Biomechanical Research Of Pedicle-lengthening Osteotomy Treatment For Lumbar Spinal Stenosis

Lumbar spinal stenosis is a common spinal disease.Laminectomy and spinal canal decompression are primary surgical therapy,but changes of lumrbar stability may lead to bad long-term results after operation,Pedicle-lengthening osteotomy is a new theory which had been proposed recently.After cutting off the pedicle,length adjustable pedicle screws were used to lengthen the pedicles.Then the volume of spinal canal and intervertebral foramen were enlarged,and the nerve compression was relieved.Pedicle-lengthening osteotomy is a new minimally invasive spine surgery with a prosperous application because it keeps lumbar posterior structure intact.However,there are still a lot of questions unanswered.Does the pedicle-lengthening length is individual?How the pedicle-lengthening osteotomy would change the lumbar kinematics and motion stability?Although this new technique maintain the spinal structure,it changes the structure of posterior part of lumbar spine,thus does this change would affect the distribution of stress in discs,load on facet joints and load carried by each part of lumbar spine?In order to answer these questions,①we would establish 3-D models from collected MR images of LSS cases,then pedicle-lengthening length and the bulge-canal volume ratio were measured and calculated,and analyze the correlation between the pedicle-lengthening length and bulge-canal volume ratio;②sixty lumbar vertebrae were used to investigate the pull out force of traditional pedicle screw,pedicle-lengthening screw and pedicle-lengthening screws augmented with polymethylmethacrylate for instant pull-out and fatigue-resistance tests,the PPF normalized for bone mineral density of all groups were compared;③biomechanical test of 8 fresh cadaveric lumbar segments was performed on intact and pedicle-lengthening osteotomy of different length at L4 and L5 levels.Range of motion,facet joint loads at L3-4,L4-5 and L5-S1 level during flexion-extension,left-right bending and left-right axial rotation were recorded;④Three-dimensional finite element models of the lumbar spine,both intact and following pedicle-lengthening osteotomy were developed.After applying a moment of 10 Nm in flexion,extension,lateral bending and axial rotation,biomechanical effects,such as the range of motion,stress in the discs and pedicle-lengthening screws,contact force of facet joints at adjacent segments and spinal load-sharing were evaluated.The results of studies indicate that that lumbar pedicle-lengthening distance strongly correlates with bulge-canal volume ratio,the predictive equation L = 0.06 +9.06R can be estimated before surgery from CT images.PLS can provide the same fixation stability as TPS after cement augmentation,and can be considered as a viable option for clinical surgical procedures,and the pedicle-lengthening screw can be improved by adjust the diameter of the inner rod screw.Spinal mechanical stability can be preserved for different surgical level and different lengthening length,but to prevent any damage to the screws in post-operative patients,the recommended ROM of the lumbar spine should be limited in flexion,left rotation and right rotation.The pedicle-lengthening osteotomy did not alter the stress in discs,but affected the facet joint load at adjacent segments,the facet joint load of upper adjacent segment increased and that of inferior adajacent segment decreased,and the change was larger and the time was shorter for longer lengthening length.Part of the force developed by the disc was transferred to the facet joints following the pedicle-lengthening osteotomy in axial left and right rotation.In conclusion,pedicle-lengthening osteotomy can be used for treatment of lumbar spinal stenosis,and the pedicle-lengthening length can be calculated accroding to the equation and develop a surgical plan before operation.Although the spinal mechanical stability can be preserved,given that the large stress in screws,appropriate immobilization in the direction of flexion and axial rotation was recommended.Degeneration of facet joint after operation would be considered,especially for patients performed with longer lengthening length as the load carried by anterior column of lumbar spine transfer to the posterior column.