Gluteal silicone implants seem terrifying! How do they stay in place for an Butt Implant?

On a person of your height/weight, you are right, its very unlikely that you are a candidate for a BBL. Butt implants this day are really safe and even that all those terrifying stories are mostly true, 90 % of them are on a different technique than today's gold standard (intramuscular). You need to make sure to chose a Plastic Surgeon who is Up to Date and perform this technique. this will significantly decrease the complication rate. 
As how do they stay in place?? If placed in the right position, they should never move from inside the muscle, we are talking about one of the strongest muscles of your body, so they should stay in place and under any circuntances should fall down the back of your thigh.

I Love this procedure and its my duty as a Board Certified Plastic Surgeon to let the world know the reality of how Butt Implants have change over the years.

Regards,

Dr. Luis Redondo 

Allow me to share with you some information that you may not hear elsewhere. There are only two proven safe and relatively effective methods for Buttock Augmentation and Hip Augmentation: 1) Autologous Fat Transfer (using your own fat, transferring from one area of the body to the other) and 2) Buttock/Hip Implants (semi-solid silicone rubber implants that cannot rupture &/or leak). Both are very good options so what it comes down to, like any surgery, is proper patient selection.
Indeed because at least 50+ % of the fat transferred will melt away within a year, most patients are not good candidates because they lack an adequate amount of fat to harvest. Another tip is that if you purposely gain weight (i.e. fat) for the procedure, the fat you lose first as you get back to your baseline weight is that very same fat that was transferred into your buttock...so don't fall victim to this recommendation. Although using your own fat is relatively safe, the one serious complication that can rarely (< 1%) happen is "fat embolism" in which some of the fat gets into the blood stream and travels up into the lungs, heart, and/or brain causing serious problems. This complication is more likely to happen with the larger amount of fat being transferred. This is even more likely to happen when using fillers like PMMA and hyaluronic acids. Also fillers, when injected in large quantities, have a relatively high tendency to migrate away from the original area they were placed and tend to stimulate a lot of inflammation and a subsequent disastrous amount of scar tissue/hardening.
Thus buttock/hip implants become a very good, safe, and long term reliable option for most patients seeking buttock augmentation (at least in my practice). I prefer to insert the buttock implants through a 2 inch long incision over the tailbone (concealed within the crevice between the buttock cheeks) and the hip implants through a ~ 1 inch incision just below the beltline above the hip region. The buttock implant should always be placed under or within the gluteus maximus muscle. In this position, the implant is less palpable, less visible, and does not sag or shift/migrate over time unlike implants placed on top of the muscle. Therefore it is extremely important to seek consultation with a board certified plastic surgeon who specializes in this procedure (in fact both implants and fat transfer so that s/he is not bias). And in this case too, at least in my surgical practice, the infection rate is minimized to ~5%. Glad to help.
Depending upon the amount of your current fat reserves and their distribution, you may or may not be a reasonable candidate for fat transfer. If you are wanting to utilize the fat to further augment your buttock in addition to implants, then I caution you against this. The reason being that the fat needs to be placed into the buttock muscles for optimal survival, however, this is exactly where the implant needs to reside and thus risks infection and/or death of the fat. Because of this risk, I frequently avoid fat grafting into a buttock that has an implant. Bottom line, I recommend consulting with a board certified plastic surgeon specialist in buttock augmentation (BOTH implants and fat transfer) to determine what option is best for you. Glad to help.