Spine Research

Bio-Robotics for Testing of Spinal Stability

Traditional wire and pulley systems only have limited degrees of freedom to test spinal stability.

A robotic system generally has 6 degrees of freedom, allowing a 360 degree spinal stability analysis.

Furthermore, the robotic arm can control the loading environment in all six axis, meaning shear forces,

bending moments, and axial forces can be controlled and manipulated independently.

Three-dimensional (3D) kinematics movement of a spinal segment was analyzed using a six degrees-of-freedom (6DOF) robotic arm testing system. Knowledge of spinal joint kinematics (i.e. lumbosacral joint) is important in the diagnosis of lower back pain and other related pathologies but its three-dimensional flexibility under different loading regime is uncertain. Due to instrument limitations, previous studies have generally investigated the range of motions in only three planes; sagittal, transverse, and frontal plane. To understand the 3D kinematics of spinal joint, we utilized a 6DOF robotic arm in combination with a 6DOF force/torque transducer. In this study, a cadaveric lumbosacral joint was tested under six loading regimes (three levels of pure bending moments and two preload conditions). Under constant applied pure bending moments, the full circumferential motion paths (motion path envelopes) of lumbosacral joint under each loading regimes were mapped. It was also shown that the posterior portion of the motion paths (in transverse plane) were collinear due to restrictions by the facet joints. A comparison between preload versus no preload showed a reduction in the projected motion fields area [mm²] up to 35% mostly seen in the anterior portion of the motion field. The protocol of using a robotic arm was justified in the study to provide a simple method to understand the kinematics of a spinal joint in three-dimensional space. The movement pattern observed in this study provides a better understanding of the interplay between intervertebral disc and facets to provide the range of motion within the lumbosacral spinal level. In addition, preload condition can have a detrimental effect on loss of range of motion..

Efficacy of Intervertebral Disc Augmentation

Intervertebral disc augmentation increased the overall range of motion of the spinal

segments and yielded very smooth contour lines of the motion profiles. This analysis

was done using a 6-axis robotic arm to test the spinal segment in 3D space.

The objective of this study was to provide a biomechanical assessment of disc augmentation efficacy on spinal functional units. The test groups included treated (augmented intervertebral disc) and un-treated controls. The control parameter was an oscillating and rotating bending moment. Recorded and analyzed parameters included injection pressure, intervertebral disc pressure, axial stiffness, bending rigidity, and changes in range of motion between pre-injection and at multiple time points post injection. The functional spinal units were tested in bending along 9 different planes, thereby spanning 360 degrees circumferential range of motion. The testing system included a six axis robotic system run in hybrid control, meaning a programmed path was altered during testing with force and moment feedback. The results of this preliminary study indicated that intervertebral disc augmentation positively increased spinal range of motion. The use of injectable biomaterials for treating joint pathology showed promising results under physiological loading conditions simulated using a 6-DOF robotic arm. For the first time, full three-dimensional motion path envelopes were illustrated under different loading regimes among treated and control groups..

Adjacent Fracture Risk of Vertebrae after Vertebroplasty/Kyphoplasty

Vertebroplasty treatment resulted in significantly reduced ultimate strength (fracture load)

of the adjacent vertebrae after fracture treatment. This phenomenon is attributed to a change in

oad transfer across the intervertebral disc due to the injected bone cement. Supported is that

hypothesis by the observed fracture pattern (dominantly concave endplate fractures after

treatment versus wedge type fractures for the untreated groups.

To investigate to whether vertebroplasty lowered the fracture strength of adjacent untreated vertebrae under physiological loading conditions, the biomechanical effect of vertebroplasty on fracture strength of untreated adjacent vertebrae was examined.
There is an increasing incidence of fractures in the untreated adjacent vertebrae after vertebroplasty. These secondary fractures have been inconclusively attributed to changes in load distribution resulting in the inward endplate deflection of the adjacent vertebrae or simply to the progression of osteoporosis. To test this hypothesis, the untreated T10-T12 and treated L1-L3 three level spinal segments from six spines were tested under unconstraint axial compression where shear forces and torque were minimized using a 6 Degrees-Of-Freedom robotic arm. Fracture strengths of pretreated lumbar segments were predicted by assuming constant fracture stress along the spinal column, and were compared to post-treatment values. Radiographic X-rays were taken at every 600N increments to record the developing fracture pattern.

Five of the six treated segments experienced reductions in fracture strengths ranging from 7.7% to 45.5%. Treated segments had biconcave fractures while wedge fractures were mainly seen in the control (untreated) segments. We concluded from our study that vertebroplasty altered the load-transfer along the anterior spinal column, thereby statistically significantly increasing fracture risk of the untreated adjacent vertebrae (p<0.05). The procedure resulted in inward deflection of the adjacent vertebral endplates, producing endplate failures compared to wedge fractures seen in the control group.

Biomechanics of Preventive Vertebral Stabilization

Numerical simulations are an ideal tool to investigate the intra-vertebral loading scenario

after treatment of vertebrae with vertebroplasty. We and others have identified causes of

adjacent bone fracture. To increase biomechanical efficacy of vertebral augmentation and

reduce adjacent bone fractures, vertebrae at risk of fracturing should be treated on a

preventive basis. To allow revisions, biomaterials instead of bone cement should be used.

The effects of bone cement placement, volume, and bone density on the degree of biomechanical reinforcement on cadaveric vertebral bodies were studied using experimentally calibrated detailed finite element models. The objective of our study was to investigate the efficacy of prophylactic vertebroplasty on intact vertebral bodies with respect to biomechanical recovery and fracture risk reduction.
Vertebroplasty is a potentially effective fracture prevention treatment, but the risk of complications due to cement leakage must be minimized. Therefore, the least amount of bone cement required to improve vertebral strengths to low fracture risk levels need to be determined. For this purpose, six different polymethyl methacrylate volumes’Äî 1, 2.5, 3.5, 5, 7.5 and 9 cm³’Äîwere virtually implanted into previously validated vertebral body finite element models, following bipedicular and posterolateral vertebroplasty approaches. Stiffness and fracture load of the treated and untreated vertebral body models under uniaxial compression were determined.
Based on our simulations, greater augmentation effects were observed for vertebral bodies with average quantitative computed tomography densities below 0.1 g/cm³ injected with polymethylmethacrylate volumes higher than 20% compared to lower injection volumes and higher bone densities, as well as for the bipedicular approach versus posterolateral. Vertebral bodies at high risk of fracture required at least 20% fill of polymethyl methacrylate to improve the
mechanical integrity of vertebral bodies to low fracture risk levels, whereas 5% to 15% polymethyl methacrylate volumes were needed for the medium-risk vertebral bodies.
From our study, we concluded that preventive vertebroplasty can be effective in reducing fracture risk. However, for the polymethyl methacrylate volume (20%) required for the successful reinforcement of high-risk vertebral bodies, the risk of complications will be as high as that for current vertebroplasty procedure for fracture repair. Therefore, alternative materials have to be investigated for prophylactic vertebroplasty. Furthermore, bipedicular vertebroplasty is the recommended approach due to its higher strengthening effect and easier surgical access than the posterolateral case.