THE MATRIX OF ENDOCHONDRAL BONE DIFFERS FROM THE MATRIX OF INTRAMEMBRANOUS BONE

Osseous tissue develops via two distinctly different processes: endochondral (EC) ossification and intramembranous (IM) ossification. The present study tests the hypothesis that each type of osseous tissue contains unique inducing factors for the promotion of cartilage and bone development. Previous work suggests that subcutaneous implants of demineralized EC and IM bone matrices both induce endochondral ossification. Thus, it concludes that the bone growth promotion properties of the respective matrices are very similar. As it was unclear to us why EC and IM bone powders should possess identical osteoinductive properties, we attempted to reproduce these results. We implanted EC (femoral) demineralized bone matrix (DBM), IM (frontal) DBM, or a mixture of the two into the ventral thoracic subcutaneous tissue of 12 to 15-week-old male Sprague Dawley rats. Morphological and radiolabeling techniques in this study demonstrated that implants of EC bone matrix induce bone formation via EC ossification in contrast to implants of IM bone matrix which do not induce EC ossification. Our findings suggest that the matrix of EC bone differs qualitatively from the matrix of IM bone due to their respective abilities to induce cartilage and/or bone formation. These observations differ from those previously reported possibly because our IM DBM preparations were not contaminated with tissues of endochondral origin. In current clinical practice, EC DBM allografts are often used to induce new bone formation in defects involving both IM and EC bone. We conclude that there may be clinical settings in which it would be more appropriate to replace bone originally formed via IM ossification with IM DBM rather than EC DBM.