Progressive Additional Lenses PDF

Progressive OPHT Additional 2612 Lenses Chapter 20 27 March 2024 Introductio n: Enable the patient to have clear vision at all viewing distances Made by specifically designed front surfaces curves ― Changing curves cause the + power to gradually increase ― Beg...

Progressive OPHT Additional 2612 Lenses Chapter 20 27 March 2024 Introductio n: Enable the patient to have clear vision at all viewing distances Made by specifically designed front surfaces curves ― Changing curves cause the + power to gradually increase ― Begins at the distance portion and ends at the near portion Permits clear vision at any viewing distance merely by changing the position of the head and eyes MAJOR ADVANTAGE over single vision readers Works very well when prescribed well HOWEVER: does have some minor limitations because of technical restrictions in lens manufacturing Introductio n: Progressive additional lenses: It is a COMPROMISE(D) lens It is a choice of vanity and convenience over optics Introductio n: Zones of CLEAR VISION: 1. Bifocals: Provides a step like of clear vision Creates a gap-zone of blur vision between distance and near zones Creates an image jump 2. Single Vision: Limited range of clear vision between distance and near zone Eg. Readers Introductio n: Zones of CLEAR VISION: 3. PAL: The first progressive lens was constructed in 1959 A progressive addition lens is a one piece lens consisting of a distance and near portion, connected by a progression zone where the optical power increases from the distance to the near power Construction is different from conventional bifocals or multifocal without dividing line Progressive is not an invisible bifocal ― Fused round bifocals are invisible bifocal = demarcation line between distance and near portion is smoothed away Introductio n: Zones of CLEAR VISION: 3. PAL: The power from the midline upward corresponds to the distance rx ― The lens begin to increase in plus power as the eyes drop from the midpoint and continue to increase until the prescribed power of the near add is reached Progressive addition lenses are specifically designed with front surface curves that cause the lens to increase in plus power in an area from the distance to the near portion The design permits clear vision at any given distance merely by positioning of the head and eyes Introductio n: Zones of CLEAR VISION: 3. PAL: Surface curvature varies from the upper (distance) zone to the lower (near) zones The power increase towards the bottom of the lens is brought about by decreasing radii of curvature Introductio n: Zones of CLEAR VISION: 3. PAL: Near portion = lower central slightly nasal displaced Progressive corridor between distance and near portion is 10-16 mm length and 2.0- 2.5 mm NASAL FROM DISTANCE OPTICAL CENTRE (INSERT) The width of the corridor differ according to the design and the power of the addition PAL Lens Markings PAL Lens Markings Temporary Markings: 1. Distance reference circle (DRC) Distance lens prescription is checked in this portion This prescription is usually verified using a vertometer/lens meter 2. Fitting cross Normally when frame is aligned on the patient’s face, the fitting cross is at the center of the px pupil On the chart it is located above the prism reference point and below the DRC Used to determine the monocular PD and fitting height PAL Lens Markings Temporary Markings: 3. Prism reference point Point used to verify the amount of prism in the lens 4. Near reference circle Used to check the near rx Use a vertometer/lensmeter to obtain the rx 5. Small horizontal locator Horizontal markers Inline with the prism reference point Assists the fitter to align the lens horizontally PAL Lens Markings Permanent Markings (engraved): 1. Lateral circles Horizontally located small circles Used to help relocate temporary markings on a lens 34mm apart One on nasal side, one on temporal side 2. Lens logo and material code The lens name/type is identified using the company logo/description The material code helps to identify the type of lens material used PAL Lens Markings Permanent Markings (engraved): 3. Addition power Temporally located Add power is confirmed using the vertometer placed in the NRC NRC – DRC = add power Either the full description of the add = 100, 125, 150 to 300 OR the first two digits of the add = 10, 12, 15 to 30 PAL and Astigmatic Error All PAL designs have aberrations that are created on the sides of the progressive zone ― The area on either side of the progressive zone produces some distortions The progressive zone gives clear vision when properly fitted and displaced The distortion is mainly due to the induced cyl power ― The wider the near portion, the greater the induced peripheral astigmatic error Astigmatism is directly proportional to add power +3.00DS add = 3x cyl compared to +1.00DS add PAL and Astigmatic Error Lens Designs MONO vs MULTI ASYMMETRICAL vs SYMMETRICAL HARD vs SOFT Lens Designs: Mono vs Multi Mono Design: Single design used for all adds and base curves Same basic design over the entire add power range Near position doesn’t change with change in add Lens Designs: Mono vs Multi Multi Design: Near position changes with the add change Near goes up with the increase in the add Multi-design vs design by prescription Multi- design: adjusted for add power only ― Multi = softer designs in low add and harder design in high adds ― Softer gave more SV perception to new presbyopes ― Harder design for advanced presbyopes in higer adds Increase viewing zone Design by prescription Design adjusted in accordance to add and base curve Corridor length varies relative to both base curve and add Lens Designs: Mono vs Multi Multi Design: Lens Designs: Symmetrical vs Asymmetrical Symmetrical Asymmetrical Older design Modern design R and L lenses identical R and L lenses have own designs Astigmatic area on the side of corridor can be adjusted independently To obtain the near inset lenses are rotated Near inset obtained without rotation of lens 9 and 11 degrees Has slight nasal inclination BV and binocular field of view effected Better BV and binocular field of view Astig and power not same between nasal and temporal sides in distance portion Nasal astigmatism is higher d/2 no rotation Differences in prism, magnification and power at corresponding points in R and L lenses Lens Designs: Symmetrical vs Asymmetrical Lens Designs: Horizontal Symmetry Asymmetrical design Same power at the same distance away in the lens Mirror optics Corresponding points in the R and L lenses power differences is minimised = BV improves No difference in prism power, blur and magnification in the two eyes therefore BV comfort is better too Lens Designs: Soft vs Hard HARD DESIGN SOFT DESIGN Wider distance portion Narrower distance portion Wider near portion Narrower near portion Higher more rapidly increasing surface Lower more slowly increasing astigmatism astigmatism (closely spaced contours) (wide spaced contours) Better central/foveal vision Better peripheral dynamic vision Critical viewing, or need of very good VA Active viewing People who read a lot Professional drivers Young presbyopes Shorter narrower progressive corridor Longer wider progressive corridor Better for current BF wearers Comfort better for early presbyopes The power in the progressive channel Slower vertical change in power as the increases rapidly as the wearer looks from wearer looks from distance to near distance to near Lens Designs: Soft vs Hard Lens Designs: Soft vs Hard HARD DESIGN SOFT DESIGN They have large areas of stable optics in They have smaller distance and near zones both distance and near portion that are free but longer and wider progressive zone of astigmatism They have shorter and narrow progressive zone The power in the progressive channel A soft design has a slower vertical change increases rapidly as the wearer looks from in power as the wearer looks from distance distance to near to near Straight lines may appear more curved There is less apparent curving of straight when viewed through the lower half of the lines lens Patient takes longer period to adapt Patient adapt easy to soft design Presbyopes experience rapid increase of Presbyopes experience gradual increase of unwanted astigmatism when moving eyes unwanted astigmatism when moving eyes from near zone to the peripheral zone from near zone to the peripheral zone Unwanted astigmatism is in smaller areas Unwanted astigmatism spread over larger Lens Designs: Soft vs Hard Advantages and Disadvantages of PAL Advantages Disadvantages 1. The segment is ‘invisible’ – cosmetically 1. Straight lines appear curved appealing 2. Decreased width at intermediate and 2. Good acuity is possible at any distances near 3. Viewing more closely resemble pre- 3. Increased in eye and head movements presbyopia 4. Expensive 4. No image jump 5. Peripheral aberrations 5. Improved appearance 6. No distracting borders 7. Continuous vision 8. Looks like a single vision lens 9. No demarcation lines 10.No forward and backward movement of the head when looking at object 11.One pair of spectacle to use at all distances 12.Simulate a person normal vision 13.They are light, comfortable and Today’s PAL They have wide distance, near and intermediate zones They have gradual increase of astigmatism on the sides of progressive zone They have a shorter progressive zone Today’s PAL Choices galore! Most companies have anywhere from 5 up to 11 different designs You can choose the corridor length – long vs short Individualised lenses – freeform You can add-on a multitude of coatings Today’s PAL Corridor length: Chapter 20 (pg.476) Freeform PALs: Referred to as direct or digital surfacing A process that is capable of producing complex surface shapes ― Aspheric, atoric and progressive addition surfaces Typical process begins by generating the lens surface using a three- axis, computer numerically controlled generator With three possible axes of movement, single-point cutting tools can produce virtually any lens surface shape with a high degree of accuracy and smoothness ― Almost no limitations Using free-form surfacing, a laboratory can directly surface a variety of lens designs directly onto a semi-finished lens blank in addition to the prescription curves Head and Eye Movements Physiological function of a progressive lens is to compensate for the presbyopic loss of accommodation ability ―PAL helps to restore the eyes ability to view objects at near to intermediate distance while preserving clear distance viewing When the eyes are viewing through progressive lens, both the head and eye movement must be executed ―The movement allow the wearer to locate objects in the field of view The PAL design must ensure that the images produced by the right and left lens form on corresponding retinal points The progressive zone must follow the visual axis of the eyes on downward gaze as they converge for the near objects. The power variation found at corresponding points on the right and left lenses should be complimentary for the images to be equal in the two eyes The desirable PAL design has to minimize head movements and the swimming effect will be smaller A good PAL will allow the wearer to read with a more natural and comfortable posture PAL Considerations Back vertex distance: A relatively small vertex distance must be used Increase field of view through the intermediate and near zones Reduces effect of peripheral aberrations such as oblique astigmatism and chromatic aberrations Small vertex distance brings back the peripheral areas of the lens away from the line of sight as the patient looks at the object to the side of the vision Pantoscopic tilt: A reasonable tilt must be 10 – 12 degrees. Increased tilt brings near and intermediate zones closer to the eye therefore increase the field of view Impacts surface astigmatism Facial wrap: The frame must not be too flat A positive face form is recommended Increased facial wrap brings closer various zones to the eye ― It moves the temporal peripheral areas of the lens away from the patient line of sight PAL Considerations Occupational or visual needs: Relative need for distance / intermediate / near vision Field of view requirements Direction of gaze requirement Prescription: Emmetropia presbyopia (distance and near) Bifocal wearers (hard or soft design) Progressive wearer Myopia Hyperopia Previous correction: Be cautious when changing progressive wearer to bifocals and vice versa Cosmetic purpose Contraindications: Field of view: Architects needs large field of view Graphic artists working on large computer screen Surgeon need a large field of view to perform well Direction of gaze: Airline pilots Librarians Head movements and mobility: Sport like tennis, cycling and squash demand body activity with rapid head and eye movement NB: specialized PAL Common Errors caused by optometrist: 1. Poor frame adjustment or frame choice 2. Errors during measurement of the fitting height or monocular PD 3. Change in lens design / type or base curve 4. Unsuitable lens type 5. Occupation visual requirements 6. Errors in prescription due to poor eye examination 7. Lack of knowledge regarding PAL 8. Dispensing of pal by unqualified personnel Steps to solve PROBLEMS: Some patients: I don’t want peace, I WANT PROBLEMS Common Problems: Having to hold a book to the side to read: Monocular PD is wrong Tilt the head back to read: Fitting height is too low Reading portion is too low Tilt the head back to see at distance: Distance Rx is over minused Head tilt forward to read: Near addition is too strong and patient try to read through progressive zone Head tilt forward to see at distance: Fitting height is too high Steps to solve PROBLEMS: Step 1: QUESTION Question the patient in detail about the problems she/he experienced with PAL Questions must be open-ended (Avoid using terminology) Step 2: Re-mark the lenses and confirm ordered heights and PD measurements. Step 3: CHECK Check frame fitting on the patient face Check fitting cross position, vertex distance, pantoscopic tilt and facial wrap Step 4: OBSERVE Observe patient when wearing spectacles Let the patient hold a reading material at the position where they see clearest Ask the patient to look at distance object and walk around Ask the patient to inform you when the vision is blurred and when is clear Observe the head and eye position Steps to solve PROBLEMS: Step 5: Find the solution / remedy to patient problem and make patient happy Remake ― Wrong Rx ― Wrong fitting height, PDs ― Advance to entry-level lens TLC patients Remind patient that they have to adjust their environment to fit with their new glasses ― Laptop work issues: adjust your seat/table top Self-Review: 1. Troubleshooting progressive addition lens problems PAGE 466 TABLE 20-1 2. Spherical and Aspherical Upper Halves: In the original progressive lens design = maintained an upper half (distance portion) having a spherical front surface For the new design (1974) varilux = intention was to reduce amount of unwanted peripheral astigmatism Small amount of astigmatism could be tolerated in the periphery of the distance portion The new design is aspherical in the upper and lower portion of the lens surface https://www.youtube.com/watch?v=FBJKirbCA2I EN D!