Neodymium:YAG laser damage to UV-absorbing poly(methyl methacrylate) and UV-absorbing MMA-HEMA-EGDMA polymer intraocular lens materials

Stephen H. Johnson, M.D., Charmaine Henderson, B.S., B.A.

ABS RACT \lemm', L 'm s and -pol, (meth I meth. 'r)late) intraocular

I 'Ill, ''i, \ hich lier ,d as 'ontrol'i, were in(1i iduall 'pos ,d to twent)' 5 mJ shots of the d:Y Glaser fo u'ied onto th post rior sud'ace \\-hil· placcd in a 'it'I'ilc holdcr containing 5 ml of Se line. The

f 'mol'} Lcns is a n '\ foldable il traocul. r I'ns (IOL) manufacturcd from a poly mer consisting of III ,thy I III thacr) lat ' (\(\1 \ ), 2-hydro y­, th I m ·tha 'r lat (II <,\rl\), and ' lh)I'n gl '01 dim thacr. lat· (ECD fA) with a V ahsorb '1'. D, mag' to the lenscs was ~ ssessed hy s 'anning cI ' ·tl'On mi 'roscopy. C toto icit t ·o;t ... design ,d to d -te ,t pot 'n tially to.i(' substanc s r -leased duril g la'i'r damag > w 'I' > conducted with the saline <;ollltion h) ineuh, ting it with mous' flhrohhl'itS. -\dditionalt sts of th saline 'iolution \: -r - condu ,t·d hy high p 'rformanee liquid 'hmmatography at d sp etl'Ophotom · tl'). D~ mage induced h tl e c1: YAG las 'r to the V-pol '(methy I m ·thacl' late) IOLs wa.'i simih r to thut r 'port d b} other authors. Damage to the \1 'mm') L 'ns appeal' ,d I 'is scvcr' and showed no radial str • ., .. cracking. Cy totoxicit} t,.,ls w 'I' • n 'gnti e for both typ's of 10 s. High p rforman ,\il/uid chrome togn ph tsts wel'e n -gn­tive rOl' I"sidual I on om n (. f fA , IIE\IA , ~G I \<fA) and sp - '­trophoton etr did not I'e eal "'sidual V ahsorber.

c)' Words: intnwclllar I('ne.; f\1 \-TlE\IA EC))\I \ pol) llH'l, d: Y \C las('r p()l~ (111 ·th~ IIlH'thacr~ \at ,). l'\ -ahsorhing

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A large percentage of the intraocular lenses currently implanted are posterior chamber lenses. l

To preserve the integrity of the eye, most surgeons prefer to leave the posterior capsule intact f()llowing extracapsular cataract extraction. Recent literature2

reports that up to .50% of the posterior capsules can opacify as early as six months or as late as five years. Visual restoration requires a discission of the opaque posterior capsule.

the close proximity of the intraocular lens (IOL) to the posterior capsule can result in damage to the IOL at the time of Nd:YAG laser capsulotomy.·'3 Uninten­ti(mal damage to the IOL has been reported to occur in up to 30% of patients having this procedure. 4 The damage, which appears as pits and/or cracks in the implant, is usually insignificant when minimal but may induce glare and other undesirable visual disturbances as the lens damage increases.

\Vith the introduction of the neodymium:YAG (Nd:YAG) laser, noninvasive discission of the opaque posterior capsule has become possible. However,

Substantial research by surgeons and manufac­turers has been conducted to determine whether laser-damaged IOLs might release substances that

Reprint reqllests to Step/leI! H. Johnson. M.D., 1441 l\t;ocado Avenue, Suite 206. Newport Beach, California 92660-77.53.

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are toxic to intraocular tissues. Previous studies have attempted to isolate and quantify known potential toxins such as plastic monomers or UV-absorbing chromophores. Since the thermomechanical energy from a laser burst could produce a variety of known and unknown substances in addition to simply releasing residual monomer, biological assays have been used to test for toxic leachables. 5 While these studies failed to reveal safety hazards for materials available at the time of these investigations,6 the safety of new materials must be similarly tested.

This report compares the effect of the Nd:YAG laser on a new IOL material and on cast-molded, UV­absorbing poly(methyl methacrylate) (PMMA) IOLs. The new material is a polymer consisting of methyl methacrylate (MMA), 2-hydroxyethyl meth­acrylate (HEMA), and ethylene glycol dimethacry­late (EGDMA) with a UV absorber. The IOL made from this new material is called MemoryLens. Both lenses were manufactured by Optical Radiation Corporation.

MATERIALS AND METHODS

Five MemoryLenses and five UV-PMMA lenses suitable fi)r implantation were exposed to energy bursts from the Nd:YAG laser. Each lens was placed in a holder inside a sterile glass vial containing 10 ml of saline and intentionally damaged by twenty 5 mJ shots from a Nd:YAG laser. The energy fro~l1 the Nd:YAG laser was fi:lcused on the posterior surface of the IOLs.

The damaged IOLs were examined by scanning electron microscopy. The saline from each vial, as well as from control vials without lenses, was sent to an independent testing laboratory where it was

Fig. 1. (Johnson) Scanning electron micrograph (x.57 magni­fication) of MemoryLens j()llowing exposllre to hWllty • '5 mJ bursts of the Nd:YAG laser.

incubated in vitro with mouse L929 fibroblast cell populations and observed for cell lysis at 24 hours. Following incubation, the culture was macro­scopically examined for cell decoloration and any decolored zone was microscopically examined for cell lysis.

Saline from the MemoryLens vials and UV­PMMA lens vials was also assayed to determine the presence of diffusihle or leachable toxic chemicals from the materials. Two vials of saline without lenses were tested as controls. Determination of the resid­ual MMA, EGDMA, and HEMA was made by high performance liquid chromatography. Determination of the residual UV absorber was made using a Perkin­Elmer Lambda 3B UV NIS spectrophotometer.

RESULTS

Scanning electron micrographs of the Memory­Lens material, presented in Figures 1 to 4, revealed surf~lCes that were remarkablv smooth and llnif(ml1. At 5 mJ, the MemoryLens poiymer showed minimal pitting without raised molten edges or extruded fragments. At 500 times magnification, the pitting had a slight molten appearance hut there were no visible cracks. Each pit appeared to be somewhat jagged without defined sharp edges. At over 5,000 times magnification, the pitting had an irregular edge. There were no deep pits and no radial fracture 1il~es seen. Each pit was similar in type and severity of damage. '

There was no apparent gaseous formation during the laser damage as evidenced hy the lack of visible bubbles in the saline. Cytotoxicity testing of the saline from the MemorvLens vials with mouse L929 fibroblast cells indicat~d no toxic effects; i.e., there

Fig. 2, (Johnson) Scanning l'ledron micrographs of l'vfemo­ryLl'ns at x .500 magnificatioll .

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Fig. 3. (Johnson) Scanning electron micrograph of Memory­Lens at x I, L50 magnification.

was no detectable cell lysis for the saline from the experimental or control vials. No residual EGDMA, MMA, or HEMA monomers were detected when the saline was assayed using high performance liquid chromatography. No UV absorber MOBP was de­tected with the spectrophotometer.

Scanning electron micrographs of the UV-PMMA lens material are presented in Figures 5 to 8. These SEMs revealed patterns of damage that were quite different from that induced in the MemoryLens. At 500 times magnification, the characteristic crater pattern with raised irregular melted edges is ob­served. Some radial cracking is visible. At over 1,000 times magnification, the raised molten edges with multiple extruded fragments were observed. Each site was similar in type and severity of damage.

Fig .. 5. (johnson) Scanning electron micrograph (x 5.5 magni­fication) of lJV-PMMA 10L /cJllowing exposure to twenty.5 mJ bursts of the Nd:YAG laser.

Fig. 4. (Johnson) Scanning electron micl-ograph of Memhry­Lens at x .5,500 magnification.

During the procedure, there was occasional intra­ocular bubbling into the saline. Cytotoxicity testing of the saline from the UV-PMMA lens vials with mouse L929 fibroblast cells showed no cell lysis. When the saline was assayed using high perfor­mance liquid chromatography, there was no MMA residual monomer detected nor could any UV­absorber be detected by spectrophotometry.

DISCUSSION

The MemoryLens polymer and the UV-PMMA lens material were both damaged when exposed to 100 m] (twenty 5 m] shots) of energy from the Nd:YAG laser. The damage appeared less severe for the MemoryLens and gaseous formation was seen

Fig. 6. (Johnson) Scanning electron micrograph of UV­PMMA 10L at x 540 magnification.

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Fig. 7. (Johnson) Scanning electron micrograph of UV­PMMA IOL at X 1,100 magnification.

only with the UV-PMMA lens. The majority of the damage sites, which consisted of craters and/or pitting in each of the materials, exhibited the characteristic morphology. Radial stress fracturing was apparent in the UV-PMMA lens only. The findings with the UV-PMMA lenses are consistent with previously published information. H)

There was no evidence that toxic substances were released from either MemoryLens or UV-PMMA in the saline as a consequence of Nd:YAG exposure. When the saline taken from the vials of laser damaged lenses was incubated with mouse fibroblast cells, there was no toxic reaction, as assessed by cell lysis. To confirm the lack of impurities, the saline was assayed for the presence of monomers. No resid­ual monomers (MMA, HEMA, EGDMA) were de­tected by high performance liquid chromatogra­phy and no residual UV absorber was detected with spectrophotometry.

The results of this study demonstrate that discis­sion of the posterior capsule with the Nd:YAG laser in patients with Memory Lenses should be similar to that in patients with UV-PMMA lenses. The Memo­ry Lens appears to sustain less physical damage them UV-PMMA and neither lens type appears to release any toxic substances.

Fig. 8. (Johnson) Scanning electron micrograph of UV­PMMA IOL at x 6,900 magnification.

REFERENCES

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3. Terry AC, Stark WI. Manll1(,lwe AE. Fagadau W. Neodvm­ium-YAG laser fill' posterior capsulotoll1Y. Am J Ophthilllol 19H3; 96:716-720

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