unit 7 Flashcards
Lens Selection
- Power (vertexed)
- Base curve
◦ Relate K readings to contact lens BC - Diameter
◦ Check HVID
Average HVID = 11.8 mm
◦ Significantly larger or smaller diameters may require custom lenses
Characteristics of a Good Fit
- Corneal coverage
◦ Extends 1 mm past limbus - Lens centration
- Dynamic fit
◦ 0.25 to 0.5 mm of movement with blink
◦ Easy push up movement
◦ Recenters easily - Alignment
◦ Not indenting conjunctival vessels - Comfort
Evaluation of Fit
- Slit lamp evaluation
◦ Corneal coverage
◦ Centration
Horizontal lens lag (Figure 4)
Upgaze (Figure 5)
◦ Primary gaze post-blink movement
◦ Push-up test
◦ Lens edge evaluation - Comfort
- Vision
Refinement of Contact Lens Fit - Poor centration
◦ Lens too large/small
◦ Loose lens
◦ Tight lids
* Edge alignment
◦ Loose lens/tight lens
◦ Peripheral lens design
Refinement of Contact Lens Fit
Primary gaze movement (less than 0.25mm)
◦ Tight lens
Flatten base curve (ex: 8.3 to 8.7)
Decrease lens diameter (loosens the fit)
* Primary gaze movement (more than 0.50mm)
◦ Loose lens
Steepen base curve (ex: 8.9 to 8.6)
Increase lens diameter (tightens the fit)
Refinement of Contact Lens Fit- Discomfot
◦ Foreign body
◦ Lens thickness
◦ Loose lens
◦ Edge standoff
* Vision
◦ Incorrect power
◦ Loose lens
Check monocular visual acuity
◦ Use occluder
* Check spherical over-refraction (SOR)
◦ Use contact lens flippers
◦ Show +0.25
◦ Show -0.25
Recording Data
- Centration
◦ Centered OU; slight nasal decentration - Movement
◦ +0.50 mm OD; moves easily with push up and +1.00 mm movement in upgaze - Periphery
◦ Flat edges OU - Corneal
◦ No corneal staining - Visual acuity
◦ OD VA 20/25 SOR -0.25 20/20
Criteria for Use of Soft Toric Lenses
- Degree of Astigmatism
◦ 0.75D or more tend to be fit with soft toric lenses - Cylinder Axis
◦ Uncorrected cyl with an oblique axis tends to cause greater degradation - Ocular Dominance
- Other Alternatives
◦ Glasses, RGPs - Visual Needs
Toric Design
* Optics
◦ Toric back surface with spherical front surface
◦ Spherical back surface with toric front surface
Toric Stabilization
- Toroidal back surface
◦ Not sufficient to achieve lens stabilization - Prism ballast
◦ BD prism added to bottom of lens
◦ Thick lens base = reduced oxygen - Truncation
◦ Bottom sliced off lens
◦ May be uncomfortable against lid margin
◦ Rarely used today
Dynamic Stabilization
- Most commonly used method of stabilization
- Thin zones on top and bottom of lens
- Eyelid action on thin zones stabilizes lens
◦ “Watermelon seed” - Lens can be placed in any orientation on eye
◦ Will orient with blinking
Toric Lens Markers
- Allow degree of rotation to be assessed
◦ Laser trace, scribe lines, engraved dots - Location
◦ 6 o’clock
◦ 3 and 9 o’clock - Does NOT indicate the cylinder axis
◦ Indicates the lens orientation required for the indicated prescription
Evaluation of Fit
- Comfort
- Vision and spherical over-refraction (SOR)
- Slit lamp evaluation
◦ Centration
◦ Movement
◦ Push-up test
◦ Upgaze
◦ Horizontal and vertical lens lag
◦ Lens edge evaluation - ROTATION
Evaluation of Rotation
- Find toric marking on lens
- Estimate
◦ 30 degrees between each clock hour - Assess rotation using slit lamp beam
◦ Narrow beam
◦ Marking at 6 o’clock
Check if vertical
Turn light tower to angle beam
Read number of degrees
◦ Marking at 3 and 9 o’clock
Turn light tower to create horizontal beam
◦ Identify as rotated right or left (or clockwise/counterclockwise)
