Joost Peerbooms

12 Chapter 1 theoretically giving each form of PRP unique properties. Most published studies do not provide a complete characterisation of the PRP compositions used, so reliable comparison between studies remains a challenging problem. 18 Several PRP preparation classification systems have been developed to facilitate comparison between studies and promote the standardisation of the PRP preparation process. However, there is currently no world-wide consensus on which classification system is to be used. 19 Clinical Effectiveness of PRP Injection The scientific literature has shown that the activated growth factors released from the platelets in PRP injections have several potential effects in a laboratory setting. 18 De Mos et al. demonstrated that both platelet-rich clot release and platelet-poor clot release increased tenocyte cell count and collagen production in vitro. 20 Another in vitro study by Zhang and Wang showed that PRP activated by calcium chloride promotes the differentiation of tendon stem cells into active tenocytes. 21 However, the available clinical research on the use of PRP has produced inconsistent results. 22,23 Caution is advised when translating in vitro results to a clinical setting, and randomised controlled clinical studies of high methodological quality should be conducted to determine the appropriate indications for PRP. TENDINOPATHY The pathophysiology and origins of pain in chronic tendon pathologies are not yet entirely clear. However, tendinopathy is known to be a degenerative process for which inflammation is generally not observed. Recently, however, Millar et al. 24 , reported an increased number of macrophages and the presence of specific interleukins, suggesting some inflammatory response. Diseased tendons are characterised by disorganisation of collagen fibres, often excessive production of extracellular matrix (ECM) proteins, an increase in micro-vascularisation and sensory nerve innervation, increased immune cells and inflammatory mediators, and improved cellular apoptosis. 24,25 Tendinopathy clinically presents as a combination of pain, swelling, and reduced function. A variety of tendons can be affected in humans. NSAIDs offer short-term pain relief but may have a negative effect on tendon structure. 3 In the past, corticosteroid injections were a frequently used treatment for chronic tendinopathy. These injections offer effective short-term pain relief but become less effective after three months; they may also cause tendon rupture by decreasing tendon cell proliferation and inducing degenerative changes in the tendon. 26-28 PRP injection has been suggested as a suitable replacement for NSAIDs and corticosteroid injections. Accordingly, there is a growing interest in the use of PRP for the treatment of tendinopathy, although high level clinical evidence for its effectiveness in this regard is limited.