## 4 Discussion  
The results of this systematic review highlight the growing body of evidence supporting the use of PRP in the treatment of tendon and ligament injuries in horses. Among studies with a low risk of bias and robust methodology (e.g., refs. [44; 45; 48; 49; 50]), PRP demonstrated consistent improvements in lameness scores, ultrasonographic appearance, and return-to-competition rates compared to untreated controls. However, these findings were less conclusive in studies with a moderate-to-high bias risk, underscoring the need for a cautious interpretation. On the other hand, PRP has been found to promote tissue healing, stimulate neovascularization, and improve collagen fiber alignment, particularly in experimental models [48; 49; 50; 51; 52; 53]. However, despite these positive findings, there are significant limitations that must be considered, particularly regarding the study design, PRP preparation protocols, and variability in outcome measures. The results of this systematic review strongly support the safety of intralesional PRP injections in horses. No significant adverse effects such as infection or severe inflammation were reported following PRP administration in the 22 studies reviewed. This suggests that PRP is a well-tolerated treatment option for equine tendon and ligament injuries. The safety profile of PRP is further supported by experimental studies in which no evidence of cytotoxicity, excessive fibrosis, or adverse immune responses were observed [48; 49; 50; 51; 52; 53; 54]. However, the lack of standardized criteria for reporting mild adverse events or transient inflammatory responses is a limitation that should be addressed in future clinical trials. The studies reviewed lacked sufficient information on the concentration of leukocytes in PRP. Only nine of the twenty-two studies [37; 38; 40; 45; 48; 50; 51; 55; 56] reported the concentrations of these cells, allowing these PRPs to be classified as L-PRP, and four studies [38; 40; 51; 55] classified PRP as P-PRP [20; 21]. In general, the clinical outcomes of these studies were positive, although one study [56] had a neutral result using an L-PRP preparation. The information obtained from the analysis of these studies does not allow a conclusion as to which type of PRP (either P-PRP or L-PRP) may be more appropriate for the treatment of tendon and ligament injuries in horses. These findings may contradict a previous in vitro study of equine tendons, where P-PRP may be the optimal preparation to stimulate superior healing without scar tissue formation [57]. This highlights the ur  
Figure 6: (**A**) Traffic light plots of the domain-level judgments for each study. Each study is color-coded to indicate the risk of bias across various domains. (**B**) Weighted bar plots showing the distribution of risk of bias ratings within each domain for experimental studies of horses treated with PRP and mesenchymal stem cells. The bars represent the proportions of studies judged according to each domain. gent need for comparative studies that specifically evaluate the influence of the leukocyte concentration on PRP efficacy, particularly under clinical conditions. Significant heterogeneity in platelet and leukocyte concentrations was observed among the twenty-two studies reviewed, with only nine [37; 38; 40; 45; 48; 50; 51; 55; 56] reporting specific leukocyte counts. This variability makes it difficult to draw definitive conclusions regarding the optimal platelet and leukocyte concentrations in PRP for the treatment of equine tendon and ligament injuries. In addition, two studies with negative clinical outcomes reported conflicting results: one used a semi-automated kit to prepare P-PRP [46], while the other used a semi-automated kit to prepare L-PRP [47]. These results suggest that the concentrations of platelets and leukocytes may not be the only critical factors influencing treatment efficacy. Instead, other aspects, such as the appropriate selection of patients for PRP therapy, may play a more important role in determining clinical outcomes.