Purpose
Abundant, accessible and autologous, adipose tissue represents the ideal filler for use in plastic surgery. Reports on fat graft survival, however, have been conflicting and equivocal. Currently, the mechanisms responsible for fat graft survival have not been elucidated, and much debate has arisen over the role of adipocytes, progenitor cells and cytokines in this process. Recent studies have focused on the effects that different harvesting and processing techniques have on the cellular viability of fat before injection. In this study, we hypothesize that centrifugation creates graded densities of fat with varying cellular and biological characteristics that influence survival post-injection.
Methods
Aliquots (10cc) of syringe-assisted lipoaspirate harvested from human subjects undergoing liposuction were centrifuged for 3 minutes at 300(x)g. The blood fraction was drained, and the oil fraction was decanted and wicked for removal. From the resulting processed lipoaspirate, 1.5cc of the highest density (HD) and lowest density (LD) fat was separated for either injection or baseline analysis. Progenitor cell number and adipokine concentrations were analyzed in each fraction using FACS and ELISAs, respectively. The relative number of functional adipocytes was measured using a glycerol-3-phosphate dehydrogenase (G3PDH) assay. HD or LD fractions injected into the dorsal subcutaneous tissues of adult FVB mice using the lipostructure technique were harvested at 2, 4 or 6 weeks post-injection.
Results
Baseline analysis revealed that HD fractions contained more progenitor cells/g than LD fractions (1.9±0.1-fold increase). Additionally, vascular endothelial growth factor (VEGF) and stromal cell-derived factor-1α (SDF) concentrations were elevated in HD fractions at baseline (10.3±0.8 vs. 7.7±0.7 pg/mL, p<0.05; and 298.2±35.9 vs. 195.0±10.2 pg/mL, p<0.05, respectively). Conversely, G3PDH activity was increased in LD fractions (4.37 vs. 1.55 mUnits/mL) indicating a greater percentage of functional adipocytes. At 2, 4 and 6 weeks post-injection greater percentages of HD grafts survived compared to LD grafts (83.7% vs. 77.9%; 81.0% vs. 76.4%; and 67.4% vs. 63.0%, respectively). Interestingly, LD grafts harvested at 2 weeks post-injection had significantly higher levels of SDF compared to HD grafts harvested at the same time point (2400.4±259.7 vs. 316.0±44.0 pg/mL, p<0.02).
Conclusion
Although HD fractions of centrifuged lipoaspirate contain fewer functional adipocytes than LD fractions, they survive to a greater extent following grafting. The mechanism responsible may be vasculogenic in nature, as baseline analysis found HD fractions to contain more progenitor cells/g as well as increased concentrations of several vasculogenic mediators (VEGF, SDF) compared to LD fractions. This study is the first of its kind, identifying graded densities of fat with unique characteristics that directly influence fat graft survival.