Can sleeping on your stomach make your chin implant shift?

At this point, it is unlikely you would shift your implant. However, I recommend following up with your surgeon about your concerns. I hope this helps. 

After three months of healing and full encapsulation of the implant, it is unlikely that pressure from sleeping will cause it to become malpositioned. But to be absolutely cautious a few more months of not sleeping on your stomach wil lower even this remote risk less likely.

 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".  
 Proper placement of a 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 (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  We ask all of our chin implant patients use a U-shaped pillow when sleeping for the first month following the surgery.  After that time, the implants are secured by the periosteum and should not move unless an intra-oral approach was used to place the chin implant.  I prefer placement of a  silastic chin implant through a small, curved incision under the chin (submental) instead of through the mouth for the following reasons:
*The submental approach is sterile while the intra-oral approach is not.
*The submental approach requires limited dissection as it is much closer to where the chin implant needs to be placed along the center (front) and lower most sections of the chin on each side (where the wings are placed).  The intra-oral approach by contrast requires dissection all the way down the entire section of the chin in order to reach the same areas for chin implant placement.  This leads to several issues in my humble opinion.  The attachment of the chin muscle to the bone must be cut during the tissue dissection, the sensory nerves (mental) and motor nerves (marginal mandibular nerve) that move the muscles of the lips and mouth) are all in much greater jeopardy of injury because of the added dissection required.
*The added areas of dissection allow upward migration of the implant as well as an increased risk of nerve injury.  Damage to a sensory nerve will create numbness and or an ache type discomfort while injuring the motor nerve will result in an inability to "lower" the corner of the mouth and lower lip.  While some of these symptoms can rarely be experienced in the immediate post op period as the mentalis muscle accommodates the chin implant....these symptoms should be gone within a few weeks post op.
 If you are experiencing any of the above symptoms you may want to discuss these with your Chin Implant Surgeon and perhaps even get a 3-D scan of the face or lower face that will show the position of the implant.  If some nerve injury has occurred, it can take a year or 2 for this to improve to whatever extend that will occur.  In my opinion and experience there is 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.