A biologic solution to the problem of tendon-to-bone healing in the rotator cuff remains elusive. The repair site may lack the cellular and/or molecular signals necessary to induce appropriate differentiation of transplanted cells. Further studies are needed to determine if cell-based strategies need to be combined with growth and differentiation factors to be effective.

Achieving functional graft integration with subchondral bone poses a significant challenge for orthopaedic soft tissue repair and reconstruction. Soft tissues such as the anterior cruciate ligament (ACL) integrate with bone through a fibrocartilage interface, which minimizes stress concentrations and mediates load transfer between soft and hard tissues. We propose that biological fixation can be achieved by regenerating this fibrocartilage interface on biological or synthetic ACL grafts. This study focuses on the in vivo evaluation of a stratified scaffold predesigned to mimic the multitissue transition found at the ACL-to-bone interface. Specifically, the scaffold consists of three distinct yet continuous phases: Phase A for ligament formation, Phase B for the interface, and Phase C for the bone region. Interface-relevant cell types, specifically fibroblasts, chondrocytes, and osteoblasts, will be tri-cultured on this scaffold, and the formation of cell type- and phase-specific matrix heterogeneity as well as fibrocartilage formation will be evaluated over 8 weeks in a subcutaneous athymic rat model. Acellular scaffolds as well as scaffolds co-cultured with fibroblasts and osteoblasts will serve as controls. It was found that the triphasic scaffold supported multilineage cellular interactions as well as tissue infiltration and abundant matrix production in vivo. In addition, controlled phase-specific matrix heterogeneity was induced on the scaffold, with distinct mineral and fibrocartilage-like tissue regions formed in the tri-cultured group. Cell seeding had a positive effect on both host infiltration and matrix elaboration, which also translated into increased mechanical properties in the seeded groups compared to the acellular controls. In summary, the biomimetic and multiphasic design coupled with spatial control of cell distribution enables multitissue regeneration on the stratified scaffold, and demonstrates the potential for regenerating the interface between soft tissue grafts and bone.

Macrophage depletion following anterior cruciate ligament reconstruction resulted in significantly improved morphologic and biomechanical properties at the healing tendon-bone interface, which we hypothesize are due to diminished macrophage-induced TGF-beta production.

Tendon-to-bone healing occurs by formation of a fibrous, scar tissue interface rather than regeneration of a normal insertion. Because inflammatory cells such as macrophages lead to formation of fibrous scar tissue, we hypothesized immobilization would allow resolution of acute inflammation and result in improved tendon-bone healing. We reconstructed the ACL of 60 Sprague-Dawley rats using a tendon autograft. An external fixation device was used to immobilize the surgically treated knee in 30 rats. We evaluated tendon-bone interface width, collagen fiber continuity, and new osteoid formation histologically. Immunohistochemistry was used to localize ED1+ and ED2+ macrophages at the tendon-bone interface at 2 and 4 weeks. Biomechanical testing was performed at 4 weeks. Interface width was smaller and collagen fiber continuity was greater in the immobilized group. Immobilized animals exhibited fewer ED1+ macrophages at the healing interface at 2 and 4 weeks. In contrast, there were more ED2+ macrophages at the interface in the immobilized group at 2 weeks. Failure load and stiffness were similar between groups at 4 weeks. The data suggest early immobilization diminishes macrophage accumulation and may allow improved tendon-bone integration

The widespread clinical implementation of hamstring tendon (HT) autografts for anterior cruciate ligament (ACL) reconstruction is currently limited by the unpredictable integration of the graft with subchondral bone and a lack of devices that are capable of promoting biological fixation of HT grafts to bone. The site of HT graft fixation within the bone tunnel has been identified as the weak point in the reconstructed ACL, likely due to the failure of the graft to reestablish the physiological tendon-bone interface capable of transmitting load from the ligament to bone while minimizing stress concentration at the interface. Although a fibrovascular tissue has been shown to form at the graft-bone interface, this fibrovascular tissue is non-anatomically oriented compared to the native fibrocartilage found at direct ligament to bone insertions. Interface tissue engineering embodies a new approach for graft fixation, focusing on securing tendon grafts to bone via biological fixation wherein the complex functional interface found natively at tendon and ligament junctions with bone are regenerated at the graft insertion site into the bone tunnels. This study focuses on the in vivo evaluation of a novel biomimetic, triphasic scaffold system co-cultured with relevant cell types found at the graft-bone interface, specifically fibroblasts, chondrocytes, and osteoblasts. The scaffold is intended to promote biological fixation of HT grafts to bone by guiding the reestablishment of an anatomically-oriented and mechanically functional fibrocartilage interfacial region. It was found that the cell-seeded triphasic scaffolds supported cellular interactions as well as tissue infiltration and abundant matrix production in vivo. In addition, controlled phase-specific matrix heterogeneity was induced on the scaffold, with distinct mineral and interface-like tissue regions. The results of this study demonstrate the feasibility of multi-tissue regeneration on a single graft, as well as th- e potential of interface tissue engineering to enable the biological fixation of soft tissue grafts to bone.