The third and final part of the fat grafting process is that of the injection technique. Injecting the fat into the desired site seems the simplest part of the process and has very little dependence or devices or any mechanized methods. It is a purely manual process. that represents the most ‘artistic’ part of the process. There are numerous variables to consider in fat injection including syringe and needle/cannula size, aliquot or injectate size and the injected tissue location.
There is no evidence to suggest that the size of the syringe used for injection has any impact on graft survival. Syringe size is almost exclusively controlled by the size of the needle or cannula used for injection. As a general rule, the smaller the injection diameter through which the fat is pushed the smaller the syringe has to be. The injection of fat does not create a smooth or linear flow as the fat does not behave like a liquid but more like a solid. It really flows through ‘spurts’ of solid material or clumps. It is difficult to get fat to flow through a needle/cannula that is smaller than a 22 gauge internal diameter needle or roughly 0.4mms. In the face this usually means 1cc syringes are used with these diameters. In the body, larger internal diameters are used in the 16 to 18 gauge range or .8 to 1mms with syringe sizes of 10cc to 60ccs.
The size of the injected fat has drawn a lot of attention since the fat must survive by osmotic diffusion. Experimental studies have shown that the center of the mass of injected fat will not survive if it is too big. This has helped drive the concept of the droplet injection technique which makes both intuitive and scientific sense. The smaller the droplet or push of the plunger during injection passes, the better the injected fat can be nourished. Laying down many small droplets from different lines of injections means that small balls of fat are distributed throughout the tissue, maximizing their surface area for nutrition. In the face with a 1 ml syringe, each deposition of a droplet of fat represents .1ml. Such a droplet approach becomes more impractical as larger body structures are injected. Small droplets for buttock augmentation require an injection precision that would take literally hours to do. Thus in the body 1 to 2ml droplet sizes are more practical even though they are not ideal.
No matter whether it is the face or the body, the choices of the tissue level of injection are fat, muscle or both. Some argue that fat survives better in like at the subcutaneous fat level. Other techniques involve placing the fat into muscle, presuming that it will get nutrition quicker due to the enhanced vascularity of muscle tissue. The muscle location makes the most sense but there are practical issues of the deeper location of the muscles, how much fat can be delivered into the size of the muscle, and potential adverse effects on muscle function due to the injection trauma. Since there many more superficial muscles in the face, it is a easier area to employ this concept. In the body the muscles are larger and often ore distinctly out of the desired plane of soft tissue augmentation.
Dr. Barry Eppley
Indianapolis, Indiana