A new type of transiliac minimally invasive internal fixation system for the posterior pelvic ring: design,surgery simulation and safety evaluation

Abstract

Objective To evaluate a self-designed minimally invasive interlocking fixation system for the posterior pelvic ring before it can be put into clinical experiments.Methods The CT scan images of the pelvis from 30 normal adults and software Mimics14.11 were used to measure the distance from the posterior edge of sacral canal to the horizontal line linking bilateral posterior iliac cristae,the thickness of the posterior iliac wing,and the distance between bilateral posterior iliac wings to determine the safe location (point C) and parameters for insertion of the iliac main nail.Next,in 4 adult pelvic specimens,insertion of the main nail was simulated with a proper stick,using 2 Schanz pins on each end of the stick to determine the exact angle of the 2 interlocking screws.The internal transiliac fixation system with bilateral interlocking nails was designed on the basis of the parameters measured and manufactured accordingly.After successful manufacture,surgery was simulated in 5 cadavers to evaluate the safety of the fixation system.Results The data of normal pelvic anatomy showed that the maximum distance (24.56 mm) lies at the posterior superior iliac spine and 17 mm above the horizontal line linking the posterior edge of sacral canal to the bilateral posterior iliac cristae.The average thickness of the posterior iliac wing at point C was 20 mm; the distance between 2 points C at the bilateral posterior iliac wings ranged from 114 mm to 138 mm.Accordingly,the insertion position of the main nail was at the posterior superior iliac spine,17 mm towards the head and 10 mm towards the lateral sacrum.Diameter of the main nail was determined as 11 mm and that of the interlocking nails as 5 mm.Length of the main nail was determined ranging from 120 mm to 144 mm.Safety evaluation on specimens showed that the sacral cannel and the sacral plexus were undamaged and the interlocking nails did not break the cortex of the iliac bone.Conclusion As our self-designed transiliac minimally invasive interlocking fixation system can provide a rigid fixation for the posterior pelvic ring,leading to a safe,minimally invasive operation,it has a potential to become a reliable fixation method for the posterior pelvic ring in clinic. Key words: Pelvis; Orthopedic fixation devices; Surgical procedures, minimal invasive