The most important finding of the study was that LR-PRP, which contained a higher concentration of WBCs compared to LP-PRP, induced a higher inflammation reaction in normal rabbit tendon and this effect could be alleviated by administration of NSAIDs both in acute (5 days) and chronic (14 days) phase. This result conﬁrm our hypothesis that LR-PRP induces a higher inflammatory response and this effect can be suppressed by NSAID.
The tendon is a tissue with low blood supply and has a low healing rate after injury. PRP contains many growth factors that have the ability to promote tendon stem cell differentiation  and tenocyte proliferation , and it is believed to have the potential to accelerate tendon healing. This has been widely proven by many basic and clinical studies [9, 24-26]. While most of the studies showed favorable results, some also showed contrasting results. In the study conducted by Harris et al. , PRP has been shown to induce an increased inﬂammatory response in normal rabbit muscle tissues, which includes the infiltration of lymphocytes and monocytes, edema, necrosis, and ﬁbrosis which might be harmful to tendon healing.
The reasons for this phenomenon remain unclear. One probable reason is that various cell components and cytokines play a role in tissue destruction. Leukocytes, which secrete vast amounts of TNF-α and IL-1β, are considered to be the most important source of pro-inflammatory cytokines . In a rat tendon injury model, Marsolais et al.  showed that inflammatory cells, such as granulocytes, ED1, and ED2 macrophages, rapidly accumulate in the injury site. These types of cells contain high amounts of proteases that can damage the tendon tissue and delay injury healing . Dragoo et al. compared acute inflammatory response of two commercial PRP products with different WBCs concentration. The result showed that PRP with higher WBCs concentration caused a greater inflammatory response in acute phase in tendon tissue.
Our study confirmed that LR-PRP, which had a higher concentration of WBCs, induced a higher tendon score than LP-PRP at day 5, thereby showing that WBCs cause an intense inflammatory reaction in the acute phase. This result was consistent with the previous study. In contrast, after 14 days, there was no significant difference between the LP-PRP and LR-PRP groups, showing that the inflammation caused by different PRP injections tends to be comparable in the chronic phase. Furthermore, we investigated whether the inflammation effect could be suppressed by NSAIDs, in which way the damage activity caused by inflammation could be suppressed.
In most PRP extraction methods, higher platelet concentrations often correspond to higher concentrations of WBCs. A recent study by Castillo et al.  compared the products from 3 different commercially available PRP separation systems. The authors emphasize that there was a significant difference between the WBC concentration in products obtained with different systems, as well as different platelet and growth factor concentrations. Therefore, repressing the tissue damage activity of WBCs might decrease the inflammatory response and enhance tissue regeneration.
NSAIDs are often used to alleviate the inflammation in injured tissues by inhibiting the activity of cyclooxygenase. This results in a decrease in the synthesis of prostaglandins, which are pro-inflammatory molecules that have the ability to recruit leukocytes and other immune cells at the injury site . A previous study indicated that NSAIDs can also block the adhesion molecules on WBCs, decrease their migratory abilities, and thus, reduce tissue damage . NSAIDs are reported to have 30–50% sparing effect on morphine consumption to release pain postoperatively. As a highly selective COX-2 inhibitor, which has a lower side effect compare to non-selective COX inhibitor, parecoxib is frequently used to treat many orthopedics diseases.[33-35] We selected parecoxib in the study also because it is an injectable COX-2 inhibitor, which is helpful for accurate drug administration in animal experiment.
Regarding their impact on platelet activation and aggregation, the combinatorial use of NSAIDs and PRP to treat tissue injuries was discouraged in the past. In a recent study, Anitua et al  examined if the intake of NSAIDs could affect the properties of platelets. The results showed that none of the tested NSAIDs affected platelet activation. In addition, the concentration of molecules that play important roles in tissue regeneration, such as VEGF, PDGF-AB, and IGF-1, was not altered.
Our study showed that the inﬂammatory reaction caused by WBCs can be suppressed by NSAIDs both in the acute and chronic phases, as the tendon scores were lower in the LR-PRP+NSAID group compared to the LR-PRP group, both after 5 and 14 days. Although administered only during the first 3 days, NSAIDs impact both the time points. This effect might not have only been caused by suppressing the synthesis of prostaglandins, but also by reducing the WBC recruitment ability that might account for the long-term effect.
The potential limitation of this study was that, we only observed the cellular and histological effects of WBC-induced inflammation, and further studies were required to elucidate the underlying molecular mechanisms. As mentioned above, Anitua et al  has proved that the intake of NSAIDs does not alter the biological properties of platelets in vitro, including the cytokine concentration of inﬂammation, cell proliferation, angiogenesis and cell migration. For this reason, we did not set a LP-PRP group to clarify the effect of NSAIDs on platelet. This could be another limitation of our study, in a following step, we need to test the histological effect of NSAIDs on platelet. Finally, for the reason that there is no ideal chronic tendinopathy model, only healthy tendons were analyzed in our study, a method which also adopted by previous study; therefore, future studies should also include injured tendons to better simulate the clinical situation.