Use of Regression Tree Analysis for Predicting the Functional Outcome after Traumatic Spinal Cord Injury.

Predicting the long-term functional outcome after traumatic spinal cord injury (TSCI) is needed to adapt medical strategies and plan an optimized rehabilitation. This study investigates the use of regression trees for the development of predictive models based on acute clinical and demographic predictors. This prospective study was performed on 172 patients hospitalized after TSCI. Functional outcome was quantified using the Spinal Cord Independence Measure (SCIM) collected within the first-year post-injury. Age, delay before surgery, and Injury Severity Score (ISS) were considered as continuous predictors whereas energy of injury, trauma mechanisms, neurological level of injury, injury severity, occurrence of early spasticity, urinary tract infection, pressure ulcer, and pneumonia were coded as categorical inputs. A simplified model was built using only American Spinal Injury Association Impairment Scale grade, neurological level, energy, and age as predictor and was compared to a more complex model considering all 11 predictors mentioned above. The models built using 4 and 11 predictors were found to explain 51.4% and 62.3% of the variance of the SCIM total score after validation, respectively. Severity of the neurological deficit at admission was found to be the most important predictor. Other important predictors were the ISS, age, neurological level, and delay before surgery. Regression trees offer promising performances for predicting the functional outcome after a TSCI. It could help to determine the number and type of predictors leading to a prediction model of the functional outcome that can be used clinically in the future.

The use of classification tree analysis to assess the influence of surgical timing on neurological recovery following severe cervical traumatic spinal cord injury.

STUDY DESIGN: Post hoc analysis of prospectively collected data. OBJECTIVES: Assess the influence of surgical timing on neurological recovery using classification tree analysis in patients sustaining cervical traumatic spinal cord injury. SETTING: Hôpital du Sacré-Coeur de Montreal METHODS: 42 patients sustaining cervical SCI were followed for at least 6 months post injury. Neurological status was assessed from the American Spinal Injury Association impairment scale (AIS) and neurological level of injury (NLI) at admission and at follow-up. Age, surgical timing, AIS grade at admission and energy of injury were the four input parameters. Neurological recovery was quantified by the occurrence of improvement by at least one AIS grade, at least 2 AIS grades and at least 2 NLI. RESULTS: Proportion of patients that improved at least one ASIA grade was higher in the group that received early surgery (75 vs. 41 %). The proportion of patients that improved two AIS grades was also higher in the group that received early surgery (67 vs. 38 %). Finally, 30 % of the patients that received early decompression improved two NLI as compared with 0% in the other group. Early surgery was also associated with a non-statistically significant improvement in functional recovery. CONCLUSIONS: Neurological recovery of patients sustaining cervical traumatic spinal cord injury can be improved by early decompression surgery performed within 19 h post trauma. SPONSORSHIP: U.S. Army Medical Research and Material Command, Rick Hansen Institute.

Impact of spinal rod stiffness on porcine lumbar biomechanics: Finite element model validation and parametric study.

A three-dimensional finite element model of the porcine lumbar spine (L1-L6) was used to assess the effect of spinal rod stiffness on lumbar biomechanics. The model was validated through a comparison with in vitro measurements performed on six porcine spine specimens. The validation metrics employed included intervertebral rotations and the nucleus pressure in the first instrumented intervertebral disc. The numerical results obtained suggest that rod stiffness values as low as 0.1 GPa are required to reduce the mobility gradient between the adjacent and instrumented segments and the nucleus pressures across the porcine lumbar spine significantly. Stiffness variations above this threshold value have no significant effect on spine biomechanics. For such low-stiffness rods, intervertebral rotations in the instrumented zone must be monitored closely in order to guarantee solid fusion. Looking ahead, the proposed model will serve to examine the transverse process hooks and variable stiffness rods in order to further smooth the transition between the adjacent and instrumented segments, while preserving the stability of the instrumented zone, which is needed for fusion.

Development of an instrumented spinal cord surrogate using optical fibers: A feasibility study.

In vitro replication of traumatic spinal cord injury is necessary to understand its biomechanics and to improve animal models. During a traumatic spinal cord injury, the spinal cord withstands an impaction at high velocity. In order to fully assess the impaction, the use of spinal canal occlusion sensor is necessary. A physical spinal cord surrogate is also often used to simulate the presence of the spinal cord and its surrounding structures. In this study, an instrumented physical spinal cord surrogate is presented and validated. The sensing is based on light transmission loss observed in embedded bare optical fibers subjected to bending. The instrumented surrogate exhibits similar mechanical properties under static compression compared to fresh porcine spinal cords. The instrumented surrogate has a compression sensing threshold of 40% that matches the smallest compression values leading to neurological injuries. The signal obtained from the sensor allows calculating the compression of the spinal cord surrogate with a maximum of 5% deviation. Excellent repeatability was also observed under repetitive loading. The proposed instrumented spinal cord surrogate is promising with satisfying mechanical properties and good sensing capability. It is the first attempt at proposing a method to assess the internal loads sustained by the spinal cord during a traumatic injury.

Impact of anchor type on porcine lumbar biomechanics: Finite element modelling and in-vitro validation.

BACKGROUND: Rigid posterior implants used for spinal stabilization can be anchored to the vertebrae using pedicle screws or screws combined with transverse process hooks. In the present study, a finite element model of a porcine lumbar spine instrumented with screws and hooks is presented and validated. METHODS: The porcine lumbar spine model was validated using in-vitro measurements on six porcine specimens. Validation metrics included intervertebral rotations (L1 to L6) and nucleus pressure in the topmost cranial instrumented disc. The model was used to compare the biomechanical effect of anchor types. FINDINGS: Good agreement was observed between the model and validation experiments. For upper transverse hooks construct, intervertebral rotations increased at the upper instrumented vertebra and decreased at the adjacent level. Additionally, nucleus pressures and stress on the annulus decreased in the adjacent disc and increased in the upper instrumented disc. The pull-out forces predicted for both anchor configurations were significantly lower than the pull-out strength found in the literature. INTERPRETATION: These numerical observations suggest that upper transverse process hooks constructs reduce the mobility gradient and cause less stress in the adjacent disc, which could potentially reduce adjacent segment disease and proximal junction kyphosis incidence without increasing the risk of fixation failure. Future work needs to assess the long-term effect of such constructs on clinical and functional outcomes.

Biomechanical assessment of the stabilization capacity of monolithic spinal rods with different flexural stiffness and anchoring arrangement.

BACKGROUND: Spinal disorders can be treated by several means including fusion surgery. Rigid posterior instrumentations are used to obtain the stability needed for fusion. However, the abrupt stiffness variation between the stabilized and intact segments leads to proximal junctional kyphosis. The concept of spinal rods with variable flexural stiffness is proposed to create a more gradual transition at the end of the instrumentation. METHOD: Biomechanical tests were conducted on porcine spine segments (L1-L6) to assess the stabilization capacity of spinal rods with different flexural stiffness. Dual-rod fusion constructs containing three kinds of rods (Ti, Ti-Ni superelastic, and Ti-Ni half stiff-half superelastic) were implanted using two anchor arrangements: pedicle screws at all levels or pedicle screws at all levels except for upper instrumented vertebra in which case pedicle screws were replaced with transverse process hooks. Specimens were loaded in forward flexion, extension, and lateral bending before and after implantation of the fusion constructs. The effects of different rods on specimen stiffness, vertebra mobility, intradiscal pressures, and anchor forces were evaluated. FINDING: The differences in rod properties had a moderate impact on the biomechanics of the instrumented spine when only pedicle screws were used. However, this effect was amplified when transverse process hooks were used as proximal anchors. INTERPRETATION: Combining transverse hooks and softer (Ti-Ni superelastic and Ti-Ni half stiff-half superelastic) rods provided more motion at the upper instrumented level and applied less force on the anchors, potentially improving the load sharing capacity of the instrumentation.

In-vitro assessment of the stabilization capacity of monolithic spinal rods with variable flexural stiffness: Methodology and examples.

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. To assess the stabilization capacity of such a spinal rod, in vitro experiments on porcine spine models are carried out. This paper describes the methodology followed to evaluate the effect of Ti-Ni rods compared to conventional titanium rods. Validation of the methodology and examples of results obtained are also presented.

Implementation of a 3D porcine lumbar finite element model for the simulation of monolithic spinal rods with variable flexural stiffness.

Monolithic superelastic-elastoplastic spinal rods (MSER) are promising candidates to provide (i) dynamic stabilisation in spinal segments prone to mechanical stress concentration and adjacent segment disease and (ii) to provide fusion-ready stabilization in spinal segments at risk of implant failure. However, the stiffness distributions along the rod's longitudinal axis that best meet clinical requirements remain unknown. The present study is part of a mixed numerical experimental research project and aims at the implementation of a 3D finite element model of the porcine lumbar spine to study the role of MSER material properties and stiffness distributions on the intradiscal pressure distribution in the adjacent segment. In this paper, preliminary intradiscal pressure predictions obtained at one functional spinal unit are presented. Due to a lack of porcine material property data, these predictions were obtained on the basis of uncalibrated human vertebral disc data which were taken from the literature. The results indicate that human annulus and nucleus data predict experimental porcine in vivo and in vitro data reasonably well for the compressive forces of varying magnitudes.

Monolithic superelastic rods with variable flexural stiffness for spinal fusion: modeling of the processing-properties relationship.

The concept of a monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion. The variable stiffness is conferred to the rod using the Joule-heating local annealing technique. The annealing temperature and the mechanical properties' distributions resulted from this thermal treatment are numerically modeled and experimentally measured. To illustrate the possible applications of such a modeling approach, two case studies are presented: (a) optimization of the Joule-heating strategy to reduce annealing time, and (b) modulation of the rod's overall flexural stiffness using partial annealing. A numerical model of a human spine coupled with the model of the variable flexural stiffness spinal rod developed in this work can ultimately be used to maximize the stabilization capability of spinal instrumentation, while simultaneously decreasing the risks associated with spinal fusion.

Monolithic superelastic rods with variable flexural stiffness for spinal fusion: simplified finite element analysis of an instrumented spine segment.

UNLABELLED: Rigid instrumentations have been widely used for spinal fusion but they come with complications, such as adjacent disc degeneration. Dynamic instrumentations have been tested but their efficiency (stabilization capability) and reliability (mechanical integrity of the implant) have yet to be proven. A monolithic Ti-Ni spinal rod with variable flexural stiffness is proposed to reduce the risks associated with spinal fusion while maintaining adequate stabilization. This publication presents a simplified numerical model capable of evaluating the eventual benefits of a Ti-Ni spinal rod with variable flexural stiffness. METHODS: A simplified instrumented spine segment model composed of six vertebrae and five discs has been developed. Two types of spinal rods were evaluated: Classic Ti instrumentation and Ti-Ni rods with variable stiffness. Both instrumentations were tested using two anchor configurations: pedicle screws only or a screws-cable combination. FINDINGS AND DISCUSSION: The all-screws configuration does not allow much motion with either classic Ti or variable Ti-Ni rods. The combination of a Ti rod with screws-cable anchoring allows more motion and, therefore, lower adjacent disk pressure, but puts extremely high stresses on the rod and anchors. The combination of the variable Ti-Ni rod and screws-cable anchoring leads to a significant decrease in adjacent disk pressure, without increasing stresses and pullout forces in the spinal instrumentation.

Manufacturing of monolithic superelastic rods with variable properties for spinal correction: feasibility study.

A new concept of monolithic spinal rod with variable flexural stiffness is proposed to reduce the risk of adjacent segment degeneration and fracture associated with rigid spinal fixation techniques while providing adequate stability to the spine. The concept is based on the use of Ti-Ni shape memory alloy rods subjected to different processing schedules implying local annealing, cold work, or a combination of both. A feasibility study of the concurrent technological routes is performed by comparing their potential to locally control material microstructure and properties.