Biological Background

Human blood platelet counts are approx imately 200 000/ml. PRP is an autologous concentration of human platelets above this in a small volume of plasma.2 Reports vary regarding the platelet concentration and different growth factors present in the PRP concentrate. Also, there are many preparation protocols, kits, centrifuges and methods to trigger platelet activation before use. The same is true for application methods, including using injectable activated PRP liquid concentrate versus implanting a fibrin scaffold, optimal timing of injection and the specific volume to use. Almost every major manufacturer in the orthopaedic and sports medicine world markets a different commercial kit. Some claim to produce a better quantity and quality of PRP than their competitors from the same amount of blood from the same patient. Costs vary tremendously: a commercial kit yields a PRP concentrate at the cost of several hundred dollars, but inhouse non-automatised techniques produce a PRP concentrate for approximately US$10. Each method to concentrate platelets leads to a different product with different biology and potential uses,3 with a high variation (3 to 27-fold) in growth factor concentration and in the kinetics of release.3,–,5 Most techniques yield a PRP concentrate of approximately 10% of the blood volume taken (eg, 20 ml of whole blood would result in approximately 2 ml of PRP). These differences might be of relevance to clinical management,3 although they have not been systematically studied. PRP preparations containing only moderately elevated platelet concentrations may be the ones to induce optimal biological benefit, with lower platelet concentrations leading to suboptimal effects, and higher platelet concentrations to inhibitory effects.6 7 Other authors have stated that the “therapeutic dose” of PRP would be at least four to six times higher than the normal platelet count.8 9 To complicate things, the actual growth factor content is not well correlated with the platelet count in whole blood or in PRP, and there is no evidence that gender or age affects platelet count or growth factor concentrations.10

A few studies have categorised the different platelet concentrates according to the presence or absence of white blood cells (WBC): either pure plateletrich plasma (P-PRP), in which WBC have been intentionally eliminated from the PRP, or leucocyte and platelet-rich plasma (L-PRP),3 possibly from the inability of the kit to differentiate between the WBC and the PRP layers. A positive or negative effect of WBC cannot be generalised to all tissues and clinical conditions. This issue requires further investigation, as the WBC content in the preparation injected has never been systematically studied. We do know that neutrophils promote additional muscle damage soon after the initial injury, but there is no direct evidence that neutrophils play a beneficial role in muscle repair or regeneration.4 11 This exacerbation of injury and/or delay in muscle regeneration may be of major importance for injuries managed with PRP. This, together with the improved homogeneity of P-PRP and its reduced donor-to-donor variability, supports some PRP production techniques that claim to be clinically superior.4 6 L-PRP injected for soft tissue injuries might induce more local pain than P-PRP. Platelets by themselves do reduce pain.12 L-PRP reduces postoperative pain, although here the contribution of the WBC to the overall effect observed remains unclear.13

Clinical Rationale

Platelets contain many biologically active factors, including many of the proteins responsible for haemostasis, synthesis of new connective tissue and revascularisation. They can stimulate a supraphysiological release of growth factors to jumpstart healing in chronic injuries, or speed up an acute injury repair process. The idea behind PRP treatment is that all stages of the repair process are controlled by a wide variety of cytokines and growth factors acting locally as regulators of the most basic cell functions, using endocrine, paracrine, autocrine and intracrine mechanisms.6 More than 95% of presynthesised growth factors are secreted within 1 h of activation from the a granules. After the initial burst of PRP-related growth factors, platelets synthesise and secrete additional growth factors for the remaining 7–10 days of their lifespan.9 14 Typically, blood, such as the haematoma formed in a muscle tear, contains approximately 94% red blood cells (RBC), a small amount of platelets (6%) and less than 1% leucocytes. The rationale for PRP therapy lies in reversing the blood ratio by decreasing RBC, which are less useful in the healing process, to approximately 5%, and increasing the platelet amount to 94% to stimulate recovery.4 9

The main growth factors in the PRP concentrate are the transforming growth factor β1, platelet-derived growth factor, vascular endothelial growth factor, epithelial growth factor, hepatocyte growth factor and insulin-like growth factor 1. Most of these growth factors play key roles in tendon, muscle, ligament, cartilage and bone healing by stimulating angiogenesis, epithelialisation, cell differentiation– replication–proliferation and the formation of extracellular matrix.15,–,17

Most early studies concentrated on purified isolated growth factors that were known to play a specific role in tissue healing. Only in the past decade has it been recognised and put into practice that the need to target various signalling pathways requires the administration of a balanced combination of mediators, as isolated growth factors would not be able to satisfy the multiple requirements of the injured tissue. PRP has been used to enhance the healing of meniscus defects18 and muscle injuries,19 20 and stimulate chondrocytes to engineer cartilaginous tissue,17 reduce pain and produce better and more balanced synovial fiuid in arthritic knees,21 improve outcomes after total knee arthroplasty22 and subacromial decompression,19 accelerate bone formation,23 stimulate the healing of an anterior cruciate ligament injury central defect, its primary repair or its reconstruction,24 25 improve the outcome of operated ruptured Achilles tendons,15 reduce pain in chronic tendinopathies26 and prevent and reverse intervertebral disc degeneration.27 However, the efficacy of PRP in spinal fusion has been questioned.14 It is remarkable that very few randomised controlled trials have been performed, and even fewer trials are adequately powered, use appropriate outcome measures and have decent follow-up.

The 2008 “Aspetar Consensus”, organised by the World Anti-doping Association (WADA) and the International Olympic Committee to debate possible conflicts with the WADA code, discussed the use of PRP in muscle injuries,28 concluding that further research is necessary. “The application of the WADA Therapeutic Use Exemption (TUE) process is the preferred approach when wishing to utilise growth factors technology in elite athletes, however the ability of the TUE committee to appropriately evaluate such applications is inhibited by the current level of scientific evidence.” The position statement concludes that “WADA will not be in a position to evaluate its clinical utility for either assessment of TUE applications or the prohibited list.” In section S2 of the Aspetar Consensus, concerning any autologous product that contains growth factors, the only actual factor mentioned in connection with PRP is insulin-like growth factor 1, which, while present in PRP, is systemically subtherapeutic by a factor of 500; only 1% of it is unbound, available and active, with a half life of 10 minutes.4 29

Another concern is that PRP might produce genetic instability, potentially leading to neoplasms. Growth factors act on cell membranes rather than on the cell nucleus, and activate gene expression by internal cytoplasmic signal proteins, which promote normal, not abnormal, gene expression.2 Growth factors are not directly mutagenic, and act through gene regulation and normal wound healing feedback control mechanisms. Furthermore, the systemic effects on circulating growth factors from a local PRP injection showed a very brief reduction of blood growth factors.30

The modalities of use of PRP vary. The use of non-steroidal anti-inflammatory drugs in the early post-injection period may exert an inhibitory effect on healing, and the use of local anaesthesia at the injection site is controversial.14 Extra-articular injections are performed under ultrasound guidance, and it is suggested that the haematoma, if present, should be evacuated and replaced with PRP.

Oversimplification

Over this background, the sceptics point out that, given the well concerted healing cascade that has evolved over millions of years, it is not easy to understand how a single or even a few injections of a cocktail of growth factors at variable, and at present not well codified, times from the injury will produce a lasting beneficial effect on a wide variety of conditions.

The aim of PRP injections is to achieve predictable and fast tissue repair through a new well-organised extracellular matrix, which ideally would reach the high mechanical performance and functional levels of native tissue in the shortest time possible. Despite the hype of the technique and its biological plausibility, the anecdotal nearly miraculous recovery reported in the lay press in some famous athletes, and the myriad of extremely favourable retrospective and prospective studies published, level I investigations are lacking. We prompt researchers to undertake appropriately powered level I studies with adequate and relevant outcome measures and clinically appropriate follow up.