Abstract
BackgroundThis 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).MethodsThis 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.ResultsThe 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.ConclusionThe 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.
Highlights
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
The minus values of the prediction error revealed that the Chapman's formula underestimates the pullout strength of even the cylindrical pedicle screw
This study was designed to derive the analytical formula for predicting the pullout strength of both conical and cylindrical pedicle screws
Summary
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). Transpedicular screw fixation has been extensively used for the treatment of instability due to degenerative disorders, trauma, tumor metastasis, and deformity correction. It provides immediate stability, enhances bony fusion, corrects deformity, and preserves the anatomic profile. Chapman et al [4] used an analytical formula to predict the pullout strength of cancellous and/or cortical screws inserted into the synthetic bone. The Chapman's formula was derived for the cancellous/cortical screw, and the thread design was different from the pedicle screw with conically distributed inner and/or outer diameters. The effects of the bone removal by pre-drilling a pilot hole and subsequently squeezing the bone chip into the thread surroundings were not considered in the Chapman's formula
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