Can symmastia develop years after procedure and am I at risk? Almost 4 years post op, 350cc (photos)

Symmastia is when the implant pockets are essentially touching - across the sternum. This is very uncommon if the implants are sub muscular. And also very unlikely to occur years later, unless there's been a trauma.
Your surgeon was spot on by mentioning your mild case of pectus excavatum. So your chest wall anatomy is likely the cause for your implants sitting so close together. This can cause the implants to lean in centrally. This tends to make "braless" cleavage more feasible.
I think your results looks great at 4 years and I wouldn't be overly concerned about symmastia. Follow up with your plastic surgeon to have an in person consult.

While symmastia can occur years after surgery, it is a rare occurrence. Based on your photos, your cleavage tissue appears to be well-attached to the sternum, with no signs of early-stage symmastia. If you'd like to take extra precautions, consider avoiding bras that push your breasts together.

Dr. B.

Dear Bella_209,
symmastia is a rare breast condition that occurs when soft web-like breast tissue pulls breast implants towards each other, until the breasts meet uncomfortably at the center of the chest near the sternum (the body’s midline point). Given this abnormal union, symmastia is known by many other names, including breadloafing, kissing breast implants, and the uniboob.Unfortunately, this condition causes the surrounding skin and pectoral muscles to literally lift off the breast bone as the implants are pulled together. This consequently causes pain and discomfort, as well as obvious distortion of the breasts, which can only be disguised with clothing that does not show cleavage.You should consult with a board certified plastic surgeon for proper assessment. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Yes, symmastia can develop in time. It is possible that the tissue over the sternum cannot tear and symmastia can occur.

Best Wishes,

Gary Horndeski, M.D.

Because of your pectus, your implants will tilt inward. I doubt you will get symmastia, but your breasts will touch because of your sternum. If you are concerned, you can go down in size and width.

Yes, symmastia can occur years after surgery. However judging from the photos provided you currently do not show any signs of it and without major changes will not likely have an issue with symmastia.

You have nothing to worry about at this point. What you see is what you get. After about 4 months things don't change unless you do more surgery