Main finding
The pooled results showed that intra-articular PRP injection appeared to be more efficacious than HA injection for the treatment of KOA in terms of short-term functional recovery. Moreover, PRP injection was superior to HA injection in terms of long-term pain relief and function improvement. In addition, PRP injection did not increase the risk of adverse events when compared with HA injection. The level of evidence, which was undermined by heterogeneity and/or study design limitations, was moderate or low, indicating that the degree of benefit must be studied although the benefit is conclusive. PRP is an autologous concentrate of human platelets isolated through centrifugation of the patient’s blood, containing numerous components containing variety of growth factors, cytokines, and many other bioactive proteins [
40].
Based on preclinical research, it is known that PRP ameliorates the degeneration of cartilage by stimulation of mesenchymal stem cell migration, proliferation, and differentiation into articular chondrocytes. PRP affects the progression of KOA via inhibition of inflammatory cytokines and altering the level of enzymatic expression and thus promotes cartilage repair [
41]. Moreover, several clinical trials and systematic reviews have demonstrated that PRP have the ability to relive osteoarthritic symptoms up to 12 months postinjection, including pain, stiffness, and function failure [
42].
HA is the most important component of articular fluid and responsible for the viscoelastic and lubricant capabilities in joints [
43]. It is involved in chondroprotection, proteoglycan, and glycosaminoglycan synthesis as well as anti-inflammation. In addition, intra-articular HA injection can significantly reduce the apoptosis rates of chondrocytes [
44]. Clinical researches have shown HA injection in patients with KOA has the potential to reduce knee pain, improve function, and quality of life [
39].
Interestingly, numerous studies have focused on the clinical efficacy between PRP and HA in the KOA treatment. Duymus et al. [
26] compared the efficacy of intra-articular injections of PRP with HA for KOA treatment. They found that PRP injection was more successful than HA injection in the treatment of mild–moderate knee OA. PRP application could provide at least 12 months of pain-free daily living activities. Similarly, Lin et al. [
17] investigated the discrepancy between PRP and HA in therapy of KOA and suggested that intra-articular injections of leukocyte-poor PRP (LP-PRP) improved function recovery for at least 1 year in patients with mild-to-moderate osteoarthritis of the knee. Furthermore, Ahmad et al. explored the clinical outcomes of PRP injection with changes in the ultrasonography structural appearance [
22]. They observed that intra-articular injections of PRP were associated with improved synovial hypertrophy and vascularity scores and less effusion. However, PRP injection failed to perform better efficacy than HA injection in several clinical studies. Filardo et al. [
19] found that the patients with PRP injection could not obtain a better clinical outcome than those treated with HA injection. With a long-term follow-up of 5 years, Di Martino et al. [
25] concluded that PRP injection did not provide an overall superior clinical improvement compared with HA injection in terms of functional improvement at any follow-up point. Although LP-PRP injection showed more effective in terms of clinical improvement with respect to HA injection, there was no influence on the X-ray and MRI performance of cartilage progression at 52 weeks follow-up.
Therefore, it still remains a contradiction whether PRP injection is superior to HA injection in the treatment of KOA. Previous systematic review and meta-analysis also evaluated the efficacy of PRP injection compared with HA injection in the treatment of KOA. Laudy et al. [
45] enrolled 10 trials and found that PRP injection performed better clinical outcomes than HA injection on pain reduction at 6 months postinjection. Recent meta-analysis by Han et al. [
44] pooled 14 RCTs and suggested that PRP injection might be more effective with respect to HA injection in terms of long-term pain relief and functional improvement. The biggest flaw of this meta-analysis was that they included bilateral knee OA, and thus, a large clinical heterogeneity existed. However, Zhang et al. [
13] analyzed 13 studies (10 RCTs and 3 non-randomized studies) and concluded that PRP injection was not obviously superior to HA in KOA. Pooled RCTs and non-RCTs for meta-analysis violates the PRISMA guideline for meta-analysis, and selection bias is ineluctable. The present study is, to our knowledge, the most comprehensive, up-to-date, and with the largest sample size (
n = 19; totally, 1281 patients) meta-analysis undertaken to estimate the efficacy and safety of PRP versus HA in OA.
Limitations of current meta-analyses should be noted. Due to the limited evidence available, previous meta-analysis data extracted from retrospective studies [
46] and even case series [
47], which might bring significant bias for the overall analysis. Most comparisons included only 1 or 2 studies due to the small number of clinical trials pooled for meta-analysis. More RCTs are responsible for the evaluation of the efficacy of PRP injection on pain relief and function improvement compared to HA injection. Our meta-analysis included 20 RCTs to investigate the efficacy of PRP injection on pain relief and function recovery compared with HA injection in patients with KOA. In short-term period postinjection (no more than 3 months), PRP injection resulted in better WOMAC function score at 1 month and WOMAC stiffness function score, stiffness score, and IKDC at 3 months compared with HA injection. PRP injection and HA injection had similar effects with respect to the WOMAC pain scores, WOMAC total scores, and VAS scores at 1 month and 3 months. And also, the patients with PRP injection showed similar effects in IKDC and EQ-VAS scores at 2 months, and KOOS at 3 months. In long-term period postinjection (no less than 6 months), we found that better clinical results were achieved in the PRP injection group compared with the HA injection group in terms of WOMAC pain, function, stiffness, and total scores and VAS scores at 6 months and 12 months. Moreover, the patients with PRP treatment showed better performance with respect to IKDC at 6 months and EQ-VAS at 12 months. Nevertheless, there was no significant difference between groups in terms of bias for the overall analysis. Most comparisons included only 1 or 2 studies due to the small number of clinical trials pooled for meta-analysis. More RCTs are responsible for the evaluation of the efficacy of PRP injection on pain relief and function improvement compared with HA injection.
Another issue that affects the effects of PRP is the leukocyte concentration in PRP composition, which may contain more proinflammatory cytokines and be detrimental to cartilage repair. Subgroup analyses were used to identify the potential heterogeneity of the results. A study conducted by Riboh et al. [
48] compared LP-PRP and LR-PRP in the treatment of KOA and found that LP-PRP injections resulted in significantly improved WOMAC scores compared with HA or placebo. These results were consistent with our subgroup findings.
Vilchez-Cavazos et al. [
45] conducted a meta-analysis and revealed that single injection was as effective as multiple PRP injections in pain improvement; however, multiple injections seemed more effective in joint functionality than a single injection at 6 months. Similarly, most of these results were also consistent in different PRP spinning approaches, whether with an activator use and PRP species (fresh or frozen), which suggested that these factors might have little influence on the efficacy of PRP.
Limitations
Some limitations in the current study should be interpreted. Firstly, the main limitation is that most overall analyses are accompanied with high heterogeneity. The high heterogeneity among pooled results has weakened the persuasion of the conclusion. Although we tried to compensate for methodological deficiencies by performing stratified analyses, some results remained inconclusive since several reports lacked the documentation of the key factors. Secondly, although 19 RCTs were included in this study, some indexes such as WOMAC function and stiffness score at 1 month, IKDC at 3 months, EQ-VAS at 2 months, and KOOS at 3 months were analyzed by the data extracted from only two studies. Moreover, similar to the previous meta-analysis studies, we evaluated the efficacy between PRP and HA within 1 year on the account of limited follow-up. Some RCTs explored the long-term follow-up (52 weeks by Buendia-Lopez et al. [
23] and 5 years by Di Martino et al. [
14]). However, we were unable to pool the long-term results from the limited data. Thirdly, almost all included RCTs used subjective questionnaires to deduce the treatment effects. Objective findings such as magnetic resonance and ultrasound seem to be needed in the efficacy evaluation. Finally, the administration of PRP injection was varied in the included RCTs. The present study failed to recommend the optimal administration dosage and interval because of the insufficient data. Therefore, more well-designed RCTs with long-term follow-up are still necessary.