The Mechanism Of Retropulsed Bone Fragment In Thoracolumbar Burst Fracture May Be Related To Basivertebral Foramen

Objective:To clarify the relationship between the basivertebral foramen(BF)and the retropulsed bone fragment(RBF)in thoracolumbar burst fracture(TLBF)and further explain the mechanism of RBF formation.Methods:From June 2013 to June 2016,162 patients suffering from TLBF with RBF were collected.The characteristics of RBF as well as the pararmeters of vertebral body were studied using CT reconstruction imaging.In the transverse images,the lengths of retropulsed bone fragment(RL)and vertebral body(VL)were measured.In median sagittal images,the heights and widths of retropulsed bone fragment(RH,RIW)and vertebral body(VH,VW)were also obtained.The ratios of different parameters of RBF and vertebral body(RL/VL,RW/VW,RH/VH)were calculated and then defined the position relationship between RBF and BF.Ten frozen cadaveric spine were selected and evaluated by Micro-CT scans.Each vertebral body was divided into three layers(Superior,Middle,Inferior)and each layer was further divided into 9 regions(R1-R9),named as SR1-SR9,MR1-MR9,IR1-IR9.Microarchitecture parameters of each region in each layer,including bone volume fraction(BV/TV),trabecular connectivity(Conn.D),trabecular number(Tb.N),trabecular thickness(Tb.Th),trabecular separation(Tb.Sp),bone mineral density(BMD)were calculated,and their differences were also analyzed to see if the trabecular bone distribution would be affected by BF.In vitro study,burst fractures were simulated on cadaveric spines by using bursting fracture simulator,aiming to observe the RBF morphology and imaging findings to further investigate the relationship between RBF and BF.Results:The length and height of RBF were close to half of vertebral body length and height(RL/VL:0.497±0.049,RH/VH:0.480±10.035).The width of RBF was usually one-third of vertebral body width(RW/VW:0.31410.021),which indicated that the fracture block was often located in the posterior of vertebral body above the BF.BV/TV,Tb.N in the MR2 and MR5 regions were lowest than other regions and the SMI of MR2 and MR5 was largest than others.SR5 was the lowest region in superior lawyer that was corresponded to regions most affected by burst fracture.In simulated burst fractures,the fracture line of RBF went across the vertex or upper surface of the BF and the lower boundaries of RBF were also the upper bound of the BF.Moreover,the damage sites of posterior longitudinal ligament were mainly located at the edge of the BF.Conclusion:At the bone defect region,the BF is the weakest area in the vertebral body which may affect the distribution of trabecular bone surrounding it.When subjected to vertical violence,these regions(SR5,MR2,MR5)undergo fracture first which impact the anterior and lower boundaries of RBF.Ultimately,RBF was produced upon the BF,involving all or part of the upper bound of the BF.