Five-year results from a study of the Boston keratoprosthesis type 1 implant suggest that frozen and fresh corneal donors offer similar clinical outcomes-with no significant differences in device retention, visual rehabilitation or complication rates.
Take-home message: Five-year results from a study of the Boston keratoprosthesis type 1 implant suggest that frozen and fresh corneal donors offer similar clinical outcomes-with no significant differences in device retention, visual rehabilitation or complication rates.
By Lynda Charters; Reviewed by Adam K. Muzychuk, MD
Montréal-Patients undergoing Boston keratoprosthesis type 1 implantation (KPro, Massachusetts Eye and Ear Infirmary) do not need to wait for fresh corneal graft carriers to become available, according to researchers.
A recent study with 5 years of follow-up demonstrated that use of frozen corneal graft carriers offers similar clinical outcomes to fresh tissue in terms of visual rehabilitation and complication rates. Recognition of this could impact that amount of fresh tissue available for use in other procedures, as well as increase the availability of KPro surgery worldwide.
Study investigators, led by Adam K. Muzychuk, MD, noted there has been a significant increase in the number of devices implanted annually over the past two decades.
“The vast majority of cases require allogenic tissue in the form of a donor corneal button, which acts as a peripheral device carrier and an interface to facilitate suturing of the device to the host,” Dr. Muzychuk said. “As such, optical clarity of donor tissue is not required.”
He was joined in this study by Marie-Claude Robert, MD, Stanley Dao, MD, and Mona Harissi-Dagher, MD.
Dr. Muzychuk explained that because fresh donor tissue can at times be difficult to obtain in Canada, his colleagues from the University of Montéal had previously conducted a study in which they compared frozen with fresh tissue. After a mean postoperative follow-up of 9.7 months, they found that visual rehabilitation and complication rates did not differ significantly between the two approaches.
In light of more recent data suggesting an increase in the risk of certain complications with longer follow-up, the present study set out to evaluate the clinical outcomes of fresh versus frozen carrier grafts up to the 24 months of the initial study protocol, and further looked at the long-term outcomes of fresh versus frozen carrier grafts in a subset of these patients up to 60 months postoperatively.
In this prospective, single-center, single-surgeon, non-blinded, randomized controlled trial, all patients who underwent implantation of the KPro device with an allograft carrier between October 2008 and December 2009 were included. All surgeries were performed using the standard KPro technique. The device used was the KPro type 1 threadless design with a 16-hole polymethylmethacrylate plate.
According to Dr. Muzychuk, who was a clinical and surgical fellow at the University of Montréal, the cryopreserved or frozen tissue arrived at the operating room as whole globe on the morning of the scheduled surgeries and was then thawed in a sterile container under running water. The surgeon used a Westcott scissors to fashion a corneoscleral rim that was cut into a donor corneal button using the standard trephination technique.
The primary study outcome was retention of the device at 24 and 60 months postoperatively. Secondary outcomes were the surgical feasibility, visual acuity, and complication rates.
A total of 37 eyes were enrolled in the 24-month study, and one was lost to follow-up, resulting in a retention rate of 97%. Of these, 15 patients (mean age, 59 years; median visual acuity, counting fingers) received fresh corneal tissue, and 11 patients (mean age, 58 years; median visual acuity hand motions) received frozen tissue.
Twenty-six patients were enrolled in the extension phase and none were lost to follow-up. No significant differences were seen in the baseline patient demographics; diagnostic category, (especially regarding patients with autoimmune disorders or chemical burns who are known to have worse prognoses in addition to all other indications), or past ocular history between the carrier graft groups.
This was true among all patients followed for 24 months and those in the extension phase, according to Dr. Muzychuk.
All procedures were uneventful. The total surgical times in the fresh and frozen groups did not differ significantly (51 and 56 minutes, respectively; p = 0.14). As expected, tissue preservation time differed significantly between the groups, on average 8.5 days for fresh tissue and 80.6 days for frozen tissue, respectively (p < 0.001).
At 24 months postoperatively, the retention rate of the device was 100%. At 60 months, the retention rate was 97%. When broken down by tissue type, the fresh tissue group had a 100% retention rate, and the frozen tissue group had a 91% (n = 10/11) retention rate, a difference that did not reach significance (p = 1.00). One device was replaced at 51 months after implantation due to a carrier graft melt.
The visual rehabilitation in the fresh and frozen groups was similar at 24 months postoperatively, after which time the visual acuity levels in both groups decreased.
At the 60-month evaluation, the median corrected distance visual acuity (CDVA) improved to 20/150 compared with the baseline of counting fingers in the group that received fresh tissue. In the group that received frozen tissue, the median CDVA improved to 20/400 from hand motions at baseline. There were no significant differences in the rates of complications, particularly uveitis or corneal melting.
“There were no statistically significant differences in the rates of complications such as retroprosthetic membrane, corneal melt or tissue necrosis, uveitis, glaucoma, or posterior segment complications between carrier graft groups at 24 or 60 months,” Dr. Muzychuk said.
In commenting on the use of frozen tissue, Dr. Muzychuk noted that deep-freezing corneal tissue destroys all keratocytes and endothelial cells, rendering tissue devoid of viable cells while providing a collagen scaffold onto which host keratocytes can migrate and become established.
Frozen tissue is not, however, the only option that has been explored. Other alternatives include gamma-irradiated corneal tissue and glycerol dehydrated tissue.
“The 5-year results of our study demonstrated that there are no significant differences between fresh and frozen carrier grafts in implant retention, visual rehabilitation, or complications, particularly those such as uveitis or corneal melts, which may reasonably have been expected to increase,” Dr. Muzychuk said.
He noted the limitations of the present study included a relatively small sample size, the possibility of selection bias introduced by the loss of patients during enrollment for the extension phase, and the small number of patients in the worst prognostic categories for KPro implantation.
In this study-thought to be the first of its kind to report the long-term safety and efficacy of an alternative to fresh corneal graft carriers for KPro surgery-it has been demonstrated that frozen corneal graft carriers offer similar clinical outcomes as fresh tissue, he noted.
Dr. Muzychuk summarized that use of frozen tissue may increase the availability of fresh tissue for use during other procedures and increase the accessibility of KPro surgery worldwide, without compromise to the visual rehabilitation or safety of the device.
Adam K. Muzychuk, MD
This article was adapted from Dr. Muzychuk’s presentation at the 2016 meeting of the American Society of Cataract and Refractive Surgery. The investigators have no financial interest in any aspect of this report.