How do you treat a 'bed sore' that is open and penetrating the muscle?

Be sore. This type of wound requires the expertise of a wound care team. Without knowing any further information, it is difficult to say exactly what is needed.
Bed sore treatment. This is a deep wound that often requires a combination of debridement (removal of dead tissue), treatment of underlying infection, wound care including vacuum dressings, and sometimes special skin and muscle flaps to cover the wound. This needs evaluation by a plastic surgeon.
Surgically. This needs to be addressed by reconstructive surgeon. This can not be treated medically.