The In Vivo Impact of Leukocyte Injections on Normal Rat Achilles Tendons: Potential Detriment to Tendon Morphology, Cellularity, and Vascularity

The goal of this study was to assess the morphology, cellularity, and vascularity of normal tendons after injections of different leukocyte populations. This is clinically important because of the potential to tailor future PRP injections on a patient-by-patient basis. In patients requiring an anabolic response, leukocyte-poor PRP may be the best option. In contrast, when patient pathology requires an inflammatory response to improve healing³⁶ or breakdown fibrotic tissue, as seen in tendinosis, leukocyte-rich PRP may be warranted. Further, properly controlled clinical studies are needed to validate these recommendations.

Limitations inherent in this study are those similar to other in vivo studies. First, the results of injections into rat tendons may not be translatable to human tendons. Despite this limitation, the rat is a common model for tendon research.³¹ A second limitation is that this study injected healthy Achilles tendons, rather than tendons with preexisting tendinopathy. In a naturally occurring tendinopathy, there may be other factors present that interact with PRP, and this model negates the contribution of these factors. Finally, while the IHC and morphological data show clear changes, the cellularity data are not clear in identifying the type of cells that were increased by granulocyte injection. However, the cells appeared rounded, resembling inflammatory infiltrate; a common cell type seen in tendons.² While fibroblasts are also a common infiltrate during chronic tendinopathy, they are generally flat and appear on H&E as long spindle shaped cells. The last limitation of this study is the lack of functional mechanical testing since, clinically, healing of the tendon is also related to the strength of the tendon. Thus, we believe the increased cellularity of the tendons after granulocyte injections is representative of an increase in inflammation. Moreover, our results are consistent with the increased inflammation previously reported linking leukocytes to tendon inflammation.^20,22,32 It is interesting to note that the increase in inflammation does not lead to an increase in vascularity as could be expected.

CONCLUSION

We found that the injection of leukocytes into healthy rat Achilles tendons increases inflammation, as evidenced by increased cellularity and disrupted morphology, which suggests that leukocyte-rich PRP preparations may be contraindicated in settings of acute tendonitis. However, these preparations may be useful for a specific subset of tendinopathies, including chronic tendinosis.