At present, Contact lenses have become the best alternative for glasses. Hence proper fitting of the contact lenses is important to make sure that there wont be any harm to the eye and visual health. Different types of contact lenses have different Fitting procedures. The most common types of contact lenses are Soft spherical and Soft toric and Rigid gas permeable contact lenses.
- Soft spherical contact lenses Fitting assessment:
- Coverage: The lens should cover the cornea in primary gaze and in all positions of gaze. If the CL covers whole cornea it is considered as complete coverage. If the CL does not cover whole cornea then it is considered as partial coverage.
- Recording: Complete/Partial
Complete: When cornea is completely covered
Partial: When cornea is not completely covered
- Centration: The center of lens should remain approximately center to the cornea in all positions of gaze. Recording: well centered/decentered with units of x,y
A binasal system is most commonly used method to record centration. The location of the geometric center of a contact lens, within the graph is the basis of the description of its centration. In this system nasal decentration of both eyes is taken as positive, temporal is taken as negative,superior -positive and inferior-negative.
- Movement: It is measured in
- Up gaze: Measure the CL periphery which is out of cornea inferiorly
- Down gaze: Measure the CL periphery which is out of cornea superiorly.
- Horizontal gaze: Measure the CL periphery which is out of cornea temporally and nasally.
Recording:2mm
- Tightness: This is assessed by push up test. When the eye is in primary gaze position, the practitioner moves the lens up with the pressure of thumb on the lower eyelid. The practitioner assess the relative ease with which the lens is displaced and the speed of its recovery to its original position.
Recording:Recorded in grade ranging from 0% to 100%.
100 indicates tight and 0 indicates loose fit.The optimum fit would have 40% to 60% of tightness.
- Effect of blink: If the fit is steep the vision would be clear after blink and for a flat fit the vision would be clear before blink. For an optimum fit there won’t be of much difference between before and after blink.
- Comfort: Ask the patient about his /her comfort on a scale of 1 to 10. 1 being the least and 10 being the most comfortable. This would certainly lead us to the type of fitting.
- Appearance of optimum fitted SCL
- Appearance of steep fit lens
- Appearance of flat fit lens
2. Soft toric lenses Fitting assessment
Fitting assessment of soft toric is same as soft spherical contact lenses except lens rotation.
- Reference marks: There are different reference laser marks imprinted on the toric lenses (but it does not indicate the axis/meridian of astigmatism). Location of laser marks varies from company to company. Based on laser marks, rotation of lens is assessed
- Rotation : When we insert a lens in the eye there may be two situations, It may be in the same position during the blink and after the the blink. It may displace during the blink but may retain its location after the blink. It may not be in the same position it moves because of blink effect. If it rotates, the axis may rotate, So to avoid this cylinder in unwanted axis we add or subtract the amount of rotation to the axis of the prescription so that the axis of the cylinder coincides with the axis of the eye.
LARS Rule:
- This is used to assess the rotation of lens and prescribe correct axis
- First, estimate the degree of lens rotation
- If the lens moves towards left add the degree of mis location to the Rx axis
- If the lens moves towards right subtract the degree of mis location to the Rx axis.
- Example:
Consider eye as a clock and compare the angle for movement
If the axis is at 6 o clock then there is no need of adding or substracting. In the second picture it has moved towards right (right side of the practitioner) by 10 so substract 10 from actual prescription axis. In the third picture it has moved towards left (left of practitioner) by 10 so add 10 to the prescription axis.
NOTE: Look for the movement only in the inferior position
Recording:
Rx:-3.00/-1.00×180
Left side rotation by 10
Final Rx-3.00/-1.00×10
3. Rigid gas permeable lenses Fitting assessment:
Dynamic fit:
All the fitting assessment done in he dynamic fit is done by asking the client to blink the eyes
Position
- Under the lid fit/lid attached fit: When the lens is partially covered and held in position by the upper eyelid. This is observed when lens is usually larger and BOZR is fitted either aligned with or slightly flatter than the cornea.
- Inter palpebral fit: When the resting position of the lens is between the upper and lower lids. This is observed when lens is usually smaller than interpalpebral aperture and BOZR is usually steeper than the cornea.
Centration:
It is rare for RGP to perfectly center on the cornea. Most of the times it decenters, decentration can be described as…
- High riding: The contact lens is decentered superiorly (mostly seen in under the lid fit)
- Low riding: The contact lens is decentered Inferiorly.
- During the blink if the lens moves along with lid but does not cross limbus and if the other characters are optimum it can be dispensed.
Recording: Well centered/high riding/low riding
Movement:
It is influenced by blink.
- Type:
- Smooth: A smooth continuous line of excursion following a blink. Smooth movement indicates near alignment pattern
- Apical rotation: The lens rotates about corneal apex from superior to inferior position
- Rocky: The lens tendency is to rock around the flatter meridian of the cornea. Mostly in case of tight fit .
- Speed: The speed at which a lens moves following a blink should be assessed as slow, average or fast. A steep would have less movement and flat has more speed
- Amount: between the point of inferior edge of lens on cornea when opening and the amount that the lens moves to regain its position of rest. Area of lens moved with blink taking the inferior position of lens as a reference A steep lens would move more than a flat lens
- Direction: Vertical (desirable), Oblique/diagonal and Apical
Stability:
Judge the stability only after adequate degree of adaptation has achieved.
| Steep | Optimum | Flat | |
| Position | Mostly interpalpebral | Interpalpebral/
under the lid |
Interpalpebral/
under the lid |
| Centration | central | High riding/low riding | Low riding |
| Movement | Very minimal | Optimal | Very high |
| Stability | More stable | Stable | Unstable |
Static fit (lens cornea relationship):
- It describes the physical relationship between the base curve of lens and anterior curvature of cornea. This is done while the CL is resting on the cornea without any movement. Flouroscein staining plays a very important role n static fitting. There are two words which clearly define static fitting are Pooling/Clearance, Bearing/touch.
- Pooling: Accumulation of tears in space between the RGP lens and cornea following fluroscein instillation.
- Bearing /touch: The dark zone where the RGP touches the cornea.
| Position | Steep | Optimum | Flat |
| Central | Pooling | Feathery touch | Bearing |
| Mid periphery | Bearing | Slight pooling | pooling |
| Peripheral/edge lift | Very narrow | Edge lift/clearence of 0.5 to 0.75mm | Excessive Pooling, wide edge lift/clearence |
References:
http://en.thevisioncareinstitute.cz/sites/default/files/content/cz/Soft_CL_fitting.pdf
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