To The Editor:
We read with interest the article "BMP-14 Gene Therapy Increases Tendon Tensile Strength in a Rat Model of Achilles Tendon Injury" (2007;89:1315-20), by Bolt et al. The article appears to be important in demonstrating the feasibility of this new technique, but we question some aspects of the study. We reported on a very similar experiment two years ago1, but, surprisingly, it was not cited in the article. In our study, we found some different results.
As reported in 1992 and 1993 by Rooney et al.2,3, cartilage and bone formation is essential during rat Achilles tendon healing. The important question is whether the use of BMPs facilitates this process. It should be noted that the observation period in the study by Bolt et al. (three weeks) is probably too short to find any differences between the groups. We found an increase of cartilage and bone formation in the BMP-14 (growth differentiation factor [GDF]-5) group at eight weeks after virus administration1.
Furthermore, the authors report that no adverse immunological response to the adenoviral vector was detected, but they do not describe specific methods, e.g., CD4+ or CD8+ T-cells, that might be used to confirm this conclusion. Hematoxylin and eosin staining is neither sensitive nor specific enough to clarify this question within the healing tendon callus.
The assessment of "gapping" of the healing tendon seems to be an imprecise and unconventional method to determine structural and biomechanical data in this model. Why didn't the authors measure the cross-sectional area (e.g., using a noncontact laser method), and why didn't they calculate Young's modulus? To which variable did the authors normalize their results, e.g., contralateral side, tendon thickness?
Finally, the authors should try to provide an explanation for the increase in tendon tensile strength. Is it because of the thicker tendon callus or did they find further changes within the quality of the callus? A thicker tendon does not necessarily mean a stronger tendon.