Can scar tissue cause chin implant complications 4 years post-op?

It would help to know whether your Medpore implant is a custom-made implant or "semi-custom."
An actual custom-made Medpore implant is fabricated and fitted onto a physical model of your facial bone anatomy by the manufacturer.
By semi-custom, I mean an implant manipulated and molded by your surgeon while in the OR using various methods.
Depending on the implant type and placement, there is the possibility that the edges of the Medpore implant (which are hard and relatively thin) could be causing discomfort and possible injury to the surrounding tissue when you press on it while you're asleep or when you apply force to the outside of the chin.
It is impossible to know with certainty what the current issue is without performing a thorough examination.
You have options if you ever decide to correct, replace or remove your current implant. I hope the swelling and soreness subside, and I also hope you are feeling better.

If the pain has been unchanged for four years and that it’s just the way you perceive having a medpore implant.
If it’s a recent change then it warrants and evaluation.
If any area suddenly becomes very tender than medical intervention should be had right away.
If the answer from your doctor is insufficient then I suggest scheduling a proper in person second opinion consultation where you can be examined.
Medpore implants can be a little challenging to remove compared to solid silicone implants.
Best,
Mats Hagstrom MD