More recently, injection of autologous platelet-rich plasma (PRP) has gained a lot of attention in the treatment of sports injuries including acute muscle injury [19] – [24] . The rationale for the use of PRP is the belief that the additional growth factors released by platelets would augment the natural healing process. Despite its increasing popularity as a treatment for muscle injury, there is a growing debate regarding PRP clinical efficacy [25] – [27] . The objective of this review is to explore the current literature on the effectiveness of PRP treatment for acute muscle injury.

Acute muscle injury is one of the commonest types of injury seen in athletes [1] – [3] . This injury often results in loss of training and competition time [4] – [9] . Despite of its frequent occurrence, the best treatment for muscle injury is yet to be identified. Current mode of management usually involves rest, ice, compression and elevation especially in the early stage following injury [10] – [14] . Other modalities includes anti-inflammatory medications (pain killers), rehabilitation exercise programs, electrotherapeutic modalities, hyperbaric oxygen therapy, and prolotherapy injections [15] – [18] . However, clinical evidence to support the use of these modalities is limited.

Each selected article was further evaluated for the methodological quality. Two investigators independently graded the methodological quality of each eligible article using the Physiotherapy Evidence Database Scale (PEDro) for randomized controlled trials [29] . The PEDro scale is an 11-point list using yes and no responses. The first statement pertains to the external validity of the study and is not included to compute the final score. The total score ranges from 0 to 10 and represents the number of positive answers on questions 2–11. The PEDro items are shown in Table 2 . The reliability of PEDro scale is fair to good [30] . A PEDro score of ≥6 was considered to represent a high quality study, whereas a score of ≤5 represented a low quality study [31] . Differences in opinion on any PEDro item score were resolved through discussion until a consensus was reached.

The titles and abstracts of all studies retrieved from the search were reviewed following criteria for study selection to decide if the full-text manuscripts were required for further evaluation. Each full-text manuscript were evaluated systematically according to the study’s, (1) objective/s, (2) characteristics of the study (study design, participants, age and sample size), (3) contents of intervention (intervention strategies, intervention provider, length of intervention and follow-up contacts), (4) targeted outcome/s, and (5) major findings. The outcomes extracted from the selected study were not combined and re-analysed due to the nature of this qualitative systematic review.

All controlled trials and controlled laboratory studies were considered in this review. Studies that were conducted on adults (≥18 years) diagnosed with acute muscle injury and using interventions to promote early recovery were included. The interventions could include one or combination of (1) rehabilitation program and (2) autologous blood products including PRP. No restrictions were defined regarding the type and contents of the control group. The interventions could be compared with no intervention control or minimal intervention control group. The primary outcome measure in the selected studies was the information on the duration to achieve full recovery or duration to return-to-play (DRP).

This qualitative systematic review includes the description of the criteria for study selection and the search methods for identification of studies, detailed qualitative synthesis of the selected studies and the discussion of the findings from this review. The search was conducted according to the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guideline [28] . The process of this search method included describing the data sources, search strategy, data extraction and quality assessment. The supporting PRISMA checklist is available as supporting information; See Checklist S1 .

Studies were searched electronically using the following databases: OvidMEDLINE, PubMed, EMBASE, SPORTDiscus and CINAHL. The reference lists of review articles and included studies were hand searched for other potentially eligible studies using the same selection criteria as described. Published systematic reviews on PRP were used as a source of randomized controlled trials. Peer-reviewed published articles until December 2012 were used. In view of limited resources for translation, only articles published in English were considered. No attempts were made to contact authors for additional information, however, cross-referencing on related previously published study is performed to obtain additional information. The search strategy used for OvidMEDLINE is displayed in Table 1 . Comparable searches were made for the other databases. In addition, a search through a local library for archived articles from the South East Asian region using similar selection criteria was also conducted.

Results

The initial search identified 1016 potential articles from the databases search and another 3 were found through cross-referencing. After removing duplicates, 883 articles were assessed based on titles and abstracts against the selection criteria. A total of 842 articles were excluded because the studies were not on autologous PRP and muscle injury. Of the 41 full-text articles retrieved for further evaluation, only four articles were included in the final qualitative synthesis. The remaining 37 articles were excluded because 35 of these articles were review articles (including systematic reviews) while the remaining two were case reports. Figure 1 describes the PRISMA flow diagram for the study selection. All articles were published after the year 2004, and in English language. Table 3, describes the characteristics of selected studies. Out of the final four studies selected for the review, there was only one controlled trial (CT) [32], while the remaining three were in vivo laboratory studies [32]–[34]. Consequently the discussion on human clinical trial and laboratory studies was conducted separately.

Clinical Study This pilot CT study was conducted in a clinic setting (Clinic for Sports Medicine & Orthopaedic). The participants in this study were professional sportsmen diagnosed with “moderate strains” (second degree). The diagnosis of injury was based on clinical assessment [35] as well as magnetic resonance imaging (MRI) examinations (detection of bleeding of the involved muscle). The mean age of participants and other demographics in the both groups were not available for comparisons. The intervention used in this study was intra-lesional injection of 2.5 ml autologous conditioned serum (ACS) combined with 2.5 ml of saline. The method of ACS preparation was well described. The intra-lesional injection was guided only through palpation of the affected area. Prior to administration of ACS, 5 ml of local anaesthetic (Meaverin 0.5%) was injected in portion of 1 ml to minimise the tonus of the injured muscle. The ACS injections started two days after diagnosis and were repeated every second day until full recovery. The mean number of ACS injection throughout the study was 5.4 injections per patient. Interestingly the control group in this study was a retrospective analysis of 11 patients who had been treated with local injection of Actovegin/Traumeel (3∶2) combination therapy. Actovegin is a deproteinised dialysate of bovine blood, while Traumeel is a homeopathic formulation containing both botanical and mineral ingredients. It is purported to suppress the release of inflammatory mediators and stimulates the release of anti-inflammatory cytokines. Local injection of Actovegin/Traumeel is considered a standard treatment of muscle strain in this centre [32]. The principles of administration were the same as those in the ACS group. The mean number of treatments with Actovegin/Traumeel per patient was 8.3. Participants in both groups underwent the same rehabilitation program and were given oral antipholgistics. The frequency and dosages of these treatments were not specified. The severity of muscle tears was similar between intervention and control groups. However, the extent (size) of the injured area was not documented. The ACS prepared was analysed to determine the types and quantity of growth factors present with ELISA tests. The ACS contains higher concentration of FGF-2 (750%), IL-1Ra (600%), HGF (35%) and TGF-β (31%) compared to levels in the serum [36]. The main outcome measured was the time required to resume full sporting activities. Return to full sporting activities was based on participant’s subjective impression of readiness to resume activities and physiotherapist’s standard examination, including restoration of muscle strength to at least 90% of that of the unaffected limb. The isokinetic strength test described however was not performed, as researchers were concern with the risk of re-injury during testing. The mean recovery time for participants in the ACS group (16.6 days) was significantly shorter compared to the control group (22.3 days.). In addition, MRI scans taken at 16 days in both groups demonstrated faster regression of the oedema/bleeding in the ACS group. Both treatments were considered safe, as there were no local or systemic side effects reported [36].

In vivo Laboratory Studies All studies were controlled animal studies conducted on different species of syngeneic rodents [32]–[34]. Studies differ in their methods of inducing muscle injuries. In one study muscle contusion on the animal’s gastrocnemius muscle was induced by dropping a stainless steel ball on the animal’s hind limb from the height of 100 cm [30]. Whereas Hammond et al. induced eccentric muscle injury over the tibialis anterior muscle by superimposing a single or multiple eccentric muscle contraction onto a maximally isometric contracted muscle [33]. Gigante et al. produced bilateral muscle tears on the longissimus dorsi muscle using a standard pincer technique. As myogenesis relies upon satellite cells activation, proliferation, differentiation, fusion with existing damaged muscle and maturation (increased myofiber diameter) [10]. Accordingly, all studies quantified amount of muscle regeneration via immuno-histochemical staining as one of their outcome measures. Wright-Carpenter et al. used Ki-67 labelled antibody as marker of satellite cells proliferation [30]. Whereas Hammond et al. and Gigante et al. both assayed the level of MyoD and Myogenin as markers of muscle regeneration [33]–[34]. In addition, both Wright-Carpenter et al. and Hammond et al. also quantified the percentages of centrally nucleated fibres (CNFs) presence in the injured area as an additional measure of myogenesis. Only one study assessed the functional recovery of injured muscle using maximal isometric torque test on the tibialis anterior muscle [33].

Characteristics of Interventions The intervention used in each study varies markedly. Using a method originally developed for human blood, Wright-Carpenter et al. utilised blood from 20 syngenic mice to produced autologous conditioned serum (ACS) [37]. Animals in the intervention group received 10 µl of ACS at days 0, 3, 5 and 7. While controls received 10 µl of saline injection administered at similar intervals [32]. Enzyme linked immunoassay (ELISA) tests demonstrated higher level of FGF-2 (460%) and TGF-β1 (82%) in the ACS than serum. Hammond et al. used 20 ml of blood collected from five adult male Sprague-Dawley rats to produce autologous platelet-rich plasma (PRP) using a commercial kit. The autologous PRP was later conditioned using high-frequency ultrasound to lyse the platelets and release the growth factors thus enriching the PRP prior to injection. The ELISA tests demonstrated significantly higher concentration of PDGF and IGF-1 in PRP compared to platelet-poor plasma (PPP). The level of PDGF and IGF-1 further increased (a 5-fold in PDGF and a 27% in IGF-1) upon conditioning. The intervention group was injected with 100 µl of PRP into the injured tibialis anterior while the controls received platelet-poor plasma or no treatment. All injections were administered on days 0, 3, 5 and 7 [33]. In the study by Gigante et al. platelet rich fibrin matrix (PRFM) was prepared using a commercial kit. A single administration of PRFM was filled in one side of the body while the contralateral injured muscle (control) was left untreated [34].

Effectiveness of Interventions Summary of the characteristics of each study is presented in Table 3. The primary outcome in all studies was quantification of muscle regeneration (myogenesis). In two studies this was achieved by immune-histochemical detection of Myogenin and MyoD (markers of muscle regeneration) [33]–[35]. Whereas Wright-Carpenter et al. used Ki-67 marker as indicator of satellite cells proliferation [30]. Only one study assessed muscle functional recovery in addition to the tests mentioned above. Hammond et al., measured maximal isometric contraction of the dorsiflexors before injury and again four minutes after injury (to measure force lost because of injury). Maximal isometric torque was retested at days 3, 5, 7, 14 and 21 after injury [33]. All studies demonstrated significantly greater muscular regeneration in the intervention group than controls [33]–[34]. In addition, Wright-Carpenter et al. demonstrated increased in satellite cells activation as early as 30 & 40 hours after the ACS therapy [32]. Accordingly higher number of central nucleated myofibers (larger diameter fibres) was found in PRP and ACS treated rodents. Interestingly Hammond et al. found PRP therapy had little effect on single-repetition injury protocol. Conversely, in the multiple-repetition protocol, PRP treatment significantly improved contractile function and effectively shortened the time to full recovery from 21 to 14 days [33].