Platelet rich plasma in regenerative tendon repair mechanobiological interactions and clinical application

Tendon injury prevalence in both athletic and occupational settings is on the increase. Tendon healing properties are poor, despite the complex biological process triggered by the injury, which makes those injuries incapacitating for months. A significant proportion of these injuries remain difficult to treat, and many patients suffer from decreased performance and longstanding sequelae. While mechanical stabilization has been a hallmark of tendon surgical management, orthobiologics are playing an increasing role in optimizing tendon healing. Platelet rich Plasma (PRP), which is a volume of autologous plasma having platelet concentration above baseline, has been suggested as an accelerant orthobiological agent rich in supraphysiological concentration of growth factors. However, strong evidence of its mode of action and of its clinical efficacy is lacking.
The purpose of this thesis is to identify the role of PRP in tendon regeneration in in-vitro and clinical studies. Firstly, the viability and biological components of clinically-prepared PRP were studied in novel experiments. This PRP was used in linked in-vitro studies to investigate the possible mechanism of PRP effect on the injured Achilles tendon cells and tissues. Cell count, viability, proliferation and DNA content were studied. The clinical application of PRP in Achilles tendon rupture was assessed in a randomised clinical pilot study using a combination of PROMs, objective outcome measures and a novel imaging modality called functional ultrasound elastography. This non-invasive technique was developed in a healthy-tendons volunteer study and its feasibility in ruptured tendons was assessed in the pilot trial. In another unique study, the immunohistochemical response to PRP was assessed in biopsies taken under US guidance at week 6 and compared to control to explore the possible mechanism of PRP effects.
The findings confirmed that PRP is a viable activatable autologous blood product rich in growth factors. The results also confirmed that leukocytes and platelets are present in very high concentration with reversal of lymphocyte neutrophil ratio. Elastography volunteer study confirmed that FUSE is feasible using clinically applicable ultrasound scan. The improved algorithm allowed visualisation of localised strain within the studied tissues. The clinical application of PRP in Achilles tendon rupture revealed positive efficacy signal that PRP led to faster healing, improved pain and earlier restoration of function. However, the findings of this pilot trial were indicative and not confirmative. Immunohistochemistry analysis showed that PRP enhanced the maturity of the healing tendon tissues by promoting better collagen I deposition, improved Collagen III/Collagen I ratio, reduced cellularity, better vascular structure and higher GAGs content when compared with control. The finding may explain the clinical improvement observed in these patients at week 6 onwards. Linked in-vitro studies showed that autologous PRP with its cellular components, which include platelets, leukocytes and erythrocytes, has the ability to stimulate tendon cell migration to the injury site and stimulate proliferation in the injured human tendon. Additionally, it may maintain tissue viability in the hypoxic environment that follows tendon injury. Promoting migration and proliferation of cells and maintain tissue viability may play an important role to accelerate tendon healing.
The findings of this project has informed the design of a phase II large multi-centre randomised controlled trial and helped secure major funding from the National Institute of Health Research (NIHR). This trial will set the scene for PRP use in tendon treatment.

Keywords: GrowthFactors
Platelet rish plasma
ultrasound elastography
PRP
autologous blood
tendon rupture
tendinopathy
achilles
Tendon
Advisor: Willett, K
Thompson, M
Sarah, F