Implant movement

Implant and prosthesis movement are important aspects of the overall cosmetic appearance after enucleation and are essential to the ideal objective of crafting a lifelike eye similar in all aspects to the normal fellow eye.[4][18] There are several theories of improved eye movement, such as using integrating prosthetic material, pegging the implant, covering the implant (e.g. with scleral tissue), or suturing the eye muscles directly to the prosthetic implant. The efficiency of transmitting movement from the implant to the prosthesis determines the degree of prosthetic motility. Movement is transmitted from traditional nonporous spherical implants through the surface tension at the conjunctival–prosthetic interface and movement of the fornices. Quasi-integrated implants have irregularly shaped surfaces that create an indirect coupling mechanism between the implant and prosthesis that imparts greater movement to the prosthesis. Directly integrating the implant to the prosthesis through an externalized coupling mechanism would be expected to improve motility further.[7]

Despite the reasoning stating that hydroxyapatite orbital implants without a motility peg would yield a superior artificial eye motility,[19] when similar surgical techniques are used unpegged porous (hydroxyapatite) enucleation implants and donor sclera-covered nonporous (acrylic) spherical enucleation implants yield comparable artificial eye motility.[5][7] In two studies[5][20] there were no differences in maximum amplitude between hydroxyapatite and acrylic or silicone spherical enucleation implants,[5] thus indicating that the implant material itself may not have a bearing on implant movement as long as the muscles are attached directly or indirectly to the implant and the implant is not pegged.[4] The motility of a nonintegrated artificial eye may be caused by at least two forces. (1) The rubbing force between the posterior surface of the artificial eye and the conjunctiva that covers the implant may cause the artificial eye to move. Because this force is likely to be approximately equal in all directions, it would cause comparable horizontal and vertical artificial eye amplitudes. (2) An artificial eye usually fits snugly in the conjunctival space (possibly not in the superior fornix). Therefore, any movement of the conjunctival fornices will cause a similar movement of the artificial eye, whereas lack of movement of the fornices will restrict its motility.[5] Imbrication of the rectus muscles over a nonintegrated implant traditionally was thought to impart movement to the implant and prosthesis. Like a ball-and-socket joint, when the implant moves, the prosthesis moves. However, because the so-called ball and socket are separated by layers of Tenon’s capsule, imbricated muscles, and conjunctiva, the mechanical efficiency of transmission of movement from the implant to the prosthesis is suboptimal. Moreover, the concern is that imbrication of the recti over nonintegrated implants actually can result in implant migration.[21] The recent myoconjuctival technique of enucleation is an alternative to muscle imbrication.[4][22][20]

Although it is generally accepted that integrating the prosthesis to a porous implant with peg insertion enhances prosthetic movement, there is little available evidence in the literature that documents the degree of improvement.[7] And although the porous implants have been reported to offer improved implant movement,[23] these clearly are more expensive and intrusive, requiring wrapping, and subsequent imaging to determine vascularization and pegging to provide for better transmission of implant movement to the prosthesis, and also are prone to implant exposure.[4]

Age and size of the implant may also effect the motility, since in a study comparing patients with hydroxyapatite implants and patients with nonporous implants, the implant movement appeared to decrease with age in both groups. This study also demonstrated improved movement of larger implants irrespective of material.[7]