In vitro pullout strength of screws inserted in adult equine third metacarpal bone after overdrilling a 4.5-mm threaded insertion hole.

The study investigates the strength of two different types of bone screws, 5.5-mm cortical and 6.5-mm cancellous, when inserted into the bone of adult horses. It was found that both screws had similar strength in the diaphysis (the shaft) of the bone, but the 6.5-mm cancellous screw had greater holding power in the metaphysis (the end) of the bone.

Research Design and Methods

• The study was conducted in vitro, meaning that it was performed outside a living organism. This was done on two independent cadavers, each consisting of 14 adult equine third metacarpal (MCIII) bones.
• Two 4.5-mm cortical screws were initially placed either in the shaft (diaphysis) or end (metaphysis) of the horse bones and these were then overdrilled with a 4.5-mm drill bit. Following this, a 5.5-mm cortical or a 6.5-mm cancellous screw was inserted into the overdrilled hole.
• Finally, pullout tests were performed on the new screws at a rate of 0.04 mm/second until either the screw or the bone failed, that is, could no longer sustain the pressure being applied.

Study Findings

• In the diaphyseal bone, the screws failed in all tests. The tensile breaking strength for 5.5-mm cortical screws and 6.5-mm cancellous screws was not significantly different.
• In the metaphyseal bone, the bone failed in all tests. However, the holding power for 6.5-mm cancellous screws was significantly greater than 5.5-mm cortical screws in the metaphysis.
• There was no difference in the tensile breaking strength of screws in the diaphysis between proximal and distal screw holes. However, the holding power was significantly greater in the distal, compared with the proximal, metaphyseal holes.

Conclusions and Recommendations

• Based on the results, the authors concluded that although tensile breaking strength was not different between 5.5-mm cortical and 6.5-mm cancellous screws in middiaphyseal cortical bone, the holding power of 6.5-mm cancellous screws was greater than 5.5-mm cortical screws in the metaphyseal bone of adult horses.
• Thus, if a 4.5-mm cortical bone screw strips in the shaft of an adult horse’s bone, either a 5.5-mm cortical or a 6.5-mm cancellous screw would have equivalent pullout strengths. However, a 6.5-mm cancellous screw would provide greater holding power than a 5.5-mm cortical screw in the end of the bone (metaphyseal bone).