This study was carried out to investigate the effects of partially threaded and nonthreaded tipped intramedullary (IM) pins in the cancellous bone of fractured and intact femurs in dogs.

At the time of IM pin insertion, mechanical other than biological factors affect the bone exposed to the IM pins. Fractured femurs had more damage from the partially threaded tipped pins compared to the intact femurs.

At 4 weeks, tremendous amount of new bone formation took place, growing into the threads of the pin in the fractured femurs. In spite of osteoclastic activity, there was not evidence of extensive bone resorption. Less amount of new bone formation and greater amount of bone marrow were observed with the tissue threads of intact femur. In the nonthreaded group, fractured and intact femurs had limited osteoclastic activity with fibrous tissue formation.

At 8 weeks, with partially threaded tipped pins of fractured and intact femurs, bone was removed and/or remodeled and replaced with fibrous tissue: There was also tremendous amount of tissue thread resorption (reduction in the height of tissue threads) and bone death in the fractured femur. The tissue threads of partially threaded tipped pins of intact femurs at 8 weeks appeared to have the same patterns on the tissue threads of intact femurs at 4 weeks. In the nonthreaded pin group, osteoclastic bone resorption along with replacement with fibrous tissue and bone necrosis were the characteristic pictures at 8 weeks.