Make a difference in someone s life FITTING GUIDE

Transcription

1 Make a difference in someone s life FITTING GUIDE

2 CONTENTS Introduction 3 Applications 4 Characteristics 4 Fitting philosophy 5 Mid-peripheral / limbal zone 6 Only 3 fitting variables to determine 7 Ideal Fit 7 Parameters available 7 Diagnostics lenses 7 Summary fitting guide 8 INTRODUCTION T h e m s d Mini-Scleral Design has a distinctive posterior lens surface incorporating reverse geometry with specially designed optical and posterior curves. The m s d lens allows for creation of a smooth, spherical second refractive surface to replace the irregular corneal surface, thereby giving good stable vision and automatically correcting most astigmatism. The msd thin profile and design results in minimal lens edge/lid interaction, providing excellent patient comfort and oxygen transmission to the cornea during wear. The m s d is easy to fit and is suitable for the majority o f pathology cases ranging from advanced Keratoconus to LASIK and other irregular corneas on which fitting corneal contact lenses has not lead to optimal results. What is sagittal depth (sag depth) 10 How does sagittal depth affect the fit? 10 Troublesshooting: tear meniscus and bubble formation 12 Fenestration or not? 14 Insertion bubbles

3 APPLICATIONS Applications for the msd lens are numerous and include the majority of pathology cases such as: > Keratoconus (Oval, Nipple) > Pellucid Marginal Degeneration > Keratoglobus > Post Graft > RK, PRK and LASIK induced ectasia > Any compromised and/or irregular cornea CHARACTERISTICS 1 The msd lens always centers well With reverse geometry, the sagittal depth of the lens can be changed independently of central optic zone profile and/or mid-periphera/limbal zone clearance values. Sagittal depth value, mid-peripheral/limbal zone clearance and lens power are the only parameters to specify when ordering an msd lens. A fenestration is incorporated in the design which acts as a controlled release valve for tear exchange and the flow of metabolic debris. The fenestration inhibits the msd lens from adhering to the eye and eases the removal process. msd lenses can be ordered with or without a fenestration (see section: Fenestration or not?) The thin profile and design results in minimal lens edge/lid interaction, providing excellent comfort. FITTING PHILOSOPHY A fundamental principle of GP contact lens fitting is to achieve a particular relationship between the posterior lens surface and the anterior cornea, thus creating a tear layer with specific characteristics. This is also true in fitting the msd lens with some differences as compared to corneal lens designs. Corneal lenses are fitted by manipulating base curve and diameter in order to create the optimal lens/cornea re l a t i o n s h i p. When the cornea becomes highly irregular with steep and flat areas arbitrarily placed, the fit becomes very complicated and often impossible. The underlying principle of the msd lens is not to rely or use the highly irregular cornea but rather vault the later as well as the limbus, and use the sclera as the landing area to properly position the posterior surface of the lens over the highly irregular cornea and recreate a smooth spherical second refractive surface. With this in mind, sagittal depth (see section on sagittal depth), as opposed to base curve and diameter, becomes the most comprehensive and easiest measurement in managing and optimizing the vaulting characteristics of msd lenses (see illustration below). The m s d lens, vaults the cornea as well as the limbus and uses the sclera as the landing area. 4 5

4 MID-PERIPHERAL / LIMBAL ZONE With the ideal sagittal depth value, comes an option of ordering 3 different mid-peripheral/limbal zone clearance values (Standard, Increased, Decreased), to further align the lens in that area. ONLY 3 FITTING VARIABLES TO DETERMINE The m s d fitting concept is simple and easy to grasp. You are only 3 variables away from making a diff e rence in someone's life! First, determine the sagittal depth value, then, specify the mid-peripheral/limbal zone clearance value and finally over-refract for the final lens power. IDEAL FIT I d e a l l y, the m s d should fit with apical clearance or, in the case of Keratoconus, central feather touch at the corneal apex, with the mid-peripheral/limbal zone completely vaulting the limbus and aligning on the sclera. Lens movement with the m s d is often v e ry limited and may be difficult for the practitioner to discern. PARAMETERS AVAILABLE (Diameter 15.8mm) SAGITTAL DEPTH VALUE 3.70mm to 4.80mm (0.10mm inc.) See section on sagittal depth MID-PERIPHERAL/ LIMBAL ZONE CLEARANCE Standard Increased Decreased LENS POWER Any DIAGNOSTICS LENSES Independent of the sagittal depth value, the mid-peripheral/limbal zone of the lens can be specified with either Standard, Increased or Decreased clearance values. A diagnostic set is composed of 36 lenses as follows: 12 sagittal depth values, with each having 3 mid-peripheral/limbal zone clearance values (Standard, Increased and Decreased). The diameter is 15.8 mm. and plano powers. Each diagnostic lens is clearly marked with the sagittal depth value as well as the mid-peripheral/limbal zone clearance value. 6 7

5 SUMMARY FITTING GUIDE Trial fitting with an m s d diagnostic lens is critical, as m s d lenses of known parameters will establish the initial fitting relationship. All diagnostic lenses are clearly identified as to its sag depth as well as mid-peripheral/limbal clearance values. Advanced Keratoconus (Oval, Nipple) PMD, Globus. Steep K readings above 52.00D 1 SELECT INITIAL SAG DEPTH VALUE 4.60 S (standard) 2 EVALUATE CENTRAL FIT Ignore mid-peripheral/limbal fit at this stage. A light feather touch at the apex in the case of Keratoconus, alignment with 0.10 to.20mm pooling for others. In case of central bubbles, lower the sag depth. If there is excessive touch, increase the sag depth. Moderate Keratoconus (Oval, Nipple) PMD, Globus. Moderate K readings between 42.00D and 52.00D 4.20 S (standard) Ignore mid-peripheral/limbal fit at this stage. A light feather touch at the apex in the case of Keratoconus, alignment with 0.10 to.20mm pooling for others. In case of central bubbles, lower the sag depth. If there is excessive touch, increase the sag depth. Post Graft, LASIK, RK, PRK traumatic cornea. Flat K readings 41.75D or less 3.80 S (standard) Ignore mid-peripheral/limbal fit at this stage. Look for central pooling of 0.10mm to 0.20mm. In case of central bubbles, lower the sag depth. If there is excessive touch, increase the sag depth. When a light feather apical touch (2mm) in the central zone is achieved, select next lens with higher sag depth value. Example: 4.30 S indicates light feather touch - use 4.40 S. When in boubt about 2 different sag depth values, always opt for the higher sag depth, as the lens tends to settle in after a few hours of wear. 3 EVALUATE MID-PEREPHERAL/LIMBAL FIT T h e re should be limbal clearance with a slight pool of tears in the mid-peripheral zone (bright fluoroceine band). In case of bubbles formation try decreased clearance. In case of touch, try increased clearance. 4 SCLERAL/EDGE FIT Look for sceral alignment. If there is to much edge clearance (bubbles) try increasing the sag depth. If there is scleral impingement try lowering the sag depth. 5 ASSESS POWER Perform over-refraction in normal light conditions. Start by using ± 1.00D steps and refine with 0.25D steps. Note : When the best possible alignment has been achieved but stagnant bubbles persists, consider ordering a lens without a fenestration. See section on fenestration for more inform a t i o n.

6 WHAT IS SAGITTAL DEPTH (sag depth) Sagittal depth (sag depth) is the measurement from the flat plane at a given diameter to the highest point of a concave surface of the contact lens - also described as the degree of corneal elevation for a given chord diameter. If the sag depth value is too high, it will result in central pooling (excessive clearance) with bubbles occurring upon or within a short time after lens insertion. HOW DOES SAGITTAL DEPTH AFFECT THE FIT? If the sag depth value is too low, it will result in excessive central touch with bubbles occurring in the scleral are a. The lens will exhibit excessive movement, as it is not aligning p roperly on the sclera but rocking on the cornea. The weare r may experience edge awareness. Sag depth value is critical in achieving the desired fit as it s e rves as a control mechanism for either completely vaulting the cornea or to determine the amount of corneal touch or positive pressure to be applied. The ideal sag depth value will have the m s d lens vault the c o rnea as well as the limbus and rest aligned on the sclera. 10 HIGHER sag depth values allow more vaulting, conversely, LOWER sag depth values will increase positive pressure. 11

7 TROUBLESHOOTING TEAR MENISCUS AND BUBBLE FORMATION Bubbles underneath the lens will form when the tear m e n i s c u s breaks up due to too much clearance between the cornea and/or sclera and the posterior surface of the lens. The location and size of the bubbles will dictate which l e n s parameter needs to be modified to reduce this excessive clearance. When the proper sag depth value has been determined to corre c t l y vault the cornea but bubbles persist in the mid peripheral/ limbal zone; keeping the sag depth value the same, select a lens with a decreased mid-peripheral/limbal zone clearance. Excessive mid-peripheral/ limbal zone clearance will invite the formation of bubbles in that area. Bubbles in the central zone of the lens are indicative of a sag depth value that is too high. Lower the sag depth value in this case. Ordering a lens of the same sag depth value but with decreased mid-periphera/limbal zone clearance (in this example, from 4.20S to 4.20D), will eliminate the bubbles. Although slight apical or feather touch is sometimes desired with Keratoconus, avoid excessive central touch as this will not allow the lens to land and align on the sclera properly. Bubbles in the scleral zone of the lens are indicative of a sag depth value that is too low. Increase the sag depth value in this case. Conversely, if the correct sag depth value creates excessive mid-peripheral/ limbal zone touch; select a lens of the same sag depth value with increased mid-peripheral/limbal zone clearance. Excessive touch in the mid-peripheral/limbal zone, as evidenced by a lack of fluoresceine in that are a. 12 Ordering a lens of the same sag depth value but with increased mid-peripheral/limbal zone clearance (in this example from: 4.20S to 4.20I) will provide the necessary clearance. 13

8 FENESTRATION OR NOT? Only on a few occasions could you come across a corn e a with such an extreme irregular corneal shape that it is almost impossible to closely align all sections of that cornea simultaneously and there f o re impossible to get rid of all of the bubbles. If this occurs; a lens can be ordered without a fenestration. If you have difficulties determining the best possible sag depth value due to the presence of bubbles, reinsert the diagnostic lens with the fenestration tucked under the upper lid, this prevents air from entering temporarily, giving you enough time to diagnose the proper sagittal depth value. Note: Small bubbles that move around, providing they do not cross the pupil are acceptable. Bubbles that remain and are fairly stationary are not. Some lenses without fenestration may need to be removed for cleaning during the patient s wearing schedule, as metabolic debris could migrate to the centre of the lens. INSERTION BUBBLES Due to the 15.8mm diameter of the lens, insertion bubbles are not uncommon, but should subside as the lens settles in. To avoid insertion bubbles ensure that the posterior surface of the lens is completely filled with solution prior to insert i o n to decrease the occurrence of this type of bubbles. 14

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