A WNT protein therapeutic accelerates consolidation of a bone graft substitute in a pre-clinical sinus augmentation model

AIM

Autologous bone grafts consolidate faster than bone graft substitutes (BGSs) but resorb over time, which compromises implant support. We hypothesized that differences in consolidation rates affected the mechanical properties of grafts and implant stability, and tested whether a pro-osteogenic protein, liposomal WNT3A (L-WNT3A), could accelerate graft consolidation.

MATERIALS AND METHODS

A transgenic mouse model of sinus augmentation with immunohistochemistry, enzymatic assays, and histology were used to quantitatively evaluate the osteogenic properties of autografts and BGSs. Composite and finite element modelling compared changes in the mechanical properties of grafts during healing until consolidation, and secondary implant stability following remodelling activities. BGSs were combined with L-WNT3A and tested for its osteogenic potential.

RESULTS

Compared with autografts, BGSs were bioinert and lacked osteoprogenitor cells. While in autografted sinuses, new bone arose evenly from all living autograft particles, new bone around BGSs solely initiated at the sinus floor, from the internal maxillary periosteum. WNT treatment of BGSs resulted in significantly higher expression levels of pro-osteogenic proteins (Osterix, Collagen I, alkaline phosphatase) and lower levels of bone-resorbing activity (tartrate-resistant acid phosphatase activity); together, these features culminated in faster new bone formation, comparable to that of an autograft.

CONCLUSIONS

WNT-treated BGSs supported faster consolidation, and because BGSs typically resist resorption, their use may be superior to autografts for sinus augmentation.

Biology of sinus floor augmentation with an autograft versus a bone graft substitute in a preclinical in vivo experimental model

OBJECTIVES

Compared to autografts, bone graft substitutes are slower to consolidate. If we understood why, this might open strategies to accelerate new bone formation and thus shorten the time to implant placement. In this study, we aimed at comparing autologous bone graft with a bovine bone graft substitute in a preclinical sinus lift model.

MATERIALS AND METHODS

The mouse posterior paranasal sinus served as a recipient site for grafting. Autograft from the oral cavity was compared against bone graft substitute using molecular, cellular, and histological analyses conducted on post-grafting days (PSD) 0, 9, 18, and 120.

RESULTS

Either autografts or bone graft substitutes were positioned on the sinus floor and remained in situ throughout the study. At the time of grafting and until day 9, bone graft substitutes were devoid of cells and alkaline phosphatase (ALP) activity while autografts were comprised of viable cells and showed strong ALP (mineralization) activity. Consequently, new bone formed faster in autografts compared to bone graft substitutes (140.21 ± 41.21 µm vs. 41.70 ± 10.09 µm, respectively, PSD9, p = .0143). By PSD18, osteogenesis was evident in autografted and xenografted sites. Osteoclasts identified by tartrate resistant acid phosphatase attached to, but did not resorb the bone graft substitute matrix. Autograft matrix, however, underwent extensive resorption. Transgenic mice revealed that Wnt-responsive osteoprogenitor cells originated primarily from the internal periosteum of the maxillary bone, and not from the Schneiderian membrane.

CONCLUSION

Autografts produce new bone sooner, but bovine bone graft substitutes eventually consolidate and then resist resorption. Enhancing osteoprogenitor cell recruitment to a bone graft substitute constitutes a viable strategy for accelerating bone formation in a sinus lift procedure.

Bioactivating a bone substitute accelerates graft incorporation in a murine model of vertical ridge augmentation

Objective.

Compared to autologous bone grafts, allogeneic bone grafts integrate slowly, which can adversely affect clinical outcomes. Here, our goal was to understand the molecular mechanisms underlying graft incorporation, and then test clinically feasible methods to accelerate this process.

Methods.

Wild-type and transgenic Wnt “reporter” mice were used in a vertical ridge augmentation procedure. The surgery consisted of tunneling procedure to elevate the maxillary edentulous ridge periosteum, followed by the insertion of bone graft. Micro-computed tomographic imaging, and molecular/cellular analyses were used to follow the bone graft over time. Sclerostin null mice, and mice carrying an activated form of β-catenin were evaluated to understand how elevated Wnt signaling impacted edentulous ridge height and based on these data, a biomimetic strategy was employed to combine bone graft particles with a formulation of recombinant WNT protein. Thereafter, the rate of graft incorporation was evaluated.

Results.

Tunneling activated osteoprogenitor cell proliferation from the periosteum. If graft particles were present, then osteoprogenitor cells attached to the matrix and gave rise to new bone that augmented edentulous ridge height. Graft particles alone did not stimulate osteoprogenitor cell proliferation. Based on the thicker edentulous ridges in mice with amplified Wnt signaling, a strategy was undertaken to load bone graft particles with WNT; this combination was sufficient to accelerate the initial step of graft incorporation.

Significance.

Local delivery of a WNT protein therapeutic has the potential to accelerate graft incorporation, and thus shorten the time to when the graft can support a dental implant.