Critical-Sized Bone Defects in Mandible of Canine Model

Regeneration of critical-sized bone defects (CSBD) is one of the major challengeable issues in bone tissue engineering. The minimal size of CSBD is different with regard to various bones of different animal species. It is acceptable that the rate of newly formed bone in a CSBD could not be more than 10% during the lifetime of an animal. ¹ According to the literature, segmental mandibular CSBD in canine model greater than 15 mm demonstrated minimal amount of newly formed bone from the defects edge even after 6 months. ²

However, we read with great interest the article by Liu et al. ³ entitled “Mesenchymal stem cells systemically injected into femoral marrow of dogs home to mandibular defects to enhance new bone formation.” According to this study, although mandibular bone defects had similar size with defined CSBD, the results of new bone formation in empty defects could be arguable. ³ For instance, the rate of new bone formation, calculated based on the below equation from figure 4 of this study, in empty defects was approximately about 14% in the defects only after 6 weeks: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} { \rm { New } } \, { \rm { bone } } \, { \rm { formation } } \, \, \left( \% \right) = { \frac { { A_1 } } { { A_0 } } } \times 100 , \end{align*} \end{document}

in which A₁ is the area under the curve of figure 4 and A₀ is the defect width multiplied by the defect depth. In fact, the defect form created by Liu et al. was not in accord with the segmental mandibular CSBD confirmed in the literature. ² Also, the medial cortical plate of the mandible was left intact.

Skeletal and vascular elements develop simultaneously in a soft cartilage callus environment to regenerate bone defects. ⁴ Briefly, upon injury, hematoma formation initiates a healing cascade, which supports formation of granulation tissue as a template for a cartilage callus. The importance of initial fracture hematoma for optimal healing has been addressed in animal models. ⁵ Also, prolonged inflammation following injury can restrict the bone healing process. Therefore, it can be concluded that apart from creating the accurate size of the defect, creating the correct form of the defect and interrupting the natural bone healing cascade, for example, by preventing the fibrin clot and hematoma formation, can be essential to have an accurate CSBD in animal model of the research.