BACKGROUND AND INTRODUCTION
As per the National program for control of blindness and visual impairment, 6.8 million people in India have severe visual impairment due to corneal diseases.[1] The most common causes for corneal blindness are infections, ulcerations, malnutrition, trauma, congenital diseases, hereditary diseases, chemical, and thermal injuries. The general treatment for serious corneal disease is corneal graft by penetrating keratoplasty. However, penetrating keratoplasty failure is virtually certain when the ocular surface is severely compromised. Kpros represents the only viable option for restoring sight in these patients, mainly in the eyes with endstage ocular surface disorders and those at a high risk for conventional penetrating keratoplasty. The choice of Kpros is based on the underlying etiology, anatomy of ocular surface, and status of tear film.[2]
CLASSIFICATION
Keratoprosthesis (Kpro) are largely classified into the Type 1 and Type 2 Kpros. Candidates for the Type 1 are eyes with normal eyelid, normal blink, and normal tear film without underlying immunological etiology and the prototype of Type 1 Kpro is Boston Type 1. Type 2 Kpros are done in those who lack normal eyelid, normal blink, and normal tear film with underlying immunological disorder. Boston Type 2 Kpro, modified osteoodonto Kpro (MOOKP), osteo Kpro, and pintucci Kpro are the different types of Type 2 Kpros available.[2]
DESIGN OF KPROS
Kpros consists of an optic cylinder which is made up of polymethyl methacrylate, haptic portion which supports the optic cylinder, and a supporting cover.[2] The haptic portion determines the type of the Kpro. Type of haptic, material, and supporting cover used for different Kpros are as given in Table 1.
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Table 1: Type of haptic, material, and supporting cover used for different Kpros
INDICATIONS OF BOSTON TYPE 1 KPRO
The indications for Boston Type 1 Kpro can be classified as per its prognosis as good, guarded, and very guarded. Good prognosis is seen in multiple failed grafts, aniridia, herpetic keratitis, and in silicon oilfilled eyes and guarded prognosis is seen in pediatric corneal conditions and chemical injuries. Stevens- Johnson syndrome, Ocular cicatricial pemphigoid, severe chemical injuries with severe forniceal shortening, and lid abnormalities have very guarded prognosis.[3]
INDICATIONS FOR TYPE 2 KERATOPROSTHESIS
The indication of Type 2 Kpros are severe endstage ocular surface disorders, Stevens-Johnson syndrome, OCP or mucous membrane pemphigoid, severe chemical injuries, severely keratinized surface, loss of eyelids, vascularized cornea with complete stem cell loss, and multiple failed penetrating keratoplasty.[4]
CONTRAINDICATION FOR KPROS
The contraindications for Kpros in general are unrealistic expectations, nil perception of light, advanced glaucoma or retinal conditions, unwilling or unable to report for regular follow-ups, unwilling to accept cosmetic outcome, unwilling to follow postoperative care and restrictions, and dense amblyopia. The specific contraindications for MOOKP include edentulous, poor oral hygiene, unfit for general anesthesia, and <18 years of age. The absolute contraindication for Boston Type 2 is absent eye lids. Bilateral Kpro should never be done as the other eye is always kept as a reserve in case the surgery in one eye fails.[4]
PREOPERATIVE EVALUATION[2]
Prerequisites include positive perception of light (PL) with accurate projection of rays (PR). An inaccurate PR is not a contraindication as it is often seen in advanced ocular surface disease. A detailed history regarding etiology, onset of vision loss, and previous intraocular surgeries should be taken before deciding the type of Kpro procedure. Complete ophthalmological evaluation including digital tonometry to measure intraocular pressure, A-scan for the measurement of axial length, Ultrasound B scan to rule out posterior segment pathologies, electrodiagnostic tests for the assessment of visual potential, and UBM/ASOCT for quantitative measurements of corneal thickness, anterior chamber depth, angle opening distance, and lens vault should be done preoperatively. Also, syringing is needed to rule out any focus of infection in the lacrimal apparatus. For Type 1 Kpro, it is necessary to assess the adequacy of blink and schirmer’s test is done to rule out ocular surface pathologies.
In addition, MOOKP enlisted patients need to undergo assessment of oral and dental hygiene and examination of buccal mucosa. Ideal tooth in size and shape, with best surrounding bone in upper or lower canine and in absence of canine, any single rooted tooth can be used. Clinical and radiological assessment of tooth including Orthopantomography (OPG), X-ray, and Spiral CT scan of canines is done for selection of a suitable tooth with the help of an oro-maxillo-facial surgeon.
Preoperatively, fitness for general anesthesia is to be obtained. Counseling the patient regarding the need for compliance with treatment and postoperative care, regular followups, expected cosmetic outcome, and to attend the emergency room immediately in case of unexplained drop in vision or pain is a must.
KPRO SETUP[2]
Setting up a Kpro unit involves considerable planning and execution. The team should constitute glaucoma, vitreoretinal, and oculoplastic colleagues along with anesthetists and nursing staff. First is to make a decision on the type of Kpro to be ordered based on the status of eye, whether pseudophakic or aphakic or adult (8.5mm backplate) or pediatric (7.0mm backplate). As a standby, an extra Kpro should be ordered preoperatively, in case an inadvertent loss or breakage of Kpro occurs during surgery. While ordering Type 1 Kpro, especially in aphakic eyes, axial length need to be specified for customized aphakic power calculation. For Type 2 Kpros, a more elaborate setup is required with the need for general anesthesia. Coordination with oromaxillofacial surgeons and radiologist is crucial and appropriate instruments have to be used for the dental or bone graft procedures.
PRINCIPLE, SURGICAL TECHNIQUE AND POSTOPERATIVE CARE
1. Boston KPROS
1a. Boston Kpro type 1
Principle:The assembly of a Boston keratoprosthesis consist of a donor cornea sandwiched between the front plate (optical stem) and titanium back plate of the device as shown in Figure 1.[5] The surgeon attaches the entire unit to the patient’s cornea which has been trephined to full thickness, similar to conventional penetrating keratoplasty.
Figure 1: Diagrammatic representation of Assembly of Boston keratoprosthesis
Procedure: After giving local anesthesia or general anesthesia as decided, the recipient cornea should be marked with trephine. As backplate of the Kpro is 8.5 mm, minimum donor graft size should also be the same. Another point to be noted is that the donor cornea must be oversized by 0.5 mm from the recipient.[2] Donor cornea is usually graded into optical, therapeutical, and not suitable for surgery as per the grading system adapted and modified from the cornea donor study.[6] Therapeutical grade donor cornea is preferred for assembling the Kpro and it is assembled before trephining the recipient. A central 3-mm opening in the donor cornea is trephined and optic of Kpro is placed on adhesive strip in upside-down position. Using a wrench, donor graft is slid down the stem of the optic into its slot and the back-plate is slid in place. Assembled Kpro is then locked with titanium ring and checked for a snug fit. The recipient cornea is trephined and removed, after which the assembled Kpro is sutured using 16 interrupted 90 nylon sutures. Finally, a bandage contact lens (BCL) is placed on the Kpro.[2]
Postoperative regimen:[2] Antibiotics such as fourth generation fluoroquinolone is administered four times a day for a month and it is continued twice a day indefinitely. For highrisk eyes, topical vancomycin (14 mg/mL) is given four times a day for a month, which is continued once daily indefinitely. To reduce inflammation, topical steroids are advised in a tapering dose to twice a day indefinitely or discontinued after 6 months. In addition to these drugs, topical lubricants are prescribed as required. BCL placed at the end of the surgery is changed every 3 months and at the time of BCL replacement in clinic and 5% povidoneiodine is applied.
Followup is scheduled once in 3 months, and during each visit refraction assessment, slit lamp microscopy and photography and fundus examination is done. On refraction, a hyperopic shift gives a clue that an early leak is present, whereas a myopic shift indicates raised intraocular pressure.
Slit lamp microscopy is done to assess BCL, corneal graft, and sutures. Any deposits on BCL are to be submitted for microbiological evaluation. Air bubbles beneath the optic flange and BCL is an indication for early thinning of the carrier graft. Corneal graft around the optic of Kpro is inspected for any infiltration or irregularity or presence of retroprosthetic membrane. Corneal graft to be stained with sterile fluorescein to look for the presence of any epithelial defect or leak and if any loose sutures are present, should be removed.
A meticulous fundus examination to document the optic disc and posterior pole findings using 90D lens is mandatory and digital tonometry is done to monitor intraocular pressure. Anterior segment optical coherence tomography (ASOCT) to identify early graft thinning, periprosthetic tissue loss, retroprosthetic membrane, angle details, and visual field examination are done every 6 months. B-scan ultrasonography should be done once in a year.
1b. Boston Type 2:
Principle: Boston Type 2 Kpro has got a similar structure as that of Type 1 and is a through-the-lid design that penetrates through a tarsorrhaphy and it incorporates a 2-mm anterior extension of an optical stem which is absent in Type 1. The difference between optical stem in both the types is as shown in Figure 2.[5]
Figure 2: Diagrammatic representation of difference between optical stem in Boston Type 1 and 2 Kpro
Procedure:[2] This is similar to the Boston Type 1 Kpro in terms of assembling and suturing. The difference from Boston type 1 is that the anterior nub of the Kpro protrudes by 2 mm to accommodate the skin and there is no separate titanium ring. The backplate of Type 2 Kpro is titanium and it snaps onto lock the Kpro complex. The entire conjunctival mucosa is removed from lid margin to lid margin in the recipient. Sphincterotomy and iridectomy done to keep pupil mid dilated for better visualization of fundus and to avoid retropupillary membrane. Kpro suturing is done afterwards and a pars plana vitrectomy is performed along with Ahmed glaucoma valve in all eyes. Finally, to get rid of the hair follicles in toto, the lid margins are excised and sutured in two layers around the optic of Kpro.
Postoperative regimen:[2] Topical fourth generation fluoroquinolone is given for 2 weeks, whereas topical antibiotic ointment at bedtime is continued indefinitely. Along with topical steroids, systemic steroids are also administered as these surgeries involve posterior segment. Steroids are tapered and stopped over a month. Kpro cleaning during the first postoperative week is done to prevent skin overgrowth. Lid sutures are removed on day 10 and follow-up to be done once in every 3 months.
2. MOOKP
Principle: Involves bypassing the ocular surface with a buccal mucous membrane patch and replacement of the anterior segment structures with an osteo-odonto-acrylic lamina as shown in Figure 3.[7]
Figure 3: Diagrammatic representation of placement of MOOKP in eye
Procedure[2]: MOOKP is a threestaged procedure and is performed as per the RomeVienna protocol. Based on the protocol, it can be classified into two stages namely stage 1 which includes Stage 1A + 1B + C and Stage 2. In the first stage, termed Stage 1 A, the eye is prepared for the procedure by removing the iris, with the cryolens extraction and a limited anterior vitrectomy. At this stage, a tectonic penetrating keratoplasty is performed in case of any corneal thinning.
After one month, the Stage 1 B + C is done. This involves harvesting the chosen canine tooth, preferably from maxillary area and fashioning it into an osteo-odonto alveolar lamina with the optical cylinder fixed in situ. The lamina is placed in the subcutaneous pouch of the contralateral cheek to develop its fibrovascular covering over the next 2 to 3 months. Simultaneously, the buccal mucosa measuring 3 cm diameter is harvested and draped over the ocular surface and secured to the four recti muscles.
After three months, the Stage 2 of the procedure is performed. The lamina from the subcutaneous pouch is then removed and prepared. The mucosa over the ocular surface is reflected with an inferior hinge. The central cornea is trephined as per the posterior diameter of the optical cylinder and the lamina is placed in the eye. The oral mucosa is reflected back over the lamina and sutured and a central opening is made in the mucosa for the cylinder to protrude through.
Postoperative regimen: Systemic and topical steroids and antibiotics are administered after every stage as warranted. Topical antibiotic ointment is continued once a day and topical lubricants are continued indefinitely. Follow-up is done once in every 6 months and in each visit, the health of the oral mucosa and the lamina is evaluated.[2,7,8]
3. Osteo-Kpro
The procedure is very similar to the MOOKP. The bone is harvested from the tibia and it is fashioned into an osteo-lamina, in which the optical cylinder is fixed.[2]
4. Pintucci Kpro
Principle: A biointegrated Dacron fabric skirt that allows a three-dimensional colonization by newly formed vascularized connective tissue constitutes the supporting element of this Kpro. The fabric is soft, pliable, chemically inert, and hence never subject to resorption, which can be easily cut into desired shape and sutured. Dacron mesh support is fixed to the polymethyl methacrylate optical cylinder and it becomes fully integrated with the surrounding tissues both biologically and mechanically and it acts as a barrier to microbial contamination. It can be implanted in thinned or perforated cornea and in corneas with stromal melting. The assembly of Pintucci Kpro is as shown in Figure 4.
Figure 4: Diagrammatic representation of Assembly of Pintucci Kpro
Procedure: Examination of the conjunctiva and lids is very important in cases of trichiasis and lagophthalmos as reconstructive plastic surgery is often necessary prior to the main surgery.
As per Pintucci, a two-stage surgery was performed.[9,10] In the first stage, surface reconstruction with an oral mucus membrane graft is performed by removing a large graft from mucosal side of the lower lip and the graft is placed on a flat surface for performing appropriate thinning. After 360-degree peritomy, the corneal epithelium is completely removed by superficial keratectomy and mucus membrane graft is placed over the cornea and sutured with 7-0 vicryl sutures to the conjunctiva and rectus muscles. Thereafter, a linear incision is made through the lower lid and orbicularis oculi muscle of the opposite eye, and the sterile KPro is placed under the muscle upside down with the optical cylinder vertical in the inferior orbitopalpebral sulcus pocket to achieve colonization.
Stage 2 of the surgery is performed after 3 months.[9,10] The KPro with vascularized Dacron skirt is removed with an incision through the lower lid. The mucosal graft is removed from the cornea and a central corneal trephination is performed with a 3-mm dermal punch followed by complete removal of the crystalline lens and iris with anterior vitrectomy. Finally, the KPro is placed projecting the posterior part of the cylinder into the anterior chamber and the skirt is sutured to the cornea using 7-0 vicryl sutures. The mucosal graft is placed back on the KPro and central trephination is done to expose the anterior pole of the cylinder. The mucosal graft is then sutured with 7-0 vicryl sutures. During follow-up, careful examination of the dacron tissue must be done for any erosion through the surrounding tissues, loosening, aqueous leaks, and infection.
COMPLICATIONS[2]
Sterile melts: In mild cases, cyanoacrylate glue can be applied to the area of thinning, whereas in moderate cases, a crescentic or annular lamellar graft would be required to manage the melt and in cases of extensive melts, it would be better to replace the Kpro with a new one. In addition, medical supportive measures including topical medroxyprogesterone, systemic doxycycline, and copious lubrication are given and a tarsorrhaphy is done in cases with frequent BCL displacements leading to graft desiccation.
Retroprosthetic membrane: It is one of the most common complications after Kpros surgery, more prevalent in individual with chronic inflammation such as autoimmune diseases and uveitis. On slit lamp examination, retroprosthetic membrane is seen along with opacification of the posterior plate fenestrations. Early membranectomy is done with the Yttrium aluminum garnet (YAG) laser to avoid thickening of membrane. If membrane is thick, leathery, and vascularized, surgical intervention using a two-port or three-port vitrectomy set-up may be required to create an opening in or to excise the retroprosthetic membrane. Surgical membranectomy or Nd YAG laser with a peribulbar injection of steroid may reduce the chance of recurrent membrane formation.
Glaucoma: It is a single-most serious complication with elevated intraocular pressure due to chronic low-grade inflammation, progressive angle closure, and anterior displacement of iris. Elevated intraocular pressure may be managed with topical and systemic antiglaucoma therapy and placement of a glaucoma tube shunt is indicated if in case of failed medical therapy. This needs proactive aggressive management in the eyes with pre-existing glaucoma disease.
Endophthalmitis: It is a dreadful complication following kpro surgery, presents with a painful inflamed eye and diminution of vision. Endophthalmitis was noted in 10% of eyes with the MOOKP and Boston Type 2 Kpro and fungal etiology was noted in equal number of eyes as those with bacterial endophthalmitis. It is treated with systemic and topical antibiotics along with steroids to reduce the inflammation.
CONCLUSION
Keratoprosthesis surgery is considered as a complex one and needs meticulous care at each step to ensure overall success rate and a lifelong follow-up. A multidisciplinary team is needed to provide the best care for the patients. As per literature, compared to high-risk keratoplasty, Boston Type 1 Kpro is better for wet blinking eyes with no bulbar or tarsal keratinization, whereas Type 2 Kpro is preferred for dry keratinized surface with defective or absent lid or blink mechanism.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
REFERENCES
1. Gupta N, Tandon R, Gupta SK, Sreenivas V, Vashist P, Burden of corneal blindness in India. Indian J Community Med 2013;38:198–206.
2. Iyer G, Srinivasan B, Agarwal S, Talele D, Rishi E, Rishi P, et al., Keratoprosthesis:Current global scenario and a broad Indian perspective. Indian J Ophthalmol 2018;66:620–9.
3. Saeed HN, Shanbhag S, Chodosh J, The
Boston keratoprosthesis. Curr Opin Ophthalmol 2017;28:390–6.
4. Yaghouti F, Nouri M, Abad JC, Power WJ, Doane MG, Dohlman CH, Keratoprosthesis:Preoperative prognostic categories. Cornea 2001;20:19–23.
5. Huang Y, Yu J, Liu L, Du G, Song J, Guo H, Moscow eye microsurgery complex in Russia keratoprosthesis in Beijing. Ophthalmology 2011;118:41–6.
6. Patel D, Tandon R, Ganger A, Vij A, Lalwani S, Kumar A, Study of death to preservation time and its impact on utilisation of donor corneas. Trop Doct 2017;47:365–70.
7. Hille K, Grabner G, Liu C, Colliardo P, Falcinelli G, Taloni M, et al., Standards for modified osteoodontokeratoprosthesis (OOKP) surgery according to Strampelli and Falcinelli:The Rome-Vienna Protocol. Cornea 2005;24:895–908.
8. Iyer G, Pillai VS, Srinivasan B, Falcinelli G, Padmanabhan P, Guruswami S, Falcinelli G, Modified osteo-odonto keratoprosthesis--the Indian experience--results of the first 50 cases. Cornea 2010;29:771–6.
9. Pintucci S, Pintucci F, Cecconi M, Caiazza S, New Dacron tissue colonisable keratoprosthesis:Clinical experience. Br J Ophthalmol 1995;79:825–9.
10. Maskati QB, Maskati BT, Asian experience with the Pintucci keratoprosthesis. Indian J Ophthalmol 2006;54:89–94.
