Biomechanical Characteristics of Oversized Spinal Pedicle Screw after Fatigue Loading

Objective: This study aimed to investigate the effect of oversized pedicle screw on the fixation strength after fatigue loading. Introduction: Screw-rod type spinal implantation is widely used in the treatment of spinal disorders. Despite the advances in medical technology, screw looseness remains one of the most frequent failures for this type of implantation system. The selection of appropriate screw size plays a crucial role in the success of spinal implantation, as larger screws increase the risk of pedicle failure during insertion, but smaller screws are thought to compromise the stability of the implantation. By investigating the relationship between screw diameter and the pullout strength of pedicle screw after fatigue loading, this study seeks to find quantitative biomechanical data to assist surgeon in the selection of the appropriate screw. There are two hypotheses for this research: 1) the fixation strength of larger screw will be higher than that of smaller screws immediately after implantation. 2) After fatigue loading, while the fixation strength will decrease for all screw sizes, larger screws will more effectively retain their fixation strength. Materials and methods: Twenty-seven human cadaveric thoracic spine vertebrae (T3-T8) were harvested from 5 human cadavers (2 males and 3 females, mean age: 67.4, ranged: 52-83). The mean bone mineral density (BMD) of the specimens was 0.645 g/cm2 (ranged: 0.353-0.848 g/cm2), which is indicative of severe osteoporosis. Two sizes of poly-axial screws (4.35mm x35 and 5.0mm x35) were randomly chosen and implanted into each of the two pedicles of each vertebrae by an experienced surgeon, and specimens were randomly distributed into a control group and 2 fatigue groups (5,000 and 10,000 cycles). The peak-to-peak force and loading frequency of fatigue loading were 10-100 N and 1 Hz. Specimens of the control groups were evaluated with a pullout test after natural frequency measurement. For the fatigue groups, natural frequency was measured both before and after fatigue loading. Three specimens of each group were randomly selected to be scanned with micro-CT and the other specimens were imaged using radiography. For statistical analysis, a paired t-test was conducted to determine difference in natural frequency between before and after fatigue loading. Differences in natural frequency, fatigue damage, pullout strength, and pullout stiffness between the different screw sizes were also considered. One-way ANOVA was conducted to determine difference of BMD, fatigue damage, pullout strength, and pullout stiffness between the control group and both fatigue groups. If significant differences were observed, the post-hoc tests were conducted to determine the relationship between each group. Pearson correlation was used to determine the relationship between BMD, natural frequency, fatigue damage, pullout strength, and pullout stiffness. A p-value of < 0.05 was considered to be statistically significant. Results From radiography and micro-CT images, we found that both sizes of screws cause lateral breaches at pedicle. BMD had no significant difference between each group (p=0.492). The natural frequency of larger screws was higher than that of smaller screws, indicating that the binding power of larger screws seemed to be higher than that of smaller screws. After fatigue loading, the natural frequency of the screws decreased, indicating that fixation strength decreased. For fatigue damage analysis, we discovered that some specimens had micro fracture (MF) after fatigue loading in the 10,000 cycle fatigue group. Therefore, we separated the 10,000 cycle fatigue group into two sub-groups, one with MF and the other without MF. The fatigue damage of the MF group was significantly higher than that of the non-MF group and the 5,000 cycle fatigue group (p<0.032). The pullout strength of the larger screws was higher than that of the smaller screws in the control group but in the fatigue groups, pullout strength were closer to each other. However, there was no significant difference in pullout strength between each group. Finally, according to the regression model of pullout strength against total damage, we found that pullout strength dropped sharply at first but slowly decreased as total damage increased. Conclusion: In this study, we proved the first hypothesis, but failed the second one. Two main findings were concluded in this research: 1) The pullout strength of the larger screw was higher than that of the smaller screw just after implantation. 2) After fatigue loading, both sizes exhibited similar pullout strengths. The pullout strength of larger screws is not higher than the smaller screws. This indicates that the smaller size of screws may be chosen for less risk of pedicle breakage without sacrificing fixation strength. In addition, we found if the entry point was located between the articular process and transverse process, an over insertion of pedicle screw may damage the cortex of transverse process; hence decreased the fixation strength of pedicle screw after fatigue loading.