There has recently been growing interest worldwide in biological therapies such as platelet-rich plasma injection for the treatment of knee osteoarthritis. However, predicting the effectiveness of platelet-rich plasma therapy remains uncertain. Therefore, this retrospective cohort study was performed to assess a range of predictors for the effectiveness of platelet-rich plasma therapy in treating knee osteoarthritis. The study included 517 consecutive patients who underwent three injections of leucocyte-poor platelet-rich plasma therapy from 2016 to 2019 at a single institution. The treatment outcomes, including patient-oriented outcomes (visual analogue scale score and Knee Injury and Osteoarthritis Outcome Score), were analyzed and compared according to the severity of knee osteoarthritis based on Kellgren–Lawrence (KL) grading using standing plain radiographs. Fisher’s exact test, univariate regression, and multivariate regression were used for data analysis. Patient-oriented outcomes were significantly improved 6 and 12 months after platelet-rich plasma therapy. The overall responder rate in patients who met the Outcome Measures in Rheumatology (OMERACT)–Osteoarthritis Research Society International (OARSI) responder criteria was 62.1%. The responder rate was significantly lower in patients with severe knee osteoarthritis (KL4, 50.9%) than in those with mild (KL2, 75.2%) and moderate (KL3, 66.5%) knee osteoarthritis. The multivariate logistic regression analysis revealed that deterioration of the knee osteoarthritis grade (increased KL grade) was a significant predictor of a worse clinical outcome (odds ratio, 0.58; 95% confidence interval, 0.45–0.75; p < 0.001). The relative risk for non-responders in severe (KL4) KOA was 2.1 (95% CI, 1.5–3.0) at 6 months and 2.3 (1.6–3.2) at 12 months compared with mild-to-moderate (KL2-3) KOA. The efficacy of platelet-rich plasma therapy was not affected by age, sex, body weight, or platelet count. This study revealed that the effectiveness of platelet-rich plasma therapy for the treatment of knee osteoarthritis is approximately 60% and that the effectiveness depends on the severity of knee osteoarthritis. This observation is useful not only for physicians but also for patients with knee osteoarthritis.

Objective: Platelet-rich plasma (PRP) therapy has become a popular treatment for knee osteoarthritis. We aimed to determine the outcomes of knee osteoarthritis patients following PRP therapy using magnetic resonance imaging (MRI) findings and patient-reported outcome measures (PROMs). Design: In this retrospective observational cohort study, we enrolled 161 patients (221 knees) with varus knee osteoarthritis who received multiple PRP injections at our hospital from June 2017 to June 2019. Patients underwent whole-body MRI before and 6 months after treatment. Whole-organ MRI score (WORMS) cartilage integrity and synovial fluid volume were assessed for the medial femorotibial (MFTJ), lateral femorotibial (LFTJ), and patellofemoral joints (PFJ). Pain visual analog scale and Knee Injury and Osteoarthritis Outcome scores were used as PROMs. In addition, a historical control group of 30 patients with medial knee osteoarthritis who did not receive intra-articular injections was evaluated by MRI over the same period for comparison. Results: After 6 months of PRP therapy, the mean WORMS cartilage score of the LFTJ and PFJ and the total WORMS cartilage score for all three joints improved significantly, and synovial fluid volume reduced significantly. Moreover, a reduction in synovial fluid volume correlated with improvements in several KOOS subscales but not with VAS, which may explain the lack of association with responder status. These results suggest that synovial fluid reduction reflects functional improvement but is not a direct surrogate for pain relief. In addition, the change score of WORMS PFJ cartilage correlated positively with clinical outcomes in responders. By contrast, in the control group, no compartment demonstrated improvement in WORMS cartilage scores, and several compartments showed a trend toward deterioration. Conclusions: In this retrospective observational study, PRP therapy was associated with improvements in WORMS cartilage integrity scores and reductions in synovial fluid volume, with partial correlations to patient-reported outcomes. The inclusion of a historical control group strengthens the interpretation of these findings, although definitive conclusions cannot be drawn. Further randomized controlled trials are needed to confirm these preliminary observations.

Pain-relieving drugs, such as nonsteroidal anti-inflammatory drugs and hyaluronic acid, are standard treatments for knee osteoarthritis (OA); however, no disease-modifying drugs exist for knee OA. Platelet-rich plasma (PRP) is a novel treatment with both symptom improvement and disease-modifying effects; however, the underlying mechanism remains unknown. In addition, the biologically active substances contained in PRP differ greatly depending on the purification method used. This controlled laboratory study aimed to investigate the therapeutic effects of two types of PRP with different white blood cell concentrations on knee OA using an animal model. Leukocyte-rich PRP (LR-PRP) and leukocyte-poor PRP (LP-PRP) were prepared from 10-week-old female C57BL/6 mice. A mouse model of knee osteoarthritis was generated by the unilateral transection of the medial meniscus in the right hind limb. Mice were randomly assigned to three treatment groups that received 6 μL intra-articular injections of either phosphate-buffered saline (control), LR-PRP, or LP-PRP at 2-, 4-, and 6-weeks post-surgery. Mice were sacrificed 12 weeks post-surgery and histologic analysis, immunohistochemistry analysis for CD68 and three‐dimensional micro-computed tomography (3DμCT) of knee joints were analyzed. Hind limb weight-bearing distribution was measured preoperatively and at 4- and 12-weeks post-surgery. Statistical analyses were performed using GraphPad Prism 9.0.2. P-values of <5 % were considered statistically significant. Histological analysis of the femoral medial condyle (Osteoarthritis Research Society International (OARSI) score) showed that both the LP and LR groups had significantly suppressed cartilage destruction compared with the Phosphate Buffered Saline (PBS) group (P = 0.01). The percentage of CD68-positive synovial macrophages in the lateral joint was significantly lower in the LR group than in the PBS group (PBS: 2.0 ± 2.9 %, LR: 0.6 ± 1.3 %, LP: 0.9 ± 1.9 %; P = 0.02). The affected-side load rate (%) increased in the LR and LP groups, with a significant increase observed in the LR group from week four. PBS group, Pre/4w/12w = 36.0 ± 4.8/33.8 ± 6.8/36.2 ± 7.4 % (P = 0.60); LR group, Pre/4w/12w = 31.9 ± 2.2/34.3 ± 8.3/44.2 ± 4.8 % (P < 0.01); LP group, Pre/4w/12w = 33.5 ± 7.4/36.1 ± 7.4/42.0 ± 5.2 % (P = 0.02). Conversely, no significant difference in BMD was observed between groups. Intra-articular injection of LR- and LP-PRP attenuated cartilage degeneration in the medial femoral condyle in a mouse model of knee osteoarthritis.