Is Macrolane filler injections safe for the buttocks? I already have butt implants.

Macrolane contains biocompatible hyaluronic acid which is naturally broken down (biodegradable) by the body. The clear gel is pure and stable with no animal derivatives.
Swelling and minor pain are the most common side effects experienced at the site of injection. Rare perioperative infections can occur.  Antibiotics are usually prescribed to inhibit this risk and an assessment is usually carried out before any treatment to see if the patient has any existing skin infections. 
Capsule contraction around an implant can also occur.