A Finite Element Analysis And Clinical Study On PVP And PKP For Osteoporotic Sandwich-body Fractures Of Spine

Part 1: Establishment of a finite element model stimulating anosteoporotic thoracolumbar sandwich-body fracture[Objective] To build the 3 dimensional finite element models(FEMs) of an osteoporotic thoracolumbar spine and an osteoporotic thoracolumbar sandwich-body fractures. An sandwich-body fracture was defined as the fracture of an intact vertebra between two fractured vertebrae, and it was a special kind of Vertebral Compression Fractures. The intact vertebra was called as a sandwich vertebra. The fracture of the sandwich vertebra was called as a sandwich fracture.[Methods] A healthy volunteer was selected and Computerized Tomography(CT) scanning was made for the volunteer’s thoracolumbar(T10-L2) region. The CT slices were input into a computer. The FEMs of the osteoporotic thoracolumbar spine and the osteoporotic thoracolumbar sandwich-body fracture(T11、L1 vertebrae were fractured)were constructed by combination of software package includinvg Mimics 15.0 and ANSYS 12.0. The FEMs were subjected to loads of 400 N vertically and 10 Nm at the upper surface of T10 for flexion,extension,lateral bending and axial rotation. The deformation and von Mises stress of FEMs were analyzed.[Results] The FEMs of the osteoporotic thoracolumbar spine and the osteoporotic thoracolumbar sandwich-body fracture were established. The responses were in good agreement with the published data previously. Under the preload of neutral position,flexion,extension,lateral bending and axial rotation, the deformation and stress distribution of the FEMs were obserbed. After the fracture of T11 and L1, the von Mises stress of sandwich vertebra(T12) was raised form 17.11 Mpa preoperatively to 26.16 Mpa under the preload of flexion. And the von Mises stress on L1,L2 and L1/2 intervertebral disc were raised obviously under the preload of neutral and flexion positions.[Conclusions](1) The FEMs of the osteoporotic thoracolumbar spine and the osteoporotic thoracolumbar sandwich-body fracture were established using CT scaning and software package includinvg Mimics and ANSYS.(2) After the osteoporotic sandwich-body fracture, the maximun von Mises stress on the adjacent vertebrae and intervertebral discs was raised obviously, and the risk of a subsequent sandwich fracture(the fracture of sandwich vertebra) was increased.Part 2: The biomechanical effects of PVP and PKP for theosteoporotic vertebral sandwich-body fracture with FEMs[Objective] To analyze the biomechanical effects of Percutaneous Vertebroplasty(PVP) and Percutaneous Kyphoplasty(PKP) for the osteoporotic sandwich fractures with FEMs.[Methods] Based on the previous FEMs of the osteoporotic thoracolumbar spine and the osteoporotic thoracolumbar sandwich-body fracture, the FEMs of osteoporotic sandwich-body fracture treated with PVP and PKP were constructed. The FEMs were subjected to loads of 400 N vertically and 10 Nm at the upper surface of T10 for flexion,extension,lateral bending and axial rotation.The deformation and von Mises stress of each FEMs were analyzed.[Results] The FEMs of the osteoporotic sandwich-body fracture treated with PVP and PKP were established. The FEM of the sandwich-body fracture treated with PKP showed that after PKP the maximum deformation of the whole FEM and the maximum von Mises stress of sandwich vertebra(T12) and the intervertebral disc of T12/L1 decreased obviously, especially under flexion loads. While The FEM of the sandwich-body fracture treated with PVP showed after PVP, the maximum von Mises stress of sandwich vertebra(T12) was changed litte and that of the intervertebral disc of T12/L1 was raised.[Conclusions] compared with conservative treatment for the osteoporotic sandwich-body fracture(1) The stiffness of the sandwich level could be restored obviously by PKP and changed little by PVP.(2) The maximun von Mises stress of sandwich vertebra and the intervertebral discs adjacent to augmented vertebrae decreased more obviously after PKP rather than PVP.part 3: The biomechanical effects of cement diffusion, leakage andaugmentation in advance during PKP for the osteoporoticsandwich-body fracture with FEMs[Objective] To analyze the biomechanical effects of cement diffusion, leakage and cement augmentation in advance during PKP for the osteoporotic sandwich-body fractures with FEMs.[Methods] Based on the previous FEM of the osteoporotic sandwich-body fracture treated with PKP, the FEMs of cement diffusion, leakage and cement augmentation in advance during PKP for the osteoporotic sandwich-body fracture were constructed. The FEMs were subjected to loads of 400 N vertically and 10 Nm at the upper surface of T10 for flexion,extension,lateral bending and axial rotation.The deformation and von Mises stress of each part of FEMs were analyzed.[Results] The FEMs of cement diffusion, leakage and cement augmentation in advance during PKP for the osteoporotic sandwich-body fracture were established. Under the preload of neutral position, flexion,extension,lateral bending and axial rotation, the deformation and stress distribution of the FEMs were obserbed. The FEM of cement leakage during PKP for the osteoporotic sandwich-body fractures showed that the deformations of T10-L2 as a whole under the preloads were decreased obviously. The maximum von Mises stress of the intervertebral discs(T10/11, T11/12, T12/L1, L1/2) under flexion preload were raised from 4.17, 1.74, 2.46, 1.59 Mpa in FEMs of cement diffusion to 111.83, 95.71, 61.18, 49.62 Mpa in FEM of cement leakage and were decreased to 8.21, 11.68, 6.80, 9.11 Mpa in FEM of cement augmentation in advance during PKP for the osteoporotic sandwich-body fracture. The von Mises stress of the sandwich vertebra under flexion preload were raised from 16.69 Mpa in FEM of cement diffusion to 19.75 Mpa in FEM of cement leakage during PKP for the osteoporotic sandwich-body fracture. The maximum von Mises stress of T12 under flexion preload were raised to 35.474 Mpa in FEM of cement augmentation in advance during PKP for the osteoporotic sandwich-body fracture.[Conclusions] after PKP for the osteoporotic sandwich-body fracture,(1) The diffusion of bone cements has little effect on the sandwich vertebra and intervertebral discs next to the augmentated vertebrae.(2) The leakage of bone cements increased the maximum von Mises stress of the sandwich vertebra and intervertebral discs next to the augmentated vertebrae, which would increase the risk of subsequent sandwich fracture and accelerate the degeneration of adjacent intervertebral discs.(3) After the leakage of bone cements during PKP, the cement augmentation of the sandwich vertebra in advance could reduce the von Mises stress on its endplates, which could reduce the risk of subsequent sandwich fracture.(4) After the leakage of bone cements during PKP, the cement augmentation of the sandwich vertebra in advance could reduce the maximum von Mises stress of adjacent intervertebral discs, which could delay the accelerated denegeration of adjacent intervertebral discs because of cemet leakage. Part 4: PVP versus PKP, which is more preferable for the treatmentof osteoporotic sandwich-body fractures?[Objective] The purpose of the study was to retrospectively compare the safety and long-term effects of PVP and PKP for the treatment of osteoporotic sandwich-body fractures and to investigate potential risks of subsequent sandwich fractures after cement augmentation.[Methods] The study population included a total of 90 patients suffered with osteoporotic sandwich-body fractures treated with PKP or PVP.(1) According to the operation type, the patients were divided into 2 groups: 68 patients in the PKP group and 22 patients in the PVP group. Outcomes were measured pre- and postoperatively using the visual analogue scores(VAS), Oswestry Disability Index(ODI), the kyphotic angle of sandwich level(KA).(2) According to whether the sandwich vertebra fractured after operation, the population was divided into two groups: fracture group(n=15), no fracture groups( n=75). A database was created containing patients’ gender, age, height, weight, BMD(bone mass density, BMD), follow-up duration, cumulative number of treated levels, TL junction or non TL junction(T10~L2), operation type, surgical approach, cement volumn, discal cement leakage and pre- and postoperative kyphotic angulation of sandwich level. The rate of sandwich fracture was calculated and multiple logistic regression analysis was executed as well.[Results] Both PKP and PVP resulted in significant pain relief and kyphosis reduction. The mean follow-up period was over 18 months.(1) One day after operation, the mean VAS and ODI scores improved significantly in each group(p<0.01), and this improvement was sustained at the final follow-up. There was no significant difference postoperatively on VAS, ODI, vertebrae with cement leakage(p>0.05) between the two groups except the postoperative KA(p=0.013), and the difference was sustained at the final follow-up. Significant difference was observed on the number of patients with subsequent VCFs(p=0.044) and sandwich fractures(p=0.049) except the mean time to subsequent sandwich fractures(p= 0.523) between the two groups. The subsequent vertebral fracture rate(17.6%)and subsequent sandwich fracture rate(10.96%)in PKP group were lower than the natural subsequent vertebral fracture rate with conservative treatments for sandwich-body fractures reported.(2) There’s no statistical difference for the subsequent sandwich fractures in patients’ gender, age, height, weight, follow-up duration, cumulative number of treated levels, TL junction or non TL junction(T10~L2), surgical approach, cement volumn,discal cement leakage and preoperative kyphotic angulation of sandwich level. The BMD, postoperative KA and operation type were found to make statistical differences for the subsequent sandwich fractures( P<0.05). Only the BMD(BMD≤-3.5SD)and postoperative KA(KA ≥10°)were risk factors with the subsequent sandwich fractures(P<0.05).[Conclusions](1)Both PKP and PVP were feasible for patients with osteoporotic sandwich-body fractures.(2)PKP offered a higher degree of KA reduction and resulted in less subsequent sandwich fractures than PVP.(3) The BMD(BMD≤-3.5SD)and postoperative KA(KA ≥10°)were risk factors with the subsequent sandwich fractures.(4)The subsequent fracture rate of sandwich vertebra treated with PKP was lower than that treated with conservative treatment reported.