Polyethylene is a high-density, straight-chain hydrocarbon formed by polymerisation of ethylene molecules under high temperature and pressure. Medpor is a polyporous form (150–400 μm) of polyethylene that is manufactured by heating and compacting polyethylene granules into spherical shapes of different size. This porous character enables fibrovascular proliferation of orbital tissue, reduces the risk of migration, exposure and extrusion, and minimises the risk of infection. This material is also non-toxic, non-allergenic and highly biocompatible. It is not brittle, thus allowing muscles to be sutured directly to it with no need for sclera.1–4
Many studies have reported favourable surgical outcomes after Medpor orbital implantation.17–22
Medpor has a rough surface, which tends to cause erosion of Tenon's capsule and conjunctiva and eventually implant exposure. To compensate for this defect, other types of Medpor have been introduced. Medpor SST is a further refinement of the original polyporous polyethylene (Medpor). It has a smooth, porous anterior surface, which helps minimise late-implant exposures, and the suture tunnels allow for easy attachment of the rectus muscle without the use of an implant wrap. Medpor MCOI is cone-shaped, which makes it possible to provide additional volume in the orbit with a similar diameter implant. Medpor MCOI has more utility in patients with severe phthisis bulbi. Medpor is currently a very popular polyporous orbital implant material. The other orbital implant materials include hydroxyapatite and aluminum oxide.
However, unlike hydroxyapatite implants, only relatively small case series have been published on the exposure and complication rates of Medpor orbital implants. Karcioglu et al23
reported eight cases of conjunctival dehiscence exposure, five cases of fornix contracture and three cases of inappropriate volume replacement in 37 patients who underwent enucleation and Medpor orbital implantation due to retinoblastoma. Cheng et al24
reported that implant exposure occurred in up to one-third of patients who received Medpor orbital implantation over a 2-year follow-up period, and this was particularly common after MCP insertion. Shoamanesh et al25
reported postoperative complications in 32 patients who had received Medpor implants with a 14-year follow-up period. Baek17
reported five cases of implant exposure and four cases of superior sulcus deformity in 36 patients after evisceration, enucleation, or secondary orbital implantation during 2 years of follow-up. We studied the overall postoperative outcomes in 314 patients over 10 years of follow-up.
Our study showed only a 1% (3/314) incidence rate of Medpor orbital implant infection, and these three cases required an implant exchange. This rate is similar to the infection rate of the hydroxyapatite orbital implant, which ranges from 0% to 1.5%.26
Postoperative implant infection using Medpor is rare, limited to only a few case reports,5
probably because Medpor has a hydrophobic and negatively charged surface that acts as a protective envelope to inhibit the adherence of bacteria.28
In the present study, implant exposure occurred in 9.3% of patients who underwent enucleation and in 3.5% of patients who underwent evisceration. Alwitry et al16
reported the long-term follow-up surgical outcomes (6 years) of 106 patients who underwent spherical Medpor implantation, and reported that the implant exposure rate was 6.3% (5/80) for patients who underwent enucleation and 53.8% (14/26) for patients who underwent evisceration. The original reason for the surgery was different between the study of Alwitry et al16
and our study. The most common cause of anophthalmic surgery was trauma in both studies, but its frequency was different: approximately 30% in our study and up to 50% in the study by Alwitry et al
In both studies, the surrounding tissue around the eyeball was damaged by trauma, and the degree of damage affected recovery rate and the final surgical outcome. Therefore, a simple comparison of incidence rates between the two studies has no meaning. In addition, we included data on patients who received Medpor MCOI and Medpor SST, not just the spherical Medpor, which may have influenced our results, whereas the study by Alwitry et al16
only included data on patients who received the spherical Medpor.
The results showed similar postoperative complication rates, except the rate of fornix contracture between the patients who had received enucleation (6/43, 14.0%) and secondary orbital implantation (1/42, 2.4%). This result was caused by the fact that secondary orbital implantation was mostly considered when an unfit artificial eye was detected.
Yoon et al26
reported that the rate of orbital implant exposure in 802 patients who received hydroxyapatite orbital implantation with a 15-year follow-up was 2.1%. Shoamanesh et al25
found that the rate of exposure was 6% for 432 patients who underwent hydroxyapatite orbital implantation and 6.25% for 32 patients who underwent Medpor orbital implantation. Baek17
reported a rate of exposure of 13% for 36 eyes that underwent Medpor orbital implantation; however, all 36 eyes successfully recovered with a dermograft. Custer and Trinkaus30
reported that the exposure rates were similar between hydroxyapatite (5.1%) and Medpor (4.2%) when patients with retinoblastoma were omitted from the pooled data in a meta-analysis of porous orbital implant studies. These reports show that surgical outcomes vary according to factors such as operator technique and the status of the conjunctiva around the operation site. More studies may be needed to determine conclusively whether hydroxyapatite or Medpor is superior, because few studies have focused on patients who received Medpor orbital implants.
Other postoperative complications may also occur, including conjunctival abnormalities and lid problems. Yoon et al26
reported that conjunctival cysts and conjunctival wound dehiscence occurred in 0.2% and 3.5% of patients who received hydroxyapatite orbital implantation, respectively, but they did not receive pegging. No marked differences were observed between the study of Yoon et al26
and our study, which showed rates of 0.6% and 1.3% for conjunctival cysts and conjunctival wound dehiscence, respectively. Shoamanesh et al25
found that blepharoptosis occurred in 20.1% of patients who underwent Medpor orbital implantation, and this was the most common postoperative complication. Our study showed similar results; blepharoptosis was the most common postoperative problem, and its incidence rate was 10.5%. However, most cases of blepharoptosis successfully recovered after a blepharoplasty or other corrective operation ().
Summary of the major studies on porous orbital implants
MCP insertion was performed in 10.2% of the patients at our institute, which is a relatively low rate, and most underwent this procedure before 2002. MCP has been used to improve artificial eyes, but it may increase the infection rate of an orbital implant.26
Furthermore, unskilled insertion of an MCP requires repositioning or removal and re-insertion.31
Therefore, we do not typically perform MCP insertion if the motility of an artificial eye is satisfactory and the patient does not wish to do it.
In summary, we report a large case series of patients implanted with porous polyethylene orbital implants with an extended follow-up. We highlighted the previously undocumented general postoperative complications after Medpor orbital implantation during long-term follow-up, and no marked differences in the complications between hydroxyapatite and Medpor were observed. We also successfully resolved the postoperative complications associated with Medpor. Therefore, we suggest that Medpor produces tolerable surgical outcomes as an orbital implant because of lower material cost, convenience of the operative procedure and other advantages.