Joost Peerbooms

56 Chapter 4 is often performed and is supported by literature showing that platelets can be slowly activated by exposure to tendon derived collagen alone. 17 For surgical applications, PRP is often treated with calcium chloride before application of trombine. This forms a gel- like substance, which can be applied directly. 18 Several PRP systems are now available that allow an efficient preparation for outpatient use (Table 2). Differences such as the volume of autologous blood, spin rate, activating agent, leukocyte concentration, final PRP volume, and final platelet concentration and growth factor, distinguish the systems which are available. Haematological variation between patients (e.g., the number of leukocytes, platelets) may also affect the final PRP preparation. The optimal amount of platelets and growth factors necessary for the healing of musculoskeletal injuries is still unclear and remains a matter of discussion. PRP has a clinical effect only if used at a concentration of at least four times the normal concentration of platelets. 6 However, the efficacy of PRP is shown in less concentrated amounts. 18 Given the nature of the autologous PRP, there are concerns about safety. Each injection should be prepared and given by an aseptic technique. Relative contra- indications apply to patients with a history of trombocytopenia, use of anticoagulants, active infection, tumour, metastatic disease or pregnancy. There is no documentation of carcinogenesis, hyperplasia or tumour growth associated with the use of PRP. 19 PRP activation and the pH of the PRP represents other parameters that are discussed in the literature. Trombine and calcium are traditionally used to activate platelets. This combination results in the formation of a gel that can be used in open surgery but cannot be injected. Trombine and calcium activation results in a rapid release of the contents of the platelets. This requires an immediate use of the PRP. Platelets can also be slowly activated by exposure to collagen derived from the tendon. Variations in partial activation of calcium are also examined. 20 The release of growth factors from PRP is pH-dependent. 21