Butt implants are robustly constructed. They are made of semi-solid, rubber-like silicone material. On the other hand, breast implants are not durable. A breast implant is made of a silicone shell that is filled with silicone gel or saline solution.
Furthermore, the anatomy of the buttocks is different than the anatomy of the breasts. Breast implants are designed for the breasts only, and butt implants are designed for the buttocks only. The shape of the breasts and buttocks are different, and they have different aesthetic requirements. To meet those unique requirements, different implants have been created for the buttocks and breasts.
Breast implants are prone to ruptures and leaks. The application of undue pressure or force on the breasts can trigger rupture or leak of the breast implants. Also, breast implants might have to be removed after a few years. In light of these facts, it is never wise to use breast implants to augment your buttocks.