Pressure distributions inside intervertebral discs under unilateral pedicle screw fixation in a porcine spine model

Abstract

BackgroundLittle data are available regarding the effects of pedicle screws on the intervertebral disc stress for different spinal segments. The aim of this study was to analyze the intervertebral disc stress in response to the placement of pedicle screws.MethodsT3–4, T11–12, T15–L1, L3–4, and L4–5 intervertebral disc segments from six porcine spine specimens were harvested. A compressive load of 200 N was applied both before and after the pedicle screw was implanted on the left side of each target segment; the resulting pressure was measured during vertical, 5° anterior flexion, 5° posterior extension, and 5° lateral bending.ResultsThe posterior intradiscal pressures of the intervertebral disc were significantly lower in the fixed group than in the unfixed group for all segments during vertical, 5° anterior flexion, and 5° posterior extension. The left pressures of the intervertebral disc were significantly lower in the fixation group for all segments. During 5° lateral bending, the left intervertebral disc pressures were significantly lower in the fixation group. Lower mean pressures were observed in the fixed group.ConclusionsUnilateral pedicle screws can effectively reduce the pressure of the fixed lateral intervertebral disc. Moreover, it can change the pressure distribution of the intervertebral disc and reduce the pressure of the entire intervertebral disc, especially the posterior side of the intervertebral disc.

Finite element modeling to predict the influence of anatomic variation and implant placement on performance of biological intervertebral disc implants

“…When these devices were implanted in the porcine cervical spine, failure occurred in the posterior region. This was surprising given the yield strength of PLA (50–70 MPa) 46–48 greatly exceeds the reported porcine intradiscal pressures (1–2 MPa) 49 . Furthermore, migration of the cage toward the anterior side of the vertebra was observed.…”

“…This was surprising given the yield strength of PLA (50-70 MPa) [46][47][48] greatly exceeds the reported porcine intradiscal pressures (1-2 MPa). 49 Furthermore, migration of the cage toward the anterior side of the vertebra was observed. A possible explanation From the non-uniform distribution of stress and strain of the cage seen in vivo, we expected that the geometry of the porcine vertebrae played a significant role in explaining the compressive failure of the cage.…”

“…This was surprising given the yield strength of PLA (50–70 MPa) 46 , 47 , 48 greatly exceeds the reported porcine intradiscal pressures (1–2 MPa). 49 Furthermore, migration of the cage toward the anterior side of the vertebra was observed. A possible explanation of both the failure as well as the migration is the presence of anatomical features of the porcine cervical vertebral endplate 50 , 51 that concentrate stress on implants.…”

“…A possible explanation From the non-uniform distribution of stress and strain of the cage seen in vivo, we expected that the geometry of the porcine vertebrae played a significant role in explaining the compressive failure of the cage. Intradiscal pressure of porcine cervical spine, 49 human lumbar spine, 52 and other large animals such as sheep 53 are over 1 MPa. Our FE model predicted that stress on the implants exceeded 150 MPa.…”

Effect of Strain Rates on Failure of Mechanical Properties of Lumbar Intervertebral Disc Under Flexion

“…The cause of lumbar intervertebral disc herniation is complex, while long-term load accumulation and sudden load mutation are considered to be important causes of herniation. Therefore, it is urgent to analyze the stress/strain behavior of the intervertebral disc under different loading conditions 4,5 .…”