Mesenchymal stromal cells and platelet-rich plasma promote tendon allograft healing in ovine anterior cruciate ligament reconstruction

Abstract

Purpose: The effect of bone marrow mesenchymal stromal cells (BMSCs) and platelet-rich plasma (PRP) on tendon allograft maturation in a large animal anterior cruciate ligament (ACL) reconstruction model was reported for the first time. It was hypothesised that compared with non-augmented ACL reconstruction, BMSCs and PRP would enhance graft maturation after 12 weeks and this would be detected using magnetic resonance imaging (MRI).

Methods: Fifteen sheep underwent unilateral tendon allograft ACL reconstruction using aperture fixation and were randomised into three groups (n = 5). Group 1 received 10 million allogeneic BMSCs in 2 ml fibrin sealant; Group 2 received 12 ml PRP in a plasma clot injected into the graft and bone tunnels; and Group 3 (control) received no adjunctive treatment. At autopsy at 12 weeks, a graft maturation score was determined by the sum for graft integrity, synovial coverage and vascularisation, graft thickness and apparent tension, and synovial sealing at tunnel apertures. MRI analysis (n = 2 animals per group) of the signal-noise quotient (SNQ) and fibrous interzone (FIZ) was used to evaluate intra-articular graft maturation and tendon-bone healing, respectively. Spearman's rank correlation coefficient (r) of SNQ, autopsy graft maturation score and bone tunnel diameter were analysed.

Results: The BMSC group (p = 0.01) and PRP group (p = 0.03) had a significantly higher graft maturation score compared with the control group. The BMSC group scored significantly higher for synovial sealing at tunnel apertures (p = 0.03) compared with the control group. The graft maturation score at autopsy significantly correlated with the SNQ (r = - 0.83, p < 0.01). The tunnel diameter of the femoral tunnel at the aperture (r = 0.883, p = 0.03) and mid-portion (r = 0.941, p = 0.02) positively correlated with the SNQ.

Conclusions: BMSCs and PRP significantly enhanced graft maturation, which indicates that orthobiologics can accelerate the biologic events in tendon allograft incorporation. Femoral tunnel expansion significantly correlated with inferior maturation of the intra-articular graft. The clinical relevance of this study is that BMSCs and PRP enhance allograft healing in a translational model, and biological modulation of graft healing can be evaluated non-invasively using MRI.

Keywords: Anterior cruciate ligament (ACL) reconstruction; Autopsy; Biological modulation; Bone marrow-derived mesenchymal stromal cells (BMSCs); Magnetic resonance imaging (MRI); Platelet-rich plasma (PRP).

Fig. 1

a Fibrin sealant at femoral aperture (arrow). b SDFT allograft soaking in PRP. c Intra-osseous injection of PRP into tibia

Fig. 2

Scoring grades for intra-articular graft maturity described by Howell et al. [7]. a Grade 1 b Grade 2 c Grade 3 d Grade 4

Fig. 3

Regions of interest (ROIs) for SNQ Calculation. Blue shows the graft ROI; green circle shows the PCL ROI; and orange shows the background signal

Fig. 4

Scoring grades for tendon–bone healing using femoral tunnel as an example. a Grade 1 b Grade 2 c Grade 3

Fig. 5

Autopsy photographs and corresponding sagittal MRI images of control (a, d), PRP (b, e), and BMSC (c, f) group. Autopsy scores, respectively, for graft integrity: synovial coverage: graft thickness/tension: incorporation at tunnel apertures. a = 2:2:0:1; b = 2:2:2:1; c = 2:3:2:2. In the control group inflammatory tissue is seen between split graft fibres (black arrow). In the PRP group the aperture is not sealed (black arrow) but the aperture is sealed in the BMSC group (black arrow). Regions of the graft appears to be more hypointense in the control and BMSC group (white arrow) but the graft is more homogenous hyperintense in the PRP group

Fig. 6

A box and whisker plots showing scores for each variable (showing median and IQR). n = 5, Mann–Whitney test

Fig. 7

Dot plots showing scores for each variable. a Graft integrity. b Synovial coverage and vascularisation. c Graft thickness and apparent length. d Incorporation at tunnel apertures. n = 5, Mann–Whitney test