Profiles of Intraocular Pressure in Human Donor Eyes during Femtosecond Laser Procedures—A Comparative Study

Abstract

Purpose.: To compare four different femtosecond laser devices (IntraLase FS, Zeiss VisuMAX, and Ziemer Femto LDV, and a prototype Schwind SmartTech Nanolaser) in human donor eyes with regard to their effects on IOP during femtosecond laser flap cutting. In order to get cuts parallel to the corneal surface, the cornea has to be forced into a defined shape and current femtosecond laser devices either use a flat or a curved patient interface design to achieve applanation.

Methods.: IOP was measured in enucleated eyeballs (n = 46) not suitable for keratoplasty by direct cannulation of the vitreous body. A second cannula was inserted to adjust IOP to a baseline pressure of 20 mm Hg. The eyeballs were lifted by custom made supporting stands to achieve an appropriate height and put under the femto-LASIK devices.

Results.: The flat patient interfaces gave rise to higher IOPs (IOP max = 328.3 ± 29.8, 228.8 ± 28.4, and 201.09 ± 21.4 mm Hg), whereas the curved patient interface caused lower IOPs in response to attachment and suction (IOP max = 104.9 ± 13.4 mm Hg).

Conclusions.: Based on previous findings of visual field defects after LASIK, and as a consequence of the present study, it seems feasible to design patient interfaces in a more physiologic manner to prevent high IOPs during refractive procedures.

Introduction

LASIK has, over the last years, become by far the most frequent corneal refractive surgery procedure for the correction of all types of ametropias performed worldwide today. During surgery, creation of the corneal flap with the mechanical microkeratome still remains the technical step associated with the highest risk of significant complications, such as a button hole, incomplete, or lost caps and flaps being thinner in the center than in the periphery (“meniscus”-like shaped), ^1–3 as well as long term complications, such as epithelial ingrowth or corneal ectasia. ^4,5  

Therefore, the newer technique using a femtosecond laser has rapidly been embraced by refractive surgeons over the last years with 47% of the LASIK procedures in the United States being the performed with the IntraLase (Abbott Medical Optics Inc., Irvine, CA) in 2007. Although this has led to the emergence of previously unknown complications, (e.g., transient light sensitivity syndrome [TLSS]), in the early models with only lower repetition rates available, the fast development of lasers with higher frequency and lower pulse energy has all but eliminated this finding. ⁶ In general, it is felt that the newer generations of femtosecond lasers, and the emerging competition in the field with several companies developing advanced and more sophisticated machines (e.g., 20/10 Perfect Vision; Femtec, Heidelberg, Germany; Femto LDV Laser; Ziemer Ophthalmic Systems, Port, Switzerland; and VisuMax; Carl Zeiss Meditec, Jena, Germany) has increased safety significantly, as recently reported in a large series of sub-Bowman's keratomileusis with a low intra- and postoperative complication rate of only 0.63%, ⁷ which, besides obvious patient safety, is also important for cost coverage by insurances, as discussed in several countries. ⁸  

During the creation of the superficial flap an increased IOP ¹ is induced by basically all types of suction rings used to both stabilize the cornea and increase ocular rigidity for precise cutting, be it either mechanical or with the femtosecond laser technology. 

Again, several complications have been reported to occur due to this IOP rise, such as optic neuropathy ^9,10 visual field loss, ¹¹ and cilioretinal artery occlusion. ¹² Using scanning laser polarimetry ¹³ and optical coherence tomography ^14–16 retinal nerve fiber layer thickness has been evaluated following LASIK with still ambiguous results. 

Other complications possibly related to abrupt changes in IOP during the flap cut include retinal breaks and retinal detachment, premacular hemorrhage, and central retinal artery occlusion. ^14–16 To the best of our knowledge, no increase in any of these complications has been reported in case series using a femtosecond laser system as compared with a mechanical microkeratome yet. 

Several experimental papers have been published in the peer reviewed literature studying IOP changes during mechanical or femtosecond-assisted flap creation either in animal or human donor eyes. An overview of the scientific literature is given in Table 1. 

Table 1. 

 

Overview of the Currently Available Reports of Maximum IOP during Corneal Flap Cutting with Either a Femto Second Laser or a Microkeratome

Femtolaser
Intralase/flat PI	Porcine/enucleated	119.33 ± 15.88 mm Hg27
Visumax/curved PI	Rabbit/in vivo	81.78 ± 6.55 mm Hg20
Intralase/flat PI	Porcine/enucleated	260 ± 53 mm Hg28
Intralase/flat PI	Porcine/enucleated	135 ± 16 mm Hg29
Visumax/curved PI	Porcine/enucleated	65 ± 20 mm Hg29
Femtec/flat PI	Porcine/enucleated	205 ± 32 mm Hg29
Femto LDV/flat PI	Porcine/enucleated	184 ± 28 mm Hg29
Visumax/curved PI	Rabbit/in vivo	26.8 ± 1.2 mm Hg19
Intralase/flat PI	Human/enucleated	328.3 ± 29.8 mm Hg*
Intralase/flat PI	Human/enucleated	192.6 ± 27.7 mm Hg*
Visumax/curved PI	Human/enucleated	88.9 ± 8.2 mm Hg*
Microkeratome
Amadeus	Porcine/enucleated	318 ± 59 mm Hg28
Moria M2	Rabbit/in vivo	141.02 ± 20.46 mm Hg20
Moria M2	Porcine/enucleated	160.52 ± 22.73 mm Hg27
Moria	Porcine/enucleated	113.65 ± 10.78 mm Hg30
Moria	Human/enucleated	175.8 ± 37.6 mm Hg31
Innovatome	Human/enucleated	151.8 ± 27.4 mm Hg31
Hansatome	Human/enucleated	154.7 ± 33.8 mm Hg31
BD K-3000	Porcine/enucleated	99.1 ± 6.1 mm Hg32
Universal Keratome	Human/enucleated	108.0 ± 22.1 mm Hg33
Keratek	Porcine/enucleated	360 ± 35 mm Hg34
Corneal shaper	Porcine/enucleated	140 ± 22 mm Hg34

 The table shows the instrument, the patient interface (PI) and the experimental model that were used, as well as the maximum IOP that was reached during the procedure. All microkeratomes have flat designed PIs. Details on the experimental conditions can be reviewed in the references given on the right side of the table.

*  Strohmaier C, et al. IOVS 2008;49: ARVO E-Abstract 2914.

It is the aim of this experimental study to measure pressure profiles in vitro during a complete femtosecond laser flap cutting procedure resembling a clinical set up in human donor eyes as closely as possible and to compare four femtosecond laser systems, IntraLase FS, Zeiss VisuMAX, Ziemer Femto LDV, and a prototype of the Schwind SmartTech Laser (Schwind, Kleinostheim, Germany), with identical experimental settings. 

Methods

All experiments were reviewed and approved by the ethics committee of the state of Salzburg and conducted in compliance with the Declaration of Helsinki.. 

Human donor eyes not suited for corneal transplantation were obtained from the local eye bank (n = 46, average age of donors 63.5 ± 12.3 years). For the femto-LASIK procedures the eye balls were placed on a supporting stand manufactured for this purpose. The vitreous compartment was cannulated with a 27 gauge needle and connected to a water column in order to adjust the baseline IOP before starting the measurements. For measurements of IOP, a second cannula was inserted into the vitreous compartment. The cannula was connected to a pressure transducer of an electronic data acquisition system (PowerLab; ADInstruments, Grand Junction, Colorado). Small amounts of superglue around the insertion site were used to keep the seal water tight. The measurement protocol followed the standard surgical LASIK protocols of the four femto laser instruments subjected to the present investigation (IntraLase FS, Zeiss VisuMAX, Ziemer Femto LDV, and Schwind SmartTech Laser). Figure 1 shows OCT images of the two types of patient interfaces used (flat versus curved). 

Figure 1. 

 

Differently shaped patient interfaces imaged with the Zeiss Visante OCT. (A) Flat patient interface design as used by IntraLase FS, Ziemer Femto LDV, and the experimental interface of the Schwind SmartTech Laser. (B) Curved interface design as used by the Zeiss VisuMax femtolaser. The displaced volumes of the two shapes are 250 μL for the flat design and 120 μL for the curved design during applanation.

Intralase FS Protocol

After the eye ball was cannulated, as described above, the patient interface was placed on the cornea and suction was applied with the syringe-locking thumb activator in a typical way. The setup was then moved under the laser and the surgeon lowered the laser. Before the laser was locked into the patient interface, the stop cock to the water column was closed to prevent water draining from the eye ball in case IOP would rise during the experiment. The laser was then lowered toward the eye and the pressure on the eye ball was increased until a green light indicated that the device was ready to engage the laser (low green condition). The laser was triggered by the surgeon and after the cutting procedure was over, the laser and the patient interface were removed from the eye. In a second series of experiments the laser was lowered until a red light indicated that the pressure exerted on the eye ball was too high. The device was then withdrawn until the red light changed to green and the laser was engaged (“high green” condition). An example of this protocol is shown in Figure 2. 

Figure 2. 

 

IntraLase FS femto procedure. (A) Single tracing of IOP during a femto LASIK procedure. Starting IOP after closure of the stop cock to the water column (1), positioning of the patient interface and application of suction vacuum (2), lowering of the laser onto the patient interface and forcing the interface against the eye, the down force was adjusted to meet the cutting criteria (green light) (3), cutting period (4), withdrawal of the laser with release of suction vacuum (5), spontaneous IOP after procedure with closed stop cock (6). (B) Average IOP values during the selected periods mentioned above. Black bars represent the pressures measured during “low green” conditions (n = 8), white bars represent pressures during “high green” conditions (n = 7) (for details see Methods section). Asterisk indicates P ≤ 0.05.

Zeiss Visumax Protocol

After the eye ball was instrumented as described above, the patient interface was mounted on the laser and the suction tubing was connected to the built in vacuum port on the VisuMax. Before the patient interface touched the eye, the stop cock to the water column was closed to prevent water from leaving the eye ball in case IOP would rise during the experiment. In order to make contact, the patient platform was elevated toward the laser/patient interface. After contact, suction was applied to the interface. When a stable vacuum was achieved, the trigger was cleared and the surgeon engaged the laser cutting procedure. After having finished the cut, the vacuum to the interface was released and the interface was removed from the eye. An example of this protocol is shown in Figure 3. 

Figure 3. 

 

Zeiss VisuMax femto procedure. (A) Single tracing of IOP during a femto LASIK procedure. Starting IOP after closure of the stop cock to the water column (1), lowering of the laser onto the patient interface and forcing the interface against the eye (2), application of suction vacuum, the down force was automatically adjusted by the device (3), cutting period (4), withdrawal of the laser with release of suction vacuum (5), spontaneous IOP after the procedure with closed stop cock (6). (B) Average IOP values during the selected periods mentioned above (n = 11).

Ziemer Femto LDV Protocol

After the eye ball was instrumented as described above, the hand held laser delivery system with the patient interface was advanced toward and placed onto the cornea. Before the patient interface touched the eye, the stop cock to the water column was closed to prevent water from leaving the eye ball in case IOP would rise during the experiment. After contact, suction was applied to the interface. When a stable vacuum was achieved the trigger was cleared and the surgeon started the laser cutting procedure. After having finished the cut, the vacuum to the patient interface was released, the laser delivery system dislodged and the interface removed from the eye. An example of this protocol is shown in Figure 4. 

Figure 4. 

 

Ziemer Femto LDV procedure. (A) Single tracing of IOP during a femto LASIK procedure. Starting IOP after closure of the stop cock to the water column (1), lowering of the hand held laser delivery system with the patient interface onto the eye, forcing the interface against the eye (2), application of suction vacuum (3), cutting period (4), withdrawal of the laser with release of suction vacuum (5), spontaneous IOP after procedure with closed stop cock (6). (B) Average IOP values during the selected periods mentioned above (n = 11).

Schwind Smarttech Laser

After the eye ball was instrumented as described above, the experimental flat design patient interface was mounted on the laser and the suction tubing was connected to the built in vacuum port of the Schwind SmartTech Laser. Before the patient interface touched the eye, the stop cock to the water column was closed to prevent water from leaving the eye ball in case IOP would rise during the experiment. In order to make contact, the patient interface was lowered onto the eye. After contact, suction was applied to the interface. When a stable vacuum was achieved, the trigger was cleared and the surgeon started the laser cutting procedure. After having finished the cut, the vacuum to the patient interface was released and the interface was removed from the eye. An example of this protocol is shown in Figure 5. 

Figure 5. 

 

Schwind SmartTech procedure with experimental flat patient interface. (A) Single tracing of IOP during a femto LASIK procedure. Starting IOP after closure of the stop cock to the water column (1), lowering of the patient interface onto the eye and forcing the interface against the eye (2), application of suction vacuum, the down force was automatically adjusted by the device (3), cutting period (4), withdrawal of the laser with release of suction vacuum (5), spontaneous IOP after procedure with closed stop cock (6). (B) Average IOP values during the selected periods mentioned above (n = 6).

All data are shown as the mean ± 95% confidence intervals. For statistical analysis of effects two way repeated measurements ANOVA was used. Differences between groups were calculated using a post hoc strategy with Bonferroni adjustments. 

Results

Table 2 shows the mean IOP values measured in the different phases of the femto laser cutting procedure for all four devices. A corresponding, representative tracing for each single device is given in the Figures 2 to 5, respectively. 

Table 2. 

 

IOPs during Four Comparable Periods of the Experiment

	IOP (Start)	IOP (Max)	Average IOP (Cut)	IOP (Stop)
IntraLase FS; (low green)	19.3 ± 1.3	192.6 ± 27.7	180.6 ± 21.6	10.12 ± 3.8	8
IntraLase FS; (high green)	19.3 ± 0.6	328.3 ± 29.8	285.6 ± 17.2	7.87 ± 3.2	7
Zeiss VisuMax	20.5 ± 0.6	104.9 ± 13.4	84.9 ± 7.3	14.6 ± 2.2	11
Ziemer Femto LDV	17.7 ± 2.9	228.8 ± 28.4	150.9 ± 17.2	5.08 ± 1.5	14
Schwind SmartTech	20.65 ± 1.9	201.09 ± 21.4	166.80 ± 17.2	14.22 ± 1.5	6
P value	n.s.	<0.001	<0.001	n.s.

 The differences between the four protocols are highly significant at the maximum IOP during the procedure and the average IOP during the cutting procedure. There is no significant difference between the protocols at IOP Start and at IOP Stop. All data are shown as mean ± 95% confidence interval.

Discussion

Why Is It Important to Know IOPs during LASIK Procedures?

As mentioned in the introduction, several pathologies of the retina and the optic nerve head were reported in relation to the creation of corneal flaps using patient interfaces with suction rings and flat designs, both with mechanical microkeratomes as well as femtosecond laser devices. However, it is not clear, if IOP alone is the reason for the observed pathologies after LASIK. Distortion and shear stress of the eye ball as caused by the suction interface with resulting changes in the biomechanics also seem possible explanations for retinal tears and destruction of nerve fibers in the lamina cribrosa. In addition, patients eventually report the experience of fading light and vision turning dark in the treated eye during LASIK and femtosecond laser surgeries, a sensation that is experienced after the application of suction and pressure to the patient interface (personal observations). This could be a result of IOP increased beyond perfusion pressure. As a result, ocular blood flow might be reduced below a critical level, ^17,18 or possibly direct transient stress to the optic nerve fibers in the lamina cribrosa might cause the sensations reported by the patients. 

As a consequence of the published side effects reportedly linked to LASIK surgery, it was the goal of the present investigation to measure IOP during corneal flap creation using different femto laser systems. The study was performed in human donor eyeballs from the local cornea bank, not suited for corneal transplantation. 

Comparison of the Results of the Study

All femto laser procedures caused a significant increase of IOP at every stage of the cutting sequence and the results of this study make it obvious that the curved patient interface (Zeiss Visumax) elevates IOP less than the flat shaped patient interfaces of the other lasers do. At the “high green” force, the IntraLase FS laser delivered the highest IOP values (328.3 ± 29.8 mm Hg) of all setups tested in the present study. 

These findings are similar to the literature as it is summarized in Table 1. Although IOP values vary, due to differences in species (ocular rigidity coefficients), displaced volume within the eye, interface sizes used and also IOP measurement methods, a common finding is a smaller increase in IOP when using curved interface designs and a higher increase in IOP when using flat interfaces. 

In addition, and for comparability reasons, it needs to be pointed out that in the publications shown in Table 1, the baseline IOPs were different across the studies. Besides species differences this is important, because the ocular pressure–volume relationship as described by the Friedenwald equation (Equation 1) follows an exponential course. As opposed to a linear relationship, the increase in IOP depends not only on the increase in volume, but also on the starting pressure at baseline. As an example, the papers by Ang and Chaurasia used the same type of patient interface and species (rabbits), but different starting pressures and subsequently reported different pressures. ^19,20 In their study, Ang et al. ¹⁹ started at a baseline IOP of 9 mm Hg, which increased to 28 mm Hg during femto LASIK flap creation, whereas Chaurasia et al. ²⁰ started at a baseline IOP of 16 mm Hg and reported peak IOPs of 60 mm Hg. While differences in anesthetic protocols might account for the different starting IOPs in these studies, the reported values for IOP in conscious and anesthetized rabbits are 20 to 25 mm Hg and 16 mm Hg, respectively, ^21,22 so the higher pressures reported by Chaurasia et al. ²⁰ reflect IOP raises under physiologic conditions more closely. 

As a consequence, in the present study, the baseline IOPs in all groups was kept within a narrow range to eliminate the bias that would be introduced otherwise. No significant difference in baseline IOPs was found between groups (Table 2). 

How Do Human Donor Eyes Compare with Living Human Eyes?

When choosing a model to investigate the situation in living humans, it is important to consider the shortcomings of the model. In terms of its relevance to its pressure–volume relationship the main difference between dead and living eyes is the lack of blood flow and blood pressure on the arterial as well as on the venous side of the circulation. This is supported by previous investigations clearly showing marked differences in ocular rigidity between eyes before and after enucleation. ^23,24 Figure 6 shows the course of IOP in a rabbit eye in response to continuous infusion of balanced salt solution into the vitreous cavity as shown by Kiel before. ²⁵ The graph clearly demonstrated the marked difference between the IOP course predicted by the Friedenwald equation (Equation 1) and the IOP course in a living eye where IOP increases in a polyphasic manner.  

 

The Friedenwald equation describes the pressure–volume relationship in relation to the rigidity of the eye (rigidity factor k). The higher the rigidity of the eye, the higher the increase in IOP in response to a certain increase in intraocular volume.  

Figure 6. 

 

Pressure–volume (P/V) relationship in a rabbit eye. The long dashed line shows the P/V relationship as calculated by the Friedenwald equation in post mortem eyes using an ocular rigidity coefficient of k = 0.02. The solid line shows the P/V relationship in a living rabbit eye. Two fundamental differences can be observed between the curves. The Friedenwald equation describes an exponential increase in IOP for additional aliquots of volume added to the eye, however, the real world IOP response to a continuous infusion of fluid results in a polyphasic behavior of IOP (solid line, infusion rate 120 μL/min). The differences are primarily caused by the fact that in living eyes, the intraocular vasculature is filled with blood, pressed into the eye by the local arterial and venous blood pressure. Increasing IOP acts against the blood pressure and the volume of the vasculature will be expelled from the eye accordingly (solid line). Once the vasculature is empty (at IOP > MAP), the PV relationship follows an exponential function. Since in human donor eyes the intraocular vasculature is not filled with blood, no fluid can be expelled by increasing IOP and consequently the pressure volume relationship follows an exponential function as predicted by the Friedenwald equation (dashed line).

Based on the assumption that the pressure volume relationship in the living and the dead eye is different, and that the same volume added to a dead eye causes a much higher increase in IOP than in the living eye, one can use the pressure volume relationship from the literature ²⁶ and try to estimate IOP during applanation with a patient interface. The difference between the corneal shape and the shape of the patient interface defines a volume, which can be estimated as the intraocular volume change that causes the rise in IOP. The displaced volume cannot be estimated accurately because the peripheral shape of the patient interface where the cornea, conjunctiva, and sclera are sucked into the device is complex, and the extent to which the anatomical structures follow the complex shape of the interface is only vaguely known. However, a rough approximation with a simplified model gives some insight into the pressures, which might be expected in living human eyes. 

The pressure rise using flat interface designs is generally higher than the IOP caused by a curved interface (Table 2). Figure 7 shows a pressure/volume relationship of a human donor eye as used for the present investigations. As predicted by the Friedenwald equation, the response of IOP to a continuous infusion of physiologic saline solution follows an exponential function (IOP = 16.09 + 0.73 × dV + 0.0024 × dV² [R ² = 1, P < 0.05]); dv indicates volume increment. In order to achieve an increase in IOP of 250 to 300 mm Hg, a fluid volume of roughly 200 to 250 μL has to be infused into the eye. Given the fact that the uveal blood volume is estimated to be approximately 250 μL, the rise in IOP should be limited to the value of mean arterial pressure. From our own observations during femtosecond LASIK procedures we know that most patients describe fading of the light and complete loss of light perception. The varying descriptions might be caused by differences in the ocular rigidity coefficients and probably also the differences in arterial pressures of the individual patients under these stressful surgical conditions. In addition it is unknown if the whole blood volume of the living eye is expelled or if it is only a fraction of the total volume. The question seems reasonable given the fact that the pressure rise occurs much faster when the patient interface is applied as opposed to the slow infusion model we used in the present study. 

Figure 7. 

 

Pressure Volume relationship in a human donor eye. The exponential function of a dead human donor eye is well fit by a simple polynomial equation: IOP = 16.09 + 0.73 × dV + 0.0024 × dV ² (R ² = 1, P < 0.05). Based on these functions and the estimation of intraocular blood volumes it is possible to try to estimate the real pressure in living human eyes during femto laser procedures with flattened corneas by patient interfaces. Depending on the donor eye, an increase of IOP to 250 to 300 mm Hg is caused by adding roughly 200 to 250 μL of physiologic saline solution to the intraocular volume (arrow).

The IOP values obtained in the present study can, therefore, be regarded as the upper limit of the expected IOP range during the femto LASIK procedure, while the individual mean arterial blood pressure of the patient is the lower limit of the expected pressure range during the cutting procedure. The range of possible IOPs explains the variable sensations of patients in our daily practice. 

Conclusions

Based on the present data it seems safe to conclude that substantial pressure elevations occur during femto-LASIK procedures and these findings are in accordance with the results published by others. Furthermore, the results of the present study clearly demonstrate that curved patient interface designs cause a smaller increase in IOP than flat designs. To the best of the author's knowledge, no systematic longitudinal observations of the nerve fiber layer, visual fields, and the optic disk have been performed. However, considering the present and previous results on IOP elevations by several, widely used patient interfaces, the data clearly provide the rationale for such investigations. 

Footnotes

 Supported by grants from the Austrian Academy of Sciences (DOC 22926 [CS]), the Paracelsus Medical University (FFF R09/01/001-REI), the Fuchs Foundation for Research in Ophthalmology (G27), the Adele-Rabensteiner Foundation of the Austrian Ophthalmological Society, and an institutional grant from the University Eye Clinic Salzburg.

Footnotes

 Disclosure: C. Strohmaier, None; C. Runge, None; O. Seyeddain, None; M. Emesz, None; C. Nischler, None; A. Dexl, None; G. Grabner, None; H.A. Reitsamer, None