Joris van Dongen

117 tSVF acts anti-inflammatory on chondrocytes in vitro block the degrading effect of MMPs on cartilage. 24 The effects of TIMPs as well as IL- 1 β , COX2 and other pro-inflammatory signaling molecules might be reversed by tSVF and could possibly slow down the progression of osteoarthritis in the affected joints and thereby reducing pain. Multiple recently published studies use cSVF as a treatment of osteoarthritis in vitro . 42-45 In these studies, however, cSVF is obtained enzymatically by time-consuming isolation procedures. 42-45 These enzymatic procedures disrupt all cell-cell interactions including ECM. 8,16 Although, several studies published about the therapeutic effect of cSVF on osteoarthritis, the regenerative role of ECM is often underestimated. 25-31 In tSVF, the ECM is preserved holding stromal cells e.g. ASCs in their local niche. 16 In this way, stromal cells have a higher retention rate after injection and thus probably a prolonged regenerative effect. Besides a cell retaining function, ECM influence cells in a complex mechanical behavioral way. 46 A time-dependent response of cell to loading or deformation, called viscoelasticity, has become widely accepted as a concept wherein mechanical properties of e.g. ECM stiffness have an effect on cell proliferation and differentiation. 46 ECM stiffness seems to regulate developmental, homeostatic and regenerative processes. 46 Another important function of ECM is the binding of a plethora of factors and ensuring controlled slow release of these different factors over time. 8,16 A natural slow release scaffold might contribute to a prolonged regenerative effect of tSVF as compared to cSVF. Alimitationof this studyis theuseof a two-dimensional culture systemwitha single layer of culture chondrocytes. It is well-known that cells behave significantly different in a two-dimensional culture system as compared to a three-dimensional culture system. 47,48 Athree-dimensional culture systemmimics processes in vivo more accurate. 47,48 In a two- dimensional culture system, the role of synovial inflammation and subchondral bone remodeling and their interaction with chondrocytes cannot be investigated. 47 In 2013, a three-dimensional osteochondral system to mimic the pathogenesis of osteoarthritis was developed which involved mechanical injury, pro-inflammatory cytokine influence and cartilage degeneration. 49 This might be of importance in the translation of results from in vitro to in vivo . 47 To date, the complex multifactorial pathophysiology of osteoarthritis is not yet fully addressed. 47-50 A second limitation of this study is that it predominantly observes the effect of tSVF on chondrocytes, while subchondral and synovial processes are known to play an important role in OA as well. 20,24 However, both chondrocytes and synovial tissue cells produce pro-inflammatory cytokines with similar underlying mechanisms. 51 Therefore, it is likely that synovial cells will respond the same in terms of downregulation of inflammatory factors in response to tSVF.