Penetrating keratoplasty has become the treatment of choice in patients with blindness due to loss of corneal transparency. However, transplantation of cadaveric corneal tissue has severe limitations in chronic inflammatory diseases such as pemphigoid, Stevens-Johnson syndrome, and chemical burns with frequently devastating clinical and visual outcomes. The osteo-odontokeratoprosthesis (OOKP) according to Strampelli with the modifications by Falcinelli provides good long-term results in restoring vision in those patients who are not amenable to any other type of conventional corneal transplantation. 1–3 The keratoprosthesis (Kpro) consists of an optical cylinder made of polymethylmethacrylate (PMMA) and a supporting element prepared from a single-rooted tooth with its surrounding alveolar bone (Fig. 1). This osteodental lamina is covered either with buccal mucosa or with the patient's own lid skin in a transpalpebral approach (only used in cases in which sufficient and healthy mucosa is not available for grafting) (Fig. 2 A,B). The surgical technique was described in detail previously. 4–7 The most critical feature for the long-term stability of this kind of implant seems to be the maintenance of the lamina because reduction in dimensions, if sufficiently severe, can result in aqueous humor leakage, inflammation, and extrusion of the optic cylinder. Its stability and preservation in this series of patients were therefore studied with spiral computed tomography (CT).
MATERIAL AND METHODS
Computed tomography of the orbit was performed in nine patients after successful OOKP surgery. Indication for surgery included ocular mucous membrane pemphigoid (three patients), Lyell's syndrome (two patients), severe dry eyes in graft versus host disease (one patient), and severe chemical burns (three patients). Eight patients received an autologous osteodental lamina, whereas in one patient, the tooth was harvested from a blood-related donor (daughter) followed by long-term postoperative immunosuppression with cyclosporin A. In four eyes, the osteodental lamina was covered with a graft from the buccal mucosa and in five eyes with the patient's own lid skin after removal of the tarsal plate using a “transpalpebral” approach. Mean time from surgery to examination was 4 years (range, 1–6). Five patients were female, and four were male. The age of the patients ranged from 32 to 75 years (mean, 48.4 ± 16.1) (Table 1).
Spiral CT scans (Toshiba X-press/SX, Toshiba Corporation, Tokyo, Japan) were performed in the transaxial plane from the level of the floor to the roof of the orbit. The patients were scanned in the supine position using a 2-mm slice thickness and a 2-mm table feed (pitch = 1). The images were reconstructed in 1-mm increments.
The volumetric data were postprocessed on a Toshiba Xtension workstation (Sun Sparc Station 20, Sun Microsystems Inc., Palo Alto, CA). Two-dimensional (2D) data sets in an axial projection were acquired in all patients in a bone window (Fig. 3A). Three-dimensional data sets were postprocessed by using a shaded surface display technique (matrix, 512 × 512).
In each patient, a 3D surface reconstruction of the OOKP was performed, not only including the orbital walls (Fig. 3B) but also after segmentation of the orbital walls (Figs. 4 and 5A–E). The 3D images were rendered in six standardized projections: anterior, right, left, craniocaudal, head-feet, and posterior. The dimensions of the osteodental laminae were measured (Fig. 4) and compared with measurements taken at the time of surgery.
According to the degree of bone reduction, the laminae were classified using a score that grades bone reduction (Table 1): osteodental laminae that presented with no reduction compared with preoperative measurements were classified as belonging to group 0, and laminae with minor reductions were classified as group 1. Laminae with moderate reductions of dentine and bone tissue but still displaying a tight fixation of the optic cylinder were considered group 2. Those laminae showing severe destruction of tissue and therefore being at high risk of a spontaneous loss of the optic cylinder were scored as belonging to group 3.
In all patients, spiral CT scans with 3D surface reconstruction of the OOKP were feasible with an impressively high resolution, making visible even details of the osteodental lamina as well as the Baerveldt glaucoma implants (in patients 4 and 9;Fig. 4). Minor reduction of the lamina could be found in practically all the patients, mainly in the anterior and inferior part (Table 1). Two patients showed almost no reduction of the dentine and bone tissue compared with intraoperative measurements (patients 2 and 7 in group 0). Two implants were classified as belonging to group 1, displaying minor reduction of the osteodental lamina. Evaluation of the CT images of three implants indicated a moderate reduction of their biologic support, still preserving sufficient stability and integrity of the lamina-cylinder complex. Two patient showed remarkable destruction of bone and dentine (group 3): One patient (75 years old, ocular pemphigoid, own tooth, lamina covered with lower lid skin, follow-up time of 2 years) showed a complete resorption of the inferior half of the osteodental lamina (patient 8;Fig. 5E). A 38-year-old male patient displayed more “diffuse” bone resorption 3 years after OOKP surgery, having a history of skin retraction around the optic cylinder, followed by fistulation and retinal detachment (patient 6;Fig. 5C). No correlation of the degree of reduction with age, diagnosis, or length of follow-up could be detected by statistical analysis.
The implantation of a Kpro is usually performed as a last surgical attempt to restore vision in patients not amenable to conventional lamellar or penetrating corneal transplantation or other types of ocular surface reconstruction procedures such as amniotic membrane or stem cell grafting. A vast number of designs and materials of Kpros with basically three different methods of insertion has been developed and implanted in patients over the past two centuries with quite variable (and often unreported) long-term success. The Dohlman-Doane Kpro is one of the few devices that are quite commonly and successfully implanted nowadays, consisting of a double-plated device made of PMMA. 8 Postoperative complications such as glaucoma, endophthalmitis, or extrusion of the implant, however, are still quite frequently encountered. 9 In the past decade, soft, flexible materials resembling native cornea in many aspects (e.g., dimensions) have been evaluated, tested, and further developed to reduce biomechanical stress at the points of attachment as much as possible because it is thought that this could promote stromal melting. The AlphaCor artificial cornea (Argus Biomedical, Australia), a Kpro made of poly-2-hydroxyethylmethacrylate, displays acceptable margins of safety in a recently presented human clinical trial with a follow-up up to 2.5 years 10 but seems to be contraindicated in patients with a history of ocular herpes simplex infection 11 or conditions of chronic inflammation, such as Stevens-Johnson syndrome. The OOKP, as reported by Strampelli and modified by Falcinelli, however, shows good long-term results for these groups of patients also. 12–14 Nevertheless, the surgeon must always be aware of eventual signs of resorption of the osteodental lamina. The potential risk associated with a reduction of this biologic support calls for a continuous assessment of its state. B-scan echography and examination with an ultrasound biomicroscope (UBM) may represent valuable instruments in the evaluation of the anterior segment prior to OOKP surgery but are of limited value only in the postoperative follow-up because the lamina causes sound attenuation and shadowing. 15 Magnetic resonance imaging (MRI) 16 as well as conventional CT 17 make 2D reconstruction of transaxial or coronal planes possible, allowing a vague impression of the condition of the implant. Up to the present, no clinical method was available that allowed 3D surface reconstruction and precise measurements of the dimensions of the osteodental lamina in vivo. Thus, spiral CT seems to be a new and excellent diagnostic tool for this purpose.
Two patients in this series were considered to be suffering from severe reduction of the dentine and bone tissue. Both had OOKP surgery performed with the transpalpebral technique. As Strampelli had postulated, it seems probable that the firm connection between the buccal mucosa covering the alveolar bone and attached to the alveolodental ligament provides a tight seal similar to the situation at the gingival-dental contact zone. 7 Covering the osteodental lamina with lid skin seems to have less stability in the long term compared with the use of buccal mucosa, probably due to a reduced skin-periosteal attachment leading to earlier skin retraction that allows easier epithelial downgrowth between the optic cylinder and lamina. Invasion of microorganisms is then facilitated, finally leading to tissue destruction. In patient 6, 30 months after OOKP surgery, this cascade was followed by fistulation and retinal detachment, reducing his visual acuity from 20/20 to perception of light only. This development was partly the consequence of very poor compliance, also caused by the underlying psychiatric disease of the patient. The 3D CT scan displayed diffuse, irregular dissolution of the osteodental lamina 38 months after OOKP surgery (Fig. 5C). Twelve months later, the optic cylinder spontaneously extruded.
Patient 8 also had a history of skin retraction around the optic cylinder, exposing the upper surface of the lamina. Repair of the skin had to be performed three times postoperatively. Smears taken from the area of the skin defect at the time of repair showed growth of enterococci and staphylococci. In the CT scan, complete dissolution of the inferior half of the lamina was apparent, whereas the upper half displayed no changes in dimensions and thickness. At time of this examination, the PMMA cylinder was still tightly fixed. Four months later, however, the patient was admitted after acute vision loss due to extrusion of the optic cylinder. In this case, the best corrected visual acuity of 20/60 was regained by implantation of a Pintucci Kpro and maintained for more than 14 months of follow-up until the present 18 (because this patient lacked any more teeth usable for a second attempt at OOKP surgery).
Histopathologic analysis of explanted OOKPs confirms the crucial role of a chronic inflammation process in dissolution of the osteodental lamina. 19–21 The described moth-eaten aspect of the alveolar bone and dentin is in agreement with the CT findings of patient 6 who had a similar appearance. Comparable findings were recently reported by our group 22: bone tissue was found focally replaced by connective tissue, dentine was reduced, and a layer of immature woven bone surrounded the remnants. The preservation of the alveolodental ligament seems to play a pivotal role in the preservation of the osteodental lamina itself because in places where the ligament was found to be preserved in a stable manner, no destruction of bone tissue or dentine could be detected. This ligament probably serves as a kind of seal against epithelial downgrowth between the optical PMMA cylinder and the tooth. 21
However, in some cases with focal reduction of dentine and bone tissue (Fig. 5A,B), no clinically significant indication for an inflammatory origin could be detected. In our series, this resorption mostly occurred in the lower part of the lamina. This could partly be due to the fact that the prosthesis was always implanted with the thicker part located cranially to keep more bone tissue in the upper half should a general reduction of the lamina take place. On the other hand, a predominance of osteoclastic bone resorption over osteoblastic bone formation could be effective in the inferior part of the lamina: we presume that in the upper part of the lamina, induced by the special arrangement of the alveolodental ligament fibers, the pressure created by the upper lid is transformed into tension acting on the bone tissue. Assuming that this kind of stress is necessary for induction of bone formation, a balance between formation and resorption is rather likely in the upper part of the lamina in contrast to the lower part, where lack of tension might rather promote a shift toward a gradual resorption of bone tissue.
Despite the excellent results of OOKP surgery reported by Falcinelli et al. 1 and independently confirmed by others, 14 our results underline the necessity of life-long follow-up for these patients. Because ultrasound scan cannot be used for accurate evaluation, spiral CT seems to represent a practicable diagnostic tool for the evaluation of laminar reduction in OOKP.
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