It has been shown in numerous experimental studies that fat graft survival and the volume retention that results is from the number of fat cells (adipocytes) surviving the transplantation process. While accompanying stem cells and other companion cells may have a contributing role in creating some of the augmentative effect (preadipocyte conversion), this cellular event is more minor than often touted. Thus harvesting and preparation of fat grafts must focus on how best to ensure the that the fat cells, hardy as they may be, are not extensively damaged in the process.
The absolute best methods to handle fat grafts are both multifactorial and incompletely understood. But considerable effort from experimental studies to surgeon’s anectodal and clinical experiences are beginning to unravel the onion layers of the process. The important steps include donor site selection, local anesthetic infiltration, harvesting cannula size and pressures, transfer exposure, and graft concentration techniques.
The least understood of these factors is the influence of the donor site which may eventually turnout to be one of the most important. Subcutaneous fat is now known to have subtle biologic behavior differences. Abdominal fat is different, for example, from inner knee or neck fat. as evidenced by how it responds to weight gain and loss. Whether this makes it more or less hardy is speculative. On a practical basis, the donor site is often driven by the volume needed. Facial fat grafting can be more selective in choosing a donor site, most body augmentations (breast and buttocks) can not.
Liposuction is commonly performed using a tumescent solution of which the epinephrine in it controls bleeding and subsequent bruising. Since epinephrine has an alpha-adrenergic effect, it has long been presumed that this has a potential lytic effect on fat cells on contact. Thus some surgeons will not harvest fat infiltrating the donor area with epinephrine-containing local anesthetic solutions. While this can be used for small volume harvests, it remains impractical for larger onees. The very low and dilute concentrations of epinephrine in tumescent solution (1:1,000,000) are accepted as a necessary ‘evil’ as some of the agent is eventually removed by other processing methods.
Ideally fat should be harvested by low pressure vacuum or syringe aspiration to prevent extensive disruption of fat cell walls by extreme differences in intra- vs extracelluar pressures. While negative 1 atmosphere (-20cms H20) is a common vacuum pressure in standard liposuction extraction, lower pressures should be used in harvesting reducing them closer to half that commonly used in fat removal that is going to be discarded.
In harvesting fat, small cannulas are usually preferred. Not because they traumatize the fat less, but because they reduce the fat particulate to a size that is more amenable to smoother injection through small cannulas later.
An open vs a closed method of transfer is a matter of current debate. It ideally would seem that never exposing the fat to environmental air from leaving the harvest site to introduction into the recipient site would be more physiologic and reduce the risk of contamination. There are a large number of devices that are commercially available to do exactly that (e.g., PureGraft) and I have probably used most of them over the past several years. They can go into canisters, bags and syringes, all of which have some mechanism to provide simultaneous concentration from the effects of gravity, centripetal force to filters. Most of these devices can be shown by histology that they do a good job of causing less fat cell damage and are effective at reducing extraneous fluids and contaminants. It is presumed that an improved purity of the graft translates into better graft take and volume retention.
While many of these fat harvesting devices have their merits, when it comes to cost, efficiency of time and material management, I have taken a de-evolutionary step. I have evolved back to low tech simple bronchial washing traps for small harvests and a sterile glass jar for very large harvests. There use does make them an open system for transfer but the potential risk for contamination seems to be very low and not a clinical problem that I have seen in terms of infection.
Dr. Barry Eppley
Indianapolis, Indiana