Effects of fracture fixation stability on ossification in healing fractures.

Temporal distribution of intramembranous and endochondral bone formation was studied in experimental fracture defects in rats under different stability of fracture fixation and fracture environments. Animals were surgically treated with a specially developed external fixation construct: Group 1 had 42 rats with a 0-mm fracture gap with bone ends touching corresponding to an axial stiffness of 265.00 +/- 34.00 N/mm and Group 2 had 42 rats with a 2-mm fracture gap corresponding to an axial stiffness of 30.38+/- 2.07 N/mm. From each group, six animals were sacrificed at 4 days and 1, 2, 3, 4, 6, and 12 weeks. Qualitative histologic and morphometric analyses revealed that less fixation rigidity and increased fracture gap induces a later response of bone formation and greater endochondral bone formation leading to prolonged time for full ossification. Furthermore, in the early phase of fracture healing temporal distribution and histologic characteristics of periosteal and intramedullary bone formation are similar and not influenced by rigidity and fracture environment. Results also showed that if tissues associated with the intramedullary region are preserved, intramedullary bone formation is substantial. Finally, histologic data indicate that woven bone might be a prerequisite for the differentiation process of endochondral bone formation.