Can barb thread design improve the pullout strength of bone screws?: a biomechanical study and finite element explanation

Abstract

Aims: To draw a comparison of the pullout strengths of buttress thread, barb thread, and reverse buttress thread bone screws. Methods: Buttress thread, barb thread, and reverse buttress thread bone screws were inserted into synthetic cancellous bone blocks. Five screw-block constructs per group were tested to failure in an axial pullout test. The pullout strengths were calculated and compared. A finite element analysis (FEA) was performed to explore the underlying failure mechanisms. FEA models of the three different screw-bone constructs were developed. A pullout force of 250 N was applied to the screw head with a fixed bone model. The compressive and tensile strain contours of the midsagittal plane of the three bone models were plotted and compared. Results: The barb thread demonstrated the lowest pullout strength (mean 176.16 N (SD 3.10)) among the three thread types. It formed a considerably larger region with high tensile strains and a slightly smaller region with high compressive strains within the surrounding bone structure. The reverse buttress thread demonstrated the highest pullout strength (mean 254.69 N (SD 4.15)) among the three types of thread. It formed a considerably larger region with high compressive strains and a slightly smaller region with high tensile strains within the surrounding bone structure. Conclusion: Bone screws with a reverse buttress thread design will significantly increase the pullout strength.

Keywords

• pullout strength
• buttress thread
• barb thread
• reverse buttress thread
• biomechanical study
• finite element study