anatomyofeye heading

menu

 

 

Search PubMed:   

 

 

Customisation based on Wavefront measurements :

Measurement of ocular abberations can be done in many ways.  a Shack Hartman wavefront sensor, a Tscherning wavefront sensor, a spatially resolved refractiometer or even some of the many new systems (Tracey)  coming on the market. These techniques measure the eye is a wavefront aberrations including second (sphere and cylinder), asymmetric or coma like( third, fifth order), symmetric or spherical like aberrations (fourth ,six )order.  While all the systems utilise a ray tracing in one form or another, each system has developed a unique manner of accessing the displacement of a ray of light from its ideal position which defines the slope of the wavefront and then by computation ,the actual wavefront.  The difference between the actual wavefront and an ideal wavefront defines the aberrations of an eye.  Assessing the aberrations of the eye they find the form of an ideal or a customised laser treatment

 

 


  567

Click the picture to enlarge

 

 

Advantages of Optical wavefront guided Lasik using the Hartman Shack abberometer.

There are some disadvantages of Ocular wave front guided Lasik


• Although the aberrometer fogs the target by +1.5 diopter; with the intention to guide the patient to a far view; the measurement depends on accommodation.
• Restricted pupil size leads to a restricted application of a working zone.
• In dark pupils extreme caution to extrapolate dilated data to application.

 

There are a few points of caution which need to be exercised with Ocular Wave front ablations.

Extreme caution with the pupillary zone. Max. optical treatment zone is related to the pupil diameter (max. diameter which is analyzed by the system). As far as possible no medication before measurement as the use of a cycloplegic  will lead to an improper calculation. If the pupil is too small, 2.5% Neosynephrine would seem to give the best result with the least optical problems. Also one must remember that this  is a  pure aberrometer measurement, you d’ont know the location of the aberrations. They might be related to the cornea, to the lens or other optical parts of the eye.
Pupillary  diameter is  a constant: An important point often forgotten is that it is vital that refraction between manifest and measured ocular refraction should be compared at the same pupil diameter (RZ = refractive zone). Commonly during the determination of the manifest refraction the pupil diameter is around 4 mm (photopic conditions). If this manifest refraction should be compared to the wavefront aberration a similar analyzing diameter (e.g. 4 mm) should be used at the aberrometer. No pachymetric measurements is permitted with corneal contact directly prior to measurement with these diagnostic systems. This might affect the result.
Contact lens discontinuance is mandatory before any measurement , be it corneal wavefront or ocular wavefront . : Soft contact lenses  are best discontinued at least two weeks prior .  Hard 4 weeks prior to the measurement.

Combining ocular and corneal wave front  Ablations.


  If the wave aberrations of the complete eye and  the cornea are available the relative abberations  of the different ocular surfaces to the retinal image quality can be successfully evaluated.  The wavefront aberration of the internal ocular optics can be estimated simply by direct subtraction of the ocular and corneal aberrations. However in practice since the ocular and corneal aberrations are obtained with two different instruments a problem arises on how to determine the correct reference entering four registration.  Customarily the pupil is selected however in normally adumbrated eyes even very small misalignment would probably introduce errors which may be difficult to estimate.
The simplest application of this combined procedure is to carry out  each one separately.  In a post-keratoplasty case with gross corneal  irregular  astigmatism, a corneal topography-based LASIK done as a primary procedure,  subsequently followed by ocular wavefront guided LASIK total three months later would give excellent results . Many results have shown that wavefront guided LASIK cannot remove all the higher order aberrations on an average but it can certainly reduce aberrations compared to conventional ablations .

 

 

 

References

  1. Marcos S. Aberrations and visual performance following standard laser vision correction. J Refract Surg 2001; 17:S596–S601.
  2. Holladay JT, Dudeja DR, Chang J. Functional vision and corneal changes after laser in situ keratomileusis determined by corneal sensitivity, glare testing, and corneal topography. J Cataract Refract Surg 1999; 25:663–669.
  3. Alessio G, Boscia F, La Tegola MG, et al. Topography-driven photorefractive keratectomy: results of corneal interactive programmed topographic ablation software. Ophthalmology 2000; 107:1578–1587.
  4. Alessio G, Boscia F, La Tegola MG, et al. Topography-driven excimer laser for the retreatment of decentralized myopic photorefractive keratectomy. Ophthalmology 2001; 108:1695–1703.
  5. Applegate RA, Nuñez R, Buettner J, et al. How accurately can videokeratographic systems measure surface elevation? Optom Vis Sci 1995; 72:785–892.
  6. Knorz MC, Jendritza B. Topographically-guided laser in situ keratomileusis to treat corneal irregularities. Ophthalmology 2000; 107:1138–1143.
  7. Knorz MC, Neuhann T. Treatment of myopia and myopic astigmatism by customized laser in situ keratomileusis based on corneal topography. Ophthalmology 2000; 107:2072–2076.
  8. Alió JL, Belda JI, Osman AA, et al. Topography-guided laser in situ keratomileusis (TOPOLINK) to correct irregular astigmatism after previous refractive surgery. J Refract Surg 2003; 19:516–527.
  9. Huang D. Physics of customized corneal ablation. In: MacRae SM, Krueger RR, Applegate RA, eds. Customized corneal ablation: the quest for supervison. Thorofare, NJ, Slack; 2001:51–56.
  10. Huang D, Arif M. Spot size and quality of scanning laser correction of higher-order wavefront aberrations. J Cataract Refract Surg 2002; 28:407–416.
  11. Krueger RR. Technology requirements for customized corneal ablation. In: MacRae SM, Krueger RR, Applegate RA, eds. Customized corneal ablation: the quest for super vision. Thorofare, NJ, Slack; 2001:133–147.
  12. Mrochen M, Seiler T. Influence of corneal curvature on calculation of ablation patterns used in photorefractive laser surgery. J Refract Surg 2001; 17:S584–S587.
  13. Pallikaris IG, Kymionis GD, Panagopoulou SI, et al. Induced optical aberrations following formation of a laser in situ keratomileusis flap. J Cataract Refract Surg 2002; 28:1737–1741.
  14. Mrochen M, Donitzky C, Wüllner C, et al. Wavefront-optimized ablation profiles: theoretical background. J Cataract Refract Surg 2004; 30:775–785.
  15. Gatinel D, Hoang-Xuan T, Azar DT. Volume estimation of excimer laser tissue ablation for correction of spherical myopia and hyperopia. Invest Ophthalmol Vis Sci 2002; 43:1445–1449.
  16. Martinez CE, Applegate RA, Klyce SD, et al. Effects of pupillary dilation on corneal optical aberrations after photorefractive keratectomy. Arch Ophthalmol 1998; 116:1053–1062.
  17. Oliver KM, Hemenger RP, Corbett MC, et al. Corneal optical aberrations induced by photorefractive keratectomy. J Refract Surg 1997; 13:246–254.
  18. Oshika T, Klyce SD, Applegate RA, et al. Comparison of corneal wavefront aberrations after photorefractive keratectomy and laser in situ keratomileusis. Am J Ophthalmol 1999; 127:1–7.
  19. Moreno-Barriuso E, Merayo Lloves J, Marcos S, et al. Ocular aberrations before and after myopic corneal refractive surgery: LASIK induced changes measured with laser ray tracing. Invest Ophthalmol Vis Sci 2001;42:1396–1403.Thibos LN. The prospects for perfect vision. J Refract Surg 2000; 16:S540–S546.
  20. Williams D, Yoon GY, Porter J, et al. Visual benefit of correcting higher order aberrations of the eye. J Refract Surg 2000; 16:S554–559.
  21. Seiler T, Mrochen M, Kaemmerer M. Operative correction of ocular aberrations to improve visual acuity. J Refract Surg 2000; 16:S619–S622.
    McLellan JS, Marcos S, Prieto PM, et al. Imperfect optics may be the eye’s defence against chromatic blur. Nature 2002; 417(6885):174–176.
  22. Brunette I, Bueno JM, Parent M, et al. Monochromatic aberrations as a function of age, from childhood to advanced age. Invest Ophthalmol Vis Sci 2003; 44:5438–5446.

previouspage

ejo