PRP Injections for the Treatment of Knee Osteoarthritis: The Improvement Is Clinically Significant and Influenced by Platelet Concentration: A Meta-analysis of Randomized Controlled Trials

Abstract

Background:

Platelet-rich plasma (PRP) has emerged as a promising therapeutic intervention for knee osteoarthritis (OA), attracting substantial clinical and research attention. However, the clinical relevance of the treatment benefit remains controversial.

Purpose:

To evaluate the effectiveness of PRP compared with placebo in patients with knee OA in terms of minimal clinically important difference (MCID) and to investigate the possible influence of platelet concentration on the clinical outcome.

Study Design:

Meta-analysis. Level of evidence 1.

Methods:

The search was conducted on 5 databases (PubMed, Cochrane Library, Scopus, Embase, Web of Science) using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. Inclusion criteria were randomized controlled trials comparing PRP and placebo injections to treat knee OA, written in the English language, with no time limitation. The effects were quantified at 1-, 3-, 6-, and 12-month follow-up points. Visual analog scale (VAS) for pain and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) scores were used, with subanalyses based on platelet concentration performed using a 1,000,000 ± 20% platelets/µL cutoff. The MCID values (VAS, 1.37; WOMAC, 6.4) were used to interpret clinical improvement. The articles’ quality was assessed using the Revised Tool for Risk of Bias in Randomized Trials and the Grading of Recommendations Assessment, Development and Evaluation guidelines.

Results:

Among the 5499 articles retrieved, 18 randomized controlled trials (1995 patients) were included. PRP presented statistically superior improvements in VAS and WOMAC scores compared with placebo at all follow-up points, exceeding the MCID at 3- and 6-month follow-up points for VAS and at all follow-up points for WOMAC. The subanalysis based on platelet concentration showed that high-platelet PRP provided clinically significant pain relief with the improvement exceeding the MCID compared with placebo at 3-, 6-, and 12-month follow-up points. In contrast, low-platelet PRP failed to offer a clinically perceivable benefit in terms of VAS score. WOMAC results showed that both products provided a clinically significant improvement at 3 and 6 months of follow-up. This benefit was maintained up to the 12-month follow-up in the high-platelet group but not in the low-platelet group, where the improvement compared with placebo did not reach statistical significance.

Conclusion:

This meta-analysis showed that PRP offered clinically relevant functional improvement at 1-, 3-, 6-, and 12-month follow-up points and pain relief at 3- and 6-month follow-up points compared with placebo for the treatment of knee OA. Platelet concentration was found to influence treatment efficacy, with high-platelet PRP providing superior pain relief and more durable functional improvement compared with low-platelet PRP.

Keywords

platelet-rich plasma PRP knee osteoarthritis OA placebo

Knee osteoarthritis (OA) is one of the most common orthopaedic diseases and represents a major cause of pain and disability in older adults.^41,43,55 The prevalence of knee OA is currently increasing, placing a heavy burden on health care systems worldwide.^14,36,74 OA is a multifactorial disease characterized by progressive deterioration and loss of articular cartilage with concomitant structural and functional modifications in the entire joint, including synovial inflammation, which is a target of several therapeutic interventions.^32,51,52 However, despite the considerable therapeutic advances made in recent years, unfulfilled medical needs persist for knee OA treatment.^10,18,32,54 Among the nonoperative treatments proposed in clinical practice to improve the altered joint environment, platelet-rich plasma (PRP) has emerged as a promising therapeutic intervention for knee OA, attracting substantial clinical and research attention.⁵

PRP has shown the potential to modulate the intra-articular environment, reducing inflammatory distress and stimulating anabolism in different tissues and exploiting the high concentration of growth factors, cytokines, and bioactive molecules stored in platelet α-granules.²² The anti-inflammatory, immunomodulatory, and anabolic properties may mitigate the degenerative processes of knee OA and improve symptoms and function of the treated joints.⁶ Given the safety of PRP and the simplicity of the preparation technique to obtain its biologically active content, PRP is now considered among many treatment options in clinical practice.³⁷ However, the efficacy of PRP remains a subject of debate, with conflicting evidence reported in the current literature: Although positive results have been documented in several trials, other studies have questioned the real benefit of PRP and its claimed superiority compared with placebo.^{3,12,38,39,66} Moreover, although some outcomes have shown statistical superiority, it is not clear whether the benefit of PRP reaches a minimal clinically important difference (MCID)⁶² and, therefore, a real improvement perceived by patients compared with the benefit of placebo.

The aim of this meta-analysis was to evaluate the effectiveness of PRP compared with placebo in terms of MCID in order to give clear indications on the real benefits offered by PRP injections in the management of knee OA.

Methods

Data Source, Search, and Study Selection

The study was registered on the International Prospective Register of Systematic Reviews (PROSPERO registration No. CRD42023466146). The following databases were systematically searched on September 26, 2023, with no time limitation and without any filters: PubMed, Cochrane Library, Scopus, Embase, and Web of Science. The following string was used in the search: ((PRP OR platelet rich plasma OR plasma rich in growth factors OR PRGF OR platelet derived growth factor OR platelet derived OR platelet gel OR platelet concentrate OR PRF OR platelet rich fibrin OR ACP OR autologous conditioned plasma OR APS OR autologous protein solution OR platelet lysate) AND (knee osteoarthritis)).

Duplicates were removed, and subsequently, all records were checked for eligibility by titles and abstracts. The full-text article was read if insufficient information could be retrieved from the abstract. The following inclusion criteria were used: randomized controlled trials (RCTs) of level 1 or 2 comparing PRP injections with placebo, published in English, and on humans. The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines were used.⁵³ The article selection process was independently performed by 2 authors (A. Bensa, A.S.), with disagreement on study eligibility resolved by a third author (A. Boffa).

Data Extraction and Quality Assessment

Data extracted on trial method from all eligible trials included level of evidence, study design, PRP manufacturing method, and PRP characteristics. Data extracted from all eligible trials on characteristics of the study population included the number of patients, OA level, sex, age, body mass index, inclusion and exclusion criteria, activity level, previous surgical treatments on the index knee, associated lesions, clinical scores, adverse events, and radiological results. Two authors (A. Bensa, A.S.) independently extracted the trial information using a standardized extraction form. When possible, data were collected from the records; otherwise, corresponding authors were contacted. The risk of bias was assessed using the Revised Tool for Risk of Bias in Randomized Trials (RoB 2.0) approved by the Cochrane Collaboration Group, which defines 3 categories: low risk, some concerns, and high risk.⁶⁹ The overall quality of evidence for each outcome was graded according to the Grading of Recommendations Assessment, Development and Evaluation (GRADE) guidelines for high, moderate, low, and very low levels of evidence.⁶⁴

Study Outcome and Statistical Analysis

The primary outcome of this meta-analysis was the overall Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score 12 months after the injection. Secondary outcomes were overall WOMAC score 1, 3, and 6 months after the injection and pain measured on the 0- to 10-point visual analog scale (VAS) at 1-, 3-, 6-, and 12-month follow-up points. WOMAC subscale scores (pain, stiffness, and function) were analyzed as well. The mean differences (MDs) between treatment groups were compared with the MCID reported in the literature for each score: 6.4 of 96 for the overall WOMAC score, 1.5 of 20 for the WOMAC pain subscale score, 0.6 of 8 for the WOMAC stiffness subscale score, 4.6 of 68 for the WOMAC function subscale score, and 1.37 of 10 for the VAS pain score.^1,65 The effect of PRP was assessed using a z test on the pooled MD for continuous variables. To account for study heterogeneity, we used a random-effects model. A subanalysis based on the platelet concentration was performed. To investigate further potential sources of heterogeneity, we analyzed the injection schedule (considering both injection number and timing) of the administered PRP and risk of bias of the included studies. A meta-regression based on PRP-injected volume was performed. Because a commonly accepted cutoff to define platelet concentration is 1,000,000 ± 20% platelets/µL,²⁴ the high–platelet concentration group included PRP above the cutoff of 1,000,000 ± 20% platelets/µL, whereas PRP <800,000 platelets was considered to have low concentration; the third group was represented by the articles not reporting platelet concentration.

A P value of .05 was considered significant for all the analyses. The statistical analysis was performed using meta (Version 4.9-7) and metafor (Version 2.1-0) packages in RStudio, R Core Team (Version 1.2.5019).

Results

Characteristics of the Included Studies and Patients

A total of 18 placebo-controlled RCTs were included out of 5499 records retrieved (Figure 1). Since the first report in 2013, the publication trend increased over time, reaching a peak in 2021 (Figure 2). One study was triple-blinded,³ 15 studies were double-blinded,^‖ and 2 studies were single-blinded.^66,73 In the included studies, 1047 patients underwent PRP injections, whereas 948 patients were included in the control groups. Among these, 1010 (96.5%) and 891 (94.0%) patients in the PRP and control groups, respectively, were followed until the last follow-up of the related studies. The male-to-female ratio was 0.71 in the PRP groups and 0.75 in the control groups. The patients’ mean age ranged from 49.8 to 67.6 years in the PRP groups and from 46.6 to 68.0 years in the control groups. The mean body mass index ranged from 24.0 to 30.9 in the PRP groups and from 24.1 to 31.2 in the control groups.

Figure 1.

PRISMA flowchart of the study selection process. OA, osteoarthritis; PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analyses; RCT, randomized controlled trial.

Figure 2.

Number of randomized controlled trials published over time on the comparison of platelet-rich plasma and placebo for knee osteoarthritis.

Nine studies used low-platelet PRP,^¶ 6 studies used high-platelet PRP,^{12,17,19,28,56,78} and 3 studies did not report platelet concentration.^36,61,66 The number of injections varied from 1 to 3, with 8 studies reporting a single-injection protocol,^{26,38,56,59,72,73,76,78} 2 studies performing 2 injections,^17,58 and 10 studies performing 3 injections.^# OA severity was evaluated using the Kellgren-Lawrence (KL) or Ahlbäck classifications. In the PRP group, 110 patients were classified as KL-1, 375 as KL-2, 294 as KL-3, 62 as KL-4, 92 as Ahlbäck-1, 43 as Ahlbäck-2, and 14 as Ahlbäck-3. In the placebo group, 112 patients were classified as KL-1, 379 as KL-2, 262 as KL-3, 56 as KL-4, 43 as Ahlbäck-1, 36 as Ahlbäck-2, and 14 as Ahlbäck-3. Further studies, patients, and treatment details are reported in Appendix 1 (available in the online version of this article).

Platelet-Rich Plasma Versus Placebo

The meta-analysis on the total WOMAC score showed a statistically and clinically significant superiority of PRP after 1 month (MD, –8.15; 95% CI, –13.75 to −2.54; P = .004), 3 months (MD, –15.90; 95% CI, –22.98 to −8.81; P < .001), 6 months (MD, –15.32; 95% CI, –21.94 to −8.69; P = .001), and 12 months (MD, –14.69; 95% CI, –25.89 to −3.50; P = .01) (Figure 3). The analysis of WOMAC subscale scores confirmed a statistically and clinically significant advantage of PRP. In particular, the improvement in terms of WOMAC pain was 2.29 (95% CI, –3.78 to −0.79; P = .003) after 1 month, 4.00 (95% CI, –5.97 to −2.02; P < .001) after 3 months, 3.97 (95% CI, –5.79 to −2.14; P < .001) after 6 months, and 3.88 (95% CI, –6.25 to −1.51; P = .001) after 12 months. Regarding WOMAC stiffness, the improvement was 0.84 (95% CI, –1.37 to −0.31; P = .002) after 1 month, 1.57 (95% CI, –2.18 to −0.95; P < .001) after 3 months, 1.41 (95% CI, –2.09 to −0.72; P < .001) after 6 months, and 1.34 (95% CI, –2.57 to −0.11; P = .03) after 12 months. Finally, for WOMAC function, the improvement was 5.56 (95% CI, –10.74 to −0.37; P = .04) after 1 month, 10.52 (95% CI, –16.88 to −4.17; P = .001) after 3 months, 10.90 (95% CI, –16.77 to −5.03; P < .001) after 6 months, and 8.40 (95% CI, –14.68 to −2.13; P = .008) after 12 months.

Figure 3.

Forest plot of the individual studies and pooled weighted mean difference (MD) for Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) improvement, including 95% CI. Platelet-rich plasma (PRP) produced statistically and clinically superior improvement compared with placebo at all follow-up points, with these improvements exceeding the 6.4 minimal clinically important difference (MCID). The subanalysis based on platelet concentration showed that both products provided a clinically significant improvement at 3- and 6-month follow-up points, but this benefit was maintained up to the 12-month follow-up only in the high-platelet group. The middle diamond for the 1-month follow-up point and the top diamond for the 12-month follow-up point indicate not statistically significant; all other diamonds indicate statistically and clinically significant (MD > MCID).

According to the meta-analysis on the VAS score for pain, the advantage of PRP over placebo was statistically significant at all of the follow-up points, whereas the MCID superiority was reached at the 3- and 6-month follow-up points. In particular, the difference favoring PRP over placebo was 0.62 (95% CI, –1.13 to −0.11; P = .01) after 1 month, 1.43 (95% CI, –2.16 to −0.71; P < .001) after 3 months, 1.65 (95% CI, –2.33 to −0.97; P < .001) after 6 months, and 1.12 (95% CI, –1.99 to −0.25; P = .01) after 12 months (Figure 4).

Figure 4.

Forest plot of the individual studies and pooled weighted mean difference (MD) for visual analog scale (VAS) pain improvement, including 95% CI. Platelet-rich plasma (PRP) produced statistically superior improvement compared with placebo at all follow-up points, with these improvements exceeding the 1.37 minimal clinically important difference (MCID) at 3- and 6-month follow-up points. The subanalysis based on platelet concentration showed that the improvement of high-platelet PRP compared with placebo exceeded the MCID at 3, 6, and 12 months of follow-up, whereas low-platelet PRP failed to offer such clinically perceivable benefits. The top and middle diamonds for the 1-month follow-up point and the top diamond for the 3- and 12-month follow-up points indicate not statistically significant; the top diamond for the 6-month follow-up point and the bottom diamond for the 1- and 12-month follow-up points indicate statistically but not clinically significant (MD < MCID); the middle diamond for the 3-, 6-, and 12-month follow-up points and the bottom diamond for the 3- and 6-month follow-up points indicate statistically and clinically significant (MD > MCID).

A further analysis was performed on the influence of platelet concentration on the clinical outcome. Further investigations on the possible role of injection schedule, volume, and risk of bias can be found in Appendix 2 (available online).

Subanalysis Based on Platelet Concentration

In terms of WOMAC total score, the subanalysis based on platelet concentration showed no difference at 1 month for high-platelet PRP. However, high-platelet PRP provided a statistically and clinically significant improvement compared with placebo at 3 months (MD, –10.68; 95% CI, –18.01 to −3.36; P = .005), 6 months (MD, –13.42; 95% CI, –25.36 to −1.48; P = .01), and 12 months (MD, –21.49; 95% CI, –31.66 to −11.32; P < .001) of follow-up. The improvement of the low-platelet concentration subgroup compared with placebo was statistically and clinically significant at 1 month (MD, –10.69; 95% CI, –19.00 to −2.38; P = .01), 3 months (MD, –22.28; 95% CI, –29.39 to −15.16; P < .001), and 6 months (MD, –19.50; 95% CI, –28.00 to −11.13; P < .001) of follow-up (Figure 3).

In terms of VAS pain score, the subanalysis based on platelet concentration showed that high-platelet PRP had no statistically significant results at 1 month but provided a statistically and clinically significant improvement compared with placebo at 3 months (MD, –1.94; 95% CI, –2.82 to −1.06; P < .001), 6 months (MD, –2.10; 95% CI, –3.22 to −0.98; P < .001), and 12 months (MD, –1.95; 95% CI, –3.29 to −0.60; P = .01) of follow-up. In contrast, the improvement of the low-platelet concentration subgroup compared with placebo was statistically significant (but not clinically significant) only at the 6-month follow-up (MD, –1.28; 95% CI, –2.44 to −0.12; P = .04) (Figure 4).

Risk of Bias and Quality of Evidence

The evaluation using the RoB 2.0 tool showed that 7 studies had a low risk of bias, 9 studies had “some concerns,” and 2 studies had a high risk of bias. A summary of the risk of bias assessment of the included RCTs is provided in Figure 5.⁴⁸ The GRADE showed that the level of evidence of the results was moderate in almost all the outcomes, with only 2 outcomes having a low level of evidence. A summary of the quality of evidence assessment of the meta-analysis outcomes is provided in Table 1.

Figure 5.

Cochrane risk of bias tool for randomized trials Version 2 (RoB 2.0). D, Domain; +, low risk; −, some concerns; X, high risk. RoB, Revised Tool for Risk of Bias in Randomized Trials.

Table 1

Grading of Recommendations Assessment, Development and Evaluation^a

Outcomes	Risk of Bias	Inconsistency	Indirectness	Imprecision	Publication Bias	Other	Quality of Evidence
VAS, very short-term	No	Serious	No	No	No	No	Moderate
VAS, short-term	No	Serious	No	No	No	No	Moderate
VAS, midterm	No	Serious	No	No	No	No	Moderate
VAS, long-term	No	Serious	No	No	No	No	Moderate
WOMAC pain, very short-term	No	Serious	No	No	No	No	Moderate
WOMAC pain, short-term	No	Serious	No	No	No	No	Moderate
WOMAC pain, midterm	No	Serious	No	No	No	No	Moderate
WOMAC pain, long-term	No	Serious	No	Serious	No	No	Low
WOMAC stiffness, very short-term	No	Serious	No	No	No	No	Moderate
WOMAC stiffness, short-term	No	Serious	No	No	No	No	Moderate
WOMAC stiffness, midterm	No	Serious	No	No	No	No	Moderate
WOMAC stiffness, long-term	No	Serious	No	Serious	No	No	Low
WOMAC function, very short-term	No	Serious	No	No	No	No	Moderate
WOMAC function, short-term	No	Serious	No	No	No	No	Moderate
WOMAC function, midterm	No	Serious	No	No	No	No	Moderate
WOMAC function, long-term	No	Serious	No	No	No	No	Moderate
WOMAC overall, very short-term	No	Serious	No	No	No	No	Moderate
WOMAC overall, short-term	No	Serious	No	No	No	No	Moderate
WOMAC overall, midterm	No	Serious	No	No	No	No	Moderate
WOMAC overall, long-term	No	Serious	No	No	No	No	Moderate

VAS, visual analog scale for pain; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index.

Discussion

The main finding of this systematic review and meta-analysis is that PRP offers a clinically relevant functional improvement at 1, 3, 6, and 12 months of follow-up and pain relief at 3 and 6 months of follow-up compared with placebo for the treatment of knee OA. Platelet concentration was found to influence treatment efficacy, with high-platelet PRP providing superior pain relief and more durable functional improvement compared with low-platelet PRP.

Intra-articular injective therapies represent a well-established nonoperative approach to address knee OA, especially in the early to moderate stages. Available standard intra-articular injective treatments, including steroids and hyaluronic acid, provide only limited clinical benefits, with an effect that often is not completely satisfactory, decreases over time, and varies among patients. In this scenario, PRP has gained attention thanks to its biological properties and its disease-modifying potential to address joints that have OA.^6,7,70 The present study focused on the clinical relevance of the literature findings. In fact, although previous meta-analyses described statistically significant differences, the real effect on patient symptoms and function remained to be elucidated. This up-to-date analysis quantified these benefits on a large number of studies demonstrating statistically and clinically significant benefits of PRP over placebo. To this purpose, the most suitable MCID values available in the literature were selected for WOMAC overall scores and subscale scores and for VAS pain scores, based on the similarity with the present study regarding the type of pathology and treatment on which the MCID values were calculated.^1,65 The results were found to have a decisive clinical effect on patient-reported outcome measures, with WOMAC and VAS improvements exceeding the MCID at all follow-up points and at 3- and 6-month follow-up points, respectively. This represents a crucial aspect because statistically significant results do not always translate into clinically appreciable benefits for patients.⁵⁸ This meta-analysis demonstrated that PRP provided not only statistically significant improvements but also, and most importantly, clinically relevant benefits in terms of pain relief and functional improvement compared with placebo.

The comparison with placebo represents a crucial aspect in the evaluation of clinical improvement generated by injective therapies because the presence of a robust placebo effect, especially for biologic trials where patients perceive they are receiving a “regenerative medicine,” has been reported.²³ A meta-analysis by Previtali et al⁶⁰ demonstrated that the placebo effect is an important component of the clinical effect of injective treatments for patients with knee OA, with saline injections providing relevant and long-lasting results not only in terms of pain relief but also with respect to stiffness resolution and function improvement. Despite the proven role of placebo in PRP injections, the results of the present study based on high-level placebo-controlled studies showed that the benefits generated by this orthobiologic approach were both statistically and clinically superior to the mere placebo effect.

The overall positive findings, however, do not guide physicians in the choice of PRP among the several formulations, with considerable heterogeneity complicating the field of PRP injections. In fact, protocols differ in terms of blood volume harvested, use of anticoagulant, number and speed of centrifugations, final volume obtained, leukocyte content, overall number, integrity, and activation methods of platelets, the possibility of cryopreserving platelets or using fresh products, and the variability in application modalities such as single injections or injection cycles and different volumes and concentrations.^11,31,63 In this scenario, the presence of leukocytes has been one of the most debated aspects regarding PRP efficacy, and it has been used as one of the main discriminants to distinguish PRP products.³⁷ Some preclinical evidence suggests that leukocytes may play an important role in PRP efficacy, with some studies showing that leukocytes impair the overall effects of PRP and other findings supporting the use of leukocytes owing to the release of beneficial cytokines.^6,8,9,33,45 However, despite the claimed potential implications of the leukocyte content on PRP efficacy, a recent high-level RCT reported no differences in the clinical results of leukocyte-rich and leukocyte-poor PRP, suggesting that the effects of leukocytes shown in vitro may not be translated into clinically perceivable differences of PRP injections.¹⁶ This prompts the investigation of further aspects that may influence the clinical benefit offered by platelet concentrates.

An important yet overlooked parameter differentiating the various blood-derived products is the platelet concentration, which might substantially influence the quantity of growth factors released after platelet activation, accountable for tissue repair and regeneration.²⁰ This represents a potentially key aspect contributing to the benefit of PRP injections, with a substantially increasing number of both preclinical and clinical studies suggesting the importance of growth factor dosage.^2,25,44 A large portion of these studies consider a high or very high concentration as the most beneficial for regenerative aims. A number of in vitro studies addressed the effect of PRP concentration on the proliferation of several cell types of mesenchymal origin, such as mesenchymal stem cells, chondrocytes, osteoblasts, or fibroblasts, reporting higher therapeutic potential for products with higher platelet dose.^{29,30,42,46,50,67} Platelet concentration also influenced tenocyte proliferation and migration and matrix gene expression and synthesis in a dose-dependent manner.^4,34 Looking specifically at joint degeneration, investigators examined different doses of platelet concentrates in a rat knee OA model, showing a dose-dependent efficacy in relieving symptoms and preventing cartilage degeneration.⁷⁷ A faster healing time was also observed with higher platelet concentrations in horses with overuse musculoskeletal injuries.⁷¹ Other studies support the use of moderate platelet dose to avoid a cell death phenomenon,²⁷ and a different effect on bone regeneration depending on platelet concentration was observed in a rabbit model, with a positive effect on bone regeneration only within an “intermediate” range.⁷⁵ Overall, these findings support the importance of investigating the effect of the platelet concentration when treating musculoskeletal targets.

These findings have not yet been investigated in the clinical setting. The normal human range of platelet concentration is 150,000 to 450,000 platelets/µL of whole venous blood.⁴⁹ Concentrations of platelets in PRP differ widely, ranging from 2.5 to 8.0 times the concentration of platelets found in whole blood.¹⁵ The range of platelets and concentration methods used lead to products with a high- or low-platelet concentration. Reportedly, the clinical benefit of platelet concentrates occurs more predictably when a 4-fold increase in platelet concentration is achieved.¹³ Accordingly, in the present study, a subanalysis was performed comparing low- and high-platelet PRP using a previously reported cutoff of 1,000,000 ± 20% platelets/µL.^24,47 The results of the current meta-analysis showed that high-platelet PRP provided clinically significant pain relief, with the MD of the improvement compared with placebo exceeding the MCID at 3, 6, and 12 months of follow-up, whereas low-platelet PRP failed to offer such clinically perceivable benefits in terms of VAS. In fact, the pain improvement generated by low-platelet PRP compared with placebo failed to exceed the MCID at all follow-up points. Functional results showed that both products provided a clinically significant improvement at 3 and 6 months of follow-up. However, a difference was documented: This benefit was maintained up to the 12-month follow-up in the high-platelet group but not in the low-platelet group, where the improvement compared with placebo did not even reach statistical significance, suggesting the ability of the former to provide more durable, clinically relevant, functional improvements.

These results offer valuable insights for both research interpretation and clinical practice. Patients receiving PRP with more platelets can experience more substantial improvements in pain reduction and functional outcomes, overcoming the MCID threshold more reliably compared with patients receiving PRP with fewer platelets. Therefore, the use of PRP with high-platelet concentrations should be considered over PRP with low-platelet concentrations, although direct comparisons in high-level studies, specifically focusing on investigating the influence of platelet concentration, are needed to confirm these suggestions. From a research perspective, studies should always report the platelet concentrations of the used PRP in order to facilitate comparisons and a more accurate interpretation of reported results. Nevertheless, these findings add important insight in the scientific discussion on the best type of products for knee OA, although these findings should still be considered with caution and only as a contributing factor in a multifactorial explanation to understand the differences in treatment results. In fact, low-platelet PRP provided better results at short-term assessment, which is probably due to the interplay of platelets and leukocytes. Low-platelet PRP also presented a smaller number of leukocytes, which has not been shown to alter the long-term results but still caused more pain and swelling after the injections and thus could be responsible for the lower short-term improvements.²¹

Several factors likely interact and contribute to the treatment effect, and a possible role may be played by other aspects like injection schedule and injected volume. However, current literature presents a limited number of studies supporting each subanalysis, which prevents us from drawing definitive conclusions on these aspects, which should be further investigated. Although the available data do not allow us to determine whether one factor has a dominant role over the others, this meta-analysis underlines the importance of not focusing PRP investigations solely on the presence of leukocytes, which has been one of the main research targets in the last decade and whose importance was only recently questioned in an RCT.¹⁶ Future studies should explore how to combine these findings, testing whether low-leukocyte and high-platelet PRP products could lead to better results at both short- and long-term follow-up. Finally, it is important to note that the current study’s conclusion could be affected by patient-specific factors, such as patient characteristics, severity of knee OA, and individual response to treatment, and future studies should investigate all these factors to optimize the use of PRP for the treatment of knee OA.

This meta-analysis presents some limitations that require consideration. First, the available RCTs presented substantial variability in terms of characteristics and dose of the injected PRP, reflecting the considerable heterogeneity in the field of PRP injections and the literature addressing this topic. This heterogeneity was also encountered in the analysis of placebo, where the number of injections ranged from 1 to 3, adding another element of variability to the analysis. Second, the selected RCTs lacked standardization in data collection and reporting of outcome measures and associated follow-up times, reducing the amount of data available for the meta-analysis. These aspects convey the urgent need for more high-level studies to identify standardized protocols that can optimize PRP administration, as well as the identification of patient subgroups that could obtain the greatest benefit from this treatment. Third, the MCID is primarily intended as a measure of clinically significant improvement in a patient undergoing a specific intervention, and the high variability in the MCIDs reported in the literature⁵⁷ suggests some caution when considering MCID in regard to the mean change in a heterogeneous population. Nonetheless, the MCID is increasingly used to interpret the relevance of the difference documented in a quantitative synthesis and is a useful tool to evaluate the clinical significance of obtained results.³⁵ Despite these limitations, the present meta-analysis provided important results, shedding new light on the complex and multifactorial field of PRP injections and suggesting the clinical relevance of platelet concentration for PRP injection efficacy in the treatment of knee OA. Far from allowing a definitive conclusion on this matter, these findings provide an important reference point for patients and physicians considering intra-articular injection of PRP, allowing them to form realistic expectations on the potential of this biological approach and offering a new perspective for future PRP investigations to optimize the clinical management of knee OA.

Conclusion

This meta-analysis underlined the potential of PRP as an effective intra-articular treatment for knee OA. This orthobiologic approach provides not only statistically significant improvements but also clinically meaningful benefits. Moreover, the differentiation of PRP formulations by platelet concentration suggests that formulations with higher platelet concentrations may be more advantageous. This finding has implications for both research and clinical practice, prompting efforts to select and develop more effective PRP treatments and allowing a more informed discussion between clinicians and patients regarding the optimal choice of PRP formulation for the treatment of knee OA.

Supplemental Material

sj-pdf-1-ajs-10.1177_03635465241246524 – Supplemental material for PRP Injections for the Treatment of Knee Osteoarthritis: The Improvement Is Clinically Significant and Influenced by Platelet Concentration

Supplemental material, sj-pdf-1-ajs-10.1177_03635465241246524 for PRP Injections for the Treatment of Knee Osteoarthritis: The Improvement Is Clinically Significant and Influenced by Platelet Concentration by Alessandro Bensa, Davide Previtali, Alessandro Sangiorgio, Angelo Boffa, Manuela Salerno and Giuseppe Filardo in The American Journal of Sports Medicine

Supplemental Material

sj-pdf-2-ajs-10.1177_03635465241246524 – Supplemental material for PRP Injections for the Treatment of Knee Osteoarthritis: The Improvement Is Clinically Significant and Influenced by Platelet Concentration

Supplemental material, sj-pdf-2-ajs-10.1177_03635465241246524 for PRP Injections for the Treatment of Knee Osteoarthritis: The Improvement Is Clinically Significant and Influenced by Platelet Concentration by Alessandro Bensa, Davide Previtali, Alessandro Sangiorgio, Angelo Boffa, Manuela Salerno and Giuseppe Filardo in The American Journal of Sports Medicine

Footnotes

The authors declared that they have no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

An online CME course associated with this article is available for 1 AMA PRA Category 1 Credit™ at https://education.sportsmed.org/Public/Catalog/Home.aspx?CourseSearch=1&Criteria=9&Option=25. In accordance with the standards of the Accreditation Council for Continuing Medical Education (ACCME), it is the policy of The American Orthopaedic Society for Sports Medicine that authors, editors, and planners disclose to the learners all financial relationships during the past 12 months with any commercial interest (A ‘commercial interest’ is any entity producing, marketing, re-selling, or distributing health care goods or services consumed by, or used on, patients). Any and all disclosures are provided in the online journal CME area which is provided to all participants before they actually take the CME activity. In accordance with AOSSM policy, authors, editors, and planners’ participation in this educational activity will be predicated upon timely submission and review of AOSSM disclosure. Noncompliance will result in an author/editor or planner to be stricken from participating in this CME activity.

ORCID iDs

Manuela Salerno

Davide Previtali

Alessandro Sangiorgio

Notes

References

Angst

Aeschlimann

Stucki

Smallest detectable and minimal clinically important differences of rehabilitation intervention with their implications for required sample sizes using WOMAC and SF-36 quality of life measurement instruments in patients with osteoarthritis of the lower extremities. Arthritis Rheum. 2001;45(4):384-391.

Assirelli

Filardo

Mariani

, et al. Effect of two different preparations of platelet-rich plasma on synoviocytes. Knee Surg Sports Traumatol Arthrosc. 2015;23(9):2690-2703.

Bennell

Paterson

Metcalf

, et al. Effect of intra-articular platelet-rich plasma vs placebo injection on pain and medial tibial cartilage volume in patients with knee osteoarthritis: the RESTORE randomized clinical trial. JAMA. 2021;326(20):2021-2030.

Berger

Centeno

Steinmetz

NJ.

Platelet lysates from aged donors promote human tenocyte proliferation and migration in a concentration-dependent manner. Bone Joint Res. 2019;8(1):32-40.

Boffa

Filardo

Platelet-rich plasma for intra-articular injections: preclinical and clinical evidence. Methods Mol Biol. 2023;2598:381-390.

Boffa

Salerno

Merli

, et al. Platelet-rich plasma injections induce disease-modifying effects in the treatment of osteoarthritis in animal models. Knee Surg Sports Traumatol Arthrosc. 2021;29(12):4100-4121.

Boswell

Cole

Sundman

Karas

Fortier

LA.

Platelet-rich plasma: a milieu of bioactive factors. Arthroscopy. 2012;28(3):429-439.

Braun

Kim

Chu

Dragoo

JL.

The effect of platelet-rich plasma formulations and blood products on human synoviocytes: implications for intra-articular injury and therapy. Am J Sports Med. 2014;42(5):1204-1210.

Cavallo

Filardo

Mariani

, et al. Comparison of platelet-rich plasma formulations for cartilage healing: an in vitro study. J Bone Joint Surg Am. 2014;96(5):423-429.

10.

Charlesworth

Fitzpatrick

Perera

NKP

Orchard

Osteoarthritis—a systematic review of long-term safety implications for osteoarthritis of the knee. BMC Musculoskelet Disord. 2019;20(1):151.

11.

Chouhan

Dhillon

Patel

Bansal

Bhatia

Kanwat

Multiple platelet-rich plasma injections versus single platelet-rich plasma injection in early osteoarthritis of the knee: an experimental study in a guinea pig model of early knee osteoarthritis. Am J Sports Med. 2019;47(10):2300-2307.

12.

Chu

Duan

, et al. Intra-articular injections of platelet-rich plasma decrease pain and improve functional outcomes than sham saline in patients with knee osteoarthritis. Knee Surg Sports Traumatol Arthrosc. 2022;30(12):4063-4071.

13.

Cole

Seroyer

Filardo

Bajaj

Fortier

LA.

Platelet-rich plasma: where are we now and where are we going?

Sports Health. 2010;2(3):203-210.

14.

Cui

Wang

Zhong

Chen

Global, regional prevalence, incidence and risk factors of knee osteoarthritis in population-based studies. EClinicalMedicine. 2020;29-30:100587.

15.

de Mos

van der Windt

Jahr

, et al. Can platelet-rich plasma enhance tendon repair? A cell culture study. Am J Sports Med. 2008;36(6):1171-1178.

16.

Di Martino

Boffa

Andriolo

, et al. Leukocyte-rich versus leukocyte-poor platelet-rich plasma for the treatment of knee osteoarthritis: a double-blind randomized trial. Am J Sports Med. 2022;50(3):609-617.

17.

Dório

Pereira

RMR

Luz

AGB

Deveza

de Oliveira

Fuller

. Efficacy of platelet-rich plasma and plasma for symptomatic treatment of knee osteoarthritis: a double-blinded placebo-controlled randomized clinical trial. BMC Musculoskelet Disord. 2021;22(1):822.

18.

Drummer

McAdam

Seay

, et al. Osteoarthritis progression: mitigation and rehabilitation strategies. Front Rehabil Sci. 2021;2:724052.

19.

Elik

Doğu

Yılmaz

Begoğlu

Kuran

The efficiency of platelet-rich plasma treatment in patients with knee osteoarthritis. J Back Musculoskelet Rehabil. 2020;33(1):127-138.

20.

Everts

Mazzola

Mautner

Randelli

Podesta

Modifying orthobiological PRP therapies are imperative for the advancement of treatment outcomes in musculoskeletal pathologies. Biomedicines. 2022;10(11):2933.

21.

Filardo

Kon

Pereira Ruiz

, et al. Platelet-rich plasma intra-articular injections for cartilage degeneration and osteoarthritis: single- versus double-spinning approach. Knee Surg Sports Traumatol Arthrosc. 2012;20(10):2082-2091.

22.

Filardo

Kon

Roffi

Di Matteo

Merli

Marcacci

Platelet-rich plasma: why intra-articular? A systematic review of preclinical studies and clinical evidence on PRP for joint degeneration. Knee Surg Sports Traumatol Arthrosc. 2015;23(9):2459-2474.

23.

Filardo

Previtali

Napoli

Candrian

Zaffagnini

Grassi

PRP injections for the treatment of knee osteoarthritis: a meta-analysis of randomized controlled trials. Cartilage. 2021;13(1)(suppl):364s-375s.

24.

Gazzetta Ufficiale della Repubblica Italiana . Provisions relating to blood quality and safety requirements and blood components. December 28, 2015. Accessed July 13, 2023. https://www.gazzettaufficiale.it/eli/id/2015/12/28/15A09709/sg

25.

Gentile

Garcovich

Systematic review—the potential implications of different platelet-rich plasma (PRP) concentrations in regenerative medicine for tissue repair. Int J Mol Sci. 2020;21(16):5702.

26.

Ghai

Gupta

Jain

Goel

Chouhan

Batra

. Effectiveness of platelet rich plasma in pain management of osteoarthritis knee: double blind, randomized comparative study. Braz J Anesthesiol. 2019;69(5):439-447.

27.

Giusti

D’Ascenzo

Mancò

, et al. Platelet concentration in platelet-rich plasma affects tenocyte behavior in vitro. Biomed Res Int. 2014;2014:630870.

28.

Görmeli

Ataoglu

Çolak

Aslantürk

Ertem

Multiple PRP injections are more effective than single injections and hyaluronic acid in knees with early osteoarthritis: a randomized, double-blind, placebo-controlled trial. Knee Surg Sports Traumatol Arthrosc. 2017;25(3):958-965.

29.

Gruber

Karreth

Kandler

, et al. Platelet-released supernatants increase migration and proliferation, and decrease osteogenic differentiation of bone marrow-derived mesenchymal progenitor cells under in vitro conditions. Platelets. 2004;15(1):29-35.

30.

Han

Meng

Tang

Chen

ZB.

The effect of different platelet-rich plasma concentrations on proliferation and differentiation of human periodontal ligament cells in vitro. Cell Prolif. 2007;40(2):241-252.

31.

Harrison

The use of platelets in regenerative medicine and proposal for a new classification system: guidance from the SSC of the ISTH. J Thromb Haemost. 2018;16(9):1895-1900.

32.

Jang

Lee

JH.

Recent updates of diagnosis, pathophysiology, and treatment on osteoarthritis of the knee. Int J Mol Sci. 2021;22(5):2619.

33.

Jayaram

Mitchell

PJT

Shybut

Moseley

Lee

Leukocyte-rich platelet-rich plasma is predominantly anti-inflammatory compared with leukocyte-poor platelet-rich plasma in patients with mild-moderate knee osteoarthritis: a prospective, descriptive laboratory study. Am J Sports Med. 2023;51(8):2133-2140.

34.

Kim

Yoon

Shin

Platelet-rich plasma stimulates cell proliferation and enhances matrix gene expression and synthesis in tenocytes from human rotator cuff tendons with degenerative tears. Am J Sports Med. 2012;40(5):1035-1045.

35.

Johnston

Thorlund

da Costa

Furukawa

Guyatt

GH.

New methods can extend the use of minimal important difference units in meta-analyses of continuous outcome measures. J Clin Epidemiol. 2012;65(8):817-826.

36.

Kan

Chan

Chiu

, et al. Non-surgical treatment of knee osteoarthritis. Hong Kong Med J. 2019;25(2):127-133.

37.

Kon

Di Matteo

Delgado

, et al. Platelet-rich plasma for the treatment of knee osteoarthritis: an expert opinion and proposal for a novel classification and coding system. Expert Opin Biol Ther. 2020;20(12):1447-1460.

38.

Kon

Engebretsen

Verdonk

Nehrer

Filardo

Clinical outcomes of knee osteoarthritis treated with an autologous protein solution injection: a 1-year pilot double-blinded randomized controlled trial. Am J Sports Med. 2018;46(1):171-180.

39.

Lewis

Merghani

Robertson

, et al. The effectiveness of leucocyte-poor platelet-rich plasma injections on symptomatic early osteoarthritis of the knee: the PEAK randomized controlled trial. Bone Joint J. 2022;104-B(6):663-671.

40.

Lin

Yang

Hsu

Yeh

Renn

. Intra-articular Injection of Platelet-Rich Plasma Is Superior to Hyaluronic Acid or Saline Solution in the Treatment of Mild to Moderate Knee Osteoarthritis: A Randomized, Double-Blind, Triple-Parallel, Placebo-Controlled Clinical Trial. Arthroscopy. 2019;35(1):106-117.

41.

Litwic

Edwards

Dennison

Cooper

Epidemiology and burden of osteoarthritis. Br Med Bull. 2013;105:185-199.

42.

Liu

Kalén

Risto

Wahlström

Fibroblast proliferation due to exposure to a platelet concentrate in vitro is pH dependent. Wound Repair Regen. 2002;10(5):336-340.

43.

Loeser

Goldring

Scanzello

Goldring

MB.

Osteoarthritis: a disease of the joint as an organ. Arthritis Rheum. 2012;64(6):1697-1707.

44.

Louis

Magalon

Jouve

, et al. Growth factors levels determine efficacy of platelets rich plasma injection in knee osteoarthritis: a randomized double blind noninferiority trial compared with viscosupplementation. Arthroscopy. 2018;34(5):1530-1540.e1532.

45.

Mariani

Pulsatelli

Cattini

, et al. Pure platelet and leukocyte-platelet-rich plasma for regenerative medicine in orthopedics—time- and preparation-dependent release of growth factors and effects on synovial fibroblasts: a comparative analysis. Int J Mol Sci. 2023;24(2):1512.

46.

Markopoulou

Markopoulos

Dereka

Pepelassi

Vrotsos

IA.

Effect of homologous PRP on proliferation of human periodontally affected osteoblasts: in vitro preliminary study. Report of a case. J Musculoskelet Neuronal Interact. 2009;9(3):167-172.

47.

Marx

RE.

Platelet-rich plasma (PRP): what is PRP and what is not PRP?

Implant Dent. 2001;10(4):225-228.

48.

McGuinness

Higgins

JPT

. Risk-of-bias VISualization (robvis): an R package and Shiny web app for visualizing risk-of-bias assessments. Res Synth Methods. 2021;12(1):55-61.

49.

Miller

Bachowski

Benjamin

. Practice Guidelines for Blood Transfusion: A Compilation From Recent Peer-Reviewed Literature. American Red Cross; 2007:56.

50.

Mishra

Tummala

King

, et al. Buffered platelet-rich plasma enhances mesenchymal stem cell proliferation and chondrogenic differentiation. Tissue Eng Part C Methods. 2009;15(3):431-435.

51.

Mobasheri

Batt

An update on the pathophysiology of osteoarthritis. Ann Phys Rehabil Med. 2016;59(5-6):333-339.

52.

Mobasheri

Rayman

Gualillo

Sellam

van der Kraan

Fearon

The role of metabolism in the pathogenesis of osteoarthritis. Nat Rev Rheumatol. 2017;13(5):302-311.

53.

Moher

Liberati

Tetzlaff

Altman

DG.

Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. Ann Intern Med. 2009;151(4):264-269, w264.

54.

Mora

Przkora

Cruz-Almeida

Knee osteoarthritis: pathophysiology and current treatment modalities. J Pain Res. 2018;11:2189-2196.

55.

Nelson

AE.

Osteoarthritis year in review 2017: clinical. Osteoarthritis Cartilage. 2018;26(3):319-325.

56.

Nunes-Tamashiro

Natour

Ramuth

Toffolo

Mendes

Rosenfeld

Furtado

RNV

. Intra-articular injection with platelet-rich plasma compared to triamcinolone hexacetonide or saline solution in knee osteoarthritis: A double blinded randomized controlled trial with one year follow-up. Clin Rehabil. 2022;36(7):900-915.

57.

Olsen

Bjerre

Hansen

Tendal

Hilden

Hróbjartsson

Minimum clinically important differences in chronic pain vary considerably by baseline pain and methodological factors: systematic review of empirical studies. J Clin Epidemiol. 2018;101:87-106.e102.

58.

Page

Beyond statistical significance: clinical interpretation of rehabilitation research literature. Int J Sports Phys Ther. 2014;9(5):726-736.

59.

Patel

Dhillon

Aggarwal

Marwaha

Jain

. Treatment with platelet-rich plasma is more effective than placebo for knee osteoarthritis: a prospective, double-blind, randomized trial. Am J Sports Med. 2013;41(2):356-64.

60.

Previtali

Merli

Di Laura Frattura

Candrian

Zaffagnini

Filardo

The long-lasting effects of “placebo injections” in knee osteoarthritis: a meta-analysis. Cartilage. 2021;13(1)(suppl):185s-196s.

61.

Qamar

Mohsin

Siddiqui

Naz

Danish

Effectiveness of Platelet Rich Plasma for the Management of Knee Osteoarthritis: A Randomized Placebo Controlled Trial. Pakistan

Journal of Medical & Health Sciences. 2021;15(7):1553-1556.

62.

Redelmeier

Lorig

Assessing the clinical importance of symptomatic improvements: an illustration in rheumatology. Arch Intern Med. 1993;153(11):1337-1342.

63.

Roffi

Filardo

Assirelli

, et al. Does platelet-rich plasma freeze-thawing influence growth factor release and their effects on chondrocytes and synoviocytes? Biomed Res Int. 2014;2014:692913.

64.

Ryan

Hill

How to GRADE the quality of the evidence. December 2016. Accessed July 13, 2023. https://colorectal.cochrane.org/sites/colorectal.cochrane.org/files/public/uploads/how_to_grade.pdf

65.

Saltzman

Leroux

Meyer

, et al. The therapeutic effect of intra-articular normal saline injections for knee osteoarthritis: a meta-analysis of evidence level 1 studies. Am J Sports Med. 2017;45(11):2647-2653.

66.

Saraf

Hussain

Bishnoi

Azam

Habib

Serial platelet-rich plasma intra-articular injections in Kellgren and Lawrence grade IV knee joint osteoarthritis: a prospective blinded placebo-controlled interventional study. Indian J Orthop. 2022;56(10):1722-1728.

67.

Slapnicka

Fassmann

Strasak

Augustin

Vanek

Effects of activated and nonactivated platelet-rich plasma on proliferation of human osteoblasts in vitro. J Oral Maxillofac Surg. 2008;66(2):297-301.

68.

Smith

. Intra-articular Autologous Conditioned Plasma Injections Provide Safe and Efficacious Treatment for Knee Osteoarthritis: An FDA-Sanctioned, Randomized, Double-blind, Placebo-controlled Clinical Trial. Am J Sports Med. 2016;44(4):884-91.

69.

Sterne

JAC

Savović

Page

, et al. RoB 2: a revised tool for assessing risk of bias in randomised trials. BMJ. 2019;366:l4898.

70.

Sundman

Cole

Karas

, et al. The anti-inflammatory and matrix restorative mechanisms of platelet-rich plasma in osteoarthritis. Am J Sports Med. 2014;42(1):35-41.

71.

Torricelli

Fini

Filardo

, et al. Regenerative medicine for the treatment of musculoskeletal overuse injuries in competition horses. Int Orthop. 2011;35(10):1569-1576.

72.

Tschopp

Pfirrmann

CWA

Fucentese

Brunner

Catanzaro

Kühne

Zwyssig

Sutter

Götschi

Tanadini

Rosskopf

. A Randomized Trial of Intra-articular Injection Therapy for Knee Osteoarthritis. Invest Radiol. 2023;58(5):355-362.

73.

Tucker

Goetz

Duncan

, et al. Randomized, Placebo-Controlled Analysis of the Knee Synovial Environment Following Platelet-Rich Plasma Treatment for Knee Osteoarthritis. PM R. 2021;13(7):707-719.

74.

Wallace

Worthington

Felson

, et al. Knee osteoarthritis has doubled in prevalence since the mid-20th century. Proc Natl Acad Sci U S A. 2017;114(35):9332-9336.

75.

Weibrich

Hansen

Kleis

Buch

Hitzler

WE.

Effect of platelet concentration in platelet-rich plasma on peri-implant bone regeneration. Bone. 2004;34(4):665-671.

76.

Hsu

Chang

Chen

. Effects of Platelet-Rich Plasma on Pain and Muscle Strength in Patients With Knee Osteoarthritis. Am J Phys Med Rehabil. 2018;97(4):248-254.

77.

Yan

Zhou

Xie

, et al. Chondroprotective effects of platelet lysate towards monoiodoacetate-induced arthritis by suppression of TNF-α-induced activation of NF-κB pathway in chondrocytes. Aging (Albany NY). 2019;11(9):2797-2811.

78.

Yurtbay

Say

Çinka

Ersoy

Multiple platelet-rich plasma injections are superior to single PRP injections or saline in osteoarthritis of the knee: the 2-year results of a randomized, double-blind, placebo-controlled clinical trial. Arch Orthop Trauma Surg. 2022;142(10):2755-2768.

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