Breast reconstruction using tissue expanders and implants are done under the pectoralis muscle. This is either with or without the use of acellular dermal matrix.
If tissue expanders and implants must be used as your mode of reconstruction, as opposed to using your own tissues, the appropriate technique is ONLY to go under the muscle. This will provide the most tissue coverage and support for the implant. Having the appropriate tissue coverage is key for the best long-term result cosmetically-speaking, as well as to protect you from contamination, infection, skin necrosis, and extrusion of the implant.