The Comparison Of Various Internal Implants For Stabilizing Posterior Pelvic Ring Disruption By Three-dimensional Finite Element Analysis

The sacrum is the base of the spinal column as well as the keystone of the pelvic ring.Vertical fractures of the posterior pelvic ring have been associated with high rates of morbidity and haemorrhage.Thus,the disrupted sacroiliac complex is an absolute indication for operative treatment.With the development of fixation technology in the past ten years,the treatment of pelvic fractures with internal fixation is the most common method.Meanwhile,the pelvis is the center of a musculoskeletal biomechanical system that facilitates the weight transference from the upper part of the body to the lower extremities.The reconstruction of pelvic comprehensive biomechanics after injury is the primary consideration of clinical management.Generally,the clinical efficacy and biomechanical features of the implants used for pelvic injuries should be evaluated through biomechanical experiments in vitro.However,irregular geometry and material heterogeneity of the pelvis often make mechanical experiments challenging4.In the last decade,the finite element(FE)analysis method has been frequently utilized in the research of human pelvic biomechanics5-8.Compared to clinical trials and mechanical experiments in animal models,FE analysis provides full field data more easily and may be used to investigate the effects of many variables more quickly and economically.In clinical,various internal implants have been used to control these injuries,including iliosacral screws(ISS),tension band plates(TBP),and transiliac sacral bars and so on.ISS have become a common technology in the treatment of pelvic posterior ring disruption.However,the ISS technique requires great surgical skill and continuous fluoroscopic guidance for appropriate screw insertion,and the risk of neurovascular injuries was shown to be higher than that for other techniques.A TBP placed in the posterior pelvic ring can provide sufficient stability for Tile C fractures.However,the TBP technique has some disadvantages,including the necessity of pre-bending the plate before fixation.furthermore,it is associated with a high rate of skin infection and symptomatic hardware.Additionally,it is difficult to avoid repeated bending,which could reduce the plate strength and lead to the screw holes threads damage in the locking compression plate.To address the above-mentioned limitations and improve the reduction ability of posterior osteosynthesis,we introduced a novel minimally invasive adjustable plate(MIAP)for treating posterior pelvic ring disruption.The MIAP consists of two Z-shaped brackets at each end of an adjustable connection bar.The Z-shaped bracket is a single entity,consisting of three parts: an upper wing,a lower wing,and a web plate.The connection bar is comprised of a hexagonal tube and two custom-made eye bolts.The hexagonal tube is threaded on the inside to allow a bolt to be screwed in from each end.The distance to which the eye bolts are screwed into the tube can be adjusted,and the length of the connection bar changes accordingly.We used the three-dimensional FE analysis technique to simulate ISS,TBP,and MIAP fixation for a posterior pelvic ring disruption with high instability of the posterior pelvic ring.According to the following three parts,we aimed to: 1)observe the effects of boundary conditions and connect conditions on biomechanics predictions in FE pelvic models.;2)analyse the influence of various internal implants on load transmission in posterior pelvic ring disruption;and 3)observe the efficiency of the stabilization of these internal fixation techniques for treating posterior pelvic ring disruption and provide references for their clinical application.Part 1 The influence of different boundary conditions in finite element analysis on pelvic biomechanical load transmissionObjective: To establish and validate an intact pelvic FE model.Then observe the effects of boundary conditions and connect conditions on biomechanics predictions in FE pelvic models.Methods: An embalmed specimen of adult female pelvis was used,removing soft tissue but retaining intact spines from the fifth lumbar vertebra to the proximal one third of both femurs,intact pubic symphysis,bilateral sacroiliac joints and ligaments,bilateral hip joints,bilateral sacrotuberous ligaments and bilateral sacrospinous ligaments.The specimen was placed in the biomechanical machine at a standing neutral posture.Vertical load of up to 500 N was applied and the strain was recorded by strain gauge.Three FE pelvic models were constructed to analyze the effect of boundary conditions and connect conditions in the hip joint: an intact pelvic model assumed contact of the hip joint on both sides(Model I);and a pelvic model assumed the hip joint connecting surfaces fused together with(Model II)or without proximal femurs(Model III).The model was validated by bone surface strains obtained from strain gauges in an in vitro pelvic experiment.The 100-500 N vertical load was simulated in the FE model.Rusults: There was a strong correlation between the FE analysis results of Model I and the experimental results(R2=0.97);meanwhile,the correlation coefficient and the linear regression function increased slightly with the increasing of load force(R2=0.854 to R2=0.988).Comparing the three models,the stress values in the point near the pubic symphysis in Model III were 48.52% and 39.1% lower,respectively,in comparison with Models I and II.Furthermore,the stress values on the dome region of the acetabulum in Models II and III were 103.61% and 390.53% less than those of Model I respectively.Besides,the posterior acetabular wall stress values of Model II were 197.15% and 305.17% higher than those of Models I and III,respectively.Conclusions: These findings suggest that the effect of the connect condition in the hip joint should not be neglected,especially in studies related to clinical applications.Part 2 Biomechanical comparison of various internal implants for stabilizing posterior pelvic ring disruption-a three dimensional finite element analysisObjective: This study aimed to compare the biomechanical stability and compatibility of two ISS,a TBP,and a MIAP for treating Tile C pelvic fractures.Methods: Three groups of the intact pelvis FE models,including the main ligament and the proximal one-third of both femurs,were developed to simulate vertical sacral fractures and treated with the three above-mentioned internal fixation techniques.Three load forces were 500 N,500 N vertical load combined 10 Nm bending or flexion rotational moments was applied respectively,and the von Mises stress and vertical displacement were recorded.Results: The results showed that all three internal fixation techniques effectively restored the biomechanical transmission of the injured pelvis.MIAP reduced the stress concentration in sacroiliac joint area of 2nd sacral vertebral level than other two groups.The maximum Von Mises stress on the implants in the TBP model was 167.47% and 53.41% higher than that in the ISS model and MIAP model,respectively;there were no significantly difference in the maximum Von Mises stress on the injured sacrum among three models,and the stress shielding phenomenon of the TBP model was more obvious than in the other two models.Meanwhile,the vertical displacement of implants and sacrum were no significantly difference among three models.However,the maximum Von Mises stress between the fracture interfaces in the TBP fixation models was apparently higher than that in the other two models.Under bending and flexion status,the trend of the results was similar to that in the stand condition.Conclusions: Based on our results,we believe that the stability of Tile C pelvic fractures fixed with MIAP was similar to that of fractures fixed with ISS and TBP,but the stress shielding phenomenon and safety of implants in the TBP models were inferior to those in the MIAP and ISS fixation models.Part 3 Three-dimensional finite element analysis of the efficacy of three internal fixations for unilateral sacroiliac joint dislocationObjective: This study aimed to compare the biomechanical stability and compatibility of two ISS,a TBP,and a MIAP for treating unilateral sacroiliac joint dislocation combined with pubic symphysis dislocation.Methods: Three groups of FE models of the intact pelvis,including the main ligament and the proximal one-third of both femurs.All the stabilizing ligaments of the hemipelvis and pubic symphysis ligaments were removed to create a vertically and rotationally unstable model and treated with the three above-mentioned internal fixation techniques.A vertical load of up to 500 N was applied,and the von Mises stress and vertical displacement were recorded.Results: The results showed that all three internal fixation techniques effectively restored the biomechanical transmission of the injured pelvis.MIAP reduced the stress concentration in sacroiliac joint area of 2nd sacral vertebral level than the other two groups.The maximum Von Mises stress on the implants in the ISS model was 13.6% and 21.12% higher than that in the TBP model and MIAP model,respectively.The maximum Von Mises stress on the sacrum in the ISS model was 26.11% and 35.35% higher than that in the TBP model and MIAP model,respectively.However,the stress shielding phenomenon of the ISS model was slightly higher than in the other two models.Meanwhile,there were no significantly difference in the vertical displacement of implants and sacrum among three models.In ISS model,the maximum Von Mises stress on the anterior pelvic ring plate was 22.07% and 20.96% higher than TBP and MIAP,respectively.We compared the flexion stability of pelvic proneness among three groups,the TBP model flexion angle was 288.18% and 256.56% higher than that in the ISS and MIAP model.Conclusions: Based on our results,we believe that the stability and biomechanical compatibility of unilateral sacroiliac joint dislocation combined with pubic symphysis dislocation fixed with MIAP was similar to that of fractures fixed with ISS and TBP;however,the risk of breakage of implants and the stress distribution of anterior pelvic ring in the TBP and MIAP models were inferior to those in the ISS fixation models.Meanwhile,the stability of flexion in TBP model was inferior to those in the ISS and MIAP fixation models.