positions around the lens.
4. Use an ACT option to steepen the fit/edge lift at 6 o’clock only.
In some cases a combination of the factors above will be required to resolve this problem.
Q: At the first 2-weekly check, I am now getting some residual astigmatism which I did not measure at the fitting or when I dispensed the lens. What could be causing this and how should I correct this?
A: It is important to determine where the residual astigmatism is arising from. Is it from the front of the lens or internally? Check this by doing a topography or keratometry over the lens in situ. If the astigmatism is arising internally, then the front of the lens will still be spherical. In this case simply ask the lab to incorporate the cylinder onto the front of the lens.
However, if the front of the lens shows some toricity, this will cause residual astigmatism. The ROSE K2 XL design is manufactured to a minimum centre thickness of 0.14mm to keep lens bulk to a minimum and allow maximum oxygen transmission to the cornea. However, in a small number of cases the lens will warp slightly over an astigmatic cornea giving rise to some residual astigmatism. There are two ways this can be corrected:
1. Increase the centre thickness of the lens to prevent the warping – the lab can advise you what centre thickness is required. This has the disadvantage of making the lens a little more bulky and reducing the oxygen transmission slightly, or
2. Leaving the lens the same centre thickness allowing it to warp slightly over the first week or so of wear, and incorporate the residual astigmatism onto the front of the lens. Please note that this lens may not give the maximum acuity until it has been worn for a few days. There is no disadvantage to the cornea, allowing the lens to warp slightly so it becomes slightly toric.
Q: I am noticing some corneal staining just inside the limbus where the lens is bearing, after several days of wear. How can I eliminate this?
A: Corneal staining as you describe here is caused by the lens not moving sufficiently. Changes are required to the fit to increase the lens movement. They are:
1. Steepen the base curve
2. Increase the edge lift
3. Increase the diameter
or a combination of these.
If the staining still persists after these changes, then transferring some of the weight bearing area from the cornea onto the conjunctiva may also assist. To do this:
1. Increase the diameter
2. Decrease the edge lift
Q: If I change the edge lift, will this change the fit over the cornea?
A: In most cases a change in the edge lift will not affect the fit over the cornea; however, there is one exception. If the trial lens shows a very tight edge lift and consequently the lens is bearing mainly on the conjunctiva and not on the cornea, then increasing the edge lift will cause the lens to sit down closer to the cornea.
To compensate for this a BC 0.1 to 0.2 mm steeper will be required to give the same central fit.
Q: In your XL fitting guide, you describe achieving “feather touch” at the highest point on the cornea when assessing the correct BC choice. Is there a more definitive way you can describe feather touch so I know I have achieved the correct fit?
A: Yes there is. Choose a BC trial lens, until you can see obvious touch at the highest point on the cornea. Then steepen the BC in 0.1 steps until no touch is obvious. This lens can then be described as the first Apical Clearance lens. Feather touch can then be defined as a lens with a BC 0.1 flatter than this lens.
Q: I am unsure in some cases where I have different fits in different quadrants outside the limbus, whether I should use ACT, a Toric Periphery (TP), or a quadrant specific edge lift. Can you please give me some guide on this?
A: 1. ACT. ACT gives the biggest change in sag height/AEL at the tuck. The steepening starts just outside the back optic zone of the lens, at 3 and 9 o’clock. This is ideal to use when you have a good edge lift at 12, 3 and 9 but lift off (excessive lift) at 6 o’clock. PMD is a typical case where ACT is often required.
2. Toric Periphery. You need symmetry here in two meridians. Usually the lens is tight at 3 and 9, if you have WTR astigmatism, and loose at 12 and 6. When using ACT the fit would have been fine at 12. However with TP you tighten the fit equally both at 6 and 12 and loosen the fit equally at 3 and 9, if you have WTR astigmatism. With XL you can split the toricity unequally so for example you can have a 1.6 TP, whereby you tighten the fit at 12 and 6 by 1.0 and loosen it at 3 and 9 by 0.6. TP starts well inside the limbus, so gives more AEL lift change than simply changing the edge lift but not as much change as ACT.
3. Quadrant specific edge lift. This option changes the AEL the least of all 3 options, as the change only occurs from just inside the limbus, so use it when you want to change the edge lift in one or more quadrants differently. The most common mistake is leaving the BC too flat so that there is more than a feather touch at the highest point on the cornea. Excessive touch can cause epithelial erosion, and reduces the movement on blinking considerably, reducing wearing time and lens comfort. This may also cause lens binding.
Usually a TP locates very well and does not require prism ballast. However for ACT and quadrant specific option always include 1D prism at 270 (6 o’clock). Most labs will include this automatically, but always stipulate it on your order.
Q: If I incorporate some residual astigmatism onto the front of the lens, will this have any effect on the way the lens fits or performs?
A: You can confidently incorporate residual astigmatism on the front of the XL design, knowing that it will not affect the fitting or performance of the lens at all. This is in contrast to corneal lenses where adding residual astigmatism to the front of the lens plus base down prism to rotate the lens correctly, can cause the lens to sit lower or move differently than a lens without prism. With the XL front surface toric design, 1 dioptre of base down prism will be automatically incorporated to orientate the lens to the correct position. This can be increased to up to 2 dioptres if required.
Q: After 2 weeks of wear with the ROSE K2 XL lens, my patient has an over Rx of -2.00 / -1.00 x 110 and -2.50 / -1.50 x 60 respectively. This gave 6/6 VA in each eye. Without the RA corrected, VA was 6/7 and 6/9.
Without having to order front surface toric lenses, would there be any benefit in ordering a slightly thicker lens (about 0.05 mm thicker)?
My reasoning being that there would be less lens flexure with a thicker lens hence less residual astigmatism.(RA)
A: To answer this you first need to establish where the RA is arising from, the front of the lens or internally?
You can do this easily by doing a topography or keratometer reading over the XL lens on the eye. If the front surface of the XL lens on the eye shows that it is spherical, then you can assume the RA is arising internally. There will be no benefit at all going to a thicker lens, but this RA can be easily corrected by using an XL front surface toric.
However if it shows that the front surface is toric and this corresponds approximately to the amount of RA you have measured, then yes, making the lens thicker can help reduce or eliminate the RA.
Q: I am used to fitting mini-sclerals with a lot of clearance over the cornea but you advocate feather touch, which effectively means only a minimum clearance of around 20 microns over the highest point on the cornea. Is this correct?
A: This concept of a minimum clearance over the highest point on the cornea does not follow the normal high clearance semi-scleral models, where over 200 microns of tear layer is frequently advocated. However, tear layer thickness under the lens does affect oxygen availability to the cornea as the tear layer has a Dk of only around 80, which effectively becomes a barrier to oxygen reaching the cornea. Please refer to the article on the website that I wrote recently regarding oxygen and ROSE K2 XL. Horizons Article
Also, ROSE K2 XL is a much thinner lens than most SS lenses, having a centre thickness for lenses over -7 of 0.14mm which also greatly enhances oxygen transmission.
Q: When we fit semi-sclerals we usually factor in an expectation of 90µm of settling in after 2 weeks. It doesn't sound like you recommend that this is necessary with the XL -- is this correct?
A: Because the majority of bearing with XL is on the cornea, there is very little settling down as the corneal epithelium will not compact much at all. Usually most of the settling takes place within 20 minutes of insertion. In the guide I recommend getting the correct clearance, and then checking it 20 minutes later as any settling down will have normally occurred by then. This makes it a very time-effective design to fit.
Q: With regards to the edge, if I flatten the BOZR does this increase the edge lift on XL like a corneal lens or like many SS designs, or are the peripheries constant relative to the horizontal plane and unaffected by changes in BOZR?
A: From approximately 10.5 mm out to the edge of XL, there are some constant tangent angles which do not change with BC. Therefore in theory changing the BC should not affect the edge lift. However, it is always important to judge the edge lift value once you have selected the correct base curve as changing the BC may cause the lens to settle slightly differently on the cornea which may affect the edge lift pattern. By the way, when you are judging the amount of edge lift by observing the fluorescein pattern outside the limbus, it is important to judge this within a minute or two of inserting fluorescein as it will flush out from under the edge rather rapidly which can then be misinterpreted as a tight edge lift.
Q: Having fitted a couple of the ROSE K XL lenses, it seems to me that most fittings would benefit from a toric periphery. Is this normal?
A: In my experience, probably not most, but certainly up to about 20%. When you are using a toric periphery, you must have some symmetry in the two meridians, most commonly tight at 3 and 9 o’clock, and loose at 12 and 6 o’clock. Also, when you state the amount of toricity you require, this is normally split equally by the lab in the two meridians, flattening it in one meridian and steepening by the same amount in the other meridian. You can however split the toricity asymmetrically, so you can loosen or tighten it in one meridian more than you do in the other meridian.
Q: The flat axis of the eye where the lens impinges on the sclera, lifts the lens in such a way that “lift off” is increased in the steepest axis increasing the chance of discomfort or “bubble swallowing at this axis”. As soon as the flat axis is flattened (lifted), the steep axis drops too. Do we have to consider this when we order a toric periphery as wouldn’t this affect the central fit as the lens would sit back closer to the eye?
A: Yes, some small adjustment to the BC should be made, to allow for the lens sitting back closer on the eye with the TP, compared with a spherical trial lens of the same BC. This adjustment would depend on the degree of toricity, but normally a BC steepening of 0.05 is sufficient to compensate for this.
Q: I would like to know what is the minimum difference in the 2 toric peripheries axes, that will still “lock” the lens in position?
A: ROSE K2 XL offers toric peripheries between 0.8 and 2.0, with the standard TP being 1.2, so that the lens is steepened 0.6 in one meridian and tightened 0.6 in the other.
On insertion it is not required that the lens be placed at the correct axis, as this design rotates into position very effectively, and a 0.8 mm difference in the two meridians should be sufficient to achieve good location and “lock on”. If you do have problems with location, you can incorporate base down prism at 270 to assist the location but this is seldom required.
Q: For a full back surface toric XL lens, what difference would be required in the back toric radii to keep the lens stable?
A: This depends on many factors including the condition you are fitting, the regularity of the astigmatism, the corneal shape and lid tension to mention but a few. However, generally an 0.8 toricity difference in the two meridians, is sufficient to rotate the lens to the correct position and for it to remain stable. XL also offers a design whereby you can independently have a different degree of toricity over the central part of the lens compared to the periphery.
Q: Which lens design from your range of lenses would you recommend for radial keratotomy cases? I do not know whether you have them in your country as well, laws being much stricter for new surgical procedures, but we certainly do.
A: RK cases can vary dramatically in the shape they present, however in most cases there is a very oblate central cornea with the highest points on the cornea being above the radial scars. This makes it extremely difficult to fit with a corneal lens where the lens would rest on those scars. ROSE K2 XL would be my first choice for an RK case.
Q: Which of your designs would you recommend for astigmats of 2.50 and up who cannot get stable vision with soft torics?
A: This is dependent on many factors so I cannot give you a blanket answer here. However, if a patient has worn a soft lens they may not tolerate a corneal lens well, so my first choice would be XL which initially gives greater comfort than a corneal design.
Q: I have a patient that has a lot of particles in the tear film and gets debris building up behind the lens after just 10-15 minutes of the lens being on the eye. The fit looks good and the patient is comfortable. Do you have any tips on how we can reduce or eliminate the debris from accumulating behind the lens?
A: Four things I can suggest to try in order of preference:
1. Going tighter in the edge lift
2. Going flatter in the BC. Keep tear layer thickness to an absolute minimum
3. Changing to a lower Dk material
4. Adding a non-preserved drop of wetting solution on insertion
Q: Would you please brief me how to order a toric periphery with the ROSE K2 XL? Sometimes the periphery is good in one meridian however it is tight in the opposite meridian. My question is, “what should be the value of the toricity, and does it not matter at which axis we have the toric periphery”?
A: Toric periphery (TP) XL lenses are for cases where you have symmetry in two meridians, most commonly the cornea being flatter in the 0/180 meridian and steeper at 90/270, causing the lens to be tight in the horizontal meridian and loose in the vertical meridian, although in PMD it is the opposite. So in the TP design the flat and steep meridians are always 90 degrees apart. The standard TP toricity value is 1.2mm, so in this case the lens is steepened in one meridian by 0.6mm and flattened in one meridian 0.6mm. However, you can also order an asymmetrical split, so if we take the example of a 1.2 TP, you could choose to flatten the lens in one meridian by, for example, 0.8 and steepen by 0.4 in the other to give you the 1.2 difference. The lens will rotate to the correct position, so the axis does not need to be stated, just the amount of toricity you require. Although prism can be added to this design, it is very rarely required to stabilise the TP lens, and should never be used initially. So on your order you would simply state the amount of toricity required, noting that it will be automatically split equally in the two meridians unless you state otherwise.
Q: I recently had an advanced case where the 6.0 steepest XL trial lens was still too flat. Would going to a larger diameter help solve this problem?
A: Although the trial set only goes as steep as a 6.0 BC, you can order an XL lens as steep as 5.6 which should be steep enough to fit by far the majority of corneas. Going bigger in XL will not increase the sagittal height over the cornea at all, so a larger diameter would not help in this case. Because the need for a larger semi-scleral lens is very infrequent, I have made the conscious decision not to design a larger SS lens at this time, as the requirement for a larger lens is so small. XL with a range of diameters up to 16mm, should fit by far the majority of corneas.
Q: I am getting some superficial light punctate epithelial staining with fluorescein after 8 hours of daily wear. This also results in feeling of dry and tired eyes after lens removal; sometimes red eyes too. What could be the problem here?
A: 1. If the staining corresponds with the highest point on the cornea, the BC is too flat. Fit a steeper BC.
2. Are the lenses moving on blinking? If not, then increase the movement by steepening the BC and/or increasing the edge lift.
3. Check to make certain the patient is using non-preserved saline for filling the lens before insertion. Any solutions with preservatives must be avoided.
4. The BC is too steep. You should fit with only 25 to 30 microns of clearance at highest point on the cornea. This should show as a feather touch with fluorescein at this point. To check this simply go 0.1 steeper and you should see no touch.
Light diffuse superficial punctate epithelial staining is not uncommon with semi-scleral lenses and often causes no long-term issues. However, patients should be monitored every 3 to 6 months.
Because semi-scleral lenses have reduced tear exchange, if you have eliminated the above fitting issues, then the most likely cause is a change in the nature of the tear layer trapped beneath the lens.
For example there may be a build-up of waste products from the cornea which could change, for example, the pH of this tear layer. Removing the lens every 3 to 4 hrs and refilling the lens with fresh saline solution may help the staining. Also in some cases adding one drop of a non-preserved dry eye solution to the saline solution when filling the lens before insertion may also assist.
Q: I would like to ask you about the ACT system with ROSE K lenses.
In most of the cases, when I order ACT, I receive the lenses with a laser mark at 6 o’clock (270°). However, in some cases the lens rotates and the laser mark will not sit at 6 o’clock. With my last patient, the laser mark stayed at 12! And definitely, this will change all the fit specially the periphery…What can I do to prevent the lens rotating?
A: 1. Increase the amount of base down prism by 0.50 D. This often is all that is required.
2. Make certain that the ACT axis is exactly where you require it. It does not need to go at 270°. You can for example order the ACT axis at, say, 250° and the prism at 270°.
3. Make sure the amount of ACT is adequate. To check this, rotate the lens until the marker sits correctly at 270 degrees. Then observe the fluorescein pattern at 270 to make sure you have an adequate degree of ACT. This can be increased or decreased as required.
4. Make sure you do not require 2 ACTs. You can have ACT at two different axes. If a lens is turning upside down as you mention in this case, then make sure you do not require an ACT at 12 as well as it is unlikely the lens would orientate 90 degrees off and stay there, unless the back surface of the lens and the cornea were aligning in this position. This suggests you may also require a second ACT at 12 o’clock as well.
Q: I have noticed in some rare cases where XL has been fitted optimally and the fit looks excellent after the 20-minute settling time at the fitting, that after longer use the lens has bound to the cornea and shows no movement. Between cornea and lens there appears to be almost no tear layer at all. The lens becomes uncomfortable and the patient needs to remove the lens, refill it with saline and reinsert it to regain comfort and lens movement.
A: 1. In some cases the lens will continue to settle back on the cornea even after the 20 minutes recommended in the fitting guide. This often seems to be associated with dry eye. In these cases you will need to fit the BC a little steeper and maximise the edge lift using more lift than is initially indicated. To maximise the lift, increase it until the lens just becomes uncomfortable but with no bubbles entering under the edge of the lens. Alternatively you can leave the lens on the eye longer than the recommended 20 mins to accurately evaluate the base curve and edge lift.
2. XL is a thin lens to maximise oxygen transmission. However, in some cases the lens can warp, particularly on a very toric cornea/sclera. If lens warpage occurs the lens will settle back further on the cornea and often bind. To check lens warpage simply do a topography over the lens in situ. If the lens has warped, you will see one or both of the following:
a. Some irregular astigmatism showing in the map. In this case thicken the lens to about 0.25mm (std. thickness is 0.14)
b. Some regular astigmatism along the two major meridians. This will also cause residual astigmatism on refraction. Thickening the lens may stop the lens warping but not always. A preferable option is to reorder the lens with a toric periphery (TP). Regular astigmatic warpage of the lens cannot occur unless the cornea/sclera is toric. Therefore, including toricity on the back surface of the lens in the form of a toric periphery so there is better alignment with the lens back surface and the cornea, prevents the lens warping. The standard TP is 1.2 but to estimate the degree of toricity required, simply use the difference in mm in the two primary meridians shown in the topography.
Q: Are fenestrations used with the XL design?
A: This can be done but the position of the hole is critical otherwise it can cause bubbling. The fenestration must be placed where the lens lands on the cornea. However, in by far the majority of cases this is not required and should never be used in the first instance.
Q: Are there any special recommendations when fitting THE XL lens design over intacs rings?
A: Make certain that there is a tear layer of at least 20 to 30 microns over the rings between the cornea and the back surface of the lens. Any corneal touch over the rings will cause issues to the corneal epithelium.
Q: When a toric periphery is ordered do both meridians change and do they change equally?
A: When you order a standard toric periphery (TP) the toricity amount is split equally (symmetrically) in the two meridians. By far the majority of TP’s are done like this. So if we take XL for example where the standard TP is 1.2 toricity, one meridian will be flattened by 0.6 outside the back optic zone while the other meridian is steepened by 0.6, giving us a 1.2 difference between the two meridians. So if your spherical trial lens was tight in the horizontal meridian (with the rule astigmatism), and loose in the vertical meridian, the TP lens will loosen the horizontal meridian and tighten the vertical meridian.
However, you can split the toricity asymmetrically if required, so instead of flattening and steepening the TP lens equally in the two meridians, you may (let’s take the 1.2 XL standard TP again) flatten it more in one meridian than you will steepen it in the other. So for example, you could put 1.0 toricity in the flat meridian and 0.2 toricity in the steep meridian still giving the 1.2 split. In this case the TP lens will significantly loosen the lens fit in the flat 0/180 meridian and tighten it only very slightly in the vertical meridian.
Q: I have done a trial fitting with a standard lift Rose K2 XL lens on a keratoconus patient and end up with residual cyls of -1.50 x 90, which I wish to incorporate as a front surface toric. Also the edge lift with the trial lens is tight at 6 and 12 and shows lift off (loose) at 3 and 9 so I will also include a toric periphery (TP on the back surface) to accommodate this corneal and scleral astigmatism.
Now the question is, do I stay with a standard edge lift (as was the trial lens) and simply order a toric periphery, or do I need to increase the edge lift because I am now incorporating a TP?
A: On refraction if you find a residual cylinder, then always do a topography over the lens in situ to make sure the lens is not warping (it will show some toricity). If there is astigmatism showing on the front of the lens this indicates it is warping and will cause the residual cyl.
Regarding the edge lift: Because the edge is tight at 6 and 12 this indicates the condition is most likely PMD, not keratoconus, because the eye is flatter in the vertical not the horizontal meridian.
Using a TP will cause the lens to sit closer to the eye than the spherical trial lens, because the lens is flattened in the flattest meridian, allowing the lens to sit closer. This will have the effect of decreasing the effective edge lift in all meridians. Therefore, if a normal spherical trial lens has been used in the fitting and you wish to incorporate a TP in the ordered lens, always increase the edge lift by +1.00 to negate the effect of the TP on the edge lift.
Q: Will the toric periphery (TP) and front surface toric options work well together in terms of stabilising the lens?
A: Yes, very much so. In most cases a TP will locate extremely well without including a base down prism.
Incorporating a front surface toric with a TP will not affect the lens rotation. However, please note that the flattest meridian of the TP will locate along the flattest meridian of the cornea. If this is at 180 for example (with the rule astigmatism) and the residual cyl is at 90, then there will be 90 degrees between the TP flattest axis on the back surface of the lens and the minus cyl correction on the front surface. The lab will normally assume these two axes are 90 degrees apart unless you state otherwise in your order. If they are not 90 degrees apart then it is necessary to include in your order the flattest axis of the cornea (where the TP markers on the ordered lens will locate) and also the axis of the front surface cyl.
For example, if the flattest axis on the cornea is 20 degrees and the residual cyl axis on the front of the lens is 90 degrees, then the angle between these two axes is not 90 but 90 - 20 = 70. The lab must know these details otherwise the front cyl axis will not be placed correctly on the lens.
Q: There is a question that always arises with contact lens fitters: how to perform a better centring of the Rose K2 XL lens. Most people think that the diameter should be increased and the base curve steepened, but in Rose K2 XL to make the lens center better and to reduce movement, we make the base curve flatter. Can you explain more about this please?
A: For a large lens which covers the cornea and has very little tear exchange or movement, the thinner and more parallel you can make the tear film the greater the forces are between the posterior surface of the lens and the cornea. The more you “misalign” these two surfaces the less the capillary force. Think of two sheets of normal flat glass. If you put some water between them, and then push them together it is impossible to directly pull them apart because of the capillary force created. You would have to slide them sideways to get them apart. So if the tear film is thin and as parallel to the cornea as possible, you create more capillary force which assists in centring the lens and also in reducing the movement. In some cases increasing the diameter also assists in centring the lens, particularly if the diameter is insufficient. With a corneal lens where you have significant movement and tear exchange these rules do not apply. Hope this helps.
Q: The patient is currently wearing an XL lens. We have flattened the BC already in an attempt to control the bubble that forms in the mid periphery and the bubble has significantly reduced, however now we are approaching touch at the apex of the cone. Aside from further flattening of the BC are there any better ways to control the bubble?
The patient is very happy with the vision and comfort. He was unable to tolerate a corneal lens due to comfort and stability.
A: Bubbles with XL are not controlled by varying the BC. Flattening the BC may make the bubble smaller but will never eliminate the bubble issue. A steep BC in a corneal lens can certainly cause bubbles, and certainly once the tear layer gets over 100 microns, but NOT with XL.
The bubble can only come about by entry at some point around the edge where the edge lift is excessive. This is most commonly from 6 o’clock where the lens lifts away, particularly in PMD. Please ask the fitter to insert the lens (no bubble) and then observe where the bubble enters from. It cannot magically appear under the lens but has to enter at some point around the edge. Once this is determined they can then decide how they can reduce the edge lift at this point by either using ACT, TP, or a QSEL option.
Q: When fitting Rose K lenses, the Base Curve is always the first parameter that I must address. When selecting the optimum Base Curve for Rose K2 XL, do I still leave the fluorescein pattern showing the same degree of "feather" touch that I see with corneal Rose K lenses or should I leave it steeper?
A: Feather touch can be interpreted quite differently by contact lens fitters. When the fluorescein pattern shows a “feather touch”, there should still be about 20 to 30 microns of tear layer at this point, but this is insufficient fluorescein depth to produce fluorescence. Also the luminance of the viewing light and whether a yellow filter is used are also variables that can change the appearance of the fluorescein pattern. Leaving the XL design even slightly flat, can lead to apical staining whereby if the fit is slightly steep this causes no major issues. One of the most common issues I see with fitting XL, is fitters leaving the BC flat. Therefore, with this in mind, I have now recently updated the XL fitting guide advocating that the optimum BC should be the flattest Base Curve that shows no apical touch. This will then indicate approximately a 40 to 50 micron clearance over the cone. If checking this with an OCT, please make sure you check the tear layer depth over the cone apex or where the tear layer is the least, as this may not be centrally over the pupil e.g. corneal graft or a decentered cone.