Predifferentiated Mesenchymal Stem Cells for Osteochondral Defects: Letter

Dear Editor: We read the recent article ‘‘Repair of Chronic Osteochondral Defects Using Predifferentiated Mesenchymal Stem Cells in an Ovine Model’’ by Zscharnack et al with great interest. Clinical results of matrix-associated chondrocyte transplantation (MACT) in the treatment of chondral defects are satisfying and this technique can be seen as gold standard in the present therapy of large chondral defects. In comparison with microfracture, it offers the possibility of regeneration with hyaline-like tissue. However, advantage toward microfracture (MFX) is dearly bought by the necessity of invasive harvesting of autologous chondrocytes, leading to defect healing of donor sites and requiring an additional surgical procedure. However, recent efforts combining mesenchymal stem cells (MSCs), obtained by MFX, with artificial matrices to improve results of the MFX procedure, resulted in inferior regeneration compared with MACT. The current study directly addresses the subsequent and, in our opinion, absolutely relevant question, if in vitro predifferentiation of MSCs helps improve results. The authors attached importance to designing a study, with results that are transferable to humans, employing a chronic osteochondral defect model in large animals. Furthermore, they worked out a convincing treatment procedure that seems to be transferable to clinical application in patients. The extensive in vitro part of the work, investigating the conditions for the subsequent in vivo part, contributes markedly to the quality of the article. However, the work of the in vivo part disappointed us with regard to a few facts. All conclusions from the costly in vivo study were drawn from histologic examination. In our opinion, a biochemical, and even more a biomechanical, evaluation of the in vivo regenerate would have based the conclusions on a far more stable fundament. These additional data would characterize function of the regenerate and thereby give hints about its long-term survival. One of the most critical points in the present study is the press-fit anchoring of the gels during the surgical procedure. It is doubtable that the gels are securely prevented from dislocation during mobilization of the specimens under full weightbearing. Previous studies in comparable models have reported that a critical number of gels dislocate even when they are additionally fixed by fibrin clue or sutures. Even in clinical application of MACT, where patients avoid weightbearing for 6 weeks after surgery, delamination of the matrix is one of the major complications. Furthermore, Figure 1 of the article shows that cartilage of the sheep’s condyle is rather thin compared with that of humans. This even more brings into question press-fit stability under full weightbearing. Also, if the authors have shown successful survival of implants in situ for 6 weeks in single sheep employing quantum dot-labeling (Figure 6), toluidine staining in Figure 8 of the article supports the assumption that some gels were dislocated after the surgical procedure. Here, the worst results of each treatment group show no residual matrix, resulting in persistence of full cartilage defects. This finding surely contributes to the general high standard deviation in the present study. On the other hand, the best results shown in Figure 8, with full defect regeneration in all groups, including untreated controls, make it questionable if the 7-mm diameter represents a critical-size defect. Here, the result of the predifferentiated MSC-seeded gel group appears as good as native cartilage, raising the question if the histological slide has missed the regenerate. Another critical limitation of the study design is creation of 2 randomized treated defects in 1 knee. Application of 2 different treatments in 1 joint cavity creates danger of their interaction. Although the authors state that they are conscious of that danger and quote that no obvious influence between the different defects was observed, it is well known that hydrogels loaded with cells release cytokines and cells to their environment and therewith to the joint cavity. Here, it is likely that stem cells, either differentiated or undifferentiated, released from gels into the joint critically interacted with healing of defects, either treated with unloaded gels or even if left empty. On the other hand, anabolic cytokines released from MSCs may have improved healing in defects treated by other methods in the same compartment. Additionally, chondrogenic differentiated MSCs incorporated to matrices may have paracrinely influenced undifferentiated MSCs applied to the same joint cavity. Thus, it is likely that interactions of the different treatment methods have contributed to the high standard deviation observed within the groups. Regarding these facts, we are wondering why the authors have not randomized the knees (instead of randomizing the single defects), meaning same treatment in both defects of 1 knee and thereby avoiding uncontrolled interaction. An additional factor surely influencing the different treatment regimens is the depth of the set defect. How did the authors confirm, that each defect was 2 mm in depth? Did the drill have a mechanical stop? The authors note that the subchondral bone layer was not opened during drilling the procedure to avoid migration of MSCs to the defect. Nonetheless, in Figure 1 the subchondral layer seems to be affected, which is in line with hypertrophy of subchondral bone in several treatments represented in Figure 8. This bone hypertrophy is frequently occurring The American Journal of Sports Medicine, Vol. 39, No. 6 DOI: 10.1177/0363546511410382 2011 The Author(s)