S. Campana

Relationships between viscoelastic properties of lumbar intervertebral disc and degeneration grade assessed by MRI.

Biomechanical modelling of the spine is of high clinical significance, either for implant evaluation or for surgery planning. Nevertheless, assessment of patient specific material properties still remains an issue, especially the viscoelastic characteristics of lumbar intervertebral discs (IVD). MRI, a dedicated system for IVD examination, provides a signal that is correlated with the biochemical content of the disc. Since IVD composition and its mechanical properties are related, the objectives of this study were to investigate how MRI could inform about viscoelastic properties of lumbar discs, determined from creep experiments. For that purpose, an in vitro protocol was carried out regarding 14 human L1-L2 IVDs; each unfrozen specimen was imaged using MRI and biomechanically tested with 10 min creep under 400 N load. Three-parameter rheologic models were used to fit the experimental curves. Additionally, geometry was obtained and degeneration was assessed using both MRI grading and physical inspection (destructive analysis). Mean creep displacement was 0.19 mm after 10 min. MRI scaling categorized elastic modulus and viscosity of the IVDs in 2 clearly distinct groups without overlaps according to degeneration: mean values for elastic modulus were 12.9 MPa and 5.7 MPa, respectively for mildly and severely degenerated IVDs; mean values for viscosity were 5.7 GPa s and 2.2 GPa s, respectively for mildly and severely degenerated IVDs. Classification derived from physical inspection did not reveal a clear discrimination. MRI could hence provide a quantification of IVDs viscoelastic properties, leading to in vivo direct estimation of material characteristics necessary for patient specific modelling.

CT-based semi-automatic quantification of vertebral fracture restoration

Minimally invasive surgeries aiming to restore fractured vertebral body are increasing; therefore, our goals were to create a 3D vertebra reconstruction process and design clinical indices to assess the vertebral restoration in terms of heights, angles and volumes. Based on computed tomography (CT)-scan of the vertebral spine, a 3D reconstruction method as well as relevant clinical indices were developed. First, a vertebra initial solution requiring 5 min of manual adjustments is built. Then an image processing algorithm places this solution in the CT-scan images volume to adjust the models nodes. On the vertebral bodys anterior and posterior parts, nine robust heights, volume and endplate angle measurement methods were developed. These parameters were evaluated by reproducibility and accuracy studies. The vertebral body reconstruction accuracy was 1.0 mm; heights and volume accuracy were, respectively, 1.2 and 179 mm3. In conclusion, a 3D vertebra reconstruction process requiring little user time was proposed as well as 3D clinical indices assessing fractured and restored vertebra.

Biomechanical Effect of an Interlaminar Device on Ranges of Motion, Intradiscal Pressure, and Centers of Rotation

Introduction. The IntraSPINE is a new interlaminar device that has been proposed with the aim to decompress the spinal canal without reducing the extension motion. The purpose of this study was therefore to evaluate the biomechanical behavior of L4-L5 spinal units implanted with this interlaminar device, in terms of ranges of motion, intradiscal pressure, and centers of rotation. Material and Methods. Six human lumbar spines were harvested within 10 days after death. A specific spine testing device was used to apply moments up to 10 Nm in flexion-extension, lateral bending (left-right flexion) and left-right axial rotation (torsion), with measurement of vertebral 3D motion and of intervertebral disc pressure. Protocol was repeated for each specimen in 5 configurations: intact specimen; after L4-L5 bilateral medial hemifacetectomy and both yellow ligament resection; after implantation of the interlaminar device at the L4-L5 level; after removal of the L4-L5 supraspinous ligament, resection of the posterior third of the disc and addition of an artificial ligament; after device and artificial ligament removal. Results. The implant reduced increases in segmental flexion seen following injury particularly when applied with the artificial ligament. Intradiscal pressure reduced following application of the implant without reducing extension range. A small posterior shift of the Mean Centers of Rotation (MCR) was noticed after instrumentation. Torsion and lateral bending range was unaffected by the interlaminar device. Conclusion. This biomechanical study yields a better understanding of this interlaminar implant effect. A large clinical trial with follow-up would be required to evaluate and confirm in vivo the observed in vitro biomechanical behavior of the device.