How are PALs made?
Prism thinning - Rx is ground into the lens
Change in radii across the front surface of the lens
What is eccentricity?
The rate of flattening from the centre to the periphery of a lens
What are some benefits of eccentricity?
Eliminated distortion, coma and spherical aberration
What are some benefits of PALs?
No image jump
What are the two types of lens designs?
Where will most distortion be placed?
Hard and Soft lens designs - Like a bean bag chair, same amount of distortion but they are spread out differently
Most of the distortion will be pushed to the inferior nasal portion of the lens - this is the least used portion of any lens
What are some pros and cons of Hard design PALs?
Pros
- Wider distance portion
- Wider reading area
- Shorter corridor
Cons
- More distortion/astigmatic error as the eye drops ∴ longer to adapt
- Narrower corridor
- Patient’s complain of curved straight lines
What are some pros and cons of soft design PALs?
Pros
- Longer and wider intermediate corridor
- Less noticeable astigmatic error as the eye drops
- Less curving of straight lines
- Faster adaptation period
Cons
- Narrower distance
- Narrower near
Define Corridor length
Distance from the fitting cross to the reading area
What are pros and cons to short corridor PAL designs?
Pros
- Less head and neck movement
Cons
- More astigmatic error as the eye drops (more distortion squished into a smaller area)
- Narrower intermediate
What are pros and cons to a long corridor PAL design?
Pros
- Less astigmatic error
- Longer intermediate
Cons
- More head and neck movement to find the “sweet spot”
What are the steps to trouble shoot a PAL fit?
- Verify the OD Rx
- Compare to previous Rx
- Inspect the lenses
- Neutralize
- Compare PDs to Frame PDs
- Segment (on the patient and compare to old glasses)
- Base Curve
- Vertical Prism individually
- Vertical Prism imbalance
- Lens Material
- Adjustment
- Vertex Distance
- Face form
- Pantoscopic tilt
- Lens design and patient’s needs
- Re-educate lens use
- Return to Doctor
- New Frame
What are the 8 requirements for fitting Bifocals an Trifocals?
- Transpose to minus cyl
- Lens design and frame choice suitability
- Pre-adjust the frame
- Measure Vertex Distance
- 8-10º of pantos
- Slight face form
- Mono distance and near PDs
- Measure Seg heights
Where should you measure a seg height for ST-28?
1 mm below lower lid
Where should you measure a seg height for Executive/Kryptok?
At the lower lid
Where should you measure a seg height for a previous wearer?
Same height as the original pari (so long as there are no complaints)
Where should you measure a seg height for Children?
1 mm above the lower lid
Where should you measure a seg height for Trifocal?
1 mm blower the bottom of the pupil (dot between the bottom pupil and lower limbus)
What are the 11 steps to fit a PAL?
- Transpose to minus cyl
- Lens design and frame choice suitability
- Know the fitting cross height options
- Pre-adjust the frame
- Measure vertex distance
- 12-15º of pantos
- Positive face form
- Mono distance and near PDs
- Monocular fitting cross heights
- Measure seg height (@ fitting cross)
- Use template to ensure the add is not cut off
Where do you measure the seg height for a PAL?
At the pupil centre
What Base Curve should you use for
- Plus lenses (regardless of index)
- Regular index (-4.00 to +4.00)
- High index minus prescriptions… why?
- +6.00
- +4.50
- +1.25 - myopes do not like magnification and they like flatter curves
Complaint: Eye lashes are rubbing
- What is the problem?
- How do you fix it?
- Too little vertex distance, the glasses are sitting too close
- Move notepads away from the frame front
Complaint: Frames are sitting on cheeks
- What is the problem?
- How do you fix it?
- Frame is sitting too low, or there is too much pantos
2. Move notepads together to raise frame or bend temples up at the end pieces
Complaint: Red marks behind ears
- What is the problem?
- How do you fix it?
- Too much bend at the temple tips
2. Lessen the bend and follow ear contour
Complaint: Red marks on nose
- What is the problem?
- How do you fix it?
- Nose pads are not sitting flush or they are too small
2. Change nose pads to larger size and bend guard arm so notepads sit flush
