The straightforward, "simple" answer to your question is that no, generally the weight (or mass) of silicone implants is not exactly equivalent to their volume, because silicone implants are very slightly more dense than water. This may vary depending upon the exact type of gel you are talking about, too - highly cohesive gel implants (so-called "gummy bear") may be more dense than other types of gel implants, but in general, silicone gel implants are ever so slightly more dense than saline. This would mean that a 375 cc silicone implant "weighs" MORE than 345 gm, in fact, it "weighs" slightly more than 375 gm too, because something that is more dense will weigh more at the same volume than something that is less dense. There was a study performed in Australia in 2013 that looked at this very issue and compared silicone gel implants to water (approximating saline implants), and while the two were very close, they found that the density of silicone implants was more like 1.05 gm/cc, whereas water, and therefore saline, by definition, has a density of 1 gm/cc. The outer shells of both saline and silicone implants are identical, so for the same size gel implants and saline implants, the shells should have identical mass and volume, and have no impact on any differences in density, mass, or volume of the implants as a whole. Thus, in your case, you would think that at the very least, your 345 gm implants should have a volume of at least 345 cc, but in reality, they may actually have a slightly smaller volume, more like 328 - 329 cc, based upon the study I quoted, because a silicone gel implant takes up takes up slightly less room for its weight (mass) than a saline implant. Conversely, a 375 cc silicone gel implant would be expected to "weigh" approximately 394 gm, based again on the above quoted study. Therefore, based strictly on the information that you have provided, and the available studies and definitions of volume, mass, and density, there appears to be a discrepancy at least in what you were planning as your implant size/volume, and what seems to be recorded on your documents. This is something you'll have to discuss in more detail with your surgeon.The more complicated issue to bring up here, and quite likely the explanation that your surgeon will give you for all of this, is that what is written on your documents depends to a large degree on the type of implants you have and who the manufacturer is. In all of my experience, I have not encountered, at least in the modern inventory currently used in the US, a measuring/sizing scheme for implants that is based upon their mass, or weight, in grams. All US manufacturers use a volumetric (cc) and dimensional (cm) sizing schema, and grams has not factored into the picture here. There are other parts of the world where implants are sized by grams, mainly in Europe and mainly with regard to the anatomically shaped highly cohesive gel implants (McGhan/Allergan in Ireland is one that comes to mind), but I'm not familiar with any of the ones approved in the US doing so. Closely related to this issue is the fact that the manufacturers may label an implant with a certain mass (weight) in grams, based upon the specifics of their manufacturing process, that is slightly more or less than the actual mass if you were to carefully measure it in a lab. Likewise, this may happen with the labeled volume too. This can explain some of the discrepancy, but not all of it. The other thing is that you are flip-flopping between planning based upon volume (375 cc) and implanting an actual implant based upon mass (345 gm), and this may have been the closest "size" implant to the 375 cc preoperative goal that you had, taking all factors into consideration, that your surgeon had available for that type of implant. Furthermore, in the event that you do have anatomically shaped implants, much of the sizing for those depends upon what we call "dimensional planning," meaning we don't rely as much on volume or mass with those at all, as we do on dimensions like base width, vertical height, and projection. I would say that in the end, these differences between grams and cc are all very subtle, and the real important consideration will be how the implanted breasts actually look and whether or not you are happy with the result. If so, the rest of this is numbers and academic mumbo-jumbo. If you aren't, however, you will need to discuss this more with your surgeon to figure out where the discrepancy between planning and execution occurred and what can be done to ultimately make you happy. This issue in general is something important to check out with your surgeon, because they will be able to tell you precisely what size and type of implant was used and how that came to be based upon your preoperative sizing and goals. I hope I haven't confused you too much here, best of luck!
