Global OK-Vision

  1. GOV
  2. About GOV
  3. GOV Fitting Guide
  4. High Myopia and Myopic Mulifocal
  5. Hyperopiaand Multifocal
  6. Low Myopia
  7. FAQ


• Mean K instead of Flat K is used for selection of trial lenses.
• KM measurement: Manual type KM is most reliable, but any calibrated auto-k or Sim-k obtained from Topography could be acceptable. Both the steepest and flattest Ks are required.

• Calculate mean K to select the first K-code for initial trial lens.
• Fine tuning, by observing flourescent pattern, to obtain the final K-code.

• Typical Bull’s eye with proper edge lift should be the goal of trial fit. (Whereas central pooling is standard for H & HP lens.)

• Spherical equivalent and corneal astigmatism is used to determine targeted power.

• Over refract or calculate to acquire the Power code.

• Determine lens diameter (OAD) by about 93-97% of HVID.

• Default OAD varied with K-code:40.00 - 42.25 / 11.2mm42.50 - 43.75 / 10.80mm44 - 47.50 / 10.40mm


All OrthoK has essentially a multifocal type effect which needs to seal for compression, get some touch on alignment zone, some edge lift and must be uniform or lenses can displace.

1) Do normal eye health / relevant checks and treat any eye health issues, assess patient suitability etc. Consider relevant patient history eg more plus for presbyopes eg +0.50 / minus for younger etc, and treat any ocular conditions before starting.

2) Going from a good centring treatment with another lens to GOV maybe fine but any doubts patient management / washout is always recommended.

3) Washout if in doubt – even high Modulus /thick soft disposables /custom which could need 1-2 days and RGP’s 5 day to a few weeks (as long as it takes for you to be comfortable the cornea has gone back to normal curvature)

4) Get patient to commit to aftercares / lens cleaning and other important eye health points you discuss.

5) Take good geometrically centred maps (Medmont preferred), get patient to look nasally one or more rings on both eyes (right eye look left to yellow ring/ left eye look right to yellow ring), hold down lower lid with eye max open to capture quality rings / topography. Poor maps don’t change Sim K’s much, but does affect the sag height calculations dramatically and gives accurate corneal curvature.

6) View patient topography, check placedio rings (adjust “colour map opacity” to zero). check quality of topography.

7) Check HVID / VVID using Medmont ruler (ruler in “Annotate” section) to assess if standard large / small cornea size (ordering a diameter different to GOV trial kit K- Code may be required). Measure along oblique to vertical axis from grey to grey through the centre of placedio rings. This ensures we don’t miss oval shaped corneas and order too larger lenses which reduces paracentral compression / centration. Removing the lids totally by getting the patient to lower their bottom lid while the optometrist uses a eg surgical tool to raise the upper lid. Getting the correct diameter (95% of VVID) in this way may result in much smaller diameter than expected, but results have shown better centration and bigger central treatment zones. This is even more critical on Hyperopic OK patients and can improve your results massively.

8) Other watch points: large pupils and corneal cyl higher than spec cyl induces ATR cyl n other meridian, you can check by putting on an RGP Sphere.

9) Before starting around mean K with your GOV trial, you must check for inferior / superior elevation and / or flat “e” see 10) and 11).

10) Check elevation maps for inferior / steep blue elevation go approx. 0.50D flatter and for superior steep blue elevation go approx. 0.50D steeper than mean K (GO custom tab bottom of map scales, press custom and adjust “max value” to 50 for best result). This applies when there is around 20 micron difference between steep and flat meridians around 4mm from the centre.

11) Check flattest “e” value, if flatter than .5 (GOV alignment aspheric curve is .5 “e”) you will likely need to go flatter eg .6 “e” approx 0.75D flatter, / .75 “e” approx 1.25D flatter etc than mean K (add elevation / flat “e” calculation to mean K to get closer fitting K- Code trial to start).

12) After choosing appropriate first GOV trial lens, rub lens to get wetting using RGP cleaner / conditioning solution, then flush with preservative free saline and fill the back with it (PFS) eg Lens Plus and add fluorescence now too. This is critical or corneal staining is likely with RGP conditioning / wetting solutions creating toxicity in large reverse zones pools and fluorescence won’t get behind the lens if done afterwards. Popular solutions used: For cleaning AOsept / Progent , while for insertion Lens Plus / Tera tears. Sustane Balance for lenses sticking in the morning and comfort probs can assist some patients (lipid component assists?)

13) Anaesthetic is also used by most practitioners to reduce tear reflex and improve patient initial comfort.
14) Insertion of the filled lens face down is critical or air bubbles will be trapped (won’t blink out) and treatment area will get affected.

15) Lens should move 1-2mm on blink when first inserted. Allow lens to settle for at least 5 minutes

16) Assess fluro fit, edge lift, eccentration, alignment curve seal off etc.

17) O/R with lens on patient’s eye to gauge likely final target power

18) If riding low consider a flatter K-Code or riding high a steeper K-Code (may need smaller / larger diameter depending on HVID / VVID)

19) If fit looks good, have the patient sit with eyes closed for 15 - 20 minutes in waiting room.

20) Remove lens and redo topography

21) Use Tangential Power Map to check 360 treatment zone has started centrally, Refractive power map to see treatment size difference (click on custom tab below scales, click custom and adjust “Max or Min value” scales to match patient scales for clearer differences in colour topography eg “Max value” may need to be reduced from eg 50 to 47.0 or 45 etc).

22) If all looking good get patient to take lens home, insert for sleeping hours only/ REMOVE on waking, and MUST return next morning for detail assessment.

23) Do correct insertion teach, basic lens care and maintenance, take out if uncomfortable etc .Reinforce importance of aftercares eg 1st morning, few days /1 week, 2 weeks, 1 month, 6 months (this can vary slightly)

24) When the patient returns next morning (after sleeping with lenses in for 6 – 8 hours /patient MUST remove lenses upon waking), assess treatment.

25) Again use Tangential Power Map to check 360 treatment zone is central.

26) Use Refractive power map and click on compare with original and treated map to see treatment power mm size achieved (50-75% of trial power expected). Myopic can be higher eg 80%, Hyperopic maybe lower at 50% but H can take a week for treatment to start in some cases (if 360 central seal / central happening may need few days / week). Higher target powers may take even longer for full treatment to occur.

27) Test refraction

28) If totally happy you can order lens or wait and assess as required. Note trial lens may be too strong / weak when full moulding occurs and this needs patient management / explanation (as per any vision correction option).

29) If not happy with treatment get patient to washout and retrial using GOV fitting lenses.

30) Register on for patient fitting blogs and / or email Innovative Contacts for technical assistance.

31) Custom OK torics may start at 1.00D corneal cyl (or lower) if notable irregular corneal curvature / “e”s / elevations present.


Other Worthwhile Points To Note:

A) To check whether a toric is needed, go into “elevation” map, click 4 mm out from centre at steep vertical point (WTR) and horizontal flatter point. If 25 to 30 micron difference between these points a custom toric is needed (bottom right “Elev” shows micron change / subtract). In this case a Innovative Contacts custom toric would be needed (just send maps / spec RX etc)

B) If patient has no spec cyl and but has corneal cyl, you will induce ATR cyl. The best way to check what / if any ATR cyl is put on a standard RGP sphere to assess / manage with patient.

C) If changing K-Code from trial fit used, going 0.25D flatter in K-Code = 0.25 in power change (0.25D steeper add -0.25, 0.25D flatter = +0.25 to power code).

D) Going 0.8mm smaller diameter than trial consider going 0.25 steeper K-code, or if larger 0.25 flatter approx.

E) When to consider MP … as all OK is a M/F effect most patients will get around +1.25 benefit depending on treatment zone size etc. Consider MP when the patient gets closer to +2.00 add if good centration is achieved (HP more common / likely needed).

F) GOV Artmost calculator is available which calculates mean K (K-Code) and best spec sphere no vertex plus half corneal cyl for Power Code.

G) When emailing maps, press and hold "Cltr" or "Command" (Mac) key and copy all maps you wish to send so when we upload them they all come at once rather than doing one at a time.

H) The best way to check the centration and size of the treatment zone in relation to the pupil. …RET all your OK patients at AC's.

I) Axial (Sagittal) Maps illustrate the optics of the cornea, and is a closer representation to what the observer ‘see’s’. Axial maps are good for assessing types of astigmatism (eg. central vs limbal-to-limbal). You can also use axial maps instead of refractive power maps when comparing refractive power changes with OrthoK.