Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

sis using the ZyWave aberrometer (BAUSCH & LOMB, CHIRON Technolas GmbH, Doranch, Germany) together with the elevation topography of the Orbscan II (Orbtek, Bausch & Lomb Surgical, Orbscan II corneal topography, Salt Lake City, Utah, USA) to correct ametropia.

To the moment the system is under trial, and is only applicable to regular virgin corneas. With the proper development of the technique, we think that it would provide us with the real customized ablation necessary not only for our desperate irregular astigmatism patients but also for obtaining a super vision for ametropes who are to be treated for the first time.

Other Surgical Procedures

Automated Anterior Lamellar

Keratoplasty

This technique was originally designed to treat superficial stromal disorders, but it has also been used for the treatment of difficult cases of irregular astigmatism, with very poor results 32. The surgeon performs phototherapeutic keratectomy or a microkeratome lamelar resection to 250-400 mm stromal depth, followed by trasplantation of a donor lamella of the same dimension on to the recipient bed33. We have limited experience with this subject. We think it is a good option for patients with thin corneas, and with the preservation of the Descemet’s membrane, the complications of rejection should be extremely minimized if not eliminated. However, the subject is out of the scope of discussion in this chapter.

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

Chapter 13

33.Melles GRJ, Remeijer L, Geerards AJM, Beekhuis WH. The future of lamellar keratoplasty. Curr Opin Ophthalmol 1999; 10: 253-259.

34.Ruckhofer J, Stoiber J, Alzner E, Grabner G. Intrastromal corneal ring segments (ICRS, KeraVision Ring, Intacs): clinical outcome after 2 years. Klin Monatsbl Augenheilkd 2000; 216:133-42 (abstract).

35.Holmes-Higgin DK, Baker PC, Burris TE, Silvestrini TA. Characterization of the aspheric corneal surface with intrastromal corneal ring segments. J Refract Surg 1999; 15: 520-8.

36.Zadnik K. Contact lens management of patients who have had unsuccessful refractive surgery. Curr Opin Ophthalmol 1999; 10: 260-263.

Prof. Jorge L. Alió, M.D.

Instituto Oftalmologico de Alicante

Avda. de Denia 111, 03015 Alicante – Spain

Tel. +34-96-5150025

E-mail: jlalio@oftalio.com

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

Figure 14-3- Compound myopic astigmatism

meridian (40.5 D x 180º). To do this, the ablation is performed with a negative cylinder of –4.5 x 180º. Ablation is performed in the center of the cornea by carving a cylinder that opens onto the steep meridian such that this axis is flattened until it becomes emmetropic.

Compound Myopic Astigmatism

In compound myopic astigmatism, both focal lines are projected in front of the retina. Following the example, figure 14-3 shows a compound myopic astigmatism of refraction -2 –4.5 x 180º. In this case, the steep meridian (45.0 D x 90º) has a myopic power of –6.5 D and projects an image in front of the retina; and the flat meridian (40.5 D x 180º) has a myopic power of –2.0 D and also projects an image in front of the retina.

To treat this astigmatism we need to flatten both meridians, but one more than the other. For this we perform the ablation with a negative cylinder (–4.5 x 180º) to flatten the steepest axis (45.0 x 90º) from –6.5 to –2.0 D. Then, on the spherical cornea obtained, we perform a central spherical ablation of –2.0 D that flattens both axes until emmetropy.

Simple Hyperopic Astigmatism

In simple hyperopic astigmatism, one focal line is projected behind the retina and the other is brought to focus on the retina. Following the example, figure 14-4 shows a simple hyperopic astigmatism of

Figure 14-6-A- Mixed astigmatism

the example, figure 14-5 shows a compound hyperopic astigmatism of refraction: +1.5 +4.5 x 90º (or its transposition +6 –4.5 x 180º). In this case, the steep meridian (45.0 D x 90º) has a hyperopic power of +1.5 D and projects behind the retina; and the flat meridian (40.5 D x 180º) is of hyperopic power +6.0 D and also projects images behind the retina.

To treat this type of astigmatism, we need to steepen both meridians, but one more than the other. To do this we perform the ablation using a positive cylinder (+4.5 x 90º) to steepen the flat axis (40.5 x 180º) from +6.0 to +1.5 D and, once the cornea is spherical, we perform a spherical peripheral ablation of +1.5 D, which steepens both axes until they become emmetropic.

Mixed Astigmatism

In mixed astigmatism, one focal line is projected in front of the retina and the other is projected behind the retina. Following the example, figure 14-6-A shows a mixed astigmatism of refraction: +2 –4.5 x 180º. In this case, the steep meridian (45.0 D x 90º) has a myopic power of –2.5 D and projects an image in front of the retina; and the flat meridian (40.5 D x 180º) is hyperopic (+2.0 D) and projects an image behind the retina.

To treat this astigmatism, we need to flatten the steep axis (45.0 x 90º), which is myopic, and steepen the flat hyperopic axis (40.5 D x 180º). This may be done according to several treatment models

Figure 14-6-B- Treatment models for mixed astigmatism

(figure 14-6-B): negative cylinder with positive sphere, positive cylinder with negative sphere or bitoric treatment, where positive and negative cylinders are combined. Based on clinical cases, these three treatment models are examined below.

Negative Cylinder Ablation to Treat Mixed Astigmatism

Figure 14-7-A shows an example of mixed astigmatism of refraction: +4 –5 x 170º and a topographic Sim K of 43.5 x 80º / 38.6 x 170º. In this case, the steep meridian (43.5 D x 80º) has a myopic power of –1.0 D and projects an image in front of the retina; and the flat meridian (38.6 D x 170º) is hyperopic (+4.0 D) and projects images behind the retina. This astigmatism was treated using the negative cylinder

Figure 14-7-A- Negative cylinder ablation to treat mixed astigmatism

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?

Figure 14-7-B- Differential map of treatment of mixed astigmatism with negative cylinder ablation

ablation. We programmed the ablation for a negative cylinder of -5 x 170º to flatten the steep axis (43.5 x 80º) from -1 to +4 D. Once the cornea was spherical, we performed a spherical peripheral ablation of +4 D to steepen both axes until emmetropy. The post-operative refractive outcome was +0.5 –1 x 170º.

The differential map (figure 14-7-B) shows that this treatment induced a flattening of 0.75 D of the steep axis (43.5 x 80º), which had a myopia of –1 D and a steepening of 3.25 D of the flat axis (38.6 x 170º), which was hyperopic (+4.0 D).

Positive Cylinder Ablation to Treat Mixed Astigmatism

Figure 14-8-A shows an example of mixed astigmatism of refraction +2 –4 x 20º (or its transposition –2 +4 x 110º) and a topographic Sim K of 43.1 x 110º / 39.4 x 20º. In this case, the steep meridian (43.1 D x 110º) shows a myopic power of –2.0 D and projects an image in front of the retina; and the flat meridian (39.4 D x 20º) has a hypermetropy of +2.0 D and projects behind the retina. This time, treatment was performed using the positive cylinder program. The excimer laser was programmed for an ablation of a positive cylinder of +4 x 110º. This steepened the flat axis (39.4 x 20º) from +2 to -2 D. Next, we performed a spherical central ablation of - 2 D on the now spherical cornea which flattened the axes until they became emmetropic.

Figure 14-9-A- Bitoric ablation to treat mixed astigmatism

hyperopic to the extent of +2.25 D and projects behind the retina. In this case, treatment was performed using the positive plus negative cylinder ablation. First, we programmed an ablation of a negative cylinder of –1.75 x 90º which flattened the steep axis (43.2 D x 180º), taking it from –1.75 D to emmetropy, followed by ablating a positive cylinder of +2.25 x 180º which steepens the flat axis (40.0 x 90º) from +2.25 to emmetropy. The postoperative refractive outcome was –0.5 x 40º.

The differential map in figure 14-9-B shows that treatment leads to a flattening of 2.25 D of the steep axis (43.2 x 180º), which had a myopia of –1.75 D and a steepening of 2.25 D of the flat axis (40.0 x 90º), which showed a hypermetropy of +2.25 D.

Using the three treatment models, the effect induced by ablation was similar: flattening of the steep axis and steepening of the flat axis. Nevertheless, the bitoric ablation method was most direct and resulted in the loss of least stromal tissue.

When a bitoric approach is used, it is important to precisely know along which meridian we wish to perform each cylinder ablation and the dioptric power to be treated at each axis. We use the “Cigales-Hoyos bitoric rule” to calculate the two cylinders. The bitoric rule states:

1- Record the negative cylinder refraction.

2- Note the positive cylinder refraction (its transposition).

3- Cross spheres and axes of the formulae (see figure 14-10) to obtain the cylinders to be treated.

Figure 14-9-B- Differential map of bitoric treatment of mixed astigmatism

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Figure 14-10- Cigales-Hoyos bitoric rule

Figure 14-10 shows the application of the bitoric rule to the previous example. We apply this rule by performing the negative cylindrical ablation first.

The bitoric ablation method was developed by

Chayet to treat mixed and myopic astigmatism. Help ? Chayet, however, introduces a correction factor in

his bitoric nomogram to compensate for his observation of hypermetropy induced by the negative cylinder along the meridian that is at right angles to it.

A further form of treatment using a two cylinder method, is the cross-cylinder program described by Vinciguerra. This treatment was proposed for mixed and myopic compound astigmatism whereby half the amount of the cylinder is ablated along the steepest meridian and the remaining half along the flattest

meridian. This is followed by central ablation of the spherical equivalent.

Both these authors perform positive cylindrical ablation first.

Results of Lasik in Mixed Astigmatism

Evaluation was made of the visual and refractive outcomes recorded in a series of 36 eyes of 24 patients with mixed astigmatism undergoing LASIK. Follow-up was at least one year. Three study groups were established according to the ablation method used:

-Group 1 (negative cylinder plus positive sphere): 1 eye.

-Group 2 (positive cylinder plus negative sphere): 12 eyes.

-Group 3 (bitoric treatment): 23 eyes. Keratectomy was performed using the Auto-

mated Corneal Shaper microkeratome (Chiron Vision, Claremont, CA) using the 160 micron thickness plate. Ablation was performed with the broad beam Apollo laser (Apollo Vision Inc., California, CA). Negative cylinder ablation was performed for an optical zone of 4.5 mm and a transition zone of 6.5 mm. Positive cylinder ablation was performed for an optical zone of 5.5 mm and transition zone of 8.0 mm. Treatment was performed according to the “Cigales-Hoyos bitoric rule”, with negative cylinder ablation performed first.

Refractive outcomes are shown in figure 14-11. The technique yielded predictable results in the treatment of mixed astigmatism using the three ablation models. No significant differences were shown among the methods. By performing the ablation according to the bitoric rule, we recorded no induced hypermetropy of the negative cylinder. Thus, the deviation noted by Chayet probably depends on the laser used and the ablation parameters, which should be tailored to suit each patient.

Visual acuity values are provided in figure 14-12. Each group showed improved uncorrected and best-corrected visual acuity. No eye lost lines of vision. No significant difference in visual acuity was

Figure 14-11- Bitoric treatment of mixed astigmatism: refractive outcomes

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

shown between groups, although in group 3 (bitoric treatment), mean postoperative uncorrected visual acuity was better than best-corrected preoperative vi-

sual acuity.

Help ?

Conclusion

LASIK is an effective method of treating mixed astigmatism according to any of the three ablation models proposed. The bitoric form of treatment is the most physiological and results in the removal of

least stromal tissue when we need to flatten one of the meridians and steepen the other. The “CigalesHoyos bitoric rule” is useful for calculating the two cylinders when the bitoric procedure is applied.

Suggested Readings

-Chayet AS, Magallanes R, Montes M, Chavez S, Robledo N. Laser in situ keratomielusis for simple myopic, mixed, and simple hyperopic astigmatism. J Refract Surg 1998 Apr;14 (2 Suppl):S175-6.

-Vinciguerra P, Sborgia M, Epstein D, Azzolini M, MacRae S. Photorefractive keratectomy to correct myopic or hyperopic astigmatism with a cross-cyl- inder ablation. J Refract Surg 1999 Mar-Apr;15 (2 Suppl):S183-5.

-Cigales M, Hoyos JE, Hoyos-Chacón J, Pradas J, Rodríguez-Mier F. Bitoric-LASIK for mixed astigmatism. ARVO Annual Meeting (abstract book). Fort Lauderdale (Florida-USA). Abril 30 – Mayo 5 de 2000, S690.

LASIK IN MIXED ASTIGMATISM

Contents

Section 1

Section 2

Section 3

Section 4

Section 5

Section 6

Section 7

Subjects Index

Help ?