Objective Evaluate and compare the mechanical resistance and the fatigue behavior associated with the use of three different modalities of vertebral fixation system rod connectors through in vitro pre-clinical mechanical tests developed specifically for this application (linear, lateral with square connector and lateral with oblique connector). Methods Cobalt chromium rods 5.5 mm in diameter were used and coupled with three types of connectors: a) side rod with oblique connector, b) side rod with square connector, and c) rod and linear connectors. Quasi-static mechanical four-point bending and fatigue tests were performed. The variables measured were (I) the bending moment at the yield limit, (II) the displacement at the yield limit, (III) the rigidity of the system in flexion and (IV) the number of cycles until system failure. Results The linear system presented the greatest force and the greatest moment at the yield limit, as well as the greatest stiffness equivalent to bending. All specimens with square and oblique connectors endured 2.5 million cycles in the minimum and maximum conditions of applied moment. The specimens with linear connector endured 2.5 million cycles with fractions of 40.14% of the bending moment at the yield limit, but failed with levels of 60.17% and 80.27%. Conclusions Systems with linear connectors showed greater mechanical resistance when compared to systems with square and oblique connectors. All systems supported cyclic loads that mimic in vivo demands. Level of evidence V; In vitro research.
The effectiveness of Ilizarov external fixators in osteosynthesis of fractured bones depends on the pre-tension applied to the transosseous wires. The pre-tension on these wires depends on the initial torque applied to the slotted bolts, which occasionally fracture during clinical setup. There is no standard method to determine the risk of failure of these bolts. This study aimed to develop a test to evaluate the mechanical strength of these bolts by imitating a bolt-wire-ring-nut configuration of an Ilizarov fixator. The fracture micromechanisms of the slotted bolts were investigated to understand the mechanical forces involved. This resulted in a reference group (Group III). Then, with these fracture characteristics, a new bolt-wire-ring-nut test was developed to mimic use in an Ilizarov fixator. Eleven slotted bolts were tested (Group II). The new test results were compared with a standardized screw torsion test (Group I) according to ASTM F543, Standard Specification and Test Methods for Metallic Medical Bone Screws. The mechanical strengths of Groups I and II were analyzed statistically. Additionally, the fracture micromechanisms of all groups were investigated. The bolts tested according to ASTM F543 (Group I) showed values with less dispersion compared with those in Group II; however, there were no statistically significant differences between their maximum torques. The bolts submitted to the new test (Group II) showed similar fracture surfaces as those in Group III, distinct from Group I. The results indicate that the new torsion test is more clinically relevant and reliable when checking whether the slotted bolts are liable to fracture during clinical setup.
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