Rotator cuff tears are a source of debilitating pain that commonly affects more than 40% of our aging population. Despite advances in surgical treatment, the failure rate of rotator cuff repairs is as high as 20-90%. Extracellular matrix (ECM) derived scaffolds have recently been investigated as augmentation devices for rotator cuff repairs, but none has yet demonstrated both the appropriate biological and mechanical properties for mitigating re-tears and enhancing healing.

This dissertation proposes to engineer the mechanical properties of allograft fascia lata in a manner that will allow its use as an augmentation device for rotator cuff repairs. This dissertation also aims to develop a simple quasi-linear spring-network model for rotator cuff repairs to elucidate the basic biomechanics of these repairs. The central hypothesis is that engineered fascia lata will have suture retention strength similar to that of human rotator cuff tendon (~250N), even after in vivo implantation. The specific aims are to engineer the mechanical properties of allograft fascia lata ECM and to subsequently evaluate the host response and concomitant mechanical properties of the engineered (reinforced) fascia in a rat model. Further, this dissertation will also develop and validate a spring-network model for simplified rotator cuff repairs.

Studies presented in this dissertation demonstrate stitching as a technology to engineer the suture retention and stiffness of allograft (human derived) fascia lata ECM. Stitching fascia ECM with braided, resorbable, polymer fibers in a unique, controlled manner increased the suture retention load of reinforced fascia scaffolds by six fold over non-reinforced fascia. Additionally, the suture retention properties of reinforced fascia scaffolds were comparable to that of human rotator cuff tendon (~250N) at time zero and even after in vivo implantation for twelve weeks. Except for the increased presence of foreign body giant cells in areas concentrated around the polymer fibers, the host response of the reinforced fascia scaffolds were comparable to the non-reinforced fascia at the time points investigated. The spring-network model predicted that the scaffold component carries ~20-30% of the total load on the repair. Parametric sensitivity analysis predicted that greatest improvements in the force carrying capacity of the repair may be achieved by improving the properties of the tendon-to-bone repair. Parametric simulation studies suggested that in the clinical setting of a weak tendon-to-bone repair, scaffold augmentation could significantly off-load the repair and largely mitigate the poor construct properties. However, engineering a scaffold with supra-physiologic stiffness would not translate into stiffer or stronger repairs.

The results of this dissertation show that reinforced fascia scaffolds may have and possibly maintain mechanical properties comparable to the suture retention properties of human rotator cuff tendon. This suggests that reinforced fascia scaffolds may be able to provide mechanical augmentation to rotator cuff repairs and also modulate tendon retraction in a manner that reduces the incidence of tendon re-tear. The spring-network model provides a starting point to develop more clinically relevant models for rotator cuff repairs.