A Tissue Engineering Solution for Segmental Defect Regeneration in Load-Bearing Long Bones

Abstract

The reconstruction of large defects (>10 mm) in humans usually relies on bone graft transplantation. Limiting factors include availability of graft material, comorbidity, and insufficient integration into the damaged bone. We compare the gold standard autograft with biodegradable composite scaffolds consisting of medical-grade polycaprolactone and tricalcium phosphate combined with autologous bone marrow-derived mesenchymal stem cells (MSCs) or recombinant human bone morphogenetic protein 7 (rhBMP-7). Critical-sized defects in sheep--a model closely resembling human bone formation and structure--were treated with autograft, rhBMP-7, or MSCs. Bridging was observed within 3 months for both the autograft and the rhBMP-7 treatment. After 12 months, biomechanical analysis and microcomputed tomography imaging showed significantly greater bone formation and superior strength for the biomaterial scaffolds loaded with rhBMP-7 compared to the autograft. Axial bone distribution was greater at the interfaces. With rhBMP-7, at 3 months, the radial bone distribution within the scaffolds was homogeneous. At 12 months, however, significantly more bone was found in the scaffold architecture, indicating bone remodeling. Scaffolds alone or with MSC inclusion did not induce levels of bone formation comparable to those of the autograft and rhBMP-7 groups. Applied clinically, this approach using rhBMP-7 could overcome autograft-associated limitations.

The rational use of animal models in the evaluation of novel bone regenerative therapies

“…Sheep tibia models are considered to be valid and reliable for the evaluation of bone regeneration, with the advantage of having a maximal weight bearing scenario and long bone dimensions in adult animals that are suitable for the testing of human implants and prostheses [165][166][167][168]. However, sheep are seasonal breeders so their bone metabolism changes during the year which presents a significant hurdle for bone metabolism studies.…”

“…The bone mineral density and bone strength in sheep is increased relative to human. Moreover, sheep models of critical size defects are extensively applied to evaluate cell-based therapeutic approaches using autologues, allogeneic or xenogeneic MPCs in combination with growth factors and different types of scaffolds to enhance bone regeneration showing significant advantages compared with cell-unloaded (empty) scaffolds [43,53,166,[169][170][171][172][173][174][175][176].…”

Mechanotransduction and Growth Factor Signalling to Engineer Cellular Microenvironments

“…[35] Sobel et al identified the heparin II binding region of fibronectin (FNIII [12][13][14] ) as a VEGF binding site. [36] They reported that fibronectin fragments including FNIII [9][10] (integrin binding region) and FNIII [12][13][14] endothelial cell migration, proliferation and signalling. [36] Martino and Hubbell generalized this result to show that FNIII [12][13][14] not only bound VEGF but was actually a highly promiscuous region with affinity towards GFs from different families.…”

“…functionalized with two recombinant fragments of fibronectin joined together, FNIII [9][10] , to promote integrin binding and cell adhesion, [35] and FNIII [12][13][14] , to bind GFs. [37] They showed that the system enhanced the formation of tube-like structures in endothelial cells (with VEGF-A), sprouting of smooth muscle cells (with platelet-derived growth factor (PDGF)-BB) and differentiation of mesenchymal stem cells (MSC) (with BMP-2).…”

“…[41][42][43][44] We noted that FNIII [12][13][14] (GF binding region) is located next to the integrin binding region (FNIII [9][10] ) with potential for synergistic integrin/GF signaling. However, for this to be exploited, fibronectin must be open, unfolded, to have the relevant regions available for interaction (Figure 3b).…”