In our experimental model in vitro, we have seen the outgrowth of articular chondrocytes only in those specimens which have been digested with collagenase. The articular chondrocytes from the particulated, but non-digested, cartilage fragments have not shown any tendency for outgrowth. This finding is quite opposite to the claim of some other authors, namely, the establishers of the abovementioned novel operative technique/s [
2]. According to these authors, the ability of articular chondrocytes to “escape” a cartilage has been proven in both laboratory and animal models [
2]. The enzymatic digestion of the cartilage with collagenase has been postulated as absolutely necessary for the migration of articular chondrocytes out of the cartilage and their multiplication, as done in ACI [
3]. A study showing goat articular chondrocyte outgrowth both in vitro and in vivo has been done [
4]. The presence of chondrocyte outgrowth in vitro has been evident after 15 days and increased at 1 and 2 months. The cartilage fragments in this study have been embedded in fibrin and loaded onto a scaffold composed of a hyaluronic acid (HA)-derived membrane in the lower phase and platelet-rich fibrin matrix (PRFM) in the superior phase [
4]. In our in vitro model, however, the human articular chondrocytes remained captured inside the fibrin matrix during the observed period of time, 2–5 weeks namely (Fig.
2). We have used neither HA-derived membrane nor PRFM, but still, it is a striking fact that we have not identified a single chondrocyte escaping the cartilage matrix. When digesting the cartilage fragments with collagenase overnight, the cells have escaped from the fragments, but a migration of cells into the fibrin matrix has not been observed either (Fig.
3). It has been suggested that a fibrin sealant promotes migration and proliferation of human articular chondrocytes in vitro [
5]. On the other side, it has been reported that human fibrin glue hampered the healing process in rabbits in a similar model to that previously described in the text [
4,
6]. The difference between the first named study [
5] and our study is that we have used primary cartilage explants, non-digested as well, while primary chondrocyte culture has been used in the other case [
5]. However, not even the cancellous bone-derived cells have penetrated the fibrin matrix, which speaks more in favor of fibrin-hampered chondrocyte migration rather than fibrin sealant promotion of cell migration as it has been described in the study mentioned above [
5]. Human mesenchymal stem cells (hMSCs) have been used for the repair of osteochondral defects in rabbits by seeding them on biphasic composite constructs (hydroxyapatite + platelet-rich fibrin glue) for 4 and 8 weeks, respectively [
7]. It has been postulated in this study that the group where differentiated hMSCs have been used has shown superior healing of osteochondral defects first after 8 weeks [
7]. In vivo, fibrin will be gradually degraded during wound healing by fibrinolysis and replaced by the mature extracellular matrix, wherein the proteolytic activity of a membrane-type matrix metalloproteinases (MT1-MMP) [
8,
9] and plasmin [
10,
11] locally degrades the fibrin matrix. In vivo, this happens in a matter of days or weeks, and the rate of degradation depends on many factors such as fibrin structure, its cross-linking, and the incorporation of protease inhibitors [
12‐
15]. Generally, the fibrin matrix is gradually replaced by mature collagen that is produced by invading cells [
16‐
18]. In our study in vitro, there is a lack of such endogenous factors participating in the process of fibrinolysis, and one could therefore speculate that no invading cells have been found in the fibrin matrix. The insufficient degradation of fibrin and therefore scarce cell invasion could be the reason why no significant healing have been found after 4 weeks but first after 8 weeks as mentioned in the previously named study [
5]. Fibrin is gradually degraded in vivo as it was already explained in the text, and the healing process succeeds its degradation, leading to the new tissue formation eventually. The initial inhibition of cell invasion and therefore new tissue formation that we have seen in our in vitro model have therefore probably no effect on the final clinical outcome. Indeed, the authors of this text have used routinely the fibrin matrix in our cartilage repair techniques and have seen neither poor defect fill nor poor clinical results that could be related to the use of the fibrin matrix [
19,
20].
We have not seen as well in our in vitro study that articular chondrocytes from undigested cartilage fragments have the capability to escape from the cartilage matrix and, by doing so, have an active role in new tissue formation. Whether they can do that in an in vivo situation under the influence of, for example, endogenous collagenases [
21,
22], we can just speculate. It seems that when implanting autologous chondrocytes using fibrin as a carrier, the results look fairly good [
23]. On the other side, no tissue bonding or new cartilaginous tissue formation has been identified in the cartilage fragments without enzymatic treatment in a nude mice model [
24]. Finally, two studies have shown no cell invasion into the fibrin matrix [
25], as well as direct influence of the pore size and fibrin strand thickness on the cell invasion [
26]. One recently done study has given even more evidence supporting our results in terms of chondrocyte outgrowth from adult human articular cartilage [
27].