In what event does a chin implant get displaced?

 Hi, I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  
 The silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  
 I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty.  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  
In my opinion and experience there is little if any risk of a silastic chin implant becoming displaced when placed through an incision under the chin and below the periosteum.  The implant will become secure in as little as 1 month after which it won't move.  There's no need to secure a silastic chin implants with sutures and screws when the following conditions are met:
 * Select a silastic chin implant ( I prefer the EAC) that doesn't have too much projection (thickness) as this becomes a problem stabilizing the implant under the periosteum when the implant is too thick.
 * Place the implant through a small curved incision under the chin which allows direct access to the periosteum (under the mentalis muscle) while preserving the muscle attachments.  This prevents upward migration of the implant which can occur when the intra-oral approach is used that severs the muscle attachments.
 * While the center or body of the chin implant should be placed at a perpendicular angle the chin bone in order to provide the proper forward projection to the chin, the "wings" of the implant should be at a lower level and follow the inferior (bottom) edge of the jaw line on either side of the chin.  When properly dissected and placed, this will be below the mental nerve foramen and be just wide enough of a dissection to accommodate the tapering wing.  There isn't enough room in this technique to allow the wings to migrate upward and contact the mental nerve.  In contrast if the dissection, along the sides of the chin (jaw line) are performed too high...this will place the mental nerve in danger and if the side dissection is too wide, the wings will have a space within which to migrate upward.
 * We have all of our chin implant patients avoid touching and feeling their chin implant for 1 month post op.  We also ask that they sleep on a U-shaped airline pillow for the same time.
  Our experience when following the technique described above is that the silastic chin implant does not need to be secured with sutures or screws and does not move.  I have placed silastic chin implants in military, SWAT, LE, professional fighters, as well as actors and many regular people who are active.  In addition, I have had the opportunity on numerous occasions to replace silastic chin implants (placed by others) that had been screwed into the bone.  During the replacement it was evident that the metal screws pushed right through the soft silastic implant as it was tightened down on the hard chin bone.  Which makes perfect sense.  So in the long run, these fixation methods alone are no guarantee that the implant won't move.  Proper placement, proper implant pocket creation and meticulous closure of all the tissues layers is what is required in my humble opinion.
Hope this helps.