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Proper processing of materials can result in low infection, rejection risks.
This article was reviewed by Fengju Zhang, MD
The use of lenticular tissue acquired during the SMILE procedure seems to be the same with a minimal risk of infection, and the risk of rejection can be reduced by using an appropriate preservation method.
Myopia has an extremely high incidence in China, and as a result, numerous patients undergo the SMILE procedure, according to Fengju Zhang, MD.
“Because of this, we have accumulated many transparent lenticules after the surgeries and they are being reused in a number of corneal procedures, such as patching corneal perforations and correcting hyperopia, keratoconus, and ectasia after LASIK,” said Dr. Zhang, from the Beijing Tongren Eye Center, Beijing Tongren Hospital, and the Beijing Ophthalmology and Visual Sciences Key Lab, Capital Medical University, Beijing City, China.
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However, the recycling of this tissue does not come without inherent risks. Possible infections that can take hold include herpes simplex virus (HSV), which can be latent in corneal stromal tissue for an extended period as well as bacteria, fungi, and Acanthamoeba that may be lurking in the normal conjunctival sac, she explained.
Another potential problem is immunologic rejection following implantation of the lenticules.
One Chinese study reported that at the one-year follow-up of 29 cases (53 eyes) that underwent allogeneic corneal stromal lenticule implantation to treat hyperopia, rejection of the tissue occurred in three eyes (5.66%) of two patients, Dr. Zhang noted.
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In light of these complications, Dr. Zhang and colleagues undertook a study to detect pathogens and antigens in the fresh lenticules obtained during the SMILE procedure.
A total of 167 patients who underwent the SMILE procedure from October 2018 to April 2019 were chosen randomly. Those included had no systemic diseases, no history of use of systemic hormones or immunosuppressive drugs, no ocular diseases except for a refractive error, no history of ocular surgery or trauma. Patients had a stable refractive with no change exceeding 0.5 D annually for two years.
Patients also could not have used soft spherical contact lenses within one week, toric soft contacts and hard contacts within two weeks, or orthokeratology lenses within three months before surgery.
Any eyes with or suspected of having corneal ectasia, moderate to severe dry eye, severe meibomian gland disease, or an allergy induced by contact lenses were excluded. SMILE lenticules were collected in 128 eyes of 64 patients with myopia.
The donor specimens from each patient were divided into two groups in order to detect pathogens: specimens from 64 eyes (32 left and right eyes) and specimens from 64 eyes (32 left and 32 right); in the latter group, each specimen was divided into three pieces.
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In the first group of 64 eyes, polymerase chain reaction (PCR) was performed to detect HSV-2; in the second group of 64 eyes, cultures were carried out to identify bacterial, fungi, and Acanthamoebic, Dr. Zhang recounted.
The investigators also undertook another experiment to identify antigens. Lenticules were collected during SMILE from 132 eyes of 103 patients with myopia and divided into three groups of 44 specimens each: the fresh group, the –78o C glycerol preservation group, and the 0.1% sodium dodecyl sulfate (SDS) group.
All specimens were subjected to immunohistochemistry, Western blot analysis, transmission electron microscopy (TEM), transmittance, and nanoindentation.
Pathogen, antigen detection
PCR showed negative results in all cases for detecting HSV-1 and -2, bacteria, fungi, and Acanthamoeba in both patients with and without a history of contact lens wear.
Regarding antigen detection, in the fresh group, immunohistochemistry showed a positive result for detection of HLA/B/C and HLA-DR. No positive results were seen in the –78o C glycerol preservation and the 0.1% SDS groups. Western blotting analysis reflected the immunohistochemistry findings.
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TEM showed collagen fibrils were very regular in the fresh group and the nuclei were intact; in the –78o C glycerol preservation group, the collagen fibrils were regular but the interiors of the nuclei were destroyed; and in the SDS group, the both elements were completely destroyed, Dr. Zhang noted.
Regarding transmittance, in the fresh and –78o C glycerol preservation groups, the specimens were transparent, with average transmittance values of 89.32% and 87.94%, respectively; however, in the SDS group the transmittance value was lower at 82.09%. The nanoindentation curves showed the highest Young’s modulus values in the fresh and glycerol groups, with values of 49.21 and 60.6 kPA, respectively, compared with 24.26 kPA in the SDS group.
Human corneal stromal lenticules from SMILE have a low risk of infection for reuse, Dr. Zhang said.
“HLA-I and HLA-II antigens were all expressed in human corneal stromal lenticules from SMILE, and there is a risk of transplant rejection for reuse,” she said. “An ideal method to reduce the expression of antigen in human corneal stromal lenticules is by using –78o C pure glycerol preservative.”
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In discussing the negative pathogenic findings of the study, Dr. Zhang said the study subjects were young and healthy with no systemic or ocular diseases.
“Previous studies have reported that HSV-1 was detected mainly in the limbal tissue, while the corneal stromal lenticules were located within the central 6.5-mm diameter of the anterior stroma, which brings us to a advantage of the SMILE procedure,” she said. “Because no knife is used, no corneal flap is created, and there is no exposed matrix bed, SMILE is unlike the bacteria-carrying corneal layers of LASIK, and the risk of microbial contamination of the conjunctival sac during SMILE is low,” Dr. Zhang said.
She said she considered the sample size to be small.
Regarding the results of antigen detection, Dr. Zhang explained that results achieved with the –78o C pure glycerol preservation method was preferred because it reduced the antigen expression by rupturing the cells.
“Based on our study results, the human corneal stromal lenticules from SMILE have a low infection risk for reuse,” she said. “HLA-I and HLA-II antigens were both expressed in human corneal stromal lenticules obtained during the SMILE procedure, and there is a risk of transplant rejection for reuse.
“The –78o C pure glycerol preservation is an ideal method to reduce the expression of antigen in human corneal stromal lenticules.” Dr. Zhang concluded.
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Fengju Zhang, MD
Dr. Zhang has no financial interest in any aspect of this report. The study was supported by the 215 High Level Talent Fund of the Beijing Health Government.