The Effect of Pressure and Shear In Autologous Fat Grafting

Purpose: Fat grafting has become routine in Plastic surgery due to low donor site morbidity, low complication rate, and fast recovery time. The optimal technique, however, has yet to be defined. One critical variable is pressure. In this study, we examined the role of aspiration and injection pressure on human fat grafts in a nude mouse model.

Methods:  Negative Pressure:  Tumescent liposuction was performed on fresh panniculectomy specimens with a standard 4mm cannula. Suction pressure was either -15 inches Hg (-0.5 atmosphere) or -25 inches Hg (-0.83 atmosphere). Lipoaspirate was centrifuged at 1200G and injected into the flanks of nude mice with a 16 gauge angiocatheter. Positive Pressure:  Fresh operating room lipoaspirate was obtained and positive pressure was applied up to 6 atmospheres for up to 3 minutes and then injected into nude mice. Shear Stress:  Lipoaspirate was centrifuged at 1200G for 3 minutes and then injected at two different speeds: fast flow rate (3-5cc/sec) versus slow flow rate (0.5-1cc/sec). After 4 weeks, the fat lobules were analyzed for weight and histology.

Results:  Negative Pressure:  There were no differences in weight or histology with high versus low suction pressures; however, the proportion of fat to tumescence fluid suctioned was much greater in lipoaspirate suctioned at high pressure. Positive Pressure:  Various positive pressures up to 6 atmospheres and various times up to 3 minutes did not demonstrate a significant difference in weight or histology at 4 weeks.  Shear Stress:  In vivo, a slow injection pressure yielded a 38% increase in weight (p<0.001) compared to fast injection. This was also seen histologically.

Conclusion: Higher aspiration pressures up to -0.83 atm did not affect fat grafts viability in vivo; however, the proportion of fat to tumescence fluid suctioned was much greater in lipoaspirate suctioned at high pressure. This allows a surgeon to achieve a targeted volume more quickly. Positive pressure up to 6 atmospheres over time (up to 3 minutes) also did not affect fat graft viability. The degree of shear stress, which is a function of flow rate, did significantly affect fat graft viability. Fat injected slowly with low shear stress significantly outperformed fat injected fast with high shear stress. These data suggest shear stress is the most important variable that significantly affects fat graft viability.