45, silicone, under muscle, implant changed shape after 18 years, do I need them replaced? (Photos)

Since your implants are 18 years old, any change should be evaluated. MRI or ultrasound can help to see if they are ruptured, and if they are ruptured, then they need to be replaced. If not, then you don't have to worry as much. I would suggest a consultation with a plastic surgeon to help guide your decision making process and also to help arrange the MRI or ultrasound.

Dear Happi2bme,
I understand your concern. However, without a proper assessment, it would be difficult to determine what is wrong. It is best that you visit your plastic surgeon for further assessment or ask for a second opinion. Only after a thorough examination, you can get proper recommendations and advice.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

One possibility is that your implant has flipped front to back. A plastic surgeon can give a better idea of the possibilities with an in-person exam, and they could confirm it with ultrasound. If this is the case, the implant could be re-rotated but if they are 18 years old it is better to replace both anyway.

I would seek out a consultation with a plastic surgeon and get a full exam.
Silicone implants can have silent ruptures - depending on the brand your risk is likely around 20-30% at this 18 year mark. Ultrasound or MRI can be used to assess the implants integrity.
If they are soft and not ruptured there's no immediate need for surgery but if there are signs of a capsular contraction ( your photos don't support this) or it is ruptured it would be recommended to replace them.

I would first ask your GP to schedule an ultrasound and possible mammogram to see if the implants are intact or ruptured. Then, go to a plastic surgeon and discuss your options. It appears that one of your implants has torn through the capsule and is poking out some.