Types of Orbital Implant
An orbital implant is buried under pink tissue lining the socket, to fill space previously occupied by an eye.
There are four broad categories of orbital implant:
- Semi integrated
- Non integrated
- Dermis fat graft.
These implants are made of porous material, meaning tissues can grow into the implant. Examples are hydroxy-apatite (coral like), polyethylene (Medpor) and Aluminium oxide (Alumina).
A recent development is coated implants such as Bioeye, where the implant has a soft coating to which the muscles can be attached.
Advantages: excellent mobility; stays in the same position; rarely migrates; can be pegged.
Disadvantages: risk of infection and extrusion is higher than non integrated; expensive.
Semi Integrated Implants
These implants are not porous but have large gaps through which the muscles are pulled and tied. The Allen implant is the best known semi-integrated implant.
Advantages: Inexpensive; good mobility.
Disadvantages: risk of exposure is higher due to the irregular surface rubbing against the artificial eye.
Non Integrated Implants
Mde of acrylic, these implants are often encased in a capsule of tissue due to the body’s healing response. There is little or no contact with the muscles. An example is the acrylic ball implant.
Advantages: Inexpensive; very low risk of exposure.
Disadvantages: Less movement; can migrate within the orbit.
Dermis Fat Graft
Fat from around the navel or buttock is used to fill space formerly occupied by the eye. The graft contains deep layers of skin and the fat below it, measuring about 2cm in length, width and depth.
Muscles cannot be attached to a fat graft.
There is no risk of rejection as this graft uses the patient’s own tissue. These grafts are often the best option when a material implant is exposed or infected.
Advantages: fat grafts grow with the child, matching growth of the other eye. Inexpensive; very low risk of exposure
Disadvantages: Less movement; when done in adulthood, they often shrink (atrophy) with time, with a less satisfactory cosmetic appearance.
Impact on the Artificial Eye
The type of implant does not affect quality of the artificial eye. Movement of the eye is better with an implant than without.
Range of movement is expressed as a percentage of movement of the other eye. Normal movement is 30% upgaze, 50% side to side, and 70% downgaze.
In some instances, a peg can be connected to the implant after enucleation, to improve mobility of the artificial eye. A small hole is drilled into the implant and a peg inserted. The artificial eye then connects to the peg, creating more natural movement via the tiny ball and socket joint.
Before this procedure, a bone scan or MRI is done to see if the socket is completely healed. If the child has a known or suspected constitutional RB1 mutation, MRI is preferable as bone scan involves radiation exposure, contraindicated in these children.