Double lumen refers to the presence of one silicone shell completely contained within another hence 2 lumens. Currently 2 types are available. The external bag contains silicone gel. The internal bag can be filled with saline. A filling tube with external reservoir is attached to the implant, traverses the outer bag & ends in the inner bag. It is used mostly when the doctor &/or patient is unsure of the final volume needed. The only time I have seen it used is in patients with developmental anomalies where one breast is much much smaller than the other. The final volume is hard to assess before surgery & the implant allows a range of filling of the inner bag with saline. There are 2 types of double lumen implants one is 50:50 saline to gel the other is 25:75 saline to gel. The valve & reservoir can be hidden under the skin for volume changes after surgery & removed in the office under local anesthesia.