Modified Osteo-odonto keratoprosthesis (MOOKP)
Dr Srinivas K Rao, DO, DNB, FRCSEd
Director, Darshan Eye Clinic, Chennai Head, Cornea Service, Rajan Eye Care Hospital, Chennai Head, Cornea Service, Sunetra Family Eye Care Center, Kolkata Visiting Ophthalmologist, Madras Medical Mission, Chennai Visiting Professor, Chinese University of Hong Kong, Hong Kong Honorary Professor, Shantou Medical University, PRC
Dr Sujatha Mohan, DO
Director, Rajan Eye Care Hospital, Chennai
What is a keratoprosthesis ?
In some eyes, severe damage to the cornea and ocular surface occurs due to drug allergy, or chemical or thermal burns – resulting in a very dry, skin-like appearance of the corneal surface. In these eyes, corneal transplantation does not work, as the dry surface is unable to support the health of the transplanted biological tissue, unlike in a normal eye. To provide the patient with useful vision, an artificial cornea or keratoprosthesis is used. A keratoprosthesis is a cylinder made of a type of plastic polymer called polymethyl methacrylate (the same material used to make intraocular lenses). The cylinder is made such that the power of the cornea is built into it, so that not only does it provide a clear corneal window, but it also supplies the refractive power needed for the eye to see well. Since plastic and living tissue cannot form a firm and safe bonding, researchers have tried over the past many years to find a way to allow integration of the two tissues, so that the long-term success rate of the procedure is enhanced.
In some designs, researchers have implanted a plastic cylinder into a central hole in a donor cornea and retained the cylinder using a nut-and-bolt design. The donor cornea is then sutured to the patient’s eye. This is relatively fast and easy to perform, but there may be some problems in the long-term retention of the prosthesis.
What is the modified osteo-odonto keratoprosthesis?
To improve the chances of the plastic prosthesis being retained in the eye, tissues from the patient’s own body are used. The tooth is ideal because it is a hard part to which the cylinder can be fixed and also it resides in the mouth where it co-exists with soft tissues, as in the eye. Originated by Prof Strampelli in Italy – this procedure has been refined and improved by Prof Falcinelli over the last 40 years. We performed the first OOKP in HK and China with his and his son Dr Johnny’s help. Since then they have traveled to HK many more times to help us with the procedure. Having performed more than 250 of these procedures, Prof Falcinelli is the most experienced surgeon for this procedure. He has also worked with Dr Rao in India 3 years ago to help start the procedure there. The procedure is at present performed only in a few centers in the world. This is a fairly complex procedure, and requires to be performed in 2 stages – each of which can take up to 4 to 6 hours. The patient can see after the second stage operation. The use of a tooth requires the help of a multi-disciplinary team including a dentist – and Dr Lee has been helping with the procedure here. The term MOOKP stands for modified - osteo (bone) – odonto (tooth) – keratoprosthesis (plastic cylinder).
This complex surgical procedure is performed in two stages:
Stage I
In the first stage, a canine tooth is harvested from the mouth of the patient after X-ray screening has determined that the tooth has a healthy and viable root structure. A surgical motorized saw is used to excise the canine root encased in alveolar bone from the jaw. The lamina is fashioned by sawing through the root of the tooth in a longitudinal fashion to expose the dentine and the root canal. The pulp in the root canal is scraped off and a hole is drilled in the widest part of the root – to a size of 3 to 4 mm depending on the width of the root at that point. An appropriately sized plastic cylinder of suitable power (determined from the axial length of the eye) is then glued to the hole using dental cement. A subcutaneous pocket is created in the tissues of the cheek and the lamina-cylinder complex is placed in it and the pocket is sutured closed after instilling antibiotic powder. In the eye, the symblephara are released, and scar tissue is excised as described earlier. A superficial keratectomy including the Bowman’s layer is performed to expose the bare corneal stroma after which a full-thickness circular piece of cheek mucosa about 4 cm in diameter is placed over the cornea and sutured to sclera, also covering the muscle insertions.
Stage II
Stage II surgery is performed 2 to 3 months later to allow time for a connective tissue cover to develop around the lamina implanted in the cheek. If required, the integrity of the lamina can be checked by performing a spiral computed tomographic evaluation. During the second stage surgery, the lamina is retrieved from the subcutaneous location and excess connective tissue is removed from the two ends of the optic cylinder, and carefully trimmed over the rest of the lamina. The mucosal graft on the ocular surface is incised superiorly and reflected from the superior sclera and cornea, in a downward direction. The inferior attachment of the mucosal graft is left undisturbed to ensure that the blood supply is retained.
A Flieringa ring is sutured in place and a 3 mm opening is created in the center of the cornea. Three radial incisions are made in the cornea extending till the limbus. The iris is disinserted at the root and removed by gently pulling on the tissue and hypotensive anesthesia is used to control the ooze. Constant irrigation with balanced salt solution also helps wash the blood away and prevents a large clot forming in the anterior chamber. The lens is then cryo extracted and the radial corneal cuts are sutured closed. A limited anterior vitrectomy is performed and the lamina is then placed over the cornea, such that the posterior part of the optic cylinder is in the anterior chamber – entering through the central corneal opening. The lamina is sutured into position using the connective tissue covering and episcleral bites. Air injection through the pars plana using a 30-gauge needle is used to maintain the intraocular pressure and avoid severe hypotony. At the conclusion of suturing, indirect ophthalmoscopy is performed to ensure that there is a good view of the disc and posterior pole, with the eye in the primary position. If this is not seen, a cylinder tilt may be responsible and the sutures are adjusted to straighten the cylinder position. Any bleeding into the vitreous cavity can also interfere with the visualization. After the cylinder and lamina are in satisfactory position, the mucosal flap is replaced and a small opening is created over the optic cylinder to allow the anterior portion of the cylinder to protrude through the mucosa. The superior edge of the mucosal flap is sutured in place and this completes the operation.
Since bleeding can occur during the ocular and oral surgical dissections, it is important that the patient is not on anti-platelet aggregators or anticoagulants. Infection is a major concern since oral tissues are used and extensive ocular surgery is performed. Intravenous antibiotics are used for the first week after surgery and oral antibiotics are continued for another week. This is more of a concern with stage I surgery. Treatment of the ocular surface mucosal graft is with antibiotic ointment and regular cleaning. Periodic follow-up is required to ensure early detection of problems with intraocular pressure and the stability of the graft. The presence of the lamina and the large mucosal graft preclude measurement of intraocular pressure using the current instrumentation. Intraocular pressure is estimated digitally and the health of the optic nerve is monitored using regular automated field measurements and visualization of the optic disc. The visual field provided by current cylinder designs is about 30 to 350 but this allows good ambulant vision to the patient. With the technique of osteo-odonto keratoprosthesis described in the previous paragraphs, prolonged retention times can be achieved with good visual function.
Complications
With a procedure as involved as the MOOKP, it is perhaps not surprising that a fair number of complications are possible, although many can be averted by careful and meticulous technique. Problems can occur during the oral and ocular parts of the procedure – both intra- and postoperatively.
During the removal of the tooth, a fracture of the tooth would render it useless as a lamina for the plastic cylinder. Sometimes, a small part of the tooth may be left behind in the bed and this would need to be extracted to prevent it from acting as a nidus for future infection. During the removal process, accidental entries into the maxillary sinus when the upper tooth is removed, and fracture of the mandible – during lower tooth removal, are possible. Both during tooth extraction and cheek mucosa excision, excessive bleeding may occur.
The fashioning of the implant is also critical. It is important to avoid damage to the periosteum and the alveolar dental ligament when the tooth is cut with the dental saw. Since the saw blades can generate a significant amount of heat, a constant stream of water must be directed at the saw to ensure that there is damage to these delicate structures. The hole that is drilled in the dentine should be perpendicular to the lamina to ensure that the plastic cylinder does not tilt. The dental cement that is used to ensure proper adhesion of the cylinder and the dentine should be used to ensure complete sealing of the hole in the lamina.
Once the tooth is removed, the absence of the canine(s) can pose both cosmetic and functional problems in the postoperative period. These stem from the fact that the oral cavity is no longer a closed chamber – due to the missing tooth – and can result in a lisp when speaking – and extrusion of liquids when eating. Neither is a major problem however, and most patients learn to cope. Occasionally, scarring of the cheek mucosal excision area may occur, although this too, is infrequent.
In the eye, the scope of the surgery and the nature of the eyes operated on, both result in the potential for problems. These damaged eyes often have preoperative glaucoma. During the surgery, care must be paid when operating in the perilimbal area to avoid excessive cautery as this can damage the aqueous collecting veins and either create or worsen glaucoma. Thinned areas of the cornea and sclera must be handled with care to avoid perforations. Removal of the iris and the lens and the anterior vitrectomy can sometimes result in bleeding in the vitreous chamber. Prolonged hypotony during the implantation of the lamina should be avoided – as this can lead to expulsive hemorrhage. The use of hypotensive anesthesia during this stage is therefore, important.
After the surgery is completed, infection remains an important concern and antibiotics are essential in the postoperative period. Inflammation in the eye – due the surgical procedure – can sometimes occur and oral steroids can help. Increased intraocular pressure is not uncommon in the early postoperative period and should be carefully watched for – intravenous mannitol may be required in addition to oral acetazolamide. Later in the postoperative period, glaucoma can sometimes occur and should be watched for using regular visual field evaluations and examination of the optic disc. Retroimplant membranes are rare – but can occur and may need excision – either with the YAG laser, or surgery. Vitreoretinal complications can also sometimes occur, and need to be treated surgically. The mucosa covering the implant, can develop infections, or can retract and must be managed appropriately. Finally, the implant can develop periostitis – often infectious and this can compromise the integrity of the lamina.
Thus, the list of complications with MOOKP is long, but with careful attention to technique and close postoperative follow-up, many of these can be prevented or appropriately managed.
Cornea fellowship programme under the guidance of author.