A Prospective Randomized Trial of Biologic Augmentation With Mesenchymal Stem Cells in Patients Undergoing Anterior Cruciate Ligament Reconstruction

Introduction

Anterior cruciate ligament (ACL) tears are a common and devastating injury among young athletes, and re-injury poses significant threats to both successful return to play and long term outcomes. Graft healing during ACL reconstruction (ACLR) is a slow biologic process, that involves graft incorporation into bone tunnel sites, as well as intra-articular graft remodeling . The intra-articular graft remodeling process is known as ligamentization, and occurs in three phases after ACLR. The first phase of early healing occurs between 0 and three months, the second phase occurs between 6 months and one year, and the final phase of maturation occurs after 1 year when the graft appears very similar to the native ACL. The studies that defined these time points biopsied human bone-patella tendon-bone (BTB) ACL autografts at different postoperative points, and described changes occurring over time in regards to vascularization, cellular aspects, and appearance of the extracellular matrix in comparison to control ACLs . Unlike animal models which display a necrotic stage, grafts in human models undergo an early vascular invasion, associated with fibroblasts that are converted to a hyper-cellular matrix with only focalized areas of necrosis . This process occurs as early as 3 weeks postoperatively, and increases over time as areas similar to the native patellar tendon with mature collagen and metabolically quiescent cells decrease . This process is the limiting factor in graft healing for bone-patella tendon-bone (BTB) grafts, as osteointegration occurs earlier .

The processes of osteointegration and ligamentization are responses to the new intrasynovial milieu and physical forces that the new graft is exposed to. During these healing and restructuring processes, failure load and stiffness of grafts decreases by up to 24% at 7 weeks, before strengthening to their potential . This contributes to the long recovery period for ACLR, and the risk of re-rupture during this period. Advances in biologic adjuncts to accelerate and improve this healing process would have a significant impact on the management of ACL injuries.

Recently, biologic preparations of various growth factors have been developed to safely and effectively treat a variety of musculoskeletal conditions, including tendinopathies and arthritis . Bone marrow aspirate concentrate (BMAC) consists of undifferentiated mesenchymal stem cells that are concentrated and applied to the injury site . Previous studies have demonstrated this treatment to offer the pluripotent potential of the cells to impact healing, regeneration, biomechanical strength, and reducing bone-tunnel enlargement . Investigators have demonstrated that mesenchymal stem cell reinforced grafts had a significantly higher failure load and stiffness as early as 8 week post-operation in a rabbit model, suggesting an added benefit of advanced healing . Using BMSCs can offer a novel method to enhance tendon graft osteointegration. While there have been numerous laboratory studies researching the effect of mesenchymal stem cells, there lacks evidence in translation to the clinical setting. Silva et al performed a study where they injected bone marrow derived stem cells (BMSCs) into patellar tendon grafts to examine for increased graft-to-bone tunnel integration. They did not find a significant difference of tunnel integration on MRI imaging, but they did not examine the intra-articular portions for the effect on the ligamentization process, nor did they examine the effect on clinical examination and patient reported outcomes .

Quantitative MRI is a non-invasive method to interrogate tissue properties and evaluate the biochemical composition of tissues. The T2* sequence gives information regarding tissue graft volume, water content, fiber alignment, and tissue density. Median grayscale values (a measure to determine tissue quality)can also be correlated to maximum failure load, yield load, and linear stiffness of grafts in a porcine model. Increases in MRI signals in ACL grafts have been found to be time dependent, becoming well established by three months, particularly at the distal intra-articular portion near the tibial tunnel . These changes have not been shown previously to be predictive of graft failure, but Investigators have recently also confirmed that normalized T2* signal intensity values of ACL grafts have significant correlation with knee instability related to ligamentization and tunnel healing.