Base Curve Nomograms?

[silentjatt]

I have finally quit Lenscrafters after working there for 3 years. Now joined an independent optical where I will be fitting contact lenses as well. The one thing that I am having trouble finding are the Base curve nomograms. I will only be fitting soft lenses. From my memory this is what I remeber to do:
if lower K is >45.00D, then fit the steeper base curve.
if lower K ranges from 41.00D to 45.00D, then fit the median base curve.
if lower K is <41.00D, then fit the flatter base curve.

Would I follow this rule with all the different soft contacts? or would I be better off trying to contact manufactures of each contact lens company and get the nomogram from them?

Also... which contacts are considered good?
In spherical dailies.. is it proclear 1 day ?
spherical bi weekely... oasys?
spherical monthly... air optix or encore 100?

torics... i guess it will depend on the cyl and the axis.

and if some1 complains of dry eyes but is good with cleaning proceduers, would u go with dailies or something like oasys that has a higher water content?

i know its a lot of questions, and i am trying to revise all my study material i have too. but these type of questions are not covered in any books...

any help would be greatly appreciated.
Thanks

[eyestrain]

I rarely look at Ks when fitting disposable contacts. If there's more than one BC, I'll grab the flattest one and go with it. If it's too loose, I'll get the steeper one. I'll take a slightly loose lens over a tight one any day. A doctor in my OD school said "If it's not on their cheek, it's not too loose."

As far as which are good, my go-to lenses currently are:

Monthly spherical - 1) Biofinity 2) Air Optix
2 wk spherical - I typically avoid like the plague, but 1) AV Oasys
Dailies - 1) Proclear 1-Day 2) Focus Dailies
Monthly Toric - 1) Proclear Toric
2 wk toric - 1) AV Oasys for Astig

[wmcdonald]

I would strongly suggest K-readings or some other form of corneal measurement on all fits. It may not be required on the front end, but is certainly valuable in follow-up. Changes in Ks provide valuable information.

[eyestrain]

I would strongly suggest K-readings or some other form of corneal measurement on all fits. It may not be required on the front end, but is certainly valuable in follow-up. Changes in Ks provide valuable information.

To be clear, I do TAKE Ks, I just don't pay much attention to them when fitting run-of-the-mill disposable lenses.

[HarryChiling]

Your nomogram looks pretty accurate for fitting soft lenses, I would just add a few other quick tips to it. The diameter should be about 2 to 3 mm larger than the diameter of the iris, with the average iris diamter measureing 11.6mm to 12.0mm in diameter, if the iris is larger add a diopter to the K's for every 0.2mm, if it's smaller subtract a diopter from the K's for every 0.2mm. Then when you convert the K's to radius add 0.3mm to the base curve for the final lens. This has always served me well.

http://onlineopticianry.com/wordpress/?p=148

[chip anderson]

Take the dam* K readings not only is it extreemly important in fitting, it is extreemly important to re-check these at follow-ups to see if you are causing changes in them. Any change is too much.

I could be more expressive on this today, but I'll try not to stir anything up.

Before you fit any contacts it would be advisable to read a book or two on the subject (see: The CLSA.com) and take a few courses.


Chip

[Shwing]

for YEARS one was taught to take the flat K, subtract 4D, and then divide that into 337.5 (corneal index of refraction).

So, by example, an average/ normal K of 42.5, less 4D, is 38.5.

Now punch in 337.5/ 38.5= 8.76 which rounds to 8.8.

So, you'd ideally fit an 8.7 or 8.8, 14.0 mm lens on a 42.50D cornea.

At the Time, Freshlook was 8.7, Acuvue's standard was 8.8 (the 8.4 was a tight fit), Focus (originally a non-disp lens called Spectrum) used 8.6, etc, etc

Notice how all base curves have steepened over the last 15 years for soft disp. lenses??

8.4 and 8.5 as standard.

So unless the 'average' cornea has steepened by a diopter...

I'm sure Chip and others with set me straight.

[chip anderson]

Actually I have been trying to find a clue on why base curves on soft lenses are getting so steep. I had a patient come back to me last week, he had a 38 diopter cornea and the good doctor decided to dispense an 8.6 mm base and everybody wondered why he couldn't see.

I suppose the steeper base curves may have to do with change in modality (hoop strength) and thinner lenses, possible aspheric posteriors, I really don't know.

I do know that Schwig's nomogram has no concideration for diameter or lens thickness (stiffness). One must go even flatter if the diameter is increased, steeper if the diameter is decreased. Stiffer lenses are more critical of sagital depth match.

Most manufacturers will furnish nomograms on request.

Chip

[silentjatt]

thanks for the replys everyone. much appreciated.
i am reading up all my notes and books from college. im also planning on inquiring about any referesher courses on contact lenses, but havent found anything yet. But since the last optician working in this store has quit, I am the only optician left here. So i kinda have to fit contacts. Now i know all the basics and i am trying to do everything by the book so i dont make any mistakes. I do take K readings for everyone and check therie eyes in slit lamp in detail.
Now that being said i have a little dillema. A customer i fit yesterday. His RX is
OD:-0.50-0.50X75
OS: -1.00

K readings:
OD: 43.00/42.75
OS: 43.00/43.25

I gave him oasys 8.4 for trial
OD: -0.75
OS: -1.00

Checking in the slit lamp, the contact in OD moved a little more than the OS. but it still wasnt an excessive movement. She says they are comfortable, but right eye is not as clear as her glasses. It goes a bit blurry sometimes, but when she focuses its better.

I am thinking its either a loose fit or is it because of her astigmatism?
What else can i try for this patient?

[chip anderson]

It's not unusual to get one lens that moves more than the other even with very similar K's. Sometimes one finds an "average" eye, say 43.50/43.50 12. 5 mm iris where what usually works, say an 8.8 14.0 fits either flatter than a pancake, or steeper than Madona's Cones. That's were "fitting" and follow-up come in.
Now as to your finding books, couses and education go to The CLSA. com, also on this site click: The CLSA University. There is more stuff here than you will be able to handle in the next decade or so.
Joining the CLSA is also a move I highly recommend, especially if your employer will spring for the various conventions and educational meetings.

Chip

[HarryChiling]

thanks for the replys everyone. much appreciated.
i am reading up all my notes and books from college. im also planning on inquiring about any referesher courses on contact lenses, but havent found anything yet. But since the last optician working in this store has quit, I am the only optician left here. So i kinda have to fit contacts. Now i know all the basics and i am trying to do everything by the book so i dont make any mistakes. I do take K readings for everyone and check therie eyes in slit lamp in detail.
Now that being said i have a little dillema. A customer i fit yesterday. His RX is
OD:-0.50-0.50X75
OS: -1.00

K readings:
OD: 43.00/42.75
OS: 43.00/43.25

I gave him oasys 8.4 for trial
OD: -0.75
OS: -1.00

Checking in the slit lamp, the contact in OD moved a little more than the OS. but it still wasnt an excessive movement. She says they are comfortable, but right eye is not as clear as her glasses. It goes a bit blurry sometimes, but when she focuses its better.

I am thinking its either a loose fit or is it because of her astigmatism?
What else can i try for this patient?

In a case like that it sounds like the astigmatism at play, unfortunately the sph equiv of -0.50 DCyl is 0.25D, which is 50% of the defocus. It's gonna be a comprimise either way here since very few lenses come in a 0.50 cyl. You could go with an RGP, but it might be overkill for this patient at such a low power, also try an aspheric lens I have found in low powers with astigmatism that patients tend to feel as though the aspherics provide crisper vision, maybe a bit more minus then their spherical counterparts. As for fit a littl movement is good, and if their is a difference between the right and left that's more than OK, just make sure you don't have excessive movement or binding, anything between is a good fit.

It's a good idea not to overthink soft lenses, they require less effort than any other fit.

[silentjatt]

aspheric lens is definitely a good idea.
what if i try the -1.00 in right eye? that should make it better i think. but want to make sure first if us opticians are allwoed to up the power a little. if my memory serves me right, we can go +/-0.50. can anyone confirm this?

edit* i am in Canada. dont know if that changes the ruleof changing the power

[HarryChiling]

aspheric lens is definitely a good idea.
what if i try the -1.00 in right eye? that should make it better i think. but want to make sure first if us opticians are allwoed to up the power a little. if my memory serves me right, we can go +/-0.50. can anyone confirm this?

edit* i am in Canada. dont know if that changes the ruleof changing the power

-1.00 is overkill on this Rx in my opinion. I would either stay with a -0.50 and drop the astigmatism correction or add the spherical equivalent which is -0.75, before you go messing with it what's the VA with the lenses on? If the VA is acceptable don't touch it. This case seems like a round peg in a square hole, you may need to go with a toric lens or RGP if the patient isn't satisfied with the VA to get a better correction. You may want to try a toric with a -0.75 cyl. For instance th AV Advance for Astig in a 8.6 -0.50 -0.75 x 70 maybe even order a trial in 60 as well. I don't know what the parameters are on the Oasys for Astigmatism but that coud be another option.

[chip anderson]

This will have nothing to do with the fit, but whenever cylinder is present in the Rx and soft lens corrected vision in either eye is less than a good sharp 20/20. Flip a Jacksonian Cross cylinder and see if the patient says "Wow." If he does, fit a toric. So many patient are running around with uncorrected cylinder that would help. Patient ends up thinking your are a genius and the doctor isn't all he could be. Especially true if the patient has been seeing a "one size fits all" practioner in the past.

Now as to the "steeper" base curves on todays "emperical" fits has everyone forgotten to take "K" readings over the contact to guard against fitting too steep? And this one will help with your fitting.

Chip

[wmcdonald]

Harry,
The astigmatism values may be equal as well in the two eyes, and the fist still vary significantly. One thing I learned over the years is that the e-value of the corneas of each eye may differ quite a bit, and have the same or similar refractive error. You may have one cornea at or near the average e-value of .45, while the other is much different. That has a bit to do with the fit as well.

Warren

[Fezz]

Is anybody else as nervous about this situation as I am?

I am all for learning and education,.....but.........................

[chip anderson]

Don't know what Fezz is nervous about. Tole me FEZZ!

I suspect the blurred at time vision is from the fit being too steen, not the cylinder and not the movement.

Chip

[sharpstick777]

I don't think K's are as important as they used to be. The newer silocone hydrogels are less sensitive to base curve than the older lenses used to be, plus they are way more comfortable (for most patients) even in the wrong BC. You will see more and more lenses only have one base curve available.

I think you have good list to start. I would add:
Proclear Monthly: Toric and Sph (for pts that don't like Silcone Hydrogels)
Air Optix for Astigmastism
Oasys for Astig
Acuvue 1 Days Moist (for patients that are very sensitive)
Night & Day (small 13.8 diameter helps on some patients, esp teens)

Many of patients love Oasys, but only for a week then they cloud (my experience as well). I personlly find the new Cooper lenses are clearer on me than most others, and that I have back down a notch in power sometimes on the Coopers.

You are off to a good start. I wish you well.

Sharpstick

[chip anderson]

My God! Think of it being too lazy to take and record "K"s. It may be true that present lenses are not as critical in fit. But not having any record of the origional shape of the cornea? This is unforgivable!

Measure the K's, record, measure the iris diameter, measure or guestimate the pupil diameter in normal room light, lid positions, lid tension, stain the cornea and record anything you see. Check for GPC or similar conditions prior to fitting and during follow-up exams. Comfort will probably always be ideal on any lens as long it is steep enough, or big enough, or too steep. But the patient will have "occasionally fuzzy vision" if it's too steep. This is revealed by taking K readings over the lens.

Are we really getting so far evolved in reverse that we think reading the side of a box constitutes a "skill level?"

Are we waiting for someone to come out with some sort of super zoom-o spectacle lens that will render PD and seg.ht. irrellivant?

Chip

[HarryChiling]

Harry,
The astigmatism values may be equal as well in the two eyes, and the fist still vary significantly. One thing I learned over the years is that the e-value of the corneas of each eye may differ quite a bit, and have the same or similar refractive error. You may have one cornea at or near the average e-value of .45, while the other is much different. That has a bit to do with the fit as well.

Warren

Good suggestion, 0.45 for the eccentricity is a good average and the nomogram above uses a 0.50 but your right their is no guarentee that the eccentricity is average or even the same between both eyes, I guess that's why it's better to take K's and perfom keratometry over CL's as chip suggested. I myself only fit about 4-5 lenses per month and on a busy month maybe 10-15, and it's only to keep my skills with the biomicroscope sharp and my keratometry sharp, the rest of the time you'll find me in the lab so I must defer to you and chip on the CL cases other than the few suggestions I can offer.

[HarryChiling]

I don't think K's are as important as they used to be. The newer silocone hydrogels are less sensitive to base curve than the older lenses used to be, plus they are way more comfortable (for most patients) even in the wrong BC. You will see more and more lenses only have one base curve available.

I think you have good list to start. I would add:
Proclear Monthly: Toric and Sph (for pts that don't like Silcone Hydrogels)
Air Optix for Astigmastism
Oasys for Astig
Acuvue 1 Days Moist (for patients that are very sensitive)
Night & Day (small 13.8 diameter helps on some patients, esp teens)

Many of patients love Oasys, but only for a week then they cloud (my experience as well). I personlly find the new Cooper lenses are clearer on me than most others, and that I have back down a notch in power sometimes on the Coopers.

You are off to a good start. I wish you well.

Sharpstick

Good choices on lenses, I like all of those lenses. I'll have to disagree with the importance of K's and the one curve fit all philosophy.

My God! Think of it being too lazy to take and record "K"s. It may be true that present lenses are not as critical in fit. But not having any record of the origional shape of the cornea? This is unforgivable!

Measure the K's, record, measure the iris diameter, measure or guestimate the pupil diameter in normal room light, lid positions, lid tension, stain the cornea and record anything you see. Check for GPC or similar conditions prior to fitting and during follow-up exams. Comfort will probably always be ideal on any lens as long it is steep enough, or big enough, or too steep. But the patient will have "occasionally fuzzy vision" if it's too steep. This is revealed by taking K readings over the lens.

Are we really getting so far evolved in reverse that we think reading the side of a box constitutes a "skill level?"

Are we waiting for someone to come out with some sort of super zoom-o spectacle lens that will render PD and seg.ht. irrellivant?

Chip

Yup Chip I think your right, "the prescriptions right on the side of the box" seems to be the mentality we've all taken to lens fitting, it's no wonder why patients purchase their lenses online, they're truly getting jipped on the fits.

The cornea is always checked by the doctor she does goldman on everyone so theirs drops in their eye's either way, on follow up fits we stain as well. I don't check K's over CL's unless the patient complians of intermittent blur or the tell tale signs like blurry when I blink then clears or clear when I blink then gets blurry. Check for and document any neo even with the new silicone lenses people abuse them so you still have to check and document. Invert the lids, I was taught every follow up you invert so checking for GPC is a no brainer. The new silicone lenses make this especially important as they don't play well with many of the solutions available and they deposit easily makign GPC a real problem if you don't look for it.

There should be a hands on course available at the expos or other venus on fitting, cause I can tell you your not gonna learn it all from a book, this is a skill that requires a practical component to become proficient.

[Newyorkoptician]

Take the dam* K readings not only is it extreemly important in fitting, it is extreemly important to re-check these at follow-ups to see if you are causing changes in them. Any change is too much.

I could be more expressive on this today, but I'll try not to stir anything up.

Before you fit any contacts it would be advisable to read a book or two on the subject (see: The CLSA.com) and take a few courses.


Chip

DIdo! Should be done on all contact fits no matter soft or not. I would suggest if your new at contact fitting that you get some training from someone like Chip, who has a world of experience and could teach you things you would never learn in a book or on this board. Chip what is your teaching fee's?

[tmorse]

OD:-0.50-0.50X75
OS: -1.00

K readings:
OD: 43.00/42.75
OS: 43.00/43.25

I gave him oasys 8.4 for trial
OD: -0.75
OS: -1.00

For a starter, I suggest your learn the proper way of recording K's... the do have an axis, you know. Only then can you guesstimate the amount of residual astigmatism present, which will help with power selection.:finger:

[GAgal]

DIdo! Should be done on all contact fits no matter soft or not. I would suggest if your new at contact fitting that you get some training from someone like Chip, who has a world of experience and could teach you things you would never learn in a book or on this board. Chip what is your teaching fee's?

Don't give him any ideas! He's supposed to help me on my advanced and I can't pay him what he's worth!:cheers::D

[silentjatt]

For a starter, I suggest your learn the proper way of recording K's... the do have an axis, you know. Only then can you guesstimate the amount of residual astigmatism present, which will help with power selection.:finger:

yea.... i should have. just so u know i do record everything. just didnt bother to write the axis on here because theres only a .25 difference between them. I thought everyone here would be able to guess it from there. but my mistake i guess.

other than that... thanks everyone for your coments. much appreciated.