Porous orbital implants and their behaviour during drilling

The use of porous spherical orbital implants enables ocular prosthesis motility to be improved if necessary by subsequent drilling of the implant and coupling it to the prosthesis with a peg. We compared the effect of drilling at different speeds on hydroxyapatite and porous polyethylene (Medpor) spherical implants. The implants were drilled at fixed speeds of between 15 and 2000 revolutions per minute. The samples were then viewed in an electron microscope and photographed. When hydroxyapatite is drilled the porous structure of the implant is maintained although a precise hole with well-defined walls was not produced. This contrasts with porous polyethylene where the porous structure is lost although a precise drill hole is created. This drilling technique has been used successfully in patients with hydroxyapatite implants, thus making possible stable epithelialisation of the drill hole. Our results confirm that the drilling of porous polyethylene using this technique is unlikely to be successful, as loss of the porous structure would prevent stable epithelialisation of the drill hole, resulting in exposure of the implant.