Stress relaxation of bone significantly affects the pull‐out behavior of pedicle screws

Abstract

The initial fixation strength of pedicle screws is commonly tested using a standard pull-out test with load applied at a constant rate. This method overlooks the cyclic nature of in situ loading responsible for clinical failure. This study was undertaken to determine the effects of stress relaxation properties at the bone-screw interface on screw fixation strength. Pedicle screws were inserted into calf lumbar vertebrae using a paired testing array. After embedding and mounting in a custom fixture, axial pull-out tests were performed at the rates of 1, 5, and 25 mm/min. For each vertebra, one screw was pulled at a continuous rate. The other screw was pulled at increments of 0.5 mm, at the same rate, with 1000 s pause between increments. Peak load, energy-to-failure, displacement-to-failure, and stiffness were calculated for each screw pull-out test. Two-way ANOVA showed that the standard pullout method yielded significantly higher peak loads (p < 0.05) at faster pull-out rates and higher stiffnesses (p < 0.05) at all rates compared to the stress relaxation pull-out protocol. These results suggest that the stress relaxation properties of bone significantly affect the pull-out behavior of pedicle screws, reducing the peak load and stiffness values observed during testing. This mode of testing may provide a better biomechanical model of screw pull-out failure and a more accurate estimate of initial fixation strength.

Effect of various factors on pull out strength of pedicle screw in normal and osteoporotic cancellous bone models

“…In the current study, there was a significant correlation between pullout strength and insertion torque of pedicle screw. The material in bone model failed primarily in the shearing of a thin region the size of outer diameter of the screw as observed in studies on synthetic, human, and animal bones [61][62][63] . The different design factors that effect the pullout strength of pedicle screws can be classified as by screw parameters, such as thread type [27,52,64] , outer diameter [9] , and inner diameter or by insertion technique, such as the effect of size of pilot holes [65] , tapping [66] , and insertion angle [23] , or by augmentation technique such as polymethylmethacrylate(PMMA) [16] , or calcium phosphate (CaP) [67] .…”

The effect of screw insertion angle and thread type on the pullout strength of bone screws in normal and osteoporotic cancellous bone models

“…Pullout strength can be used to measure screw fixation strength [3]. To date, work on screw pullouts has focused mainly on axial pullouts in cadaveric [4][5][6] and animal [7][8][9] bone and synthetic materials mimicking the properties of human bone [7,9,10]. Several studies report on the pullout of screws augmented with polymethylmethacrylate, various bone cements, calcium phosphate products and laminar hooks in OP bone [4][5][6][11][12][13][14][15].…”

Constitutive modelling of inelastic behaviour of cortical bone

“…With reference to previous works in the literature, it must be pointed out that visco-elastic constitutive models have been applied to bone tissues [6][7][8] and bone-derived materials [9] considering an isotropic configuration. Anisotropic models have frequently been proposed for the elastic response of cortical bone [5,[10][11][12].…”