Roslinah M., Wan Hazabbah W.H., Md Salzihan M.S., Suzina S.A.H.,
The use of orbital implant to replace the volume lost after enucleation or evisceration was a major breakthrough in anophthalmic socket surgery. An integrated orbital implant has been advocated to reduce incidence of postenucleation/evisceration socket syndrome PEESS . This material typically have multiple interconnecting pores which allows fibrovascular ingrowths into the implants.
Such implant should reduced incidence and severity of infection due to availability of immune responses, allow soft tissue connection between the extraocular muscles and results in better motility, reduced implant migration and extrusion .The most common porous orbital implants used today includes hydroxyapetite, porous polyethylene and aluminiumoxide. Schmidt had introduced bone dHA orbital implant in 1899. Jordan and co-worker documented that this implant has pore sizes varies from 300 to 600 µm . The pore size appears to have effect on rate of fibrovascular ingrowth which occurs more rapidly in implants with 200 µm than implants with 500 µm .Porous polyethylene implants (Medpor®) were availablesince 1991. It has pores similar to hydroxyapetite but less uniform in size and more irregular in shapes. The commercially available Medpor® has pore size 100-500 μm and 125–1000 μm.
Material and methods.
14 New Zealand white rabbits were enrolled and divided using single block randomisation. Group A (n=7) were implanted with bone dHA and group B (n=7) with Medpor®. Bone dHA implants were synthesised from local bovine femoral head and prepared at National Tissue Bank, USM. On day 42, the rabbits received euthanasia for enucleation. Implants were examined by an assigned independent examiner and prepared using undecalcified method . After fixation, the samples were dehydrated, embedded in metacrylate solution (Technovit 720 VCL, Kulzer, Wehrhein, Germany), underwent polymerisation and were sliced between 0,1–0,3 mm thick.The sections were stained with eosin and haematoxylin and examined under light microscope. The grade of inflammation and fibrovascular ingrowths maturity were evaluated based on standardised histopathological grading as described by Chung et al. (2005) and Tiennen et al. (2003) respectively [6, 7]. Five readings of fibrovascular depth were measured based on average depth/average radius of the implant as described by Jordan et al. (2004) . Statistical analysis was done with SPSS 11.0. Data was analysed with statistical significance set at p<0,05.
In both groups, tissue ingrowth were visible in between interconnecting channels but the central most area were spared in Medpor® (Fig. 1, 2).Grade 1 inflammatory reaction was observed in two (28,6%) specimens with bone dHA but none in Medpore®. Four (57,1%) specimens demonstrated grade 2 inflammatory reaction in both groups. Remaining specimens showed grade 3 inflammatory reaction. Both groups demonstrated fibrovascular ingrowth after six weeks of implantation. Bone dHA demonstrated higher degree of fibrovascular maturation while Medpor® demonstrated variable degree of fibrovascular maturation. Grade 5 maturation was seen in 5 (71,4%) and 3 (42,9%) specimens with bone dHA and Medpor® respectively. Two specimens from each group demonstrated grade 4 maturation. Lower grade of maturation seen in remaining 2 specimens with Medpor® (grade 2 (1), grade 3 (1)). However, there was no statistically significant difference in the grade of fibrovascular maturation between groups (p=0,559). All specimens with bone dHA demonstrated 100% penetration of fibrovascular ingrowth. One specimens with Medpor® demonstrated ingrowth less than 50% while the remaining had ingrowth more than 50%. Statistical analysis showed significant difference in depth of fibrovascular ingrowth between group (p=0,001).
Bone dHA group demonstrated 5 (71,4%) subjects with higher grade of fibrovascular maturation (Grade 4 and 5) compared to 4 (57,1%) subjects in Medpor® group. In contrast, 3 subjects with lower grade of fibrovascular maturation were seen in Medpor® and 2 subjects in bone dHA. As there was no statistically significant different in the grade of maturation, we suggested that this study demonstrated a similar biocompatibility of both implants. Similar finding was reported by Chung and associate . Their study demonstrated that Hydroxyapetit (HA) implant had grade 5 fibrovascular maturation at three weeks post implantation. They suggested that the pore sizes and distribution of the implant being the determinant factor. Fibrovacsular ingrowth was observed to develop in centripetal progression towards the centre of implants. All subjects with bone HA displayed more than 50% amount of fibrovascular ingrowth after six week of implantation. Six subjects (85,7%) had complete 100% fibrovascular ingrowth and one subject had 80% ingrowth. These results are consistent with report by Thakker and co-worker . They documented a complete fibrovascularization in HA implant at six weeks. In contrast, porous polyethylene implant reached complete fibrovascularization at twelve weeks. Jordan and associates reported that fibrovascularization in Medpor® appeared to occur in stepwise increment with 100% fibrovascularization at twelve week . Our result correlates with this finding as only 4 subjects (57,1%) demonstrated complete fibrovascularization at the end of study period. Hsu and colleague implanted Medpor® in rabbits and reported that the extents of fibrovascular ingrowth at six week are at average of 76,3% which is lower than our study with average of 82,9% . Rubin and co worker compared the extent of fibrovascular ingrowth in these implants. They also reported complete fibrovascularization at six weeks in HA and twelve weeks in porous polyethylene . The size and distribution of pores determine the rapidity of fibrovascular ingrowth. Generally, larger pore size will allow rapid fibrovascularization. Our study demonstrated more subjects with bone dHA achieved complete fibrovascularization after six weeks. This is in keeping with gross specimen observation which display larger pore sizes (300–600μm) in bone dHA than in Medpor® (100–500μm). However, previous reports documented that pore sizes above 700 μm, fibrovascular ingrowth is sluggish due to insufficient supporting structure, increased fragility, decreased intensity and higher risk of infection [9, 11, 12]. Rubin and co-worker analyzed the rate and extent of fibrovascular ingrowth in HA and porous polyethylene (PP). They reported that HA implants were vascularized most rapidly. Complete vascularization was observed in the large pore PP at 12 weeks of study period but not in small pore PP. In our study, the assessment of fibrovascular ingrowth was confirmed through microscopic histopathological examination which would be impossible in clinical practice. There are many reported studies that advocated the use of postcontrast magnetic resonance in assessing fibrovascular ingrowth in the porous orbital implant. They demonstrated that the MRI study was identical to the histological fibrovascularingrowth pattern [13, 14].
Locally synthesized bone dHA orbital implant did have similar biocompatibility as Medpor® implant. In view of comparable results with supportive imaging modality, we suggested an extended study with MRI imaging to assess fibrovascularingrowrth in this proposed bone dHA to provide more credit to this product. Hence, adding more option in search of most ideal orbital implant.