ABSTRACT
Objective To analyze the viscoelastic properties of adjacent segments after anterior fusion under prolonged flexion, and further reveal the mechanism of accelerated adjacent segment degeneration after intervertebral fusion. Methods The same prolonged flexion lasted 30 minutes was applied on the two-level ovine lumbar specimen before and after anterior fusion respectively, and the moment relaxation and viscoelastic deformation of adjacent segments were measured. The moment relaxation curves from two groups were then fitted to obtain the quantitative viscoelastic results. Results After fusion，the initial and final moment in two groups significantly increased by 30.68% and 34.34%, and the viscoelastic deformation of the adjacent segments increased by 28.21%. The Prony model could perfectly fit the moment relaxation curves (R2=99.50%). The integral stiffness significantly increased by 47.82% and 31.14% for two groups, while the viscoelasticity significantly decreased by 27.19% and 28.16%, respectively(P<0.05). Conclusions After intervertebral fusion, to maintain the same posture with the same time, the joints should bear larger loads than before. The viscoelastic deformation of adjacent segments becomes larger, which increases the risk of instability or injury, and further leads to the accelerated degeneration of adjacent segments. The mechanism of quasi-static daily loading on adjacent segment degeneration should be focused in clinical research.
ABSTRACT
<p><b>BACKGROUND</b>The cause of the adjacent segment degeneration (ASD) after fusion remains unknown. It is reported that adjacent facet joint stresses increase after anterior cervical discectomy and fusion. This increase of stress rate may lead to tissue injury. Thus far, the load rate of the adjacent segment facet joint after fusion remains unclear.</p><p><b>METHODS</b>Six C2-C7 cadaveric spine specimens were loaded under four motion modes: Flexion, extension, rotation, and lateral bending, with a pure moment using a 6° robot arm combined with an optical motion analysis system. The Tecscan pressure test system was used for testing facet joint pressure.</p><p><b>RESULTS</b>The contact mode of the facet joints and distributions of the force center during different motions were recorded. The adjacent segment facet joint forces increased faster after fusion, compared with intact conditions. While the magnitude of pressures increased, there was no difference in distribution modes before and after fusion. No pressures were detected during flexion. The average growth velocity during extension was the fastest and was significantly faster than lateral bending.</p><p><b>CONCLUSIONS</b>One of the reasons for cartilage injury was the increasing stress rate of loading. This implies that ASD after fusion may be related to habitual movement before and after fusion. More and faster extension is disadvantageous for the facet joints and should be reduced as much as possible.</p>
Subject(s)
Humans , Biomechanical Phenomena , In Vitro Techniques , Lumbar Vertebrae , Range of Motion, Articular , Physiology , Spinal Fusion , SpineABSTRACT
Objective To provide data for establishing, driving and validating the inverse dynamics model of AnyBody Modeling System, the simulated half squat parachute landing experiment was designed and relevant data were collected. Method The subject was required to jump from a 0.32 m high platform to simulate the half squat parachute landing. The kinematic parameter of lower extremity joint, the ground reaction force and the surface electromyogram (SEMG) of four main muscles in the lower extremity joint were measured simultaneously. Results The angle changes of hip, knee and ankle along with time in three anatomical planes, the ground reaction force of right foot and the trajectory of the center of pressure were collected within 1 second just before and after the subject landing. These data would be used to drive the muscleskeletal model, while the data for measuring electromyogram activity would be used to validate the model. Conclusions The experiment meets the requirement of muscleskeletal model analysis, which can be used for further study of half squat parachute landing.