[Perspectives of excimer laser-assisted keratoplasty]

BACKGROUND AND PURPOSE

The purpose of this paper is to demonstrate the evolution of laser keratoplasty and to outline the potential future perspectives of this technique.

METHODS

For non-contact donor trephination from the epithelial side an artificial anterior chamber has been used. Since 1989 more than 3,300 penetrating keratoplasty operations (PKP) have been performed successfully with the Zeiss-Meditec MEL60® excimer laser in Erlangen and Homburg/Saar.

RESULTS

Prospective clinical studies have shown that the technique of non-contact excimer laser PKP improves donor and recipient decentration, reduces vertical tilt and horizontal torsion of the graft in the recipient bed, thus resulting in significantly less all-sutures-out keratometric astigmatism, higher regularity of the topography and better visual acuity. Besides less blood-aqueous barrier breakdown during the early postoperative course after PKP, excimer laser trephination does not induce cataract formation and does not impair the graft endothelium. Likewise, the rate of immunological graft rejections is not adversely affected by the excimer laser. In addition, trephination of an instable cornea is facilitated.

CONCLUSIONS

Because of undisputed clinical advantages, especially in eyes with keratoconus, excimer laser trephination with orientation teeth/notches is still favored in Homburg/Saar in daily practice. The femtosecond laser (FSL)-assisted keratoplasty technique is very exciting but a prospective randomized study is required to determine potential benefits over excimer laser PKP.

Robotic system for microsurgical keratoplasty

Robotic system is developed to improve the effect of microsurgery for keratoplasty. Based on task and motion analysis in the surgery, type synthesis and size synthesis of robotic mechanism are performed to meet the requirements of surgical tasks. Force and position microsensors are integrated on the end-effector of microsurgical robot. The two digital filtering methods suitable for the PC and the sensor data board respectively are presented. A feasible method of calibration and reconstruction for surgical microscope is adopted to implement the guidance of robotic vision. The estimated coefficients by linear regression are evaluated for preciseness and robustness. The experiment results show that the robotic system for microsurgical keratoplasty can fulfill the surgical task of corneal trephination precisely.

New formula for calculating intraocular lens power after laser in situ keratomileusis

PURPOSE

To assess the validity and accuracy of a proposed formula for keratometry (K) readings after laser in situ keratomileusis (LASIK).

SETTING

The Eye Center and the Eye Foundation for Research, Riyadh, Saudi Arabia.

METHOD

This studied comprised 34 eyes that had LASIK surgery. Refraction and an automated K-reading (auto-K) were performed preoperatively. Refraction, auto-K, and K-reading assessment by the clinical history method and the proposed formula were performed 4 to 12 weeks postoperatively. The proposed formula is K(postop) = K(preop) - [(N(c) - 1) x (R(a-postop) - R(a-preop))/(R(a-postop) x R(a-preop))], where K(postop) is the K-reading after LASIK, K(preop) is the K-reading before LASIK, N(c) is the index of refraction of the cornea (1.376), R(a-postop) is the radius of curvature of the anterior corneal surface after LASIK, and R(a-preop) is the radius of curvature of the anterior corneal surface before LASIK.

RESULTS

Twenty patients (10 men, 10 women) were included in the study. The mean age of the patients was 30.58 years +/- 17.68 (SD) (range 18 to 44 years). Preoperatively, the mean spherical equivalent (SE) was -4.99 +/- 2.82 diopters (D) (range -1.12 to -15.00 D), the mean R(a) was 7.76 +/- 0.32 mm (range 7.33 to 8.50 mm), and the mean auto-K reading was 43.45 +/- 1.73 D (range 39.62 to 46.00 D). Postoperatively, the mean SE was +0.02 +/- 0.63 D (range -2.75 to +1.00 D), the mean R(a) was 8.63 +/- 0.53 mm (range 7.80 to 9.92 mm), and the mean K-reading assessed by auto-K, clinical history method, and the proposed formula was 39.17 +/- 2.35 D (range 34.00 to 43.25 D), 38.79 +/- 2.52 D (range 33.1 to 42.78 D), and 38.69 +/- 2.51 D (range 33.1 to 43.0 D), respectively. The results obtained by the proposed formula were similar to those obtained by the clinical history method (P =.098). Auto-K readings significantly overestimated the K-values (P<.0001) when compared to the proposed formula and clinical history method.

CONCLUSION

The proposed formula was simple, objective, not dependent on refraction, and as accurate as the clinical history method in determining K-readings after LASIK.

Q-Switched 2.94-microm Er:YAG laser trephination with convergent and divergent cut angles for penetrating keratoplasty

PURPOSE

To study the morphologic properties of perpendicular (P), convergent (C), and divergent (D) cut angles using different speeds of rotations during donor and recipient nonmechanical trephination for experimental penetrating keratoplasty.

METHODS

With a Q-switched 2.94-microm Er:YAG laser corneal trephination was performed in 150 enucleated porcine eyes using ceramic open masks with 8 "orientation teeth/notches" and an automated globe rotation device allowing different cut angles [0 degree (P), 10 and 20 degrees (C and D)] toward the optical axis and variation of the rotation speed [3, 7, and 11 rotations per minute (rpm)]. The regularity of the cut (I, regular; II, slightly irregular; III, irregular) was assessed by light microscopy. The area of thermal damage and the number and size of "spikes" in the stroma at the superficial, intermediate and deep level of the excision were analyzed using digital images and the Optimas image processing software.

RESULTS

The regularity of the cut was classified as I in 42%/22% of donor/recipient and as II in 41%/56%, respectively. The thermal damage was least expressed with D20 degree cut angle and donor mask (P < 0.01). With all cut angles and speeds of rotation, thermal damage at the deep level of excision was significantly smaller (P < 0.01). With different speeds of donor rotations, the thermal damage showed no significant difference. With recipient trephination, the thermal damage at the deep level was greatest with 7 rpm (P < 0.01). The number and size of spikes of thermal damage with donor and recipient masks were significantly smaller in the deep stroma (P < 0.01).

CONCLUSIONS

Q-switched Er:YAG laser trephination with appropriate settings results in low thermal damage zones at the cut margin. Different cut angles and speeds of trephination may affect the cut performance and quality of the excision. In our study, low rotation speed and divergent donor cut angles showed the best results. The cut quality and the small thermal damage with the Q-switched 2.94-microm Er:YAG laser seem to be tolerable for corneal trephination. Therefore, this modality may be a low-cost, easy-to-handle alternative for nonmechanical corneal transplantation in humans.

Measuring corneal power for intraocular lens power calculation after refractive surgery. Comparison of methods

PURPOSE

To find a more accurate and predictable method for intraocular lens (IOL) power calculation in eyes after refractive surgery.

SETTING

Department of Ophthalmology, Kangnam St. Mary's Hospital, Seoul, Korea.

METHODS

The accuracy of the following methods for calculating IOL power in 132 eyes after PRK or LASIK was compared: manual keratometry, hard contact lens, refraction-derived keratometry at the corneal plane, and the refraction-derived keratometry at the spectacle plane. Based on this comparison, the IOL power was calculated in the 2 eyes of a patient using refraction-derived keratometry at the spectacle plane with the SRK II formula. Cataract surgery with IOL implantation was then performed.

RESULTS

The largest corneal power values were obtained using a manual keratometer and the smallest using refraction-derived keratometry at the spectacle plane (P <.001). In the patient having cataract surgery with IOL implantation, near target refraction was achieved with minimal error in IOL power.

CONCLUSIONS

If the corneal power is known before refractive surgery, the use of the smallest value of those obtained using refraction-derived keratometry and the hard contact lens method is recommended. However, if the corneal power before refractive surgery is unknown, the use of the hard contact lens method is recommended.

Superficial corneal effects of experimental nonmechanical penetrating keratoplasty using a Q-switched Er:YAG laser

PURPOSE

To assess thermal effects of Q-switched Er:YAG laser trephination to corneal epithelium and superficial stroma using different mask types and materials for experimental penetrating keratoplasty.

METHODS

Laser trephination was performed in 20 freshly-enucleated porcine eyes (repetition rate 5 Hz, pulse energy 65 mJ, spot size 0.7 mm). We used flat, open-metal and ceramic masks for donor and recipient trephination placed directly onto the corneal surface. Main outcome measures as assessed by light microscopy after PAS staining of 8-microm paraffin sections included: extension of tissue thermal damage at the cut edge in the superficial and basal epithelial layers, the basement membrane and subepithelial stroma, and depth and width of epithelial/stromal involvement in the area of the donor mask contact.

RESULTS

The thermal damage in the superficial epithelium was more pronounced in donor (mean extension 61.6 +/- 15.6 microm) than in recipient (29.4 +/- 24.9 microm, p= 0.05) trephination. In donor trephination, thermal damage zone of the superficial epithelial layer was significantly smaller with ceramic than with metal masks (21.0 +/- 23.0 versus 61.6 +/- 15.6 microm, p= 0.014). In contrast, differences at basal epithelial layer (p= 0.44), basement membrane (p= 0.79), and subepithelial stroma (p= 0.2) were not statistically significant. Superficial donor involvement of the cornea adjacent to the paracentral donor mask contact zone was seen neither with ceramic nor with metal masks.

CONCLUSION

Superficial corneal alterations adjacent to the mask-cornea contact zone may be minimized by using the Er:YAG laser in a Q-switched mode. Ceramic masks, in contrast to metal masks, further reduce superficial thermal alterations at the cut edge.

Lens opacities after nonmechanical versus mechanical corneal trephination for keratoplasty in keratoconus

PURPOSE

To compare the lens opacity formation after penetrating keratoplasty (PKP) using nonmechanical excimer laser corneal trephination and mechanical motor trephination. SETTTING: University Eye Clinic, University of Erlangen-Nürnberg, Erlangen, Germany.

METHODS

Ninety-six patients with keratoconus (96 eyes) and clear crystalline lenses were randomly assigned to the nonmechanical trephination (NMT) group (n = 46; 35 men; mean age 38.2 years +/- 10.8 [SD]) or the mechanical trephination (MT) group (n = 50; 35 men; mean age 34.4 +/- 9.0 years). Suturing and postoperative treatment were identical. Dilated pupil biomicroscopy and slitlamp lens photography were performed preoperatively and postoperatively at 3 month intervals. Opacities were identified as cortical, nuclear, and posterior subcapsular and graded from 1 (mild) to 3 (severe).

RESULTS

Mean follow-up in the NMT/MT group was 3.2 +/- 1.3 years/3.4 +/- 1.1 years. Overall, incident opacities appeared in 23.9%/32.0% of eyes (4.3%/6.0% cortical; 19. 6%/26.0% posterior subcapsular; 0%/0% nuclear) (P =.833). All cortical opacities in both groups were grade 1; posterior subcapsular opacities were grade 1 in 66.6%/61.5% of eyes and grade 2 in 22.2%/30.8% of eyes. One patient in each group presented grade 3 posterior subcapsular opacities. No differences between trephination methods were seen in a 5 year Kaplan-Meier cumulative risk of lens opacity formation (P =.763 cortical, P =.530 posterior subcapsular).

CONCLUSIONS

In addition to its optical advantages, nonmechanical corneal trephination appears to have no adverse impact on cataract formation after PKP for keratoconus.

Nonmechanical excimer laser penetrating keratoplasty for perforated or predescemetal corneal ulcers

OBJECTIVE

To review the authors' results using nonmechanical excimer laser trephination in penetrating keratoplasty for perforated or predescemetal corneal ulcers.

DESIGN

Noncomparative, consecutive, interventional case series.

PARTICIPANTS

Thirty-two patients with perforated (17) or deep (15) corneal ulcers (9 bacterial, 5 acanthamoebal, 10 herpetic, 3 associated with rheumatoid arthritis, 5 other) necessitating immediate tectonic keratoplasty.

INTERVENTION

Penetrating tectonic keratoplasty with excimer laser trephination was performed along metal aperture masks in donor and recipient corneas.

MAIN OUTCOME MEASURES

Clinical results including intraoperative and postoperative complications were evaluated.

RESULTS

Trephination was possible in all eyes without perforation of predescemetal ulcers or extrusion of intraocular contents. No eyes with bacterial or acanthamoebal ulcers showed persistence or recurrence of infection. One eye showed recurrent epithelial herpetic keratitis. Best-corrected postoperative visual acuity ranged from 20/700 to 20/20 (median, 20/58), with 30 of 32 eyes achieving improvement of best visual acuity during follow-up (mean, 22.4 months) and with 12 of 32 eyes reaching a visual acuity of 20/40 or greater. Eyes with bacterial or acanthamoebal ulcers showed best results. Episodes of graft rejection occurred in nine eyes and resulted in irreversible opacity of the graft in seven cases. None of the eyes lost all vision, developed endophthalmitis, or had to be enucleated.

CONCLUSIONS

Nonmechanical trephination is a useful technical refinement of tectonic penetrating keratoplasty à chaud for perforated or deep progressive corneal ulcers. This technique greatly facilitates exact trephination under these difficult surgical conditions and might possibly improve the prognosis of this procedure.

Nonmechanical corneal trephination with the excimer laser improves outcome after penetrating keratoplasty

OBJECTIVE

To assess the impact of nonmechanical trephination on the outcome after penetrating keratoplasty (PK).

DESIGN

Prospective, randomized, cross-sectional, clinical, single-center study.

PATIENTS

A total of 179 eyes of 76 females and 103 males, mean age at the time of surgery 50.6 +/- 18.5 (range, 15-83) years. Inclusion criteria were (1) time interval from October 1992 to December 1997; (2) one surgeon (GOHN); (3) primary central PK; (4) Fuchs dystrophy (diameter, 7.5 mm) or keratoconus (diameter, 8.0 mm); (5) graft oversize, 0.1 mm; (6) no previous intraocular surgery; and (7) 16-bite double-running diagonal suture.

INTERVENTION

In a randomized fashion, eyes were assigned either to trephination with the 193-nm Meditec excimer laser (manually guided beam in patients, automated rotation device of artificial anterior chamber in donors) along metal masks with eight orientation teeth/notches (EXCIMER: 53 keratoconus, 35 Fuchs dystrophy; mean follow-up, 37 +/- 16 months) or with a hand-held motor trephine (Microkeratron; Geuder) (

CONTROL

53 keratoconus, 38 Fuchs dystrophy; mean follow-up, 38 +/- 14 months). Subjective refractometry (trial glasses), standard keratometry (Zeiss), and corneal topography analysis (TMS-1; Tomey) were performed before surgery, before removal of the first suture (15.2 +/- 4.2 months), and after removal of the second suture (21.4 +/- 5.6 months).

MAIN OUTCOME MEASURES

Keratometric and topographic net astigmatism as well as refractive cylinder; keratometric and topographic central power; best-corrected visual acuity (VA); surface regularity index (SRI), surface asymmetry index (SAI), and potential visual acuity (PVA) of the TMS-1.

RESULTS

Before suture removal, mean refractive/keratometric/topographic astigmatism did not differ significantly between EXCIMER (2.5 +/- 1.8 diopters [D]/3.4 +/- 2.8 D/4.7 +/- 3.1 D) and CONTROL groups (3.0 +/- 1.8 D/3.7 +/- 2.4 D/4.3 +/- 2.1 D). After suture removal, respective values were significantly lower in the EXCIMER group (2.8 +/- 2.0 D/3.0 +/- 2.1 D/3.8 +/- 2.6 D) than in the CONTROL group (4.2 +/- 2.4 D/6.1 +/- 2.7 D/6.7 +/- 3.1 D) (P < 0.0009). In the EXCIMER versus CONTROL group, mean VA increased from 20/100 versus 20/111 (P > 0.05) before surgery, to 20/31 versus 20/38 before (P = 0.001) and to 20/28 versus 20/39 (P < 0.00001) after suture removal. Mean spherical equivalent was significantly less myopic in the EXCIMER group before (-0.9 +/- 3.6 D vs. -2.6 +/- 3.4 D) (P = 0.01) and after suture removal (-1.4 +/- 3.1 D vs. -2.4 +/- 3.5 D) (P = 0.02). Mean SRI (P = 0.04) and PVA (P = 0.007) were significantly more favorable in the EXCIMER versus CONTROL group after suture removal (0.91 +/- 0.45 and 0.82 +/- 0.15 vs. 1.05 +/- 0.46 and 0.73 +/- 0.18).

CONCLUSIONS

Postkeratoplasty results seem to be superior using nonmechanical excimer laser trephination. Thus, this methodology is recommended as the procedure of first choice in avascular corneal pathologies requiring PK.