Many people just spell "CI" to mean "cochlear implant.
The original sign that was used for this concept was
to jab behind the ear with a "bent-V" handshape.
Some people do the sign with a "bent-U" handshape.
A short while back I noticed a 14-year old girl (who attends CSD-Fremont) using a "bent-U" on the cheekbone in-front of the ear (almost in the ear canal) to mean "HEARING AID." So I can see how using the "bent-U" could become popular behind the ear to mean "Cochlear Implant.
I notice that when I do the sign with a "bent-V" I tend to use a single movement--which causes the sign to mean "implanted." As in, "He has been implanted." Sort of makes it seem like we are talking about someone in whom aliens have taken an interest.
When I do the sign with a "bent-U" I tend to use a double movement which contributes to the sign functioning as a "noun."
Initially the use of "CI" was simply a "euphemism." As time passed, the sign "CI" gained widespread use.
Use a "bent V" handshape. Poke your head behind your ear.
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars