Callus formation after re-injury to experimental bone defect.

Osteochondrogenic callus formation after delayed re-injury at different time intervals was evaluated. Twenty young rats were operated, and a standard bone defect reaching the medulla was sawn into the left hind leg. The rats were divided into two groups and the defect was re-created after 4 or 12 days. The rats were killed 7 or 14 days after the second operation. The developing new bone was revealed with a triple fluorochrome labelling system. The areas of bone and cartilage were measured histomorphometrically. The results suggested that an external factor, re-trauma, could increase both external and internal primary osteogenic callus formation. The optimum time to potentiate bone formation in the defects was found to be earlier than identified in previous fracture studies. After a 4-day delay the external callus was significantly larger than after the 12-day delay. Cartilage formation was not enhanced despite the sudden increase in osteogenic callus formation.