Development of a scoliotic spine model for biomechanical in vitro studies

Biomechanical in vitro evaluation of the kangaroo spine in comparison with human spinal data

On the basis of the kangaroo's pseudo-biped locomotion and its upright position, it could be assumed that the kangaroo might be an interesting model for spine research and that it may serve as a reasonable surrogate model for biomechanical in vitro tests. The purpose of this in vitro study was to provide biomechanical properties of the kangaroo spine and compare them with human spinal data from the literature. In addition, references to already published kangaroo anatomical spinal parameters will be discussed. Thirteen kangaroo spines from C4 to S4 were sectioned into single-motion segments. The specimens were tested by a spine tester under pure moments. The range of motion and neutral zone of each segment were determined in flexion and extension, right and left lateral bending and left and right axial rotation. Overall, we found greater flexibility in the kangaroo spine compared to the human spine. Similarities were only found in the cervical, lower thoracic and lumbar spinal regions. The range of motion of the kangaroo and human spines displayed comparable trends in the cervical (C4-C7), lower thoracic and lumbar regions independent of the motion plane. In the upper and middle thoracic regions, the flexibility of the kangaroo spine was considerably larger. These results suggested that the kangaroo specimens could be considered to be a surrogate, but only in particular cases, for biomechanical in vitro tests.

Novel use of telescoping growth rods in treatment of early onset scoliosis: An in vivo and in vitro study in a porcine model

INTRODUCTION

Treatment of early-onset scoliosis (EOS) can be difficult. Various forms of growing rods exist to correct deformity while delaying definitive spinal fusion. The disadvantage of traditional growing rods is need for repeated surgical lengthening procedures. Telescoping growth rods (TelGR) are a prototype new, guided growth technology with a rod mechanism that allows spontaneous longitudinal growth over time without manual lengthening. We hypothesized that the TelGR system will permit unrestricted growth with limited complications through 12 weeks in vivo, and that the range of motion (RoM) in each of three directions and stiffness of the TelGR system would not be significantly different than the rigid rod system in vitro.

MATERIALS AND METHODS

In vivo: Six immature pigs were surgically implanted with TelGR with cephalad fixation at T6-7 and caudal fixation at T14-L1. Radiographs of the involved vertebral segments were measured postoperatively and after 12 weeks. In vitro: A robotic testing system was utilized for flexibility tests in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) of eight immature porcine specimens (T3-T15). Testing was performed on both dual rigid rods and bilateral TelGR with instrumentation at T4-5 and T13-14.

RESULTS

In vivo: Over the 12-week period, the rod length of the TelGR increased an average of 65 mm. In vitro: TelGR demonstrated significantly increased motion in LB and AR RoM compared with rigid rods. No difference was noted in FE RoM.

DISCUSSION

The in vivo results in this study showed expected skeletal growth with spines instrumented with TelGR. In vitro findings of increased RoM in AR and LB suggest that the TelGR system may be less rigid than traditional growing rods. Treatment with TelGR might, if proven efficacious in the clinical setting, decrease the need for repeated surgical intervention compared with traditional growing rods. This study adds to the limited body of biomechanical evidence examining guided growth technology.

Characteristic morphological patterns within adolescent idiopathic scoliosis may be explained by mechanical loading

PURPOSE

Adolescent idiopathic scoliosis (AIS) is a three-dimensional deformity of the spine which exhibits morphological changes during growth. The goal of this study was to identify morphological patterns that could be explained by different loading patterns for AIS.

METHODS

Computed tomography data of 21 patients with diagnosed AIS and 48 patients without any visual spinal abnormalities were collected prospectively. The bony structures were reconstructed, and landmarks were placed on characteristic morphological points on the spine. Multiple morphological parameters were calculated based on the distances between the landmarks. The intra- and inter-observer variability for each parameter was estimated. Differences between healthy and scoliotic spines were statistically analysed using the t test for unpaired data, with a significance level of α = 0.01.

RESULTS

Within the healthy group, an out-of-plane rotation of the vertebrae in the transverse plane was measured (2.6° ± 4.1° at T2). Relating the length of the spinal curvature to the T1-S1 height of the spine revealed that scoliotic spines were significantly longer. However, the endplate area in the AIS group was significantly smaller once compared to the curvature length. The relation between the left and right pedicle areas varied between 2.5 ± 0.79 and 0.4 ± 0.19, while the ratio of the facet articular surfaces varied within 2.3 ± 0.5 and 0.5 ± 0.2.

CONCLUSIONS

This study identified a certain morphological pattern along the spine, which reveals a distinct load path prevalent within AIS. The data suggested that the spine adapts to the asymmetric load conditions and the spine is not deformed by asymmetric growth disturbance. These slides can be retrieved under Electronic Supplementary Material.

The association of lumbar curve magnitude and spinal range of motion in adolescent idiopathic scoliosis: a cross-sectional study

Background

Spinal deformities affect the overall alignment of the spine and thus the vectors of loading on the lumbar region and intervertebral discs. Due to wedging of the disc or vertebrae of unbalanced spinal segments, alignment change may affect the range of motion (ROM) of individual spinal segments or the global spine. This is particularly important in adolescent idiopathic scoliosis (AIS) patients who may suffer from early degeneration, back stiffness and pain. Hence, this study aimed to determine the correlation between spine range of motion (ROM) and adolescent idiopathic scoliosis (AIS) curve magnitude.

Methods

Consecutive recruitment of all AIS patients with Lenke 5 (thoracolumbar/lumbar) curves within one month was performed with ROM assessments in the coronal, sagittal and axial planes using the change in C7-S1 distance on standing upright, active flexion and extension positions, change in finger-floor distance on forward bending position and lateral bending, lateral bending angles, modified Schober’s test, and trunk rotation in seating position. Patients were further stratified into two groups based on their lumbar spine curve magnitude: Group A with curves of 10 to 39 degrees and Group B with 40 degrees or greater. Univariate and multivariate analyses were conducted, with lumbar curve magnitude severity being the dependent variable.

Results

In total, 58 patients (n = 12 males, n = 46 females; mean age: 15.7 years) were recruited. The mean curve magnitudes were 25 ± 6.5 degrees in Group A and 48 ± 10.6 degrees in Group B. Mean axial rotation (Group A: 90 ± 21.7 degree; Group B: 76 ± 19.6 degrees; p = 0.038) and lateral bending ROM (Group A: 67 ± 13.4 degrees; Group B: 58 ± 14.3 degrees; p = 0.045) decreased in more severe curves. These two parameters continued to remain significant irrespective of the curve severity cut-off values.

Conclusions

This is the first study to determine associations between spinal ROM parameters with the lumbar curve magnitude in AIS patients. We found that the coronal curve severity is associated with reduced axial and coronal ROM. This is a platform for future studies assessing lumbar spine biomechanics in AIS and to determine the effects of altered spine motion in this context and its implication in patient management and outcomes.

Planning the Surgical Correction of Spinal Deformities: Toward the Identification of the Biomechanical Principles by Means of Numerical Simulation

In decades of technical developments after the first surgical corrections of spinal deformities, the set of devices, techniques, and tools available to the surgeons has widened dramatically. Nevertheless, the rate of complications due to mechanical failure of the fixation or the instrumentation remains rather high. Indeed, basic and clinical research about the principles of deformity correction and the optimal surgical strategies (i.e., the choice of the fusion length, the most appropriate instrumentation, and the degree of tolerable correction) did not progress as much as the implantable devices and the surgical techniques. In this work, a software approach for the biomechanical simulation of the correction of patient-specific spinal deformities aimed to the identification of its biomechanical principles is presented. The method is based on three-dimensional reconstructions of the spinal anatomy obtained from biplanar radiographic images. A user-friendly graphical user interface allows for the planning of the desired deformity correction and to simulate the implantation of pedicle screws. Robust meshing of the instrumented spine is provided by using consolidated computational geometry and meshing libraries. Based on a finite element simulation, the program is able to predict the loads and stresses acting in the instrumentation as well as those in the biological tissues. A simple test case (reduction of a low-grade spondylolisthesis at L3–L4) was simulated as a proof of concept, and showed plausible results. Despite the numerous limitations of this approach which will be addressed in future implementations, the preliminary outcome is promising and encourages a wide effort toward its refinement.