Finite Element Analysis Of Effects Of Minimally Invasive Decompression Procedures On Lumbar Spinal Biomechanics

Lumbar disc herniation(LDH) is a common disease causing low back pain, and surgical treatment is necessary when conservative treatment is ineffective. Traditional operation has obtained satisfying short-term effort, but the long-term effort is poor due to the excessive resection of posterior elements. In recent years, minimally invasive discectomy has been widespread applied clinically, and has the advantages of less bleeding, minor injury and quicker recovery.Stability of segment is an important criterion in evaluating the security and validity in spinal surgery. Many biomechanical studies have been done about effects of discectomy on lumbar stability previously, but the results are controversial due to different experimental methods and conditions. In previous studies, experimental methods are always used, and these methods have the shortage in reflecting stress distribution in disc. In addition, the influence of minimally invasive discectomy on lumbar spinal stability needs biomechanical evidance eagerly.Changes of three-dimensional kinematics in L4-5 motion segment were analyzed in this study in intact models and three surgically altered models using finite element(FE) method, and stress distribution in annulus was analyzed which can not be measured using experimental method. This study aims topredict the overall effects of minimally invasive decompression procedures on lumbar spinal biomechanics, so as to offer theoretical evidance for clinical practice, to guide patients in post-operation to recover properly. Additionally, Reverse Engineering techniques were investigated in establishing three-dimensional geometry model of lumbar vertebrae. Materials and MethodsL4 and L5 vertebrae specimens were selected, which were not apparently degenerative. The specimens were scanned with LACUS150B-type laser scanner at various angles, and accurate three-dimensional coordinates data of specimens surface were got after the signals were handled in computer. And then, the coordinates data was transformed into three-dimensional geometry shape of L4 and L5 vertebrae in Solidworks software.Taking the later finite element model into account, the vertebral bodies were assumed to have a cancellous core covered by a cortical shell(about 1.5 mm thick). Posterior elements were treated as independent components.L4-5 disc, superior and inferior cartilaginous endplate and facet cartilagine were drawn in Solidworks software. The ratio of the cross-sectional area of the nucleus pulposus to that of the total disc was 40%. Cartilaginous endplate and facet cartilagine was assumed 1.0mm and 0.2mm thick, respectively; and the gap of facet articulation was 0.6mm wide. Data above was derived from literature. Finally, all the components were assembled into integrated geometry model of L4-5 motion segment according to normal orientation.L4-5 geometry model was imported into Ansys7.0 professional FE software. After element types and materials properties were defined, the model was meshed manually and automatically according to principles of meshing, and then, geometry model of L4-5 motion segment was transformed into FE model. At last, main ligaments and capsular were added to the model. Linear isotropic properties were assumed for all the model components, and the data for allmaterial properties and ligamental positions were derived from the literature.Three minimally invasive decompression procedures were simulated in this study. Procedure 1 simulated discectomy through osseous lamina fenestration: An oval window(14mmX 16mm) was made on the left lamina of L4 along its longitudinal axial, with the resection of left ligamentum flavum; A round windowC 5mm in diameter) was cut in the left-posterior side of the annulus, and the nucleus pulposus was removed completely, with the facet articulation intact. Procedure 2 simulated discectomy with lateral recess decompression through osseous lamina fenestration: 1/3 of the left facet articulation inside and corresponding capsular ligament were removed, with other parts same as procedure 1. Procedure 3 simulated discectomy with lateral recess decompression through L4-L5 interlaminar fenestration: Inferior 1/3 of L4 left lamina and superior 2mm of L5 left lamina were removed, with 1/3 of the left facet articulation inside and corresponding capsular ligament removed. Other parts resected kept the same as procedure 1.The inferior endplate of L5 vertebral body was fixed in all directions, and all loads were employed evenly on the superior endplate of L4 vertebral body. Axial compressive load of 400Nm with moment of 7.5Nm in six directions were applied to the intact and surgical models, simulating various posture of the body under physiological loads, namely, flexion, extension, left and right lateral bending, left and right axial torsion. Rotational degrees of intersegment and the highest von Mises stress in annulus were examined. In contrast to the results of intact model, the percentage changes of rotational degrees and von Mises stress in surgical models was calculated. Results1. A more accurate three-dimensional finite element model of L4-L5 motion segment was developed, including posterior elements and main ligaments, and the model can be used for further biomechanical study.2. The intact model contained 81 964 elements and 129 861 nodes, including 81 820 solid elements and 144 cable elements. The ratio of cross-sectional area of the nucleus pulposus to the total disc was 40%. The model was differentiated into seven sorts of different materials.3. Under compressive load of 400N with a moment of 7.5Nm, the rotational degrees of intact model in flexion, extention, lateral bending and axial rotation was 4.52°, 2.28°, 4.05°and 1.52°, respectively. The stiffness of FE model macthed well with cadaver model reported.4. Under same loadings, stress distribution in different models had the same results: the highest von Mises in annulus in flexion, extention, left and right lateral bending, left and right axial rotation conditions located in the anterior-inferior side, posterior-inferior side, left and right posterior-inferior side, right and left posterior-inferior side, respectively.5. For procedure 1, the rotational degrees in flexion , extention, left lateral bending, right lateral bending, left and right axial rotation conditions increased by 24%, 28%, 8%, 8%, 19% and 20%, respectively; and the highest von Mises in annulus increased by 16%, 6%, 0.3%, 0.3%, 0.3% and 0.4%, respectively.6. For procedure 2, the rotational degrees in flexion , extention, left lateral bending, right lateral bending, left and right axial rotation conditions increased by 33%, 29%, 8%, 11%, 18% and 22%, respectively; and the highest von Mises in annulus increased by 17%, 6%, 0.3%, 0.4%, 0.4% and 1.2%, respectively.7. For procedure 3, the rotational degrees in flexion , extention, left lateral bending, right lateral bending, left and right axial rotation conditions increased by 31%, 39%, 8%, 12%, 29% and 37%, respectively; and the highest von Mises in annulus increased by 17%, 0.2%, 0.3%, 0.4%, 0.5% and 1.3%, respectively. Conclusions1. Reverse Engineering technique can be used to reconstruct three-dimensional FE geometrical model of lumbar vertebrae accuratelly.2. Minimally invasive discectomy has the tendency of inducing the lumbar segment unstable in flexion-extention conditions. The trend becomes more apparent after lateral recess decompression has been done.3. Minimally invasive discectomy with lateral recess decompression through interlaminar fenestration makes lumbar segment more unstable in extension and axial rotation conditions than through osseous lamina fenestration.4. After discectomy using minimally invasive techniques, the stress distribution in annulus keeps unchanged, but the stress values increase in all cases, and the percentage of increase in flexion condition is the highest.