Biomechanical comparison of pedicle screw-rod system and pelvic external fixator in models of rotatory unstable pelvic injury

Abstract

Objective To compare the biomechanical performances of pedicle screw-rod system versus pelvic ala ilium external fixator in cadaveric models of rotatory unstable pelvic injury.Methods Seven cadaveric specimens of normal adult pelvis were used for the test,retaining intact spines from the fifth lumbar vertebra to the proximal 20 centimeters of both femurs,intact pubic symphysis,bilateral hip joints,bilateral sacroiliac joints,bilateral sacrotuberous ligaments,bilateral sacrospinous ligaments,bilateral anterior sacroiliac ligaments and bilateral posterior sacroiliac ligaments.The pelvic specimens were put in an AGX biomechanical testing machine at a standing neutral posture and subjected to a vertical load of up to 500 N downward from the L5 body.The displacements of the symphysis pubis were recorded sequentially in the following 4 conditions:1.intact pelvis; 2.pelvic Tile B1 injury in simulation of rotatory unstable injury of the pelvis in which the symphysis pubis was cut off and accompanied with ruptures of ipsilateral sacrospinous and sacrotuberous ligaments; 3.pelvic Tile B1 injury fixated with the pedicle screw-rod system; 4.pelvic Tile B1 injury fixated with the pelvic ala ilium external fixator.Results Under the vertical load of 500 N,displacements of the symphysis pubis increased significantly from condition 1 (0.121 ± 0.025 mm),to condition 3 (0.656 ±0.103 mm),to condition 4 (1.512 ±0.101 mm) and last to condition 2 (4.512 ±0.391 mm).The difference between any two conditions was significant (P ＜ 0.05).Conclusion Fixation by the pedicle screw-rod system is significantly better than that by the pelvic ala ilium external fixator for rotatory unstable pelvic injury,because the former can more effectively restore the biomechanical stability of the pelvic ring. Key words: Pelvis; Bone nails; External fixators; Biomechanics