Analysis of the pull-out strength of screws and pegs used to secure tibial components following total knee arthroplasty.

The purpose of this study was to compare the pull-out forces of 3.8-mm and 6.5-mm diameter cancellous screws and a round peg 10 mm in diameter, implanted in bovine vertebral cancellous bone and two different grades of polyurethane. The relationships between the pull-out forces and the properties of the material into which they were inserted were also examined. For the same depth of insertion, the pull-out force of a 6.5-mm diameter screw was two times larger than that of a 3.8-mm diameter screw. Despite its smaller diameter, a 6.5-mm diameter screw demonstrated a pull-out force 13.6% greater than that of an optimally press-fitted 10-mm diameter round peg when both were inserted to the same depth; however, optimal compression for the peg was strongly dependent upon the size of predrilling and the modulus of elasticity of the material into which the peg was inserted. The pull-out force of a cancellous screw has a linear relationship with the shear strength of the material into which it is inserted. The pull-out force of a peg exhibits a linear relationship with the modulus of elasticity of the material into which it is inserted.