Allen implant is an ocular implant in PMMA for primary enucleation composed by a hemisphere with a fenestrated crown to allow the rectus muscles crosswise suture. In this way the implant can transmit movement to the prosthesis.
The higher limit of this implant is only the ocularist technical ability: the hard material forces the ocularist to make a prosthesis with the rear surface shaped so as to align perfectly with the front surface. A prosthesis with rear surface not properly shaped would cause an excessive traumatism of the cover tissue which would remain compressed between prosthesis and implant. It can cause a decubitus with a very high risk of expulsion of the implant.

Even if this implant is used for decades, the following reasons demonstrate that Allen implant is still a satisfactory choice for primary enucleation: satisfactory motility, low incidence of superficial tissue lesion and still lower incidence of extrusion observed in Long-Term Follow-Up.