What are the pros and cons of sub pectoral vs. sub dermal implant placement immediately following a bi-lateral mastectomy?

Thank you for your question.   First let's discuss your choice of mastectomy.  Nipple sparing mastectomy performed in clinical stage 1 tumors has proven to be safe based on studies published.  So if your breast surgeon has given you the option of nipple sparing mastectomy and your  plastic surgeon agrees that you are an excellent candidate for reconstruction given the shape and sagging of your breast then nipple sparing mastectomy is a good option for you. Next comes the incision selection and this is based on the comfort level of your breast surgeon.   I prefer inframammary fold incision as there is no scar on the central aspect of the breast.  Next, the decision needs to be made where the expander will beplaced: subpectoral (under muscle) or pre pectoral (over the muscle). I do both and it depends on the decision that the breast surgeon and your  plastic surgeon make together based on the cancer stage and proximity to the chest wall.  If a prepectoral technique is chosen then the entire expander would have to be covered with AlloDerm so additional coverage is obtained. Future fat injections may be necessary and MRI surveillance may be needed every 2-3 years to evaluate the chest wall since the implant is over the muscle.  The biggest advantage of the prepectoral technique is the lack of pain and NO muscle animation which is a problem for some patients.   Long term risks could be more sagging and implant palpability.  Therefore the risks and benefits  should be discussed and a technique should be chosen that best fits your cancer stage and lifestyle. Best of luck. 

Pre-pectoral breast reconstruction involves placement of the implant/expander on top of the pec muscle rather than the traditional under the muscle (sub-pectoral) technique at the time of mastectomy.  Pre-pectoral is also sometimes called "subcutaneous" implant/expander placement because the device is placed between the breast skin flap and pec muscle after the breast tissue has been removed.  
There are two main advantages of the pre-pectoral (subcutaneous) technique.  The first is less pain due to lack of stretch on the pec muscle during surgery and expansion.  The second is no animation deformity, which is the movement of the implant that occurs when the pec muscle is flexed that makes the presence of implants more visibly noticeable and is bothersome to some patients.  
The main advantage of the sub-pectoral technique is that it provides another layer of coverage over the implant in the absence of breast tissue.  The additional coverage acts as a barrier to exposure of the implant in the case of incision breakdown in the territory of the pec muscle.  The additional coverage also acts to conceal the appearance of the implant beneath the skin such as rippling.  However, this is less of a problem with the advent of highly-cohesive (gummy bear) breast implants and fat grafting to thicken to the subcutaneous tissue of the mastectomy flap at a second stage surgery.  Both options are good in experienced, well-trained hands when the advantages of each are weighed along with surgeon and patient preference.
Ziyad Hammoudeh, MD

I do not believe you are well informed. You need a lengthy consultation to discuss all options of breast reconstruction and the pros and cons of each. Implant reconstruction needs significant coverage to avoid implant problems. This can be done by many methods.As for the scar position, that depends on many factors, position of the tumor, size of the breast, shape of the breast ultimate results desired and a full discussion between the plastic surgeon and the breast surgeon.Is there a plan for sentinel node biopsy? any plans for radiation, chemotherapy all these factors are discussed with the breast cancer team in my practice to make a good decision

Most implant reconstructions after mastectomy will place the implant under the muscle because it needs to have additional protection.  In fact it would be highly unusual to place the implant just under the skin as it needs more coverage to limit issues with palpability, rippling, exposure, and unusual contours.  Your incision placement will depend on the comfort level and expertise of your breast and plastic surgeons.  Common incision sites are just above or below the nipple and in the inframammary fold.  The inframammary fold incision is nice in that it is the most concealed of those options.   Given your age, you may want to look into using your own tissues to recreate the breast if you have enough stomach skin and fat (e.g. a bilateral DIEP flap) just as an alternative option.  Good luck!