Importance of the critical-size bone defect in testing bone-regenerating materials

A substantial interest exists in developing substitute materials and human recombinant bone-inducing factors to enhance bone regeneration both in the craniofacial complex and in other parts of the skeleton. A persistent problem has been to find an animal model that allows for the comparison of different osteopromotive materials. The purpose of this investigation was 1) to determine whether a 5-mm calvarial defect in adult rats fulfilled the requirements of a critical-size bone defect and 2) to discuss the use of this animal model in assessing bone repair in the craniofacial region. Bilateral full-thickness calvarial defects were trephined in the calvarial bone of 56 5- to 6-month-old Wistar rats. Except for minor amounts of bone formation limited to the margins of the defect, none of the defects revealed any signs of spontaneous bone regeneration 6 and 12 months after surgery. The full-thickness 5-mm calvarial defects thus fulfill the criteria for a critical-size bone defect. This animal model allows for an experiment with a paired design, it avoids inclusion of the sagittal suture in the osseous defect, and it thereby minimizes morbidity by reducing the risk of damaging the midsagittal sinus. An adequate experimental model has been developed to evaluate the efficiency of osteopromotive materials in the healing of bone defects in the craniomaxillofacial region.