INTRODUCTION
Platelet-rich plasma (PRP), which was first introduced in the field of oral and maxillofacial surgery by Whitman et al. [
1] in 1997, has more concentrated platelets than normal plasma (approximately 150-400×10
3 cell/µL). Since it was reported that PRP accelerates the healing and regeneration of both soft and hard tissues, its scope of application has been continuously expanded to include various fields of surgery [
2-
4]. The concentrated platelets in PRP have been reported to play an important role in the wound healing process, aggregating rapidly at the damaged site and releasing a variety of growth factors and cytokines that are associated with wound healing, thereby accelerating the process of regenerating soft tissues and bone [
3]. Examples of growth factors that are secreted from α-particles by the activation of platelets include platelet-derived growth factor (PDGF), vascular-endothelial growth factor (VEGF), transforming growth factor-β (TGF-β), insulin-like growth factor (IGF), and epidermal growth factor (EGF) [
5,
6].
Based on these theoretical grounds and advantages, PRP is used as a source for autologous growth factors for various treatment purposes, such as implant placement and bone grafting, chronic wound cure, and fat transplantation [
1,
4]. However, even though PRP is widely used in clinics, the mechanism and actual effects are not completely understood. With numerous studies presenting contradictory results, the use of PRP remains controversial. Moreover, despite the importance of objectively evaluating the effectiveness of PRP through quantitative measurement of the levels of growth factors and platelets contained in the prepared PRP, there has been a lack of data and research related to the quantitative measurement of the constitutive components of PRP [
4]. Thus, in order to determine the critical mechanism of PRP, and to enable an effective application in clinical settings, we sought to determine the necessity of activating PRP by using thrombin and calcium chloride for the quantitative measurement of its constitutive components, such as platelets and growth factors, and for its clinical application.
DISCUSSION
PRP is autologous plasma containing highly concentrated platelets, and plasma containing a large amount of platelets possesses abundant growth factors, enabling healing and regeneration of tissues. The tissues of the human body, when damaged, are regenerated and healed by cell growth and its redistribution and rebuilding, and the following 3 factors play important roles therein. The first factor is a scaffold that forms a framework; the second factor is undifferentiated cells; and the third is a growth factor that is a signal protein secreted from the platelets, plasma, and white blood cells. Among the 3 factors, growth factors play the most important role. It is reported that growth factors are contained in the highest concentrations in the platelets, and that they play a pivotal role during the wound healing process [
7-
12]. For the growth factors contained in the platelets, when damages such as wounds or inflammation occur in tissues, the platelets are activated and the growth factors are secreted from the inner α-particles, including the 3 types of PDGF (PDGF-AA, PDGF-AB, and PDGF-BB) and 2 types of TGF-β as well as IGF and EGF [
5,
13,
14]. In particular, of those growth factors, PDGF and TGF-β play critical roles in the regeneration and healing of the damaged tissues.
PDGF is released from the platelets and plays important roles in relation to various cells. In our study, the level of growth factors in each experimental group was analyzed and compared, and it was found that the PDGF-AB level was 8.28 times higher in the inactivated PRP experimental group than that in the WB, and it was 5.13 times higher in the activated PRP experimental group than in the WB. Further, the PDGF-BB level was 9.12 times higher in the inactivated PRP experimental group than in the WB, and it was 6.07 times higher in the activated PRP experimental group than in the WB. However, Weibrich et al. [
15] reported that PDGF growth factors were at least 5 times higher in the activated PRP than in the inactivated PRP. However, in our study, the PDGF-AB level was increased by only 1.06 times compared with that in the inactivated PRP, showing no statistically significant difference (P>0.05). Further, the comparative analysis of the PDGF-BB value between the activated and inactivated PRP groups did not show statistical significance (P>0.05).
TGF-β, another growth factor in PRP, is a multi-functional potential cytokine involved in the control of cell growth, stimulation of matrix production, and suppression of the immune system. In our study, the analysis and comparison of the level of growth factors in each experimental group revealed that the TGF-β value was 2.55 times higher in the inactivated PRP experimental group than in the inactivated WB, and it was 2.63 times higher in the activated PRP experimental group than in the activated WB, showing statistical significance. Further, the mean TGF-β level in the inactivated PRP group was found to be 120.28±1.74 ng/mL, and that in the activated PRP group was 118.7±1.84 ng/mL, revealing that the concentration of TGF-β was rather decreased in the activated experimental group.
As shown above, our study and that of Roussy et al. [
16] gave different results, and those of other prior analysis regarding the quantitative measurement of the growth factors contained in PRP also varied [
15]. Such differences in the quantitative analysis of growth factors seems to be multifactorial, and include inter-patient variability of the amount of proteins contained in the platelets, different degrees of concentration of platelets during PRP preparation, activation, or inactivation, as well as different degrees of platelet membrane breakage and the degree of platelet activation at the time of measurement [
12].
In 2004, Marx et al. [
9] suggested that 1×10
6 platelet/µL be set as the threshold concentration of therapeutic PRP in order to ensure a therapeutically effective amount of growth factors in PRP, which are important for tissue regeneration and healing. In our study, the concentration factor of platelets was found to be about 4.25 times higher in the inactivated PRP experimental group than in the inactivated WB; in the comparative experiments between the activated WB and PRP, this value was 4.51 times higher in the PRP experimental group than in the WB experimental group. Accordingly, the efficiency of platelet collection in our study was confirmed to be consistent with the effective PRP suggested by Marx et al. [
9], and therein, it was confirmed that activation with thrombin and calcium chloride does not have any relationship with the efficiency of platelet collection. Moreover, as a result of comparative analysis of the efficiency of platelet collection and the level of PDGF-AB, PDGF-BB, and TGF-β before and after the activation of PRP in our study, it was confirmed that there was no statistical difference. This indicates that there is no influence of activation of PRP using thrombin and calcium chloride on the secretion of growth factors and on the amount of platelets concentrated in plasma. As such, it was inferred that adding thrombin and calcium chloride is not necessary for the effective preparation of PRP or the release of growth factors. Concerning the causes for the activation of platelets other than that by thrombin and calcium chloride, the centrifugal force applied for the separation of PRP is worth considering. Marx et al. [
9] suggested a 2-step separation process, and reported that the total rotation should be about 11,000 g including separating rotations and concentrating rotations, which corresponds to about one third of the force required for breaking the membrane of the platelets (30,000 g) [
17]. Taking this as a reference, the recommended rate of centrifugation in our PRP manufacturing system is a total rotation of 31,000 g consisting of the separating rotations of 2,000 g for 3 minutes and the concentrating rotations of 5,000 g for 5 minutes, which is similar to the force capable of breaking the platelet membranes as suggested by Marx et al. [
9]. Accordingly, it was inferred that the platelet membranes were damaged by the high-speed centrifugation in the absence of PRP activation using thrombin and calcium chloride, influencing the activation of platelets, and as such, the growth factors are released. Second, the process of cultivation for 1 hour at 37℃ can also be taken into consideration. It was inferred that as PRP contains fibrinogen, the amount of growth factors, like in the case where calcium and thrombin were added, can be released by cultivation for an appropriate period of time.
Through the present experiments, we quantified the platelets and growth factors in PRP and confirmed a change in the growth factors during the manufacturing process. In conclusion, the concentration of platelets was statistically significantly higher in PRP than in WB, and the levels of growth factors, i.e., PDGF-AB, PDGF-BB, and TGF-β, were also significantly higher in the PRP than in the WB. Moreover, through the analysis of the change of growth factors according to the activation using thrombin and calcium chloride, it was confirmed that no statistical relationships exist among any of the WB and PRP experimental groups. Such experimental results suggest both objective and theoretical grounds in effectively applying PRP in clinical settings, and it is believed that further studies are required to establish a standard manufacturing method that can stabilize the release of growth factors and produce an appropriate amount of growth factors to effectively stimulate the regeneration and healing of tissues in vivo.