Outcomes

9.1 Safety: MICS versus Coaxial Phaco

George H. H. Beiko

Core Messages

ßMicroincisional cataract surgery (MICS) is a recent innovation upon standard coaxial cata-

ract phacoemulsification.

ßMICS meets the safety benchmark of coaxial phacoemulsification, and surpasses it in some

aspects.

ßVisual rehabilitation and outcomes with MICS meet or surpass that of coaxial phacoemulsi-

fication.

9.1.1 Introduction

The separation of irrigation and aspiration in cataract surgery, termed “biaxial technique,” was first described in the early 1970s [1, 2]. In 1985, Shearing advocated the use of the biaxial technique for cataract extraction; however, his technique required the enlargement of the incision to 7 mm in order to insert the intraocular lens [3]. It was not until the advent of the ability to modulate

G. H. H. Beiko

Assistant Professor in Ophthalmology, McMaster University, Lecturer in Ophthalmology, University of Toronto, Hamilton, ON, Canada

e-mail: george.beiko@sympatico.ca

power in phacoemulsification that sleeveless phaco tips could be inserted safely through small incisions, and microincision surgery was born.

Microincision cataract surgery (MICS) was first coined by Professor Alio in 2001 to describe cataract surgery through sub-2.0 mm incisions [4]. The term “MICS” was patented by Alio in 2003 (Alio, personal communication).

The benefits of the biaxial approach in MICS are described in other chapters in this book, but briefly can be listed as the following [5]:

•Better control of rhexis

•Improved surgical efficiency by decreased effective phaco time (EPT)

•Better fluidic control

•Enhanced chamber stability

•Better followability through fluid dynamics by separation of irrigation and aspiration so as to minimize opposition between the two

•Access to entire anterior chamber with either irrigation or aspiration by inserting the instruments through either incision

•Ease of removal of subincisional cortex; lower posterior capsular opacification rates

•Increased ability to handle complications, including the ability to avoid areas of zonular weakness

•Ability to use the irrigating fluid as a tool to manipulate material within the capsular bag or anterior chamber

•Reduction of vitreous prolapse in cases of posterior capsular tear or rupture

•Increased suitability for complicated cases

The purpose of this chapter is to investigate the safety of MICS, by looking at visual outcome, incision damage, corneal changes and complications.

9.1.2 Visual Outcomes

The true test of a new technique or instrument in cataract surgery is visual outcome. Whenever a new lens is introduced, the point of comparison is the visual outcome compared to the current standard lens. Similarly, at the very least, MICS should be able to attain vision comparable to standard coaxial phacoemulsification; otherwise, any perceived benefit would be compromised.

Howard Fine and colleagues compared visual outcomes with bimanual phaco with those of standard phaco in a number of phaco systems and found that vision was improved similarly, irrespective of the technique [6] (see Fig. 9.1.1).

In studying the visual rehabilitation following cataract surgery, it has been reported that MICS patients gain vision faster than coaxial patients. Kurz et al. [7] studied 70 patients prospectively, equally divided between MICS and coaxial; their findings are reproduced in Fig 9.1.2 and demonstrate a smaller interquartile range in MICS.

MICS has been reported to have a significantly decreased mean total phaco time and mean EPT [7, 8] (see Fig. 9.1.3). As phaco time is believed to be directly related to corneal damage, less phaco time should translate into less corneal trauma and quicker visual recovery.

In point of fact, it has been reported that MICS brings about more rapid visual rehabilitation due to reduced postoperative inflammation as a result of decreased chamber turbulence [9–11]. Similarly, it has been reported that postoperative inflammation using flare readings, were decreased significantly in MICS eyes compared to coaxial phaco eyes at 1 week [12]. Thus, decreased inflammation should also mean faster visual recovery.

In terms of visual rehabilitation, MICS has certainly been demonstrated to be similar to coaxial phaco, and in some authors’ hands, to be superior since visual recovery occurs sooner.

*P<.01

Fig. 9.1.1 Percentage of eyes with UCVA of 20/40 or better, 2–24 h postoperatively, comparison of coaxial and bimanual phaco (from Fine et al. [6])

Post operative BVCA in Biaxial MICS compared With Coaxial Small Incision Clear Cornea Cataract Surgery15

Note. MICS: 35 eyes, final incision size 1.5 - 1.7mm; SICS: 35 eyes, final incision

size 2.75mm

Fig. 9.1.2 Visual rehabilitation following cataract surgery; comparison of MICS and coaxial techniques (from Kurz et al. [7])

EPT for Biaxial MICS versus Coaxial Phaco11

Fig. 9.1.3 Comparison of effective phaco time [EPT]; MICS vs. coaxial (Alio et al. [8]) (“mean effective phacoemulsification” was defined as the mean EPT in seconds in this study as it “represented the estimated phacoemulsification time if 100% phacoemulsification power in continuous mode had been used” [8])

9.1.3 Incision Damage

Examination of the incision should reveal the extent of trauma or injury at the incision sites. Ideally, the incision should allow access to the eye without compromising the ability of the incision to seal post-op and to minimize the amount of permanent change such as induced astigmatism.

In a scanning electron microscope comparison study of incision damage in cadaver eyes [13] (see Fig. 9.1.4), it has been suggested that incision leakage occurred in all MICS eyes under test conditions when the IOP was raised to 125 mmHg for 30 s and in none of microincisional coaxial eyes under similar conditions. It was also

reported that whitening of the incision occurred in 4 or 5 MICS eyes, suggesting that incision burn would occur. However, the criticism of this study has been made that the investigators were inexperienced with MICS and that incision construction was inaccurate (Alio, personal communication).

In a similar cadaver study, the changes in incision architecture were also studied by the authors who compared MICS with microincisional coaxial incisions [14]. It was found that both types of incisions were slightly larger post-op compared to the incision made. However, Descemet’s tear extensions were greater at the MICS phaco site than the microincisional coaxial phaco site while tears were similar at the irrigation sites in both. Also, endothelial cell loss [ECL] was similar in both, although there was a greater loss near the phaco site of microincisional coaxial than MICS and no difference at the irrigation site in both. This study would suggest that although minimal differences exist between MICS and microcoaxial incisions, there is likely no significant impact on corneal endothelial function, incision leak or induced astigmatism when the two are compared.

9.1.4 Corneal Incision Burn

Corneal incision burns (see Fig. 9.1.5) can result in significant distortion of the incision, resulting in the necessity of incision closure with sutures and significant induced astigmatism. Incision burns occur as a result of thermal damage to corneal collagen, and this occurs when tissue temperatures exceed 60°C [15, 16]. It is for this reason, that irrigating sleeves on the phaco

Fig. 9.1.5 Corneal incision burn

tip were introduced in traditional coaxial phaco. When considering conversion to sleeveless phaco for MICS, this is of paramount concern as there is a perception that the loss of the irrigating sleeve will allow friction of the phaco tip with the cornea tissue, causing incision temperatures to rise and incision burns to occur.

In MICS, it is almost a necessity for the phaco tip to make contact with the corneal tissue when micro incisions are employed. Power modulations of phaco energies through variation of burst and pulse modes have allowed the safe use of sleeveless phaco tips in direct contact with corneal tissue without the creation of corneal incision burns. The proof of this is as follows.

Steinert and Schafer [17] used a thermal camera in a laboratory setup to investigate the rise in temperature, comparing continuous ultrasound with Whitestar micropulse technology (AMO, Santa Ana, USA). Their findings were that, continuous ultrasound resulted in a phaco tip temperature of 61°C, while the Whitestar micropulse

phaco tip temperature never rose above 35°C. Thus, clinically, continuous ultrasound would induce incision burn while the Whitestar settings would prevent this due to the decrease of at least 20°C in incision temperature, and well below 60°C which would cause incision burn.

In a cadaver eye study, using sleeveless phaco, it was found that the incision temperature never rose above 41.8°C and that the incision remained clear [10].

Clinically, using a thermocouple attachment in cataract patients, it has been shown that a sleeveless phaco tip with Whitestar modulations results in temperature rises in the 24–34°C range; far short of the temperature for incision burn [18].

Similarly, other clinical studies have also determined that the wound temperature with MICS does not rise above 40°C under normal clinical conditions [19, 20]. In a series of 637 patients who underwent MICS, there was no report of corneal incision burn [21].

As corneal incision burn is dependent on the temperature, the impact of ultrasonic phaco handpieces, tip geometries and operating modes is significant. In a laboratory setting, it has been shown that there are differences between the various phaco systems currently available. Using high speed thermal imaging of phaco tips, it was found that the greatest temperature rise was with the Alcon Inifinity system with Torsional handpieces; B & L Stellaris and AMO Signature with Ellips had the least temperature rise, as demonstrated in Fig. 9.1.6 [22].

Thus, not only is there considerable laboratory and clinical evidence to support the use of sleeveless phaco tips in MICS, but also some indication that the AMO Signature Ellips system may be least likely to cause incision burn.

9.1.5 Corneal Changes

a. Clarity

Clarity of the cornea following cataract surgery is an indication of the trauma incurred during surgery. In a study comparing MICS and standard coaxial phaco, no difference was found between the techniques [3], as Fig. 9.1.7 illustrates.

Thus, MICS and standard coaxial phaco, at the very least, induce comparable minimal trauma to the eye.

b. Induced corneal aberrations

As cataract surgery has evolved over the past two decades, so have corneal incisions decreased from greater than 10 mm with ICCE, 8–10 mm with ECCE, 6.0–7.0 mm with phaco and PMMA lenses, and less than 3.5 mm with phaco and foldable IOLs. With each incremental decrease in incision size, there has been a decrease in induced corneal astigmatism. Scleral

*P<.05 for all comparisons

Fig. 9.1.7 Percentage of eyes with clear corneas, 2–24 h postoperatively (from Fine et al. [6])

Vectorial Astigmatic Change

Fig. 9.1.8 Vectoral astigmatic analysis of surgically induced astigmatism in biaxial MICS compared with coaxial cataract surgery (from Alio et al. [8])

incisions have been associated with less induced astigmatism than corneal incisions [23].

In a prospective, randomized study of 100 patients, comparing MICS and standard coaxial phaco, Alio et al. [8] analyzed the surgically induced astigmatism using vectoral analysis. The MICS group had a mean incision size of 1.71 ± 0.21 mm compared to the coaxial group of 3.1 ± 0.25 mm. They reported a mean change of 0.36 ± 0.232 D in the MICS group compared to a significantly greater increase of 1.2 ± 0.74 D in the standard coaxial phaco group. An interesting finding was that more than 50% of the coaxial group had more than 1 D of induced astigmatism while none of the MICS group had this (see Fig. 9.1.8).

Similar reports of decreased induced corneal astigmatism have been reported by other authors [21, 24, 25]. Koch reported an induced astigmatism of less than 0.25D at 4 weeks post-op with 2.0mm incision; this decreased to less than 0.075D by 6 months post-op [26]. Ke Yao et al. [25] found that the change in the simulated keratometry value was greater in the coaxial group (3.2mm incision) than in the MICS group (1.7mm incision) (see Fig. 9.1.9).

Thus, corneal induced astigmatism following MICS is significantly less than standard coaxial and approaches almost zero change [27].

Not only can lower order aberrations of astigmatism be affected by cataract surgery, but also higher-order

Comparative results on corneal astigmatism for Microincision Incision Cataract Surgery versus Small Incision

Cataract Surgery10

Note. Sim k represent the difference in power between the steep and flat meridians. MICS: 30 eyes, 1.7 ± 0.1mm; SICS: 30 eyes, 3.2mm

Fig. 9.1.9 Changes in the simulated keratometry values; comparison of MICS and coaxial cataract surgery (from Ke Yao et al. [25])

Comparative results on corneal astigmatism for Microincision Incision Cataract Surgery versus Small Incision

Cataract Surgery10

Note. MICS: 30 eyes, 1.7 ± 0.1mm; SICS: 30 eyes, 3.2mm

MTF (modulation transfer function) are the metrics to indicate retinal image quality from an incoming object

Fig. 9.1.10 Comparison of MTF with MICS and coaxial phaco (from Ke Yao et al. [25])

aberrations. A 3.2 mm superior clear corneal incision “induced consistent and significant changes in several corneal Zernicke terms (vertical astigmatism, trefoil and tetrafoil) resulting in a significantly increased overall corneal RMS wavefront error” [28]. However, the 3.2 mm incision did not induce significant changes in spherical aberration or coma terms. The amount and orientation of the aberrations induced depended on the surgical meridian and incision location.

In contrast to the 3.2 mm incision, MICS does not induce any change in total RMS value [27].

When optical quality is evaluated, it has been found that implantation of micro incision IOLs with MICS results in at least similar modulation transfer function (MTF) values as standard IOLs in coaxial cataract surgery [29]. In fact, Ke Yao et al. reported a significantly better MTF values at a spatial frequency of 0.5 MTF with MICS, in their study of MICS vs. coaxial surgery (see Fig 9.1.10) [25]. Thus, MICS provides similar or better quality of vision than coaxial.

If the corneal higher order aberrations are measured pre-op and post-op, and a comparison is made of MICS vs. 3.2 mm coaxial cataract surgery, an increase in

Corneal Coma Z3

6 mm O.Z.

Fig. 9.1.11 Comparison of total corneal coma; MICS vs. coaxial cataract surgery (from Bellucci and Morselli [30])

Corneal Horizontal Coma Z3 +1

6 mm O.Z.

Fig. 9.1.12 Comparison of horizontal corneal coma; MICS vs. coaxial cataract surgery (from Bellucci and Morselli [30]

Corneal Vertical coma Z3 -1 6 mm O.Z.

0,35

Preop Postop

induced coma has been reported in the latter group [30]. This increased coma would result in degradation of the quality of vision (See Figs 9.1.11–9.1.13).

All studies support the conclusion that smaller incision with MICS result in less induced corneal aberrations, whether of lower order (astigmatism) or higher order (coma). Lower aberrations result in better vision.

c. Endothelial cell loss

In this chapter, it has been shown that MICS is associated with decreased phaco time, decreased inflammation and less ocular trauma; in theory, this should result in lower ECL.

Before looking in depth into this aspect of MICS, it is important to understand the measurement of ECL. Measurements of endothelial cell counts are highly reproducible by a model of specular microscope but the results are not interchangeable between different models of specular microscopes [31]; thus in comparing ECL, it is essential that the same type of specular microscope is used. Cell loss of 10% is within confidence interval of measurement error with a specular microscope [32]. In multicentre studies, precision of only 8–10% can be expected; improved precision of 2% can be anticipated only if one centre and experienced staff are employed for the interpretation of the data [33].

In standard coaxial phaco, ECL of 6.4–8.8% at 90 days post cataract surgery has been reported. To be determined that MICS is safe, it must be shown that at least comparable ECL occurs. A number of studies prospectively comparing coaxial phaco to MICS have confirmed this [7, 8, 34–39]) (see Fig. 9.1.14 for summary of the data)