Cartilage Grafts: A Developmental Engineering Strategy to Promote Mandibular Repair

Objectives: Current mandible fracture therapies are based almost exclusively on studies in long bones. However, whether the mandible heals similar to long bones is unknown. Long bones develop via endochondral ossification, while craniofacial bones develop through intramembranous ossification. In long bones, fracture instability promotes endochondral ossification while stability promotes intramembranous ossification. We developed murine mandible fracture models to study the role of stability during healing. Recent evidence shows chondrocytes transdifferentiate into osteoblasts/cytes during endochondral repair in long bones. We confirmed this also occurs in the mandible and applied this information to test whether cartilage grafts could repair critical-sized mandible defects.
Methods: Unstable and stable fractures were generated by creating osteotomies or trephine defects in right mandibular rami of C57BL/6 mice. Mandibles were harvested at multiple times (n=5/time point) and healing was assessed histologically. Immunohistochemistry was used to examine osteogenic and pluripotency gene expression at day 10. Chondrocyte lineage tracing was performed using Aggrecan-Cre^ERT2::dTomato^HZE mice (n=3). Tamoxifen was injected days 6-10 and tissue harvested at day 14. Cartilage grafts were harvested from unstable tibia fractures at day 7 and inserted into 2mm mandibular defects in eGFP reporter mice. Mandibles were harvested at multiple times (n=3/time point) and healing assessed histologically.
Results: Minimal cartilage formed in stable fractures compared to unstable fractures, where maximum cartilage was observed at day 10. Hypertrophic chondrocytes at the chondrosseous junction stained positive for Oct4, Sox2, and osteocalcin. Lineage tracing revealed extensive contribution of chondrocytes to new bone. Mandibular defects receiving cartilage grafts healed by 28 days post-fracture; empty defects did not.
Conclusions: Our data confirm that motion promotes endochondral repair and chondrocytes transdifferentiate into osteoblasts by possibly acquiring a stem cell-like state in the mandible. Also, cartilage is an effective graft for healing critical-sized mandibular defects. Future studies will examine mechanisms of transdifferentiation and test novel therapies to treat non-unions.