The osteo-odontokeratoprosthesis (OOKP) according to Strampelli and modified by Falcinelli provides good long-term results in restoring vision to most patients with severe corneal opacification not amenable to conventional corneal transplantation. 1–8 The prosthesis consists of an optical cylinder made of polymethyl methacrylate (PMMA) and an osteodental support obtained from a single-rooted tooth of the patient or prepared from a histocompatible blood-relative donor (Fig. 1). Over the years, however, some of the patients can demonstrate a decentration of the initially well-aligned optical cylinder because of partial resorption and/or transformation as well as loosening of the osteodental lamina. In this paper, we describe the clinical and histopathologic features of three patients whose osteodental lamina had to be explanted for different reasons.
OOKP surgery was successfully performed on the left eye of a 67-year-old male patient because of corneal blindness caused by linear IgA bullous dermatosis. The OOKP was prepared from a canine tooth of the lower jaw and a PMMA cylinder (7.85 mm long and 4 mm in diameter) was inserted. The osteodental lamina was covered with the patient's own lid skin in a transpalpebral approach. Postoperative vision was 20/25 for most of the follow-up, but the osteodental-lamina had to be removed 15 months after surgery because of aqueous humor leakage after the resorption of the inferior half of the lamina (Figs. 2 and 3). At the time of surgery, another tooth was prepared for a second attempt to perform OOKP surgery.
The 77-year-old female patient had ocular cicatricial pemphigoid resulting in bilateral corneal blindness (projection of light only). OOKP surgery was performed on the patient's right eye, but no visual improvement could be obtained after a massive postoperative subchoroidal hemorrhage the day after surgery. The canine tooth from the patient's daughter had been used for the prosthesis because the recipient lacked usable teeth; the diameter of the optical cylinder used was 4 mm. The prosthesis again was covered with lid skin. Systemic immunosuppression with cyclosporin A was prescribed to prevent rejection but was stopped 4 days after the last step of the OOKP surgery because of no visual benefit. The optical cylinder was expelled spontaneously 30 months after implantation, and the remnants of the surgically removed osteodental lamina were used for histologic examination.
A 50-year-old male patient was referred to our clinic because of decentration and protrusion of the optical cylinder. OOKP surgery had been performed on his right eye 14 years before in Rome, Italy, after a bilateral chemical burn. Slit-lamp examination indicated resorption of the osteodental lamina in the nasal lower quadrant (Fig. 4). Because of this complication, the visual acuity of the patient had decreased to hand motion only. During the complex surgical attempt to affix the lamina to an optically centered position, the PMMA cylinder detached from the dental lamina and had to be completely removed.
More details of these three cases are given in Table 1, including the data of two children (cases 4 and 5) who showed complete dissolution of the osteodental lamina within several months after OOKP surgery (Fig. 5).
MATERIAL AND METHODS
Immediately after surgical removal, the laminae were prefixed with 5.2% buffered formaldehyde. Specimens were then placed in a decalcifying solution consisting of both formic and hydrochloric acids. The decalcifying process was completed using electrolysis. Twelve volts (1.2A) was applied for 8 days, and the solution was changed once after the first 2 days. Thereafter, the laminae were fixed in formaldehyde for 1 hour, dehydrated in a graded series of ethanol (60 to 100%) and rinsed with 100% xylene. The specimens were embedded in paraffin, sectioned (8 μm), stained with hematoxylin and eosin, and studied with a Nikon microscope (Eclipse E 800 M).
In the anterior parts of the lamina, sebaceous glands and hair follicles that extended from the overlying lid skin were detected. The optical cylinder was surrounded by keratinizing squamous epithelium. Resorption of bone tissue that had been replaced by connective tissue was quite obvious. A few osteoclasts could also be detected. At places where bone tissue was still preserved, it appeared normal, retaining its lamellar structure. Remnants of dentin were found in the inferior sections, surrounded by a small layer of immature woven bone. A chronic inflammatory reaction was present, consisting mainly of lymphocytes and plasma cells (Fig. 6).
The osteodental lamina was for the most part replaced by connective tissue that showed a massive infiltration with lymphocytes and dense vascularization. Only islands of dentin and lamellar bone tissue could be detected, containing fatty tissue and plasma cells.
The thin layer of connective tissue next to the optical cylinder was covered by a layer of woven bone. Dentin was found only in the temporal half of the specimen and was replaced by bone and connective tissue in the nasal part (Fig. 7). Adjacent to the dentin, the alveolar-dental ligament was found to be preserved in some parts; in other areas, the dentin was observed in a position directly adjacent to lamellar bone. Furthermore, sections showed fatty tissue and vascularization, but no signs of acute or chronic inflammation could be detected.
Implantation of a keratoprosthesis can certainly be considered as the last resort to restore vision in patients with profound corneal blindness not amenable to conventional keratoplasty. Several different devices were developed over the course of the past two centuries and implanted in the past decades with variable but growing success. The most critical points regarding long-time performance of the keratoprosthesis implants are both the stability and biocompatibility of the supporting element. The OOKP according to Strampelli and modified by Falcinelli makes use of a biologic support consisting of a longitudinal section of one of the patient's teeth (dentin) that is also supported by the surrounding alveolar bone tissue (Fig. 1). Compared with other devices, favorable long-term results have been reported and independently confirmed. 1–8 Over the years, however, some of the patients have demonstrated a decentration of the initially well-aligned optical cylinder owing to a slow reduction of the osteodental lamina (Fig. 4), sometimes dramatically complicated by a loss of the optic cylinder (Fig. 5;Tab. 1). The mechanisms of the watertight integration of the osteodental lamina onto the ocular surface (probably via localized, circular scar formation after the surgical fixation of the newly formed periosteum of the alveolar bone on the cornea close to the limbal region) and the changes taking place thereafter over longer periods of time are not yet fully understood because histologic examinations of functional OOKPs are lacking, and pathology of extruded or lost laminae is only rarely described. 9–11
The maintenance of the alveolar-dental ligament seems to play a pivotal role in the preservation of the lamina itself. No ligament was found to be left in cases 1 and 2, well in accordance with the teachings of Falcinelli (personal communication), with the specimens showing inflammation and remarkable destruction of bone and dentin, whereas in case 3, those sections with a preserved ligament displayed unchanged dentin and mature lamellar bone. No signs of inflammation were detectable in the explanted prosthesis of this patient. It was previously postulated that a retained ligament within the osteodental lamina serves as a barrier against downgrowth of the epithelium between the optic cylinder and the tooth. 11 This is similar to the physiologic situation in the oral cavity in which this structure prevents the ingrowth of oral mucosa between tooth and alveolar bone, making it the only place in the body with an exposed, compact, pressure-resistant, and basically cell-free surface. Considering this particular aspect, covering the osteodental lamina with a buccal mucosal graft would be more physiologic and therefore possibly a more stable condition compared with coverage with lid skin in the second, less frequently performed transpalpebral approach, when sufficient mucosa is not available for grafting. One can assume that slow epithelial downgrowth, which probably also promotes invasion of microorganisms, could be the beginning of a cascade resulting in inflammation of the tissue, finally leading to destruction of the tooth via activation of osteoclasts.
In a previously reported study using spiral computed tomography and three-dimensional reconstruction of the implants, we found a reduction of the bony laminar substance to a lesser or greater extent in almost all of our patients with OOKP, mainly in its lower (caudal) part. 12 The mechanisms that trigger this probably noninflammatory dissolution of bone and dentin and the transformation of dentin into bony tissue that we now report are not completely understood.
One possible explanation could be the change in the direction of the force that continuously acts on the tooth and bony supporting tissue: physiologically, the tooth is stabilized by the alveolar-dental ligament with pressure on the tooth being transformed into tension on the bone tissue through a special arrangement of the fibers of the ligament. This tension induces osteoblastic bone formation activity, counterbalanced by the osteoclastic bone resorption. In the special situation of OOKP, the originally vertical orientation of the ligament becomes horizontal. Accordingly, the force created by the lid pressure (mostly from above) is unevenly distributed, with a “separating tension” on the fibers in the cranial part of the ligament and a compression (and thereby reduction of stress) in the caudal half. Consequently, this could lead to a relative dominance of bone resorption.
Whether, and in what directions, these slow transformation processes are influenced by the initial diagnoses, the age of the patients, surgical techniques, and other yet unknown parameters needs further investigations in larger series of patients.
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