Comparison and prediction of pullout strength of conical and cylindrical pedicle screws within synthetic bone

Abstract

Background: This study was designed to derive the theoretical formulae to predict the pullout strength of pedicle screws with an inconstant outer and/or inner diameter distribution (conical screws). For the transpedicular fixation, one of the failure modes is the screw loosening from the vertebral bone. Hence, various kinds of pedicle screws have been evaluated to measure the pullout strength using synthetic and cadaveric bone as specimens. In the literature, the Chapman's formula has been widely proposed to predict the pullout strength of screws with constant outer and inner diameters (cylindrical screws).

Methods: This study formulated the pullout strength of the conical and cylindrical screws as the functions of material, screw, and surgery factors. The predicted pullout strength of each screw was compared to the experimentally measured data. Synthetic bones were used to standardize the material properties of the specimen and provide observation of the loosening mechanism of the bone/screw construct.

Results: The predicted data from the new formulae were better correlated with the mean pullout strength of both the cylindrical and conical screws within an average error of 5.0% and R2 = 0.93. On the other hand, the average error and R2 value of the literature formula were as high as -32.3% and -0.26, respectively.

Conclusion: The pullout strength of the pedicle screws was the functions of bone strength, screw design, and pilot hole. The close correlation between the measured and predicted pullout strength validated the value of the new formulae, so as avoid repeating experimental tests.

Figure 1

The six Pedicle Screws used in this study. Three pedicle screws (UPS-4, Diapason, and HCD) have a conical inner core, while the other three (UPS-3, CCD and Moss Miami) have a cylindrical core.

Figure 2

The setup of testing in the MTS machine, jig assembly, and synthetic bone. The synthetic bone (white color) was encased in the jig, and the screw head was firmly attached to the grip on the top.

Figure 3

The mechanism of the Integral formula to predict the pullout strength of the pedicle screw with variable distributions of outer and inner diameters (i.e., conical or cylindrical screw). The diagram shows the insertion of a conical shaped screw into the synthetic bone. The bone chips within the region ABCD was assumed to be squeezed into the region CDE. See the text for the detailed meaning of the symbols.

Figure 4

The spiral of bone chips. The spirals of bone chips were peeled from the surface of the screw threads.

Figure 5

The measured and predicted pullout strength of six pedicle screws. One standard deviation is used as the error bar of the experimental data. The 0.5 value of the equivalent diameter ratio is used in calculating the Chapman's and Lin's formulae. It is obvious that the predicted pullout strength of each screw using the new formula was closer to those under testing, while the calculated data from the Chapman's formula were very variable and could not match the experimental data.

Figure 6

The parametric analyses of the parameters of screw shape and pilot hole. The parametric analyses of the parameters of screw shape (D_o and p) and pilot hole (d_o) in the new formulae.