Reduced platelet concentration does not harm PRP effectiveness for ACL repair in a porcine in vivo model

Abstract

Enhanced primary repair of the ACL using a collagen scaffold loaded with platelets has been shown to improve the functional healing of suture repair in animal models. In this study, our objectives were to determine if lowering the platelet concentration would reduce the structural properties of the repaired ACL and increase postoperative knee laxity. Eight Yucatan mini-pigs underwent bilateral suture repair. In one knee, the repair was augmented with a collagen scaffold saturated with platelet-rich plasma (PRP) containing five times the systemic baseline of platelets (5×) while the contralateral knee had a collagen scaffold saturated with PRP containing three times the systemic baseline of platelets (3×). After 13 weeks of healing, knee joint laxity and the structural properties of the ACL were measured. The 3× platelet concentration resulted in a 24.1% decrease in cellular density of the repair tissue (p < 0.05), but did not significantly decrease the structural properties [3× vs. 5×: 362 N vs. 291 N (p = 0.242) and 70 N/mm vs. 53 N/mm (p = 0.189) for the yield load and linear stiffness, respectively]. The 3× platelet concentration also did not significantly change the mean anteroposterior knee laxity at 30° and 90° of flexion [5× vs. 3×: 3.5 mm vs. 5.1 mm (p = 0.140), and 6.1 mm vs. 6.3 mm (p = 0.764)] but did result in a lower AP laxity at 60° [5× vs. 3×: 8.6 mm vs. 7.3 mm (p = 0.012)]. The decrease in platelet concentration from 5× to 3× to enhance suture repair of the ACL did not significantly harm the mechanical outcomes in this animal model.

Figure 1

Schematic diagram depicting the primary suture repair with the collagen scaffold in place. Sutures are fixed proximally with an Endobutton device. The scaffold is threaded onto four of the trailing suture ends (RED) which are then passed through the tibial tunnel and tied over a button to provide initial knee stability. The remaining two suture ends (GREEN) are tied to the sutures in the tibial stump of the ACL. All sutures used in this study were resorbable and there was no evidence of suture material remaining at the time of knee retrieval. (Figure from Murray et al, JBJS, 201011).

Figure 2

Photomicrograph of the ACL at site three, thirteen weeks after rupture using both 3X and 5X PRP, demonstrating the differences in collagen wave crimp. Size bar 100μM.

Figure 3

Ligament Maturity Index scores for the wound area. Values are expressed as mean ± SD, where n=8. # represents a significant decrease between 5X and 3X in CELLULARITY, p<0.05 † represents a significant decrease between 5X and 3X in VASCULARITY, p<0.05