Angela D. Melnyk

Pediatric and adult three-dimensional cervical spine kinematics: effect of age and sex through overall motion

Study Design. Cross-sectional study. Objective. To determine the effect of age and sex on the three-dimensional kinematics of the cervical spine. Summary of Background Data. Spine kinematics information has important implications for biomechanical model development, anthropomorphic test device development, injury prevention, surgical treatment, and safety equipment design. There is a paucity of data of this type available for children, and it is unknown whether cervical spine kinematics of the pediatric population is different than that of adults. The helical axis of motion (HAM) of the spine provides unique information about the quantity and quality (coupling etc.) of the measured motion. Methods. Ninety subjects were recruited and divided into 6 groups based on sex and age (young children aged 4–10 years, older children aged 11–17 years, adults aged 25+ years). Subjects actively moved their head in axial rotation, lateral bending, and flexion/extension. An optoelectronic motion analysis system recorded the position of infrared markers placed on the first thoracic vertebrae (T1) and on tight-fitting headgear worn by the subjects. HAM parameters were calculated for the head motion with respect to T1. Results. HAM location in axial rotation and flexion/extension was more anterior in young females compared to adult females. Young females had a more anterior HAM location in flexion/extension compared to young males, indicating an effect of sex. For females, the HAM locations of adults were superior to those of children in flexion/extension and lateral bending whereas in males the HAM locations of adults were inferior to those of children. Age-related differences in HAM orientation were also observed in axial rotation and lateral bending. Conclusion. Cervical spine kinematics vary with age and sex. The variation in spine mechanics based on age and sex found in the present study may indicate general trends that would grow stronger in even younger children (age <4 years).

Load transfer characteristics between posterior spinal implants and the lumbar spine under anterior shear loading: an in vitro investigation.

Study Design. A biomechanical human cadaveric study. Objective. To determine the percentage of shear force supported by posterior lumbar spinal devices of varying stiffnesses under anterior shear loading in a degenerative spondylolisthesis model. Summary of Background Data. Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spinal conditions with instability. Novel spinal implants are designed to correct and maintain spinal alignment, share load with the spine, and minimize adjacent level stresses. The optimal stiffness of these spinal systems is unknown. To our knowledge, low-stiffness posterior instrumentation has not been tested under an anterior shear force, a highly relevant force to be neutralized in the clinical case of degenerative spondylolisthesis. Methods. The effects of implant stiffness and specimen condition on implant load and intervertebral motion were assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression force and a cyclic anterior shear force (5–250 N). Implants (high-stiffness [HSI]: ø 5.5-mm titanium, medium-stiffness [MSI]: ø 6.35 × 7.2-mm oblong PEEK, and low-stiffness [LSI]: ø 5.5-mm round PEEK) instrumented with strain gauges were used to calculate loads and were tested in each of 3 specimen conditions simulating degenerative changes: intact, facet instability, and disc instability. Intervertebral motions were measured with a motion capture system. Results. As predicted, implants supported a significantly greater shear force as the specimen was progressively destabilized. Mean implant loads as a percent of the applied shear force in order of increasing specimen destabilization for the HSI were 43%, 67%, and 76%; mean implant loads for the MSI were 32%, 56%, and 77%; and mean implant loads for the LSI were 18%, 35%, and 50%. Anterior translations increased with decreasing implant stiffness and increasing specimen destabilization. Conclusion. Implant shear stiffness significantly affected the load sharing between the implant and the natural spine in anterior shear ex vivo. Low-stiffness implants transferred significantly greater loads to the spine. This study supports the importance of load-sharing behavior when designing new implants.

An in vitro model of degenerative lumbar spondylolisthesis.

Study Design. A biomechanical human cadaveric study. Objective. To create a biomechanical model of low-grade degenerative lumbar spondylolisthesis (DLS), defined by anterior listhesis, for future testing of spinal instrumentation. Summary of Background Data. Current spinal implants are used to treat a multitude of conditions that range from herniated discs to degenerative diseases. The optimal stiffness of these instrumentation systems for each specific spinal condition is unknown. Ex vivo models representing degenerative spinal conditions are scarce in the literature. A model of DLS for implant testing will enhance our understanding of implant-spine behavior for specific populations of patients. Methods. Four incremental surgical destabilizations were performed on 8 lumbar functional spinal units. The facet complex and intervertebral disc were targeted to represent the tissue changes associated with DLS. After each destabilization, the specimen was tested with: (1) applied shear force (−50 to 250 N) with a constant axial compression force (300 N) and (2) applied pure moments in flexion-extension, lateral bending and axial rotation (±5 Nm). Relative motion between the 2 vertebrae was tracked with a motion capture system. The effect of specimen condition on intervertebral motion was assessed for shear and flexibility testing. Results. Shear translation increased, specimen stiffness decreased and range of motion increased with specimen destabilization (P < 0.0002). A mean anterior translation of 3.1 mm (SD 1.1 mm) was achieved only after destabilization of both the facet complex and disc. Of the 5 specimen conditions, 3 were required to achieve grade 1 DLS: (1) intact, (3) a 4-mm facet gap, and (5) a combined nucleus and annulus injury. Conclusion. Destabilization of both the facet complex and disc was required to achieve anterior listhesis of 3.1 mm consistent with a grade 1 DLS under an applied shear force of 250 N. Sufficient listhesis was measured without radical specimen resection. Important anatomical structures for supporting spinal instrumentation were preserved such that this model can be used in future to characterize behavior of novel instrumentation prior to clinical trials.

The effect of lateral eccentricity on failure loads, kinematics, and canal occlusions of the cervical spine in axial loading

Current neck injury criteria do not include limits for lateral bending combined with axial compression and this has been observed as a clinically relevant mechanism, particularly for rollover motor vehicle crashes. The primary objectives of this study were to evaluate the effects of lateral eccentricity (the perpendicular distance from the axial force to the centre of the spine) on peak loads, kinematics, and spinal canal occlusions of subaxial cervical spine specimens tested in dynamic axial compression (0.5 m/s). Twelve 3-vertebra human cadaver cervical spine specimens were tested in two groups: low and high eccentricity with initial eccentricities of 1 and 150% of the lateral diameter of the vertebral body. Six-axis loads inferior to the specimen, kinematics of the superior-most vertebra, and spinal canal occlusions were measured. High speed video was collected and acoustic emission (AE) sensors were used to define the time of injury. The effects of eccentricity on peak loads, kinematics, and canal occlusions were evaluated using unpaired Student t-tests. The high eccentricity group had lower peak axial forces (1544 ± 629 vs. 4296 ± 1693 N), inferior displacements (0.2 ± 1.0 vs. 6.6 ± 2.0 mm), and canal occlusions (27 ± 5 vs. 53 ± 15%) and higher peak ipsilateral bending moments (53 ± 17 vs. 3 ± 18 Nm), ipsilateral bending rotations (22 ± 3 vs. 1 ± 2°), and ipsilateral displacements (4.5 ± 1.4 vs. -1.0 ± 1.3 mm, p<0.05 for all comparisons). These results provide new insights to develop prevention, recognition, and treatment strategies for compressive cervical spine injuries with lateral eccentricities.

Shear force measurements on low- and high-stiffness posterior fusion devices

Low-stiffness posterior fusion devices for the lumbar spine have been developed to treat degenerative spinal conditions. However, the demands on an implant vary between a stable motion segment and one which exhibits a significant degree of sagittal plane instability. Shear motion in the antero-posterior direction is a relevant mode of instability for clinical conditions such as degenerative lumbar spondylolisthesis. Shear load-sharing between the implant and spine in conditions of antero-posterior instability has not been studied, nor have there been comparisons between traditional rigid implants and novel low-stiffness implants. The objective of this study was to develop a method to measure in vitro shear forces on three clinically relevant fusion implants when they are applied to an unstable model of degenerative spondylolisthesis in a human cadaver spine. Uniaxial strain gauges were affixed to the surface of the implants and a spine-segment-specific calibration method was used to calibrate the strain output to an applied shear force. The accuracy of the force measurements was within 3.4N for all implant types and the repeatability was within 5.4N. The force measurement technique was sufficiently accurate and reliable to conclude that it is suitable for use in in vitro experiments to measure implant shear force.

A novel sideways fall simulator to study hip fractures ex vivo

Falls to the side are the leading cause of hip fractures in the elderly. The load that a person experiences during a fall cannot be measured with volunteers for ethical reasons. To evaluate injurious loads, while considering relevant energy input and body posture for a sideways fall, a subject-specific cadaveric impact experiment was developed. Full cadaveric femur-pelvis constructs (N = 2) were embedded in surrogate soft tissue material and attached to metallic surrogate lower limbs. The specimens were then subjected to an inverted pendulum motion, simulating a fall to the side with an impact to the greater trochanter. The load at the ground and the deformation of the pelvis were evaluated using a 6-axis force transducer and two high-speed cameras. Post-test, a trauma surgeon (PG) evaluated specimen injuries. Peak ground contact forces were 7132 N and 5641 N for the fractured and non-fractured specimen, respectively. We observed a cervical fracture of the femur in one specimen and no injuries in a second specimen, showing that the developed protocol can be used to differentiate between specimens at high and low fracture risk.

Damage Identification on Vertebral Bodies During Compressive Loading Using Digital Image Correlation

Study Design. Ex vivo compression experiments on isolated cadaveric vertebrae. Objective. To qualitatively compare the fracture locations identified in video analysis with the locations of high compressive strain measured with digital image correlation (DIC) on vertebral bodies and to evaluate the timing of local damage to the cortical shell relative to the global yield force. Summary of Background Data. In previous ex vivo experiments, cortical bone fracture has been identified using various methods including acoustic emission sensors, strain gages, video analysis, or force signals. These methods are, however, limited in their ability to detect the location and timing of fracture. We propose use of DIC, a noncontact optical technique that measures surface displacement, to quantify variables related to damage. Methods. Isolated thoracolumbar human cadaveric vertebral bodies (n = 6) were tested in compression to failure at a quasi-static rate, and the force applied was measured using a load cell. The surface displacement and strain were measured using DIC. Video analysis was performed to identify fractures. Results. The location of fractures identified in the video corresponded well with the locations of high compressive strain on the bone. Before reaching the global yield force, more than 10% of the DIC measurements reached a minimum principal strain of 1.0%, a previously reported threshold for cortical bone damage. Conclusion. DIC measurements provide an objective measure that can be used to identify the location and timing of fractures during ex vivo vertebral experiments. This is important for understanding fracture mechanics and for validating vertebral computational models that incorporate failure. Level of Evidence: N /A

Reply: The effect of disc degeneration on anterior shear translation in the lumbar spine.

Dear Dr. Sandell, We thank Dr. Shen and colleagues for their interest in our research. Upon reviewing their letter to the editor, we note that they identified two main issues: (i) the effect of disc degeneration on motion may be different at different levels along the lumbar spine and (ii) in vitro and in vivo testing are different and in vivo testing may show a correlation between degeneration and excess motion. Our focus was on the L4-5 segment to assess the effect of disc degeneration on anterior shear motion. The L4-5 level is one of the most common levels to be diagnosed with degenerative spondylolisthesis, a condition that likely relates to increased shear flexibility in the spine. While our results and conclusions do not necessarily extend to other levels of the lumbar spine, including L5-S1, we suspect it unlikely that another intervertebral level would have a significant relationship between disc degeneration and shear flexibility, given the minor effects of disc degeneration on spine mechanical properties from other studies of the lumbar spine. The letter by Shen et al. claimed distinct level differences in the study by Passias et al. In that study, vertebral kinematics were compared between people with discogenic low back pain (n1⁄410) and asymptomatic controls (n1⁄48) under bending and twisting movements of the lumbar spine. They reported small, yet significant, motion differences at the L4-5 level in flexion and at the L3-4 level in lateral bending and axial rotation. The in vitro studies referred to previously suggest that spinal motion is affected modestly by disc degeneration, with small decreases in flexion-extension and lateral bending and small increases in axial rotation. Interestingly, Passias et al. show no motion differences at the L4-5 level between the degenerated and normal populations for lateral bending and axial rotation. Axial rotation is the most similar motion to anterior shear, as it involves a shearing load on the disc. Therefore, despite the differences between our studies, such as in vivo versus in vitro, both our studies support the idea that at the L45 level, under shear, the Pfirrmann disc grade is not related to motion. Applied loading is an important issue that arises when comparing in vitro and in vivo kinematic studies. In vitro tests involve applying a known load to all specimens and measuring the resulting motion. In vivo studies involve patients moving their joints to a general location (i.e. forward 45 degrees, or forward until it hurts) and motion is measured using imaging techniques. The precise loads applied to the vertebrae of the human subject are not known. For example, during forward bending, the loads applied to the lumbar vertebrae consist of a combination of flexion, shear, and compressive loading. The advantage of the in vivo studies is that living people are being studied. The strength of the in vitro studies is that the mechanical properties of the spine can be assessed using precise methodology. Therefore, the synthesis of these studies must be done with care. The letter by Shen et al. claimed that an in vivo study by Kong et al. reported a close association between vertebral motion and lumbar disc degeneration from the Pfirrmann grading system. Kong et al. found that the sample population prevalence of excess translation in static flexion-extension poses on standing MRI is positively associated with Pfirrmann disc grades, excluding grade V. They also found that the prevalence of excess motion in all the symptomatic subjects was only 10% for translation and 26% for angular rotation. Therefore, the majority of patients with low back pain did not have any excessive motion in their lumbar spine, although they had varying levels of degeneration. It is easy to see how our sample of 30 cadaveric specimens could fall in the majority of patients showing no effect of disc degeneration on motion. However, it is difficult to compare their results to ours, since we report magnitudes of motion at a specific load and they report prevalence of excess motion within a symptomatic population who would be affected by pain and muscle status. We maintain that the findings from our study support the absence of a relationship between shear motion in the lumbar spine and disc degeneration based upon the Pfirrmann scale. Of course, our study has limitations and thus there remains a need for possibly more sensitive degeneration grading scales. We encourage researchers to continue improving in vivo and in vitro kinematic methods and degeneration grading scales to untangle this relevant issue.