Biomechanical comparison of a transiliac internal fixator and two iliosacral screws in transforaminal sacral fractures: a finite element analysis.

PURPOSE Vertically unstable sacral transforaminal fractures can be stabilized with a transiliac internal fixator (TIFI) or two iliosacral screws (IS). This study was designed to compare stiffness between TIFI and IS. METHODS Using CT images finite element model of the pelvis was developed. Denis II type fracture was simulated and fixed either with TIFI or two IS. The sacral base was loaded vertically (250-500 N), displacement magnitudes on medial and lateral fracture surface and the maximum bone stress were calculated. The intact pelvis was used as a reference. Stiffness was determined by linear regression of load and displacement, computed stiffness ratio %. The von Mises stress was expressed as % ratio, evaluation of colour mapping was made. RESULTS The mean stiffness ratio medially in TIFI was 75.22%, in IS 46.54% (p = 0.00005), laterally in TIFI 57.88%, in IS 44.74% ( p = 0.03996). The von Mises stress ratio of TIFI was 139.27%, of IS 565.35% ( p < 0.00001). CONCLUSIONS Significantly higher stiffness and lower stress were found in TIFI model. TIFI provides a lower risk of over-compression of the fracture line in comparison with IS. TIFI thus exhibits superiority for fixation of trans- foraminal fractures, particularly with comminutive zone.