Joost Peerbooms

95 2-year follow-up of PRP in lateral epicondylitis and inhibition of cell proliferation and viability in vitro. 17 This may be overcome when using an autologous-derived thrombin. Collagen is an attractive alternative to bovine thrombin as it is naturally involved in the intrinsic clotting cascade. Fufa et al. 9 measured clinically relevant levels of transforming growth factor beta (TGF-ß1), platelet-derived growth factor (PDGF-AB), and vascular endothelial growth factor (VEGF) from both type I collagen–activated as well as bovine thrombin-activated PRP. During the first 2 days of tendon healing, an inflammatory process is initiated by migration of neutrophils and subsequently macrophages to the degenerative tissue site. In turn, activated macrophages release multiple growth factors, including PDGF, TGF- a and TGF-ß, interleukin-1, and fibroblast growth factor. 7 Angiogenesis and fibroplasia start shortly after day 3, followed by collagen synthesis on days 3 to 5. This process leads to an early increase in tendon breaking strength, which is the most important tendon healing parameter, followed by epithelialization and the ultimately the remodelling process. This course of repair was confirmed in a previous animal study. 1 The presence of an elevated concentration of leukocytes in the PRP is a topic of discussion nowadays. Companies that concentrate white blood cells argue that leukocytes are useful in creating an antibacterial response and have the ability to debride dead tendon tissue and jump-start healing (because they also contain growth factors). A basic study in horses showed no lengthening of the inflammation phase when L-PRP was used to treat an acute lesion of the bow tendon when compared to the control group. 4 Companies that purposely eliminate white blood cells from PRP argue that leukocytes have detrimental effects on healing tissue, because of the enzymes from the matrix metalloproteinase family that are released by neutrophils. 24 This is, however, not proven in prospective randomized controlled studies. The treatment of tendinosis with an injection of concentrated autologous platelets may be a nonoperative alternative. Injection of autologous platelets has been shown to improve repair in tendinosis in several animal and in vitro models. 14,23 The effect of 1-injection PRP is shown to last longer than 1 year, while the percentage of success after a single corticosteroid injection drops from 51% at 1 year to 40% after 2 years of follow-up. This figure resembles the number for an invasive placebo treatment. A possible explanation for the long-lasting effect of platelets could be that platelets improve the very early neotendon properties so that the cells are able to perceive and respond to mechanical loading at an early time point. 1 In our study, a single percutaneous injection of PRP or corticosteroid was performed, using a peppering technique in both groups. Repeated injections might be beneficial in patients who had suboptimal results after the initial injection, although no evidence for a beneficial effect of more than 1 injection exists. On theoretical grounds, by studying the inflammation cascade in tendon repair, a reinjection after 3 to 4 weeks seems logical because at this stage the cell proliferation and matrix deposition activity will 6