| Purpose:Pedicle screw fixation may induce abnormal activity at adjacent segment and even accelerate the degeneration of lumbar vertebrae.Dynamic stabilizers could provide an intermediate solution between conservative treatment and fusion surgery.As one kind of the dynamic stabilizations,interspinous process devices were advocated to treat patients with Grade 1 degenerative spondylolisthesis,discogenic pain,disc herniation and non-traumatic instability.Lumbar vertebral segment cephalad to instrumented fixation was the most common localization of adjacent segment degeneration.The aim of this study is to explore the use of interspinous process devices in the lumbar vertebral segment cephalad to fixation segment in changing the mechanical distribution and limiting abnormal activity of the spine.Methods:Eight specimens(L1-L5)with no known abnormalities(confirmed via X-ray)were tested in the following groups: intact group;instability group(bilateral facetectomy at L3-L4);fixation group(bilateral facetectomy and pedicle screw fixation at L3-L4);hybrid fixation group(fixation at L3-L4 and simulating interspinous device implantation of 6,8,10,12,14,16,and 18 mm at L2-L3).The measurement of spine kinematics and facet strain was divided into five steps.First,the strain gauges were fixed to the facet joints(inferior articular process of L1 and L4,the superior and inferior articular processes of L2,and the superior articular process of L3 and L5),the strain gauge signal was sampled using a static resistance strain indicator.Next,the rigid bodies was connected with three vertebral bodies(L2?L3 and L4),Optotrak Centus System was used to measure the three-dimensional coordinates of the rigid bodies with a sampling frequency of 100 Hz.Third,torque-controlled movements were applied to the specimen with simultaneous recording of kinematics,vertebral Ro M,and strain gauge outputs.Finally,instability,fixation,and hybrid construct groups were operated on in order.Range of motion,motion of vertebral body,and strain distribution change were recorded.The Interspinous process devices were manufactured using a 3D printer and were made of polylactic acid,a circled spacer,and two side platens.The spacer was designed with a height of 6,8,10,12,14,16,or 18 mm.Results:(1)Range of motion.The range of motion in extension with 16 and 18 mm hybrid constructs was significantly lower than intact,instability,and fixation groups(P<0.05).The 18 mm hybrid construct was significantly less Ro M than the 10 and 12 mm distractions of the hybrid construct in extension(P<0.05).Compared with the instability group,the fixation group and all hybrid construct groups had decreased axial rotation(P<0.05).(2)Range of motion of vertebral body.There were no significant differences in flexion,extension,lateral bending,or axial rotation motion(P>0.05).Linear regression analysis showed that the Ro M of L3 in flexion and L2 in extension are positively correlated with distraction height.(3)Resistance strain gauge.In flexion and lateral bending,the strain values of L4 inferior articular process with 18 mm hybrid construct have a significant difference compared with the other groups(P<0.05).In axial rotation,under the condition of a contralateral state,the strain values of L2 superior articular process with 18 mm hybrid construct have a significant difference compared with the intact,fixation and other hybrid construct groups(P<0.05).After treatment for instability,the strain value have a significant difference compared with 8 and 12 mm hybrid groups.The strain value of L1 inferior facet process positive correlation with distraction height in flexion(P<0.05),while the strain value of the L4 inferior articular process negative correlation with height distraction in three dimensions,except extension(P<0.05).A negative correlation between the strain value of the L2 superior articular process and distraction height was found in contralateral bending and contralateral axial rotation(P<0.05).Conclusions:(1)For the flexibility test,absolute overdistraction of the interspinous height could markedly limit the Ro M of whole lumbar spine.The “side platen” of Interspinous process devices mainly restrict the transverse movement of the circled spacer and has no effect on the Ro M in axial rotation.Overdistraction of interspinous cephalad to instrumented fixation lumbar could markedly limit the Ro M of whole lumbar,clinicians should be taken into consideration when choosing a larger or small size,meanwhile,the changed trend of Ro M should be concerned.(2)For vertebral motion,combined used single level pedicle screw fixation and interspinous distraction devices can make the Ro M of upper fixation segment and adjacent segment cephalad to fixation negative correlation with height distraction in some,this could improve the lumbar stability.Compared with the single level fixation group and intact group,under the condition of extension,the Ro M of upper fixation segment and adjacent segment cephalad to fixation were increased and in the opposite direction,and the Ro M of lower fixation segment decrease,these will accelerate adjacent segment degeneration.(3)For the strain gauge value,combined used single level pedicle screw fixation and interspinous distraction devices,can make the adjacent segment facet joint stress decreased,reducing the stress of the upper segment of non-fusion segment,thus may delay the degeneration of the adjacent segments.In the clinical work,choose an appropriate distraction height of Interspinous process devices can limit the lumbar motion and reduce stress of the upper fixation segment and adjacent segment cephalad to fixation.(4)IPDs in the lumbar vertebral segment cephalad to fixation segment can change the mechanical distribution of the lumbar facet joints and limit activity in some of segments of the spine,this maybe delay the degeneration of the adjacent segment. |
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