In order to promote solid fusion across a decompressed spinal segment, inter-body spacers/cages are used with and without posterior instrumentation to provide an initial “rigid” fixation of the segment. Inter-body spacers (cages) of various shapes (e.g., rectangular, cylindrical) and materials are currently available on the market. Important factors affecting the biomechanics of the fused segment are (i) cage shape and placement, (ii) cage material property (iii) surgical approach used –posterior vs. antero lateral (iv) cage with additional instrumentation. The objective of this study is to address change in the stability and stress patterns associated with the various factors described above.
A cadaveric study using established protocols and a finite element (FE) study were conducted. For the cadaveric study, nine fresh ligamentous lumbar spine specimens (L1-S2) were radiographed out of which six specimens were prepared for testing by fixing a base to the sacrum and a loading frame to the top-most vertebra. Each specimen was subjected to pure moment (6 Nm in steps of 1.5 Nm) in six loading modes: flexion, extension, right and left lateral bending, and right and left axial rotation. The load-displacement data was collected in a sequential manner for the following cases: 1) intact spine, 2) insertion of rectangular cages (Vertebral spacer PR, Synthes, Inc.), 3) fixation with posterior instrumentation, 4) fatiguing the instrumented spine. The relative motion of L4 with respect to L5 was calculated for all these cases.
A validated three-dimensional, nonlinear FE model of lumbar spine from L3-L5 was used. The model was modified to simulate the bilateral placement of cages alone. Contact surfaces were defined between the cages and the endplates to simulate the bone-implant interface. The cages were placed using posterior approach and left antero lateral approach to see the effect of the surgical approach on the stability of the segment. In the FE model with cage placed using posterior approach, posterior instrumentation was added. For this model the material property of the cage was changed form PEEK to titanium to study the change in load sharing and stresses on the endplates. For all the models a 6Nm moment was applied and all the six loading cases were simulated. The relative motion of L4 with respect to L5 was calculated, stresses in the implants and endplates were studied.
Results from the in vitro study indicate that the stability of the spine decreased after the stand alone placement of bilateral cage when compared to the intact for all the loading cases except in flexion. However, no statistically significant difference was seen in the stability between intact and stand alone cage placement. After stabilization with posterior fixation using the pedicle screw rod system, the stability increased in all loading cases. There was no significant change in stability after fatiguing.
The FE model predictions for the bilateral cage alone and with additional instrumentation placed at L4-L5 disc space were within 1 SD of the cadaveric data in all loading modes. There was no change in stability offered by stand alone cage placement using antero lateral approach and posterior approach. For the cage made of titanium peak Von mises stress in the endplates were twice of that for cage made of PEEK. Cages placed laterally from the mid-sagittal plane provide better stability in bending when compared to medially placed cages.
|School:||The University of Toledo|
|School Location:||United States -- Ohio|
|Source:||MAI 57/06M(E), Masters Abstracts International|
|Keywords:||Cage, Fusion, Interbody fusion, Lumbar spine, Spacer, Spine|
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