Ординатура / Офтальмология / Учебные материалы / Orthokeratology Principles and Practice 2004

provide valid elevation data. This is essential if a lens does not quite reach the refractive change required. The ability to cause greater refractive changes whilst simultaneously modifying the treatment zone (TxZ) may require fitting a specially designed second lens. The lens parameters would then be calculated from the data of the altered corneal shape. At present, this is not possible due to the inability of topographers to measure oblate surfaces accurately (Tang et aI2000).

8.The subtractive refractive power map should automatically measure the TxZ diameter.

9.Ideally, the topographer should also display aberration maps. This is already available with the Keratograph topographer. Also, a program that integrates with most of the currently available topographers and uses the data to generate Zernike maps is available from CT View (www.sarverassociates.com). This then gives the practitioner and researcher the ability to assess the quality of the unaided vision objectively. Also, Beuhren et al (2001) have shown that the incorporation of wavefront analysis can be used to correct for image tilt. If separate topography maps are performed (subtractive maps, for example), the assumption is that fixation and apex detection are identical to the original capture. This is not the case, resulting in some degree of tilt between maps. This in turn leads to inaccuracies in the subtractive difference maps.

10.The instrument and its software must be simple and user-friendly. The hardware must be dependable. A large database storage capacity is essential.

11.The practitioner must be able to observe the patient's alignment, fixation stability, and the position of eyelash, brow, and nose shadows before image capture takes place.

Some corneal topographers are simply not suitable for orthokeratology. They lack either the accuracy and repeatability of other instruments or some of the essential requirements such as subtractive axial, tangential, and refractive power maps and the derivation of apical radius and the eccentricity of the flat meridian. Practitioners

should carefully assess the instrument prior to purchase, and make sure it meets the correct requirements.

UNANSWERED QUESTIONS

There are still a lot of mysteries in orthokeratology. The following section deals with these gray areas, and the author is indebted to the following academics for their helpful suggestions of topics for future research: John Mark Jackson, Marjorie Rah, Helen Swarbrick, Ed Bennett, Joe Barr, Pauline Cho, Harue Marsden, Nathan Efron, and Milton Hom.

Efficacy and predictability

The ultimate test of the efficacy of ortho-k, as I trust you appreciate, is a large-scale, randomized, placebo-controlled, double-masked study of what is anecdotally considered to be the ultimate ortho-k lens. The problem is, of course, that the ortho-k community keeps changing its mind about what is the best lens, based on accumulated anecdotal evidence ... i.e, clinical hunches leading to other clinical hunches. The only way forward into the future is for the procedure to be properly validated by the type of study described above. Failing that, ortho-k will always remain a possible technique for the future ... (N Efronpersonal communication).

The previous statement was made by Nathan Efron, and is basically correct. The study described above is necessary particularly as the currently accepted standard for assessing the efficacy of a procedure is the evidence-based model. The author does, however, strongly disagree with the statement that the refinement of the designs is purely based on "hunches leading to other clinical hunches." In reality, the changes usually occurred as a result of the application of a specific model based on a mathematical fitting correlation between the lens and the corneal shape, trials of the model followed by statistical analysis of the results, and then modification of the model to overcome the inadequacies of the design. The application of topography-based fitting, and the mathematical formulae described

in this book did not occur through hunches: they have been an accepted part of the theory of contact lens design and fitting for over 20 years (Bibby 1976,Atkinson 1984, Young 1998).

However, Efron (2001) does raise the important point of independent validation, and this should be a priority in research areas. The difficulty is funding such a large-scale project. The other area of orthokeratology that is still in need of concentrated work is that of predictability. This is a vitally important area. The practitioner must be able to determine accurately the likely change in refraction possible before undertaking a course of treatment in order to rule out those subjects who will not achieve the ideal outcome of emmetropia or slight overcorrection and good-quality highand low-contrast vision. The predictive value of the initial corneal eccentricity has been reported in this book, with both the Mountford and Noack model and the Day surface area model. Also, both the Mountford and EI Hage et al (1999) studies found a similar relationship between corneal shape change and refractive change.

However, neither study was designed to meet the highest standards of experimentation and controls, as both were retrospective studies on an admittedly successful group of patients. What the studies did show was that, in the case of ideal responders, a correlation between the two factors existed. To date neither study has been repeated, and therefore lack independent corroboration. Similarly, those who use the Jessen factor do not have independent published proof of the accuracy of the underlying assumption. The data, when analyzed, show large variations and a relatively poor correlation coefficient. The issue of a valid and repeatable predictive tool is a necessary project.

Visual acuity

The studies on visual acuity to date have reported on the change in highand, in some cases, lowcontrast vision, but without any reference to the quality or otherwise of the postwear topography. Future research should try to correlate the topography outcomes to the unaided visual acuity, as well as the aberrations induced in the altered corneal shape and its effect on the quality of

vision. The results may then lead to modifications in lens design that could improve the visual outcomes. Tang (2001) has suggested adopting a standard protocol of which optotypes are best suited to determining the quality of unaided vision following the procedure (see Ch. 7).

Astigmatism

Vickers (2000) makes the classic statement: "the one good thing about being an optometrist is being able to talk about astigmatism without understanding what it is." The basic understanding is that astigmatism represents two different radii separated by an axis difference of 90°. Topographical interpretations of the "appearance" of astigmatism confuse the issue even further. For example, the appearance of astigmatism differs markedly between axial and tangential maps. Theoretically, tangential maps are superior at showing fine local changes in curvature, but the axial map representation of astigmatism is more representative of what we "think" it "should" look like. The immediate question (and this is important when trying to design lenses that correct astigmatism) is which elevation should the lens fit be based on: the steep or flat meridian, and is there a difference in elevation for different types of astigmatism? Also, how does the degree of astigmatism affect Ro? Areas that may be fertile grounds for investigation are the differences in eccentricity along the two principal meridians, but this is somewhat limited at present, as the basic research of evaluating the accuracy of topography data for astigmatic values has not been performed.

To date, only Jackson et al (2002) and Mountford & Pesudovs (2002) have published the results of the effects of orthokeratology on astigmatism. The results raised more questions than they answered. For example, it has been a longheld clinical anecdote that reverse geometry lenses do not perform well on significant degrees (>0.75 D) of against-the-rule astigmatism. Why? Also, why does limbus-to-limbus astigmatism respond so poorly, and why is there such a large variation in outcomes with different patients? When astigmatism is reduced with spherical lenses, the reduction is in the order of 50%, and

308 ORTHOKERATOLOGY

then only over the central 2.00 mm chord, and the predictability of the final axis is poor. The model of forces acting in orthokeratology gives some insights into these problems, but requires much more analysis before any clinical applications occur.

There is no doubt that the efficacious reduction of astigmatism will require novel lens designs. However, until the underlying effect of spherical lenses on astigmatism is better understood, the attempts at correction with more adventurous designs will remain somewhat hit and miss. The work of Kwok (1984) on the surface area of astigmatic surfaces could hold promise in this area.

What changes and why?

At first glance, this seems to be a relatively simple question to answer: the cornea changes shape, and thereby refraction due to the influence of the lens. However, the refractive changes are due to epithelial thinning and perhaps some form of corneal bending, but the exact cause of this is still in the realms of modeled speculation. The forces modeled in Chapter 10 are purely that, models, and require controlled experimentation to validate the concepts. Why do the effects last for up to 4 days in some individuals, yet others need to wear the lenses every night in order to maintain clear vision for all the waking hours? The variations in regression are not related to the magnitude of the induced changes (Soni & Horner 1993, Mountford 1998), and tend to suggest that a viscoelastic effect is at work. This would therefore indicate a stromal involvement.

However, as has been shown in studies of extended-wear RGP lenses (Ren et al2002), there is a decrease in epithelial surface cell exfoliation with time. Does this have a correlation with overnight orthokeratology lens wear and the stability of the refractive change, or does the absence of the lens during the daytime influence the changes?

These are areas of great interest, as being able to understand fully the interaction of the forces involved and the corneal response to them at all levels will lead to greater control of the outcomes with orthokeratology.

Myopia control

This was the most commonly mentioned area of interest to most of the academics approached for their thoughts on the future directions of orthokeratology research. The impetus behind it is probably due to the strongly held beliefs of some practitioners that their clinical experience supports the theory. However, there is simply no proof to support the anecdotal reports, so controlled studies must be performed to arrive at the truth. If orthokeratology were shown to be effective at controlling the rate of progression of myopia, the impact, especially in Asian countries, would be enormous. The question then arises as to how long (years?) would the lenses need to be worn in order for the effect to be maintained or stabilized, and what would happen if lens wear ceased? Would the myopia simply progress to the stage it would have reached if intervention had not occurred, or would the reductions be maintained? These questions will take years to answer properly, and in the end, the whole concept depends on orthokeratology lens wear having an effect on the eyes' axial length.

Accuracy of fit and different designs

There are obvious contradictions in orthokeratology with respect to the fitting philosophies used by different designs. The basic approach is simply to supply the laboratory with the relevant keratometry readings and spectacle prescription and the lens will be manufactured. The opposite viewpoint is that the efficacy of the procedure, especially with respect to the first-fit success rate, is totally dependent on accurate topographybased fitting and the correct interpretation of trial wear periods. Can they both be correct?

The simple approach relies on patient reports of quality of vision, whereas the complex approach treats this as only one part of the picture, in that the quality of the vision is intimately related to the quality of the postwear topography, and that other factors, such as TxZ diameter, low-contrast vision, and refraction, all combine to set the standard of success or failure. The assessment of both techniques using wavefront analysis and other objective methods of

determining unaided vision may eventually give the answers. At present, most of the designs appear to give the same refractive results, but the differences in visual quality, first-fit success rates, and time required for a satisfactory result are unknown. However, a common problem is the relatively small TxZs possible with higher refractive errors and the incidence of flare and haloes at night. Research is also required to determine if design modifications can be made that will lead to an increase in effective TxZ diameter.

Long-term patient acceptance and satisfaction

Will orthokeratology patients wear their lenses for years? What will the drop-out rate be in the long term, and what term of lens wear defines the success or otherwise of the procedure? Early indi-

REFERENCES

cations are that new orthokeratology patients rate the process highly, yet 30% still complain of poor night vision and flare. The true success of the procedure may not be known for years, but the simple fact that it has survived for 40 years, with relatively modest results, does indicate that there is a high demand for a nonsurgical means of vision correction.

CONCLUSIONS

As stated in Chapter I, orthokeratology "belongs" to optometry. However, it alone cannot answer some of the questions asked above. It will require the help of ophthalmologists, anatomists, physiologists, engineers, and other scientists. More complex questions will be raised as the simpler ones are answered. Orthokeratology is complex, not simple.

Atkinson T C 0 (1984)A re-appraisal of the concept of fitting rigid hard lenses by the tear layer and edge clearance technique. Journal of the British Contact Lens Association 7: 106-110

Beuhren T, Collins M J, Iskander D R, Davis B, Lingelbach B (2001) The stability of corneal topography in the postblink interval. Cornea 20: 826-833

Bibby M M (1976) Computer assisted photokeratoscopy and contact lens design. Optician 171(4423): 37-44; 171(4424): 11-17; 171(4426): 15-17

Edwards K, Hough T (2001) Fear and loathing in Surfers' Paradise: on the demise of rigid contact lenses - a critique. Optometry Today October 19,16-19

Efron N (2001) Contact lens practice and a very soft option. Clinical and Experimental Optometry 83(5): 243-245

EI Hage S G, Leach N E, Shahin R (1999)Controlled kerato-reformation (CKR): an alternative to refractive surgery. Practical Optometry

10(6):230-235

Hough T, Edwards K (1999)The reproducibility of videokeratoscope measurements as applied to the human cornea. Contact Lens and Anterior Eye 22:91-99

[ackson ] M, Rah MJ, Jones LA, Bailey MD, Marsden H, Barr J T (2002)Analysis of refractive error changes in overnight orthokeratology using power vectors. ARVO abstract. Investigative Ophthalmology and Visual Science

Kwok S (1984)Calculation and application of the anterior surface area of the model human cornea. Journal of Biology 108: 295-313

Mountford J A (1998) Retention and regression of orthokeratology with time. International Contact Lens Clinics 25: 60-64

Mountford J A, Pesudovs K (2002) An analysis of the changes in astigmatism with accelerated orthokeratology. Clinical and Experimental Optometry 85(5): 284-293

Ren D H, Yamamoto K, Iadage PM et al (2002)Adaptive effects of 3D-night wear of hyper O2 transmissible lenses on bacterial binding and corneal epithelium. Ophthalmology 109: 27-39

Soni P 5, Horner D J (1993)Orthokeratology. In: Bennett E, Weissman B (eds) Clinical contact lens practice. Philadelphia, J B Lippincott, ch. 49

Tang W (2001) The relationship between corneal topography and visual performance. PhD thesis. Centre for Eye Research, Queensland University of Technology, Brisbane, Australia

Tang W, Collins M J, Carney L, Davis B (2000) The accuracy and precision performance of four videokeratoscopes in measuring test surfaces. Optometry and Vision Science 77:483-491

Vickers M (2000) Chairside: the 10 great things about being an optometrist. Review Optometry October: 12

Young G (1998) The effect of rigid lens design on fluorescein fit. Contact Lens and Anterior Eye 21(2): 41-46

30 surface reconstruction, computer-assisted videokeratography 30

A

abnormal symptoms, orthokeratology treatment 244

academics 4-5

Acanihamoeba, RGP lenses 62-4 accuracy

corneal topography 18

EyeSys Corneal Analysis System 43-4

of fit, future 308-9

sag fitting accuracy 104-6, 232-3 ACP see apical corneal power aftercare 240-54

continuing suitability decision 252 emergencies 253-4

first 242-3

fluorescein fit assessment 246-7 lens movement 245-6

lens position 245-6 postremoval visual assessment

247-51 problem-solving 254 scheduling 240-2

symptoms and history 243-5 visual correction, first week 252-3

age of lenses 266

aims, this book's 12-13 algorithms

computer-assisted videokeratography 29-33

corneal profile reconstruction 31-3 alignment curve variations, reverse

geometry lenses 86

alignment peripheral curves construction 88-104 design 88-104

reverse geometry lenses 88-104 Alpins method, astigmatism 185-6 anatomical classification, corneal

topography 34

anatomical factors, patient selection 118-26

apical corneal power (ACP) 188-90 refractive changes 180

apical radius, refractive changes 207-10

aspheric mold BOZR96-102

reverse geometry lenses 80-1 asphericity

corneal changes 207-10 refractive changes 207-10

astigmatism

Alpins method 185-6 complications 291-5

corneal shape changes 291-5 corneal topography 186-8 future 307-8

limbus-to-limbus 119-20, 291-5 orthokeratology 185-6

patient selection 114-17 wedge 119-20

ATLAS 20,24

B

back optic zone diameter (BOZO) 6 reverse geometry lenses 72-80,

83-106 TxZ 123

back optic zone radius (BOZR) 2-3, 6 Dreimlens 96-102

El Hage aspheric mold 96-102 empirical fitting 141 measurement 229-30 refractive changes 98-102 reverse geometry lenses 69-106

sag fitting accuracy 104-6, 232-3 trial lens fitting 147-51 verification 228-9

back peripheral zone diameter (BPZO), measurement 231-2

back vertex power (BVP), verification 228-9

band magnifier 231

BE lens (Mountford and Noack) anterior corneal power 198 fluorescein pattern analysis

153-6

reverse geometry lenses 85-6 trial set 151

BE software 220-4 binding, lens 261-3 bleb response

endothelium 55-6 RGP lenses 55-6

blocked fenestration 251

BOZO see back optic zone diameter BOZR see back optic zone radius BPO! (first back peripheral

diameter) 6

BPR! (first back peripheral radius) 6 BPR2 (second back peripheral

radius) 6

BPR3 (third back peripheral radius) 6 BP~ (fourth back peripheral

radius) 7

BPZO see back peripheral zone diameter

bridging theory, three and 9 o'clock staining 257

bubble formation 249

E

eccentricity, corneal 120-2, 281 edge profile inspection 234 educational standards, future 304-5 efficacy, future 306-7

EI Hage aspheric mold BOZR96-102

reverse geometry lenses 8D-1 elevation maps, computer-assisted

videokeratography 37-8 emergencies, aftercare 253-4 empirical fitting

BOZR 141 methods 141

recommendations 140 d. trial lens fitting 147

endothelium

bleb response 55-6 polymegathism 56-9 RGP lenses 55-9

epithelial iron deposition 264-5 epithelial microcysts, RGP lenses 52-4,

248 Euclid Emerald

fluorescein pattern analysis 152 trial set 151

Euclid ET-800;20, 103-4 Eye-Map EH-290; 20

EyeSys Corneal Analysis System 20, 24

accuracy 43-4 sagittal maps 34-6

EZM calculator, computerized modeling 211, 213

EZM software (Gelflex) 213

F

Fargo lens 103-4

fluorescein pattern analysis 153-8

fenestration imprint 260

finite element analysis, forces affecting lenses 274-7

first back peripheral diameter (BPDj ) 6 first back peripheral radius (BPRj ) 6 Fischer-Schweitzer polygonal mosaic

258-9

fitting

empirical 140-7

philosophies, reverse geometry lenses 69-107

principles, reverse geometry lenses 70

trial lens 139-74

fluid jacket molding seesqueeze film forces

fluorescein fit assessment 246-7 fluorescein pattern analysis, trial lens

fitting 152-8 focimeters 112, 229

focusing systems, computer-assisted videokeratography 26-9

Fontana lens 2

forces affecting lenses 271-84 finite element analysis 274-7 lid force 271-2

squeeze film forces 273-4, 277-84 surface tension 272-3

formulae, lens construction, reverse geometry lenses 72-4

four-and five-zone lenses constant-surface-area concept

103-4

reverse geometry lenses 8D-6 tear layer profile 92

fourth back peripheral radius (BP~)7 Free Choice OK software 217-20 future 303-9

accuracy of fit 308-9 astigmatism 307-8 educational standards 304-5 efficacy 306-7

long-term acceptance 309 myopia control 308 predictability 306-7 topography 305-6 unanswered questions 306-9 VA 307

G

Gaussian maps, computer-assisted videokeratography 36-7

Gelflex EZM lens, trial set 151 Gelflex VMC lens, reverse geometry

lenses 83-4, 213

general principles, orthokeratology 1-16

H

high-order polynomial descriptors, corneal topography 40

history, orthokeratology 1-16 horizontal visible iris diameter

(HVID) 124-6 refractive changes 207-10

hydraulic model 270 hygiene 236-8

hypoxia, RGP lenses 50-1

INDEX 313

image editing, computer-assisted videokeratography 26-7

information, patient, patient selection 132-4

informed consent, patient selection 134-6

instruction session 238 instrumentation standards 110-12 International Orthokeratology Society,

first meeting 14-16

iron deposition, epithelial 264-5

J

Jessen's factor 8, 269-70

reverse geometry lenses 92-4

K

kappa function, computerized modeling 205-7, 209

keratoconus, patient selection 127-8 Keratograph/CTK corneal

topographer 21 keratometry 122

changes 176-8, 179-81 keratoscopes 111-12 Keratron 21, 24, 43 KR-8000P21

L

lens adherence, RGP lenses 6D-1 lens binding 261-3

lens care advice 238-40 see also aftercare

lens condition 250-1 lens delivery 236-8

lens design/ fitting software, computerized modeling 212-24

lens fitting, computerized modeling 205-25

lens movement 245-6 lens position 245-6 lens sag 8

measurement 232-3

lid force, forces affecting lenses 271-2 lid position/ tension, patient selection

126

limbus-to-limbus astigmatism 119-20, 291-5

lipoidal deposits 251

314 INDEX

long-term acceptance, future 309 long-term changes 200

loss of the effect 265-6

M

marginal dry eye, patient selection 128-9

MasterVue dual-camera system, computer-assisted videokeratography 26-7

materials companies 5-6 mean curvature maps,

computer-assisted videokeratography 36-7

measurement BOZR229-30 BPZD231-2

corneal topography 17-47 diameters 231-2

lens sag 232-3 peripheral radii 230-1 power 229

Medmont E300;22

meridional skew, computer-assisted videokeratography 30

microbial infection, RGP lenses 62-4 microbial keratitis (MK)265 microcysts 52-4, 248

microscopes 111, 229-30 MK see microbial keratitis modeling, computerized see

computerized modeling Mountford-Noack lens sag gage 232-3 multiple-arc technique, computer-

assisted videokeratography 32-3

Munnerlyn's formula 98-102,122-3 myopia control, future 308

myopia, patient selection 113-14

N

night therapy 12, 130-2

Nightmove lens, fluorescein pattern analysis 153

noninfectious ocular inflammation, RGP lenses 61-2

normal symptoms, orthokeratology treatment 244

o

objectives, this book's 12-13 oblate / sphere corneal shape,

computerized modeling 210-12

occupational factors, patient selection 126-7

one-step curve fitting, computer-assisted videokeratography 31-2

Orbscan II; 22, 29

Ortho Tool 2000;212, 215-17 orthokeratology

astigmatism 185-6 defining 1

general principles 1-16 history 1-16

outcomes, computerized modeling 205-25

oxygen transmissibility, RGP lenses 50-1

p

PAR CTS 22-3, 28-9 Paragon CRT lens

fluorescein pattern analysis 153 reverse geometry lenses 84-5 trial set 150-1

patient satisfaction 200-1, 309 patient selection 109-38

anatomical factors 118-26 astigmatism 114-17 consent, informed 134-6 contraindications 127-9, 130 corneal dystrophies 128 corneal eccentricity 120-2 corneal edema 128

corneal topography 118-20 dry eye 128-9

factors affecting 112-30 information, patient 132-4 informed consent 134-6 instrumentation 110-12 keratoconus 127-8

lid position / tension 126 marginal dry eye 128-9 myopia 113-14 occupational factors 126-7 physiological factors 127-9 PMMA lenses 117 psychological factors 129-30 pupil diameter 122-6 recreational factors 126-7 refractive factors 112-8 RGP lenses 117-18 standards,

practice / instrumentation 110-12

unstable refractive errors 117-18 VA112-13

peripheral radii, measurement 230-1

physiological factors, patient selection 127-9

physiological signs, RGP lenses 51 Placido disk 18

PMMA lenses see polymethyl methacrylate lenses

polymegathism endothelium 56-9 RGP lenses 56-9

polymethyl methacrylate (PMMA) lenses, patient selection 117

postorthokeratology topography 181-5

postremoval visual assessment 247-51 power measurement 229

practice, standards 110-12 practitioners 2-4 predictability, future 306-7 protein deposition 251

Pseudomonas aeruginosa, RGP lenses 64 psychological factors, patient

selection 129-30

pupil diameter, patient selection 122-6

qualitative descriptors, corneal topography 34-9

quantitative descriptors, corneal topography 39-40

R

R&R lens, trial set 151

radial keratotomy (RK),mean curvature maps 36, 37

radiuscope 112 BOZR229-30

rasterstereography, computer-assisted videokeratography 27-9

recreational factors, patient selection 126-7

reference points, computer-assisted videokeratography 25-6

refractive changes 175-203 ACP180

apical radius 207-10 asphericity 207-10 BOZR98-102

computerized modeling 207-10 corneal topography 178-81 HV1D207-10

reverse geometry lenses 190-2 refractive errors, unstable, patient

selection 117-18

refractive factors, patient selection 112-18

Reinhart Reeves lens, fluorescein pattern analysis 153

research 4-5

retention, corneal shape changes 188-90

reverse geometry lenses alignment curve variations 86

alignment peripheral curves 88-104 aspheric mold 80-1, 96-102 background 75

BElens (Mountford and Noack) 85-6

blocked fenestration 251 BOZO 72-80, 83-106 BOZR69-106

clearance factor 79 constant-surface-area concept

103-4

Contex OK series 75-80 CRT lens 84-5

design variables 69-107 Oreimlens 81-3

El Hage aspheric mold 80-1, 96-102 empirical fitting 140-7

fitting philosophies 69-107 fitting principles 70

formulae, lens construction 72-4 four-and five-zone lenses 80-6 history 75

Jessen's factor 92-4 Munnerlyn's formula 98-102 Paragon CRT lens 84-5 peripheral curve design 74-5 refractive changes 190-2

sag fitting accuracy lQ~ sag philosophy 70-2, 77

Scioptic EZM (Gelflex VMC) lens 83-4,213

short-term changes 198-200 tangential peripheries 86-8 three-zone lenses 75-80 WAVE lens 84

rigid gas-permeable (RGP) lenses

Acanthamoeba 62-4 bleb response 55-6 CL-SIK61-2

complications, clinical 60-4 complications summary 58-9 corneal edema 50-1, 54-5 day therapy 130-2 dimensional tolerances 236 endothelium 55--9

epithelial microcysts 52-4, 248 extended wear 49-67

hypoxia 50-I

lens adherence 60-1

night therapy 130-2

noninfectious ocular inflammation 61-2

overnight wear 49-67 oxygen transmissibility 50-1 patient selection 117-18 physiological signs 51 polymegathism 56-9

Pseudomonas aeruginosa 64 stroma 54-5

tear composition 51 tonicity /pH 51-9

ring jam 11-12

s

sag

corneal 8 lens 8, 232-3

sag fitting accuracy BOZR 104-6, 232-3

reverse geometry lenses 104-6 sag philosophy, reverse geometry

lenses 70-2, 77

sagittal maps, computer-assisted videokeratography 33, 34-6, 41, 42

SAl seesurface asymmetry index satisfaction, patient 200-1, 309 scanning slit topography, computer-

assisted videokeratography 27-9

schedules, wearing 238-40 Scioptic EZM (Gelflex VMC) lens,

reverse geometry lenses 83-4,

213 scratched lens 251

second back peripheral radius (BPRz) 6

short-term changes, reverse geometry lenses 198-200

smiley face 9, 10, 11 empirical fitting 142 forces causing 287-90

topography-based fitting 164-7, 172

software, lens design/ fitting 212-24 sphere / oblate corneal shape,

computerized modeling 210-12

squeeze film forces 273-4,277-84 SRI seesurface regularity index stability, corneal shape changes

188-90

staining see central staining; conjunctival staining; corneal staining; fluorescein fit

INDEX 315

assessment; fluorescein pattern analysis

standards

educational standards 304-5 instrumentation 110-12 practice 110-12

static state molding 295-8 stroma, RGP lenses 54-5

surface area, computerized modeling 205--7

surface asymmetry index (SAl) 38-9

surface deposit-induced staining 263 surface quality 234-6

surface regularity index (SRI)38-9 surface tension, forces affecting lenses

272-3 symptoms and history

abnormal symptoms 244 aftercare 243-5

normal symptoms 244 systems, computer-assisted

videokeratography 19-24

T

tangential maps, computer-assisted videokeratography 33, 36

tangential peripheries construction 86-8 design 86-8

reverse geometry lenses 86-8 tear composition, RGP lenses 51 tear layer profile

Oreimlens 90

four-and five-zone lenses 92 tear reservoir 7-8

terminology 6 test charts 112

thickness gages 112,233-4 third back peripheral radius

(BPR3)6

three and 9 0' clock staining 256-7 three-zone lenses, reverse geometry

lenses 75-80 tinting 235

TMS-2N (TOMEY)23, 24 tolerances, dimensional 236 tonicity /pH, RGP lenses 51-9 topography-based fitting

assessment, post-trial 162-8 bull's eye 162-4, 165, 172 refining the fit 171-3 smiley face 164-7, 172

trial lens fitting 158-68 topography, corneal see corneal

topography

316 INDEX

topography, future 305-6 topography, postorthokeratology see

postorthokeratology topography

treatment zone diameter (TxZ) 11, 122-6

trial lens fitting 139-74 BOZR 147-51

care and maintenance 151-2 d. empirical fitting 147 fluorescein pattern analysis

152-8

postwear results 168-71 topography-based fitting

158-68

trial lens sets 147-51

TxZ see treatment zone diameter

u

unanswered questions, future 306-9 unstable refractive errors, patient

selection 117-18

v

V-gage 231

VA see visual acuity vacuum molding 297-9 verification 228-9

videokeratography, computer-assisted see computer-assisted videokeratography

videokeratoscopy, principles 29-31

visual acuity (VA) 192-4 future 307

patient selection 112-13 visual assessment, postremoval

247-51

visual correction, first week 252-3

w

WAVElens, reverse geometry lenses 84

WAVEsoftware 213-15 wearing schedules 238-40 wedge astigmatism 119-20

working distance, computer-assisted videokeratography 24--5