[The Biomechanics Influence of unilateral laminectomy and discectomy surgery on the adjacent different grades of Disc Degeneration].

OBJECTIVES: To study the stress distribution of the adjacent different grades of disc degeneration underwenting unilateral laminectomy and discectomy surgery using non-linear finite element analysis. METHODS: Based on the lumbar CT scans, the finite element model (FEM) of lumbar spinal segment (L3-L5) was established. According to L3-L4 intervertebral disc degeneration, different grades of disc degeneration (healthy, mild, moderate and severe) models were established and unilateral laminectomy and discectomy surgery were also established. Physiological action such as flexion, extension, lateral bending and lateral rotation was simulated and the von Mises stress in the nucleus pulposus and annulus fibrosus matrix of L3-L4 disc was investigated. RESULTS: After unilateral laminectomy and discectomy surgery, the extremum value of von Mises stress of nucleus pulposus and annulus fibrosus matrix was maximum during extension and minimus left bending in the healthy intervertebral disc. Compared with healthy disc, the increment of extremum value was found during left bending in the mildly degenerated disc. When the value decreased in the moderately degenerated disc, but still higher than that in the healthy disc. When the adjacent disc is severely degenerated, the extremum value of nucleus pulposus decreased, in addition to axial rotation, and even lower than that of healthy disc. The value of annulus matrix decreased and still higher than that of healthy disc, especially during left bending. CONCLUSIONS: After unilateral laminectomy and discectomy surgery, avoiding lateral bending will reduce the abnormal stress in the degenerated disc and decreased the risk of accelerating disc degeneration.

Finite element analysis in adjacent segment degeneration after lumbar fusion.

BACKGROUND: Adjacent segment degeneration (ASD) following lumbar fusion has been well documented in recent years. However, the pathogenesis of ASD is not clear. To investigate this issue, we established a finite element model of segments L2-L5, simulated a single-segment posterior fixation in L3-L4 and investigated the stress variation and the effects of the instrumented lumbar posterior fixation on adjacent levels. METHODS: Models A, B and C of L2-L5 multisegment finite element intact models were established. In model A, segment L3-L4 was not fixed and was without disc degeneration in the adjacent segment (L2-L3, L4-L5); in model B there was posterior pedicle fixation in segment L3-L4 without disc degeneration in the adjacent segment (L2-L3, L4-L5); in model C there was posterior pedicle fixation in segment L3-L4 with a degenerated disc in the adjacent segment (L2-L3, L4-L5). Four levels of axial pressure, 0.3, 0.5, 1.0 and 2.0 MPa, were compared between each model of the stress variation on the discs of the adjacent segment (L2-L3, L4-L5). RESULTS: The maximum principal stress mean value of disc L2-L3 under four pressures in model A was determined. The statistical results showed that stress was not significant difference in disc L2-L3 between models A and B, but there was a significant difference in disc L2-L3 between models A and C and a significant difference between each group in disc L4-L5 under four pressure conditions. CONCLUSIONS: The preoperative degeneration of the adjacent segment of the disc is a significant risk factor for ASD.

[Finite element analysis of adolescent idiopathic scoliosis of PUMC II d2 surgical treatment with different fusion levels].

OBJECTIVE: To explore the best surgical fusion level for adolescent idiopathic scoliosis (AIS) of PUMC II d(2) with finite element model (FEM). METHODS: FEM (T(5)-S) of PUMC II d(2) idiopathic scoliosis was used to simulate upper thoracic curve, lower lumbar curve and double curve fusion manners. The pedicle of concave vertebral arch received 50, 100 and 150 N load respectively. Displacement of T(5) and T(11) on upper sagittal plane (displacement of Z axis positive value on upper sagittal plane, displacement of negative value on lower sagittal plane) and their different values were compared. T(5) displacement represented the outcomes of double curve orthopedics. T(11) displacement represented the outcomes of lower lumbar curve orthopedics. Their difference (T(5)-T(11)) represented the outcomes of upper thoracic curve orthopedics. Different fusion segments and displacement of T(5) and T(11) under different orthopedic forces were measured. RESULTS: In PUMC II d(2) lateral curvature, T(5) displacement on Z axis: fusion displacement of double curve was greater than the upper or lower curve alone (F = 8, P < 0.01). Difference of T(5)-T(11) displacement: double curve orthopedics > upper thoracic curve orthopedics alone > lower lumbar curve orthopedics alone (F = 8, P < 0.01). Displacement of T(11) on Z axis: double curve orthopedics > lower lumbar curve orthopedics alone > upper thoracic curve orthopedics alone (F = 8, P < 0.01). CONCLUSION: Fusing two curves achieves the best effect on the AIS of PUMC II d(2) in comparison with upper or lower curve fusion alone. Effects of 3 kinds of load on correction of upper thoracic curve: double curve orthopedics > upper thoracic curve orthopedics alone > lower lumbar curve orthopedics alone. Effect of 3 kinds of load on correction of lower lumbar curve orthopedics alone: double curve orthopedics > lower lumbar curve orthopedics alone > upper thoracic curve orthopedics alone. Three-dimensional finite element analysis is an effective method to analyze the biomechanics of scoliosis deformity correction and provides a virtually non-invasive verification manner. And it may optimize the surgical protocol.

[Computer-assisted versus traditional Cobb angle measurement on digital radiograph in scoliosis].

OBJECTIVE: To assess Mimics as a new measurement method in Cobb angle in a comparison with traditional measurement and try to explore the advantages and disadvantages of the new method. METHODS: Twenty X-ray plates of scoliosis were chosen randomly. Two experienced physicians measured the Cobb angles via Mimics and traditional methods twice with an interval of 1 week. And statistical analyses of Cobb angle and difference between two measurements were carried out. RESULTS: The overall correlation was 0.986 for Mimics and traditional measurements. ANOVA demonstrated no significant difference between these two methods. A significant difference existed between the first and second values of Cobb angle in Mimics and traditional measurements. However the absolute difference was small. CONCLUSION: Mimics can be a new method for measuring the digital radiographs. And its precision is superior to that of the traditional measurement.

[Finite element analysis on stress change of the lumbar disc degeneration].

OBJECTIVE: To investigate the mechanical changes of the degenerated lumbar disc with finite element analysis. METHODS: A three dimensional finite element model of a human lumbar spine at the L3-L4 disc was established by the software MIMICS and ABAQUS based on computer tomography images. Degeneration was modeled by changes in geometry and material properties. The model was loaded with 0.3 MPa in axial plane. The von Mises stress on the annulus fiber, nucleus pulposus, endplate and facet joints in healthy and degenerated discs was compared. RESULT: Compared with healthy discs, the von Mises stress of disc distributed in the side of annulus fiber, the stress of nucleus pulposus decreased remarkably, the stress of endplate distributed in the posterior part, and the stress of facet joints increased for the degenerated disc. CONCLUSION: The finite element models can provide a method of understanding the relationship between biomechanical performance of the disc due to disc degeneration.

[Finite element analysis on stress change of lumbar spine].

OBJECTIVE: To build a 3D finite element model of whole lumbar spine and verify its efficiency and analyze the biomechanical change of L3-4 motion segment. METHODS: L1-L5 segment data were obtained from computed tomography (CT) scans of the lumbar spine of a 40-year-old man with no abnormal findings. A three-dimensional finite element model of the human whole lumbar spine was built in the Mimics and the ABAQUS software. The model was composed of bony vertebrae, articulating facets, intervertebral disc and associated ligaments. The basic stress analysis of L3-4 motion segment was made under the considerations of different material properties of bone, ligaments and facet joints contacting frictional property. The stress on annulus fiber, nucleus pulposus, endplate and facet joints under axial pressure (0.3 MPa, 0.5 MPa, 1.0 MPa, 2.0 MPa & 4.0 MPa) were analyzed. RESULTS: A three-dimensional finite element model of human L3-L4 motion segment has 272, 619 elements, the stresses were higher in the posterior of annulus fiber, the Max pressure stress (S33) distributed in nucleus pulposus and the center of endplate. The stresses increased as axial pressure rose. CONCLUSION: 3D finite element model of whole lumbar spine and L3-4 motion segment were established successfully and the stress analyses were feasible and reliable.

[Finite element analysis of stress changes of posterior spinal pedicle screw infixation].

OBJECTIVE: To evaluate the mechanical response of L3-L4 segment after posterior interfixation with a transpedicle screw system. METHODS: Spiral CT machine was used to conduct continuous parallel scan on the L3-L4 section of a 40-year-old healthy male Chinese. The image data thus obtained were introduced into MIMICS software to reconstruct the 2-D data into volume data and obtain 3-D models of every element.. Pro/3-D model construction software system was used to simulate the 3-D entity of L3-L4 fixed by screw robs through spinal pedicle via posterior approach that was introduced into the finite element software ABAQUS to construct a 3-D finite element model. The stress changes on the vertebrae and screw under the axial pressure of 0.5 mPa was analyzed. RESULTS: Under the evenly distributed pressure the displacement of the L4 model was 0.00125815 mm, with an error of only 0.8167% from the datum displacement. The convergence of the model was good. The stress of the fixed vertebral body, intervertebral disc, and internal fixators changed significantly. The stress concentration zone of the intervertebral disc turned from the posterolateral side to anterolateral side. The stress produced by the fixed vertebral bodies decreased significantly. Obvious stress concentration existed in the upper and lower sides of the base of screw and the fixed screw at the upper vertebral body bore greater stress than the lower vertebral body. CONCLUSIONS: Integration of computer aided device and finite element analysis can successfully stimulate the internal fixation of L3-IA visa posterior approach and observe the mechanic changes in the vertebral column more directly.