the type 1 boston keratoprosthesis ( kpro ) is a poly ( methyl methacrylate ) device that has gained acceptance in many types of end - stage corneal diseases as a viable surgical option.13 the kpro is assembled using a carrier donor tissue , and its optics afford excellent visual acuity and field of vision4 assuming that the alignment of the kpro is optimized .
centration of the kpro is determined during preparation of the donor carrier tissue and host trephination .
the manufacturer s recommended steps for kpro preparation involve punching the donor with an 8.5 mm or greater diameter followed by a 3-mm central punch .
we have found that centration of the kpro in the carrier tissue can be difficult using this procedure , and we present a modification for minimizing centration error .
the center of the donor carrier tissue is marked using a surgical marker , and a 3-mm central punch is performed using the centration mark ( figure 1a ) .
. the 3-mm - diameter punched tissue is removed , and the edge is inspected .
an appropriate outer diameter punch size is selected , which should be at least 8.5 mm diameter if possible , and a disposable barron corneal punch ( katena eye instruments , denville , nj ) is typically used .
the donor carrier tissue is placed on the punch block and the 3-mm hole is centered between the vacuum holes of the punch block ( figure 1b ) , and the vacuum is engaged .
the punch is then carefully performed , and the kpro is assembled and sutured to the host rim ( figure 1c ) .
the type 1 boston kpro is a viable therapeutic option for many end - stage corneal diseases that are poor candidates for traditional penetrating keratoplasty , and the reported visual results of the kpro have been good.2,3 the assembly of the kpro involves preparation of a carrier tissue with two punches that are ideally concentric .
we have found difficulty in creating concentric punches using this technique ( figure 2 ) , and we believe that punching the central 3-mm hole first and then using this hole to center within the barron corneal punch s four vacuum ports affords much better control . if the manufacturer s recommendations are followed and the outer punch is performed first , then the free - hand 3-mm dermatologic punch must be placed perfectly for a concentric result .
the optical performance of the kpro on an optical bench using a 3-mm pupil and precise alignment was evaluated and found to produce a sharp , crisp image with a tight point - spread function.4 the optical cylinder , under these optimized alignment conditions , produced scattered ghost images when the 3-mm pupil was widened to 10 mm .
if the alignment of the kpro is suboptimal with the cylinder off - axis from the fovea , then the projected image will most likely be further degraded .
we believe the centration of the kpro in the donor carrier and host cornea is integral for optimal alignment of the cylinder and minimization of scatter and other optical phenomena .
our technique of punching the 3-mm inner hole first and then centering the outer punch on this hole relieves much centration uncertainty in kpro preparation .
in addition to the optical advantages of optimizing centration , suturing to the host rim is much easier with better distribution of tension when the kpro is centered .
we have not found any limitations or problems from this simple modification of kpro assembly .
 the type 1 boston keratoprosthesis preparation requires a 3-mm central punch and an 8.5 mm or larger punch in the carrier tissue . 
 these punches are ideally concentric , but we have found difficulty in achieving concentric punches when the larger punch is performed first . 
 we present a modification in the preparation procedure to help minimize centration error . 
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Discussion