## 4 Discussion
A recent systematic review reported that there is no current evidence that using PRP in ACLR improves functional outcomes [8]. This stays in accordance with results of our study, in which only a slight superiority was noted in most of subsections of KOOS scale. What is more, in the Quality of life section there was an advantage of the no-PRP group. The same applied to the IKDC scale, where the superior results of patients from PRP group did not reach statistical significance. At 18 months postoperatively, harvest site pain was rare in our patients, being present in less than 6% of patients in both groups. However, the pain intensity was greater in the no-PRP group than in PRP group, reflected also by the lower mean score in Pain section of KOOS scale in the no-PRP group. This is in agreement with the study of [PERSON] et al., who reported improved outcomes for the Victorian Institute Sport Assessment (VISA) pain scale in the PRP group at 12 months of follow-up [19]. The same authors reported no significant differences in the VAS at 12 months of follow-up [19]. What is more, in the recent double-blind randomized controlled trial by [PERSON] et al., there was no difference in the VAS between the PRP and no-PRP groups at 12 weeks, 6 months, 1 year and 2 years after surgery [20]. However, VISA pain scale seems more comparable to Pain section of KOOS used in our study, because both these scales consisted of the set of questions assessing pain during different activities [19]. There was a slight tendency towards superiority of the PRP group in dynamic test performance; however, it was not supported by the functional scales. The potential mechanism of faster maturation of the graft and better proprioception in the PRP group could explain such differences [9; 21]. However, it may be too slight to gain clinical significance. Using PRP resulted in 1.4 mm lower side-to-side difference in anterior tibial translation during the arthrometer (Rolimeter, Aircast Europa) stability test. This is comparable with the study of [PERSON] et al., who reported mean side to side difference in the PRP group to be 1.1 mm, compared with 2.2 mm in the control group [22]. Tibial tunnel widening was observed in a slightly lower percentage of patients in the PRP group; however, this difference did not reach statistical significance. However, mean widening was lower in the PRP group. This parameter is of crucial importance, because massive widening of the tunnel makes it impossible for the graft to properly incorporate into bone and remodel. Therefore, lower degrees of tibial tunnel widening can be considered as an indicator of improved graft incorporation into bone [23; 24]. The results of our study are in agreement with the literature reporting improved incorporation of graft into bone when ACLR is enhanced with PRP administration [16; 25; 26]. [PERSON] et al. assessed the formation of the tunnel wall cortical bone encircling the tibial tunnel of an ACLR, which was recognized as an indicator of graft incorporation. They found significantly higher percentages of cortical bone created at two and a half months and six months after surgery in the PRP group compared with controls [25]. [PERSON] et al. reportedbetter remodeling of ACL grafts in the PRP group than in controls at 4 months, 6 months and 12 months postoperatively [16]. In the study of [PERSON] et al., patients who underwent an ACLR with PRP administration achieved higher density of the graft at 6 months, comparable to the density of the posterior cruciate ligament (PCL) [26]. The main limitation of this study was that it was not possible to perform statistical analysis for continuous outcomes. The second limitation was that this study was retrospective with all inherent limitations of such study design. On the other hand, groups in this study were larger than in any other study included in the recent systematic review by [PERSON] et al. and we also analyzed some outcomes that were not reported before in the literature [8].