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Background: The association of lumbar spine instability between laminectomy and laminotomy has been clinically studied, but the corresponding in vitro biomechanical studies have not been reported. We investigated the hypothesis that the integrity of the posterior complex (spinous process-interspinous ligament-spinous process) plays an important role on the postoperative spinal stability in decompressive surgery.

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BackgroundTraditional one-above and one-below four-screw posterior short-segment instrumentation is used for unstable thoracolumbar burst fractures. However, this method has a high rate of implant failure and early loss of reduction. The purpose of this study was to use finite element (FE) analysis to determine the effect of treating thoracolumbar burst fractures by short-segment pedicle screw fixation using a combination of two additional pedicle screws and vertebroplasty at the level of the fracture.MethodsAn intact T11-L1 spine FE model was created from the computed tomography images of a male subject. Four fixation models with posterior fusion devices (pedicle screws, rods, cross-link) were established to simulate an unstable thoracolumbar fracture with different fusion surgeries: short-segment fixation with: 1) a link (S-L); 2) intermediate bilateral screws (S-I); 3) a link and calcium sulfate cement (S-L-C); 4) intermediate bilateral screws and calcium sulfate cement (S-I-C). Different loading conditions (flexion, extension, lateral bending, and axial rotation) were applied on the models and analyzed with a FE package. The range of motion (ROM), and the maximum value and distribution of the implant stress, and the stress in the facet joint, were compared between the intact and fixation models.ResultsThe ROM in flexion, extension, axial rotation, and lateral bending was the smallest in the S-I-C model, followed by the S-I, S-L-C, and S-L models. Maximum von Mises stress values were larger under lateral bending and axial rotation loadings than under flexion and extension loading. High stress was concentrated at the crosslink and rod junctions. Maximal von Mises stress on the superior vertebral body for all loading conditions was larger than that on the inferior vertebral body. The maximal von Mises stress of the pedicle screws during all states of motion were 265.3 MPa in S-L fixation, 192.9 MPa in S-I fixation, 258.4 MPa in S-L-C fixation, and 162.3 MPa in S-I-C fixation.ConclusionsShort-segment fixation with two intermediate pedicle screws together with calcium sulfate cement at the fractured vertebrae may provide a stiffer construct and less von Mises stress of the pedicle screws and rods as compared to other types of short-segment fixation.

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