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 R 2 = 0.93. On the other hand, the average error and R 2 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.
In Brief Study Design. The biomechanical effects of disc degeneration and hybrid fixation on the transition and adjacent segments were evaluated using a numerical approach. Objective. This study aimed to evaluate the rigidity-rising effects of the dehydrated disc and bridged fixator on the kinematic and mechanical redistribution of the transition and adjacent segments. Summary of Background Data. After static fixation, a dynamic fixator can be used to preserve motion and share loads for the transition segments. However, the hybrid use of both static and dynamic fixators and its effects on the biomechanical behavior of the transition and adjacent segments were not investigated extensively. Methods. A nonlinear and osseoligamentous lumbar model from L1 vertebra to S1 vertebrae was developed. Ligament interconnection, muscular contraction, and weight compression were all used to simulate lumbar flexion. The static fixator was instrumented at the degenerative L4–L5 segment and the dynamic fixators (Dynesys system) with different stiffness were subsequently applied to the degenerative or healthy L3–L4 segment. A healthy lumbar model was used as a reference point for further comparison and evaluation. The predicted results were validated with the cadaveric and numerical values of the literature studies. Among the 21 models, the junctional problem at the adjacent (L2/L3 and L5/S1) discs as well as the motion preservation and stress distribution at the transition (L3/L4) disc were compared. Results. Static fixation and the degenerative disc deteriorated the junctional problem at adjacent segments. On average, the hybrid fixation of the original Dynesys cord constrained the range of motion (ROM) by 65%. Furthermore, it shared 43% of the stress on the transition disc. However, this resulted in the adjacent discs increasing about 50% ROM and 40% stress. The term "trade-off stiffness" was used to express the concept that the decreased stiffness of the original cord could balance the junctional problem, motion preservation, and load protection of the transition and adjacent segments. The trade-off stiffness of the degenerative transition disc was higher than that of the healthy disc. Compared with the original design, the increased ROM and stress of the adjacent segments can be reduced by about 43% using the trade-off stiffness. Conclusion. The use of the hybrid fixator should involve a certain trade-off between the protection of the transition segment and the deterioration of the adjacent segments. This trade-off stiffness, which largely depends on both fixator design and disc degeneration, provides the improved rigidity and flexibility of the transition and adjacent segments. The use of the hybrid fixator should involve a certain trade-off between the protection of the transition segment and the deterioration of the adjacent segments. This trade-off stiffness, which largely depends on both fixator design and disc degeneration, provides the improved rigidity and flexibility of the transition and adjacent segments.
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