Lecture 8 Multifocals 2024 PDF

OPT505 Clinical Skills & Refractive Management Multifocals: Bifocals and Trifocals Claire Gorman One Time Code:GC-SP-NX Core Competencies 4.1.1 ABILITY TO ADVISE ON, ORDER AND TO DISPENSE THE MOST SUITABLE FORM OF OPTICAL CORRECTION TAKING INTO ACCOUNT DURABILITY, COMFORT, COSMETIC APPEAR...

OPT505 Clinical Skills & Refractive Management Multifocals: Bifocals and Trifocals Claire Gorman One Time Code:GC-SP-NX Core Competencies 4.1.1 ABILITY TO ADVISE ON, ORDER AND TO DISPENSE THE MOST SUITABLE FORM OF OPTICAL CORRECTION TAKING INTO ACCOUNT DURABILITY, COMFORT, COSMETIC APPEARANCE, AGE AND LIFESTYLE (MULTIFOCAL DISPENSE) 4.1.2 ABILITY TO ADJUST A SPECTACLE FRAME OR MOUNT TO OPTIMISE PHYSICAL AND OPTICAL PERFORMANCE One Time Code:GC-SP-NX  Learning Outcomes You will be able to: Describe the different types of bifocals and trifocals Explain the advantages and disadvantages of different types bifocals and trifocals Calculate ‘Jump’ Lenses used for Near Vision Define the artificial far point Discuss the impact of the working distance Describe various trifocal designs available Apply prescription analysis for different patients One Time Code:GC-SP-NX Refresher on the Near ADD Presbyopia: when the amplitude of accommodation has dropped to such an extent that near vision becomes difficult or uncomfortable As a rule of thumb we are comfortable using between ½ & 2/3 of our full amplitude of accommodation. – If this is not sufficient to view the task, additional correction may help. Presbyopia is a reference to near and intermediate vision, it is not exclusively about ‘reading’ The visual consequences of presbyopia (and correction required) depend on the patient’s – age, lifestyle, occupation, hobbies and interests etc. Onset is generally around 45 years old Remember… It is called an Add because we are adding Positive Power to the distance prescription to change the focal length The addition is the difference in vertex power between the distance and near corrections E.g. Distance Rx +3.00/-2.00x90 Near Rx +4.75/-2.00x90 Near Add = +1.75 Tentative Adds WD 33cm 40cm 50-60cm Age 45 1.25 0.75 0.50 50 1.75 1.25 1.00 55 2.25 1.75 1.50 60 2.50 2.00 1.75 Multifocal Lens Definitions Multifocal lens: a lens designed to provide correction for two or more discrete viewing ranges Bifocal lens: a lens that has two discrete viewing portions (normally for distance and near) Trifocal lens: a lens that has three discrete viewing portions (normally distance, intermediate and near) Progressive Power lenses: a lens with a surface which is not rotational symmetrical, with a continuous change in focal power over part of whole of the lens Bifocal Lenses The area of the lens used for distance vision is called the distance portion (DP) The area used for near vision is called the reading or near portion (NP) and has additional plus power The larger of these two areas is referred to as the major portion Most bifocals have the near portion in the lower part of the lens referred to as the segment Addition power = vertex NP – vertex DP – (If seg on front, flip over to focimeter) Distance Why Conventional Bifocals Are Reversed To Properly Measure The Addition Power of a Bifocal Lens (2020mag.com) Near Classification of Bifocals Fused Round Franklin Solid Segment E D Construction Shape Blended Cemented C Bifocal Classification The most popular methods of bifocal construction are Molded, Solid or Fused Plastic bifocals are Molded Glass bifocals are Solid or Fused With both Molded and Solid construction, the bifocal can be felt on either the front or back surface of the lens Less Common Bifocal Classifications Cemented bifocals. A small plus lens is essentially glued to the distance carrier lens Blended bifocals- such as blended lenticular bifocal lenses Franklin split bifocals - the OG lens Construction of Bifocals summary... Split Bifocals: 2 separate lenses glazed half and half into the frame (Franklin Split) Fused Bifocals: Near seg in a higher index and fused into a depression in the main lens. Glass and no line felt. Solid Bifocals: From 1 lens produced by changing the curvature. Glass or plastics line can be felt. Cemented Bifocal: Seg cemented or glued to back surface of the lens. No longer produced but used in Fresnel lenses.  Segment Shapes and Sizes Most segments are now glazed on the front Round Segment D or S Segment surface The most common type is solid plastic There are a range of sizes for each shape The size and shape choice varies with the E-Style C segment or patient Curve top Down Curve Segment Segment Size To determine the segment size, measure the segment diameter (use the widest point) Common diameters are 22mm, 24mm, 28mm, 30mm, 35mm, 38mm and 45 mm Shaped segments generally follow, Round, “D” or a “C” pattern Round (Downcurve segment) Less visible than flat top segments More Jump, than E- or D-segments Large range of segment diameters: – 22 mm, 24 mm, 28 mm, 30 mm, 38 mm and 45 mm Available in solid plastic, solid glass, cemented plastic, cemented glass and fused glass Round (Downcurve segment) Os is the optical and geometric centre of the segment OD is the optical centre of the distance lens Os= Radius of Segment OD Os Segment Diameter Segment Radius D Segment A truncated round segment Smaller range of segment sizes: 25, 28, 35, 40 More noticeable edge than round seg Less jump than round seg Geometry of a 28 x 19 D-seg Bifocal 5mm 28mm 19mm (segment depth) Seg top to Os = seg depth – ½ seg diameter = 19 – 14 = 5mm C Segment D-segments with a rounded top edge Easier to glaze and surface than D-segs Available in solid plastic and fused glass : 25 & 28 mm There is no standard radius for the top curve Similar optically to D-seg E line (E-style or Executive) The optical centres of the DP and NP coincide No jump Largest reading area Available in solid plastic and glass as well as Franklin split Executive only applies to lenses made by American Optical E line (E-style or Executive) The optical centre of the segment Os coincides with the dividing line Segment Terminology D segments also referred to as S segments (Straight/Flat top). Some use the following distinctions: – D segments are glass lenses – S segments are plastics lenses There was such a thing as an ‘R’ segment, so avoid abbreviating round segments to R24 etc Invisible – Describes a bifocal or trifocal lens in which every dividing line is inconspicuous Visible – Describes a bifocal or trifocal lens which has a noticeable dividing line Segment Height and Top Position Segment height is the vertical distance from the segment top to the lowest point on the main lens. Segment top position is the vertical distance from the horizontal centre line to the highest point on the segment dividing line. HCL Segment Height Segment Top Position Segment (Geometric) Inset The geometrical inset is the horizontal distance from the optical centre (centration point) on the distance portion (OD) to the geometrical centre of the segment (OS) Each segment is generally inset 1.5–2mm, as each eye converges approximately 1–3mm for near vision Plastics bifocals may be specially manufactured to correct inset by any amount required to match the patient’s convergence The accurate inset for each eye is calculated as: Inset = Mono Distance CD – Mono Near CD The Geometrical Inset of a Round Segment Bifocal OD Geometrical centre of inset Inset The Geometrical Inset of a D-Segment Bifocal Inset OD Bisector of segment top Geometrical centre of segment How we measure addition power in bifocals It depends how they have been glazed, is the near addition a front surface or back surface segment? Segment Jump Jump is the sudden introduction of Base Down Prism caused by the segment and experienced at the dividing line of the whole segment No prism at an optical centre Jump is: – greatest in round segments – reduced in D & C segments – zero in traditional E-style segments and D35’s Jump in different segs The reading segment is positive so there is base down prism above the optical centre Jump increases with reading addition power and distance from the optical centre of the segment Os It is calculated using P = cF c = distance from dividing line to optical centre of the segment Os F = reading addition P = prism Prentice’s Rule P = cF Segment Jump The Distance Rx does not affect jump There are two possible visual effects for the patient: 1. Objects viewed close to the dividing line move or “jump” to a new position 2. The segment dividing line causes an annular scotoma within which objects are completely hidden until the wearer moves their head Segment Jump Why is Jump greater in Round Segments compared to D Segments Think of the lenses as 2 prisms Positive lens Negative Lens Optical Centre for each lens  Segment Jump For a Round Seg, the For a D Seg, the optical centre optical centre is towards is higher up – near the the bottom of the segment dividing line – induces less portion –induces more jump jump Optic centre of Seg Optic centre of Seg Prentice’s Rule P = c*F where c = the distance from the optical centre of the lens. This will be bigger in a Round Seg compared to a D seg – therefore jump is greater  Segment Jump A Px with a Positive Distance Rx A Px with a Negative Distance will benefit from a Round Seg as Rx will benefit from a D Seg as the 2 lower portions the 2 lower portions cancel each other out base down cancel each other out base & base up – so experience less down & base up – so jump experience less jump  Segment Jump For a Round Seg, the For a D Seg, the optical centre optical centre is towards is higher up – near the the bottom of the segment dividing line –or the seg is portion – or seg is produced from the bottom produced from the top half half of a positive lens so of a positive lens so induces Base Up Prism induces Base Down Prism Jump examples Calculate the jump produced by R28mm round down curve segment with a +2.00D reading addition P = cF P = 1.4x2 14mm P = 2.8 28mm Remember c is in cm Miscellaneous Bifocals NP Upcurve round segment: DP – Needed for some occupations, the segment is at the top Blended: – normally round segment solid bifocals – Blends the edges of the segment into the distance portion – Poor visual performance and limited success Fresnel stick on Bifocals: – Not used much – Can be useful for unusual occupational or hobbies masks e.g. diving masks – Available in D-segs Advantages of Bifocals Clear vision over both distance and near portions Wider reading area than progressive power lenses More convenient than separate distance and near single vision spectacles Disadvantages of Bifocals Visible junction of the distance portion and the segment Dividing line Indicates patient’s age! Reflections from the dividing line can be irritating, especially to first time wearers Jump – sudden introduction of base down prism at the dividing line Trifocals Similar design to bifocals Trifocals have an additional portion of seg This part takes into account visual tasks between 40-80cm Giving an intermediate addition as well as the near Most patients from 55 years and over will need an intermediate add Trifocals Available in various types: E-type C segment - C728 or C740 D Segment- D725, D728 and D735 Occupational segments The 7 within the D725 etc. denotes the depth of the intermediate Trifocals The intermediate add is generally 50% of the near addition The Intermediate add can be stipulated in % of the near add dependent on Px’s needs Optometrist should assess the intermediate add needed in Rx Occupational Trifocals: – Double D, Double Curve and Quadrifocal – These are used for particular occupations Pilot, Mechanic, Electrician, Librarian etc. Traditional Trifocals ED Style Concentric Downcurve Band Style R-segment Occupational Trifocals Occupational Trifocals: – Double D, Double Curve and Quadrafocal These are used for particular occupations – Pilot, Mechanic, Electrician, Librarian etc. Dependant on task Px can have: – Both segs with near add or intermediate add – Upper seg with 0.50D less then near add – Upper seg with intermediate add either 50 or 60% of near Occupational Variations Up-and-downcurve Double (small) round segment Double D-segment Double E-style Quadrifocal: 2x IP sections Order writing – Bifocal/Trifocal Lenses Lens type and/or brand The full prescription including the near add & intermediate add Segment diameter or size Monocular distance centration distances Segment height or top position Inset – usually default of 1.5-2mm in each eye – this is usually fixed by the manufacturer although with a round seg the cyl axis can be recalculated to increase inset as much as you like! – however, it must be accurately calculated and stated for high plus lenses Order writing – Bifocal/Trifocal Lenses Break! The Presbyopic Challenge Accommodation – what do we need to know Working distance – requirement varies with the individual Artificial far point Range of clear vision Average Amplitude of Accomodation. Adds Calculating the required add: 2/3rds to 1/2 amplitude can be comfortably sustained add=|L|-2/3Amp or add=|L|-1/2Amp Example: – For a working distance of 33cm – 4.5 - 6D is required To view 33cm – the required power is 3D, to view comfortably, this needs to be 1/2 (6D) or 2/3 (4.5D) of your accommodation facility. Working Distance Working distance can refer to two things: 1. The focal length of the lens (e.g. in Ret) 2. The distance to task that a person wants to view clearly The patient will have to help you define what their working distance(s) is(are) for you to provide the most appropriate correction This will vary person to person in hobbies and occupations – e.g. librarian vs the pilot – e.g. needle work vs photographer Artificial Far Point When a near correction is used, there is an artificial far point created. Artificial Far Point: This is the furthest point from the eyes that an object can be viewed clearly while looking through the additional power of the lens. It is calculated simply as the focal length of the Add. f ’ = 100 F As the add increases, the artificial far point decreases Focal Length F=1 f’ = 1 f’ F f’ f’ = 1 FD Focal Length f=1 FD f Range of clear vision Range of clear vision: the range in cm, over which an object can be viewed clearly – This may vary depending upon the duration of the task – The range that can be seen comfortably (or for any duration) will be smaller than the total range This is from the Artificial Far Point to the Artificial Near Point Artificial Near Point: the closest distance an object can be held and seen clearly – This uses all available accommodation plus the power of the Add Artificial Near Point Range of Clear Vision Artificial Far Point  Range of clear vision Example 1: – Emmetropic Px 40 years old – 5.00D of accommodation (near point 20cm – (100 5.00 =20cm ) – 1/2 accom 2.50D (comfortable near point 40cm) – Maximum range of clear vision = ∞ → 20 cm – Comfortable range (using 1/2 accom) = ∞ → 40cm Range of clear vision Example 2: – Emmetropic Px 42 years old – 4.00D of accommodation (near point 25cm) – 1/2 accom 2.00D (comfortable near point 50cm) – Maximum range of clear vision = ∞ → 25 cm – Comfortable range (using 1/2 accom) = ∞ → 50cm Range of clear vision Example 3: – Emmetropic Px 45 years old – 3.00D of accommodation (near point 33.3cm) – 1/2 accom 1.50D (comfortable near point 66.6cm) – Maximum range of clear vision = ∞ → 33.3 cm – Comfortable range (using 1/2 accom) = ∞ → 66.6 cm Average ranges of clear vision Age Add Working distance (cm) Range of clear vision (cm) +1.00 40 100 to 25 45 +1.25 35 80 to 24 +1.50 30 67 to 22 +1.50 40 67 to 29 50 +2.00 35 50 to 25 +2.25 30 44 to 24 +1.75 40 57 to 31 55 +2.25 35 50 to 29 +2.50 30 40 to 25 +2.00 40 50 to 33 60 +2.50 35 44 to 31 +2.75 30 36 to 27 +2.00 40 50 to 33 65 +2.50 35 40 to 31 +3.00 30 33 to 30 +2.25 40 44 to 36 70 +2.50 35 40 to 33 +3.00 30 33 to 29 Range of clear vision Example 4: – Emmetropic Px 45 years old – 3.00D of accommodation (near point 33.3cm) – 1/2 accom 1.50D (comfortable near point 66.6cm) – Maximum range of clear vision = ∞ → 33.3 cm – Comfortable range (using 1/2 accom) = ∞ → 66.6 cm – Px wants to read at 35cm but is finding they are getting eye strain Range of clear vision Example 4: – Emmetropic Px 45 years old 1/2 accom 1.50D (comfortable near point 66.6cm) – Px wants to work at 35cm so Add=|L|-1/2Amp Add=|100| - 1.50 35 Add = 3.00 – 1.50 Add = +1.50DS Artificial Far Point = 66.6cm Artificial Near Point = 22.22cm  Range of Clear Vison with +1.50DS Add is 66.6 → 22.22cm Complex lens solutions for presbyopes - thinking outside the box… So far we have discussed standard bifocals and trifocals These suit many people but there are instances where these won’t suit the individual We need to anticipate the potential problems More importantly, we need to be aware of the many other options out there to suit different situations The standard bifocal – what’s the problem? There are only two set far points created – The distance portion will focus for ∞ – The near portion will focus for the near add far point As the add increases, the far point will decrease There is a gap in clear vision between ∞ and the artificial far point created by the segment = blurry ‘intermediate vision’ The position of the segment may not suit the user Problem solving with bifocals There are many options available to change the dispense and meet an individual’s needs with a Bifocal: – Vary segment size – Vary segment position – Invert blank to put segment at top – Dispense segment in an unusual position – Change Rx in distance and/or reading portions to suit occupational requirements Problem Solving: A full Time Presbyopic Administrator Vision needs: – View ahead of VDU 60cm ahead of them for long periods of time – View of clock on wall and people Problem with Standard D28 Bifs: – Wearing a standard D28 will restrict the size of the clear vision and will require the patient to look through the bottom portion, causing neck ache. Solution? Problem Solving: A full Time Presbyopic Administrator Vision needs: – View ahead of VDU 60cm ahead of them for long periods of time – View of clock on wall and people Solution: Think about what they are looking at most of the time- VDU 60 cm away Give an upcurve bifocal with Distance at the top and major portion for intermediate set at 60cm DV Intermediate Standard Trifocals – what’s the problem? There are now three set far points created – The distance portion will focus for ∞ – The intermediate portion will focus for a given intermediate point – The near portion will focus for the near add far point Are the corrections in the right place or the right size for their use? The reason that many require an intermediate correction is that they are carrying out a task at an intermediate distance frequently that needs to be clear The traditional position or sizes of the segment may not suit the user even through the three portions does Trifocals and their uses Although PPLs have been hugely successful, trifocals still have their place and offer valid options to some situations. These specs offer the correction for three different distances: – Traditionally this is distance, intermediate and near – The relationship between the intermediate and near portions is described as a ratio (stated as a percentage): – IP/RP ratio = IP Add X 100% RP Add Conventional Trifocals A conventional straight top trifocal Segment sizes could be 278 or 835 (Plastic) IP/RP 50% And 725, 728, 735 or 828 (Glass Problem solving: An Orchestral Oboist Vision needs: – View of music 75 cm in front of them for a long period of time – View of conductor above and ahead of them for frequently – View of instrument keys and other smaller objects close up less frequently Problem with Standard Trifocals – There is a very limited intermediate area and it will require an awkward head position not compatible with playing the instrument Solution? Problem solving: An Orchestral Oboist Vision needs: – View of music 75 cm in front of them for a long period of time – View of conductor above and ahead of them for frequently – View of instrument keys and other smaller objects close up less frequently Problem with Standard Trifocals – There is a very limited intermediate area and it will require an awkward head position not compatible with playing the instrument Solution? Other Trifocal Solutions (Plastic): S1435 Double D Intermediate add 50% or Equal adds or one 60% 66% of full add intermediate  Prescription Analysis Robert 55 Right Left Sph Cyl Axis VA Sph Cyl Axis VA +2.50 -0.75 90 6/6 +3.50 -1.00 90 6/6 ADD +2.00 N5 ADD +2.00 N5 Robert Right Left Sph Cyl Axis VA Sph Cyl Axis VA +2.50 -0.75 90 6/6 +3.50 -1.00 90 6/6 ADD +2.00 N5 ADD +2.00 N5 Presbyopic patient, around 55 years old Multiple options including PPL’s, bifocals, two pairs of single vision lenses C or round segment due to hyperopia, C28 most common Would need to warn about jump Large range of frames, need to ensure that there is enough room for segment Thickness will be in the centre so a smaller frame (BC = PDs) will minimise blank size required and keep it thin CR39 suitable or could consider 1.6 – note this will increase reflectance (MAR) and the TCA (Abbe number reduced). Need to discuss these with the px. Anita, 50 Right Left Sph Cyl Axis VA Sph Cyl Axis VA -4.50 -0.75 10 6/5 -5.00 -1.00 170 6/5 ADD +1.75 N5 ADD +1.75 N5 BVD 11mm Anita Right Left Sph Cyl Axis VA Sph Cyl Axis VA -4.50 -0.75 10 6/5 -5.00 -1.00 170 6/5 ADD +1.75 N5 ADD +1.75 N5 BVD 11mm Presbyopic patient, around 50 years old Multiple options including PPL’s, bifocals, two pairs of single vision lenses C or D segment due to myopia, D28 is most likely C less visible & used in Hi-Index Would need to warn about jump if first time wearer, E style could reduce this if a problem Large range of frames, need to ensure that there is enough room for segment – consider plastic Thickness will be on the edges so look for minimal size frame or increase index Best glazing will be if BCs match PD to maintain minimal edge thickness Recommend1.6 (or 1.67) – but this will increase reflectance and the TCA (Abbe number reduced) – will need to discuss with px  Samuel, 65 Right Left Sph Cyl Axis VA Sph Cyl Axis VA Dist +1.00 -2.00 70 6/6 +1.50 -1.50 110 6/6 N8 Inter +2.25 -2.00 70 WD 50-75cm N8 +2.75 -1.50 110 WD 50-75cm N5 Near +3.50 -2.00 70 WD 20-35cm N5 +4.00 -1.50 110 WD 20-35cm Samuel Right Left Sph Cyl Axis VA Sph Cyl Axis VA Dist +1.00 -2.00 70 6/6 +1.50 -1.50 110 6/6 Inter +2.25 -2.00 70 WD 50-75cm N8 +2.75 -1.50 110 WD 50-75cm N8 Near +3.50 -2.00 70 WD 20-35cm N5 +4.00 -1.50 110 WD 20-35cm N5 Trifocals/varifocals or combination of Bifs or SV are all options Maybe asked to calculate Adds if Rx written out in full as above IP/RP Ratio D728 is most likely choice for trifocals – large ranges of lenses available Large range of frames, need to ensure that there is enough room for segment(s) Thickness will vary – vertically will be thinner edges than horizontally- Rx is moderate so large range of frames will be suitable. Rectangular frames may be best as will even out the edge thickness. CR39 suitable – not too thick and won’t have as many off axis issues as with higher index Will need to ensure that mono PD and heights of distance Rx are met to ensure that astigmatism is fully corrected – a more sturdy frame to keep lenses steady. Recommended Reading Jalie, M. (2008) Ophthalmic Lenses & Dispensing, 3rd Ed, Chapter 4, 8, 9, 13. Jalie, M. (2001) The Principles of Ophthalmic Lenses, Chapter 11, 18, 21 Gilbert, P. (2013)Ophthalmic Lens Availability ABDO Tunnacliffe, A. (2007) Essentials Of Dispensing, 3.1,3.4,4.6,4.7, 4.8 ABDO Griffiths, A. (2003) Practical Dispensing, Chapter 17. ABDO Brooks, C (2013) System for Ophthalmic Dispensing. Butterworth and Heinemann ABDO website (www.abdo.org.uk) Library Occupational Dispensing Tunnacliffe, A. (2007) Essentials Of Dispensing, OPALs page 57-62. ABDO Griffiths, A. (2003) Practical Dispensing, Chapter 15. ABDO Freeman, S. (2011) Lifestyle Dispensing parts 1 Optician. Reed Publishing Freeman, S (2014) OMB considerations in Dispensing. Dispensing Optics. ABDO