Stiffness In Axial Rotation Research Articles

Assessment of cervical stiffness in axial rotation among chronic neck pain patients: A trial in the framework of a non-manipulative osteopathic management

BackgroundCervical stiffness is a clinical feature commonly appraised during the functional examination of cervical spine. Measurements of cervical stiffness in axial rotation have not been reported for patients with neck pain. The purpose of this study was to investigate cervical spine stiffness in axial rotation among neck pain patients and asymptomatic subjects, and to analyze the impact of osteopathic management. MethodsThirty-five individuals (17 patients) were enrolled. Measurements were carried out for left-right axial rotation using a torque meter device, prior and after intervention. Passive range of motion, stiffness, and elastic-and neutral zone magnitudes were analyzed. Pain intensity was also collected for patients. The intervention consisted in one single session of non-manipulative osteopathic treatment performed in both groups. FindingsA significant main effect of intervention was found for total range of motion and neutral zone. Also, treatment by group interaction was demonstrated for neutral-, elastic zone, stiffness in right axial rotation, and for total neutral zone. Significant changes were observed in the clinical group after intervention, indicating elastic zone decrease and neutral zone increase. In contrast, no significant alteration was detected for the control group. InterpretationsStiffness characteristics of the cervical spine in axial rotation are prone to be altered in patients with neck pain, but seem to be relieved after a session of non-manipulative manual therapeutic techniques. Further investigations, including randomized clinical trials with various clinical populations and therapeutic modalities, are needed to confirm these preliminary findings.

Do the Position and Orientation of the Crosslink Influence the Stiffness of Spinal Instrumentation?

Biomechanical study of double-level pedicle screw constructs with or without crosslinks (CL) in an unstable model. The purpose of this study is to investigate the optimal position and orientation of the CL. Several reports have described biomechanical research on such CL, but no definite consensus has been reached regarding the effects. Very few studies have examined the position and orientation of the CL. The question of where and how the CL should be clinically set remains unanswered. Ten cadaveric lumbar spines (L3-L5) of boars were used and 7 models were prepared by the sequential damage and spinal instrumentation of each specimen. Bending stiffness was measured in flexion, extension, lateral bending, and axial rotation for each model using 6-axis material tester under torque of 0 to ±3 N m. Results for each configuration were compared using analysis of variance and the Turkey-Kramer test. In flexion, extension, and lateral bending, 7 models showed similar stiffness with no significant differences. In axial rotation, stiffness increased significantly (P<0.05) in the cephalic, central, caudal, and oblique CL models in comparison with that of the no CL model, and stiffness of the horizontal 2 CL and oblique 2 CL models was significantly higher than that of cephalic, central, caudal, and oblique CL models (P<0.05). However, no significant differences in stiffness were seen between cephalic, central, and caudal CL models, between the central and oblique CL models, or between the horizontal and oblique 2 CL models. Concomitant use of CLs significantly increased axial rotational stiffness, even though stiffness in flexion, extension, and lateral bending was not increased. In addition, stiffness in axial rotation significantly improved with the use of 2 CLs instead of a single CL, and stiffness was unchanged by position and orientation of CL.

Biomechanical Analysis of Intervertebral Cement Extravasation in Vertebral Motion Segments.

Kyphoplasty is a therapeutic option for pain relief in the setting of compression fractures. Cement extravasation into adjacent disks is a common occurrence. The biomechanical and clinical consequences of cement in the disks currently are unknown. This study investigated the biomechanical effects of cement extravasation into the intervertebral disk in a human cadaveric model. Seven thoracolumbar and lumbar embalmed human cadaveric motion segments were evaluated in axial rotation, right and left lateral bending, and flexion and extension. Stiffness was calculated at baseline and following injection of 1 mL of cement into the intervertebral disk. There was a 13.4% (P=.041) increase in stiffness in axial rotation compared with preinjection motion segments. No significant difference was observed in lateral bending or flexion and extension. In this model, cement extravasation into the disk space increased stiffness in axial rotation. [Orthopedics. 2017; 40(2):e300-e304.].

Biomechanical Evaluation of the CD HORIZON Spire Z Spinal System With Pedicle and Facet Fixation.

Human cadaveric biomechanical study. The aim of this study was to evaluate the biomechanics of lumbar motion segments instrumented with the CD HORIZON Spire Z plate system (Spire Z), a posterior supplemental fixation spinous process plate, alone and with additional fixation systems. Plates and pedicle screw/rod and facet screw implants are adjuncts to fusion. The plate limits motion, improving segmental stability and the fusion microenvironment. However, the degree to which the plate contributes to overall stability when used alone or in conjunction with additional instrumentation has not been described. Standard nondestructive flexibility tests were performed in 7 L2-L5 human cadaveric spines. Spinal stability was determined as mean range of motion (ROM) in flexion/extension, lateral bending, and axial rotation. Paired comparisons were made between five conditions: (1) intact/control; (2) Spire Z; (3) Spire Z with unilateral pedicle screw/rod system (Spire Z+UPS); (4) Spire Z with unilateral facet screw system (Spire Z+UFS); and (5) Spire Z with bilateral facet screw system (Spire Z+BFS). Stiffness and ROM data were compared using one-way analysis of variance, followed by repeated-measures Holm-Šidák tests. Spire Z was most effective in limiting flexion (20% of normal) and extension (24% of normal), but less effective in reducing lateral bending and axial rotation. In lateral bending, Spire Z+BFS and Spire Z+UPS constructs were not significantly different and demonstrated greater ROM reduction compared with Spire Z+UFS and Spire Z (P < 0.001). Spire Z+BFS demonstrated greatest stiffness in axial rotation compared with Spire Z+UPS (P = 0.025), Spire Z+UFS (P = 0.001), and Spire Z (P < 0.001). Spire Z+UPS was not significantly different from Spire Z+UFS (P = 0.21), and superior to Spire Z (P = 0.013). The Spire Z spinous process plate provides excellent immediate fixation, particularly for flexion and extension. While the hybrid Spire Z+BFS screw construct afforded the greatest stability, Spire Z+UPS demonstrated considerable promise. N/A.

The effect of cross links on rotational stiffness of scoliosis constructs

The purpose of this study is to investigate what effect cross links have on scoliosis constructs and whether cross links may be used instead of pedicle screws at the apex of the deformity The rotational stiffness of six different construct designs was investigated on scoliotic sawbone models with zero, one or two cross links. In three of the constructs the screws at the apex were removed. Testing was performed to an average torque of 3Nm and ration was detected using electromagnetic motion tracking system. The stiffness in axial rotation of all constructs increased with the number of cross links, however the difference was not statistically significant. In constructs with apical screws the stiffness increased by 3.01% and 12.9% for one and two cross links respectively. In constructs without apical screws the increase was 1.64% and 14.3% for one and two cross links respectively. The total stiffness of the construct increased with the addition of apical screws by 20%, 21.7% and 18.8% for zero, one and two cross links respectively. This increase was statistically significant using a paired t-test (p= 0.01142). On the basis of these results we conclude that the use of cross links in scoliosis correction surgery is not necessary. Pedicle screws positioned at the apex of the scoliosis curve statistically increase the stiffness in axial rotation and are therefore necessary to promote an environment suitable for bony fusion.

A biomechanical investigation of dual growing rods used for fusionless scoliosis correction

BackgroundThe use of dual growing rods is a fusionless surgical approach to the treatment of early onset scoliosis which aims to harness potential growth and correct spinal deformity. The purpose of this study was to compare the in-vitro biomechanical response of two different dual rod designs under axial rotation loading. MethodsSix porcine spines were dissected into seven level thoracolumbar multi-segment units. Each specimen was mounted and tested in a biaxial Instron machine, undergoing nondestructive left and right axial rotation to peak moments of 4Nm at a constant rotation rate of 8deg. s−1. A motion tracking system (Optotrak) measured 3D displacements of individual vertebrae. Each spine was tested in an un-instrumented state first and then with appropriately sized semi-constrained and ‘rigid’ growing rods in alternating sequence. The range of motion, neutral zone size and stiffness were calculated from the moment–rotation curves and intervertebral range of motion was calculated from Optotrak data. FindingsIrrespective of test sequence, rigid rods showed a significant reduction of total rotation across all instrumented levels (with increased stiffness) whilst semi-constrained rods exhibited similar rotational behavior to the un-instrumented spines (P<0.05). An 11.1% and 8.0% increase in stiffness for left and right axial rotation respectively and 14.9% reduction in total range of motion were recorded with dual rigid rods compared with semi-constrained rods. InterpretationBased on these findings, the Semi-constrained growing rods were shown to not increase axial rotation stiffness compared with un-instrumented spines. This is thought to provide a more physiological environment for the growing spine compared to dual rigid rod constructs.

The effect of repeated loading and freeze-thaw cycling on immature bovine thoracic motion segment stiffness.

There is growing interest in the biomechanics of "fusionless" implant constructs used for deformity correction in the thoracic spine; however, there are questions over the comparability of in vitro biomechanical studies from different research groups due to the various methods used for specimen preparation, testing and data collection. The aim of this study was to identify the effect of two key factors on the stiffness of immature bovine thoracic spine motion segments: (1) repeated cyclic loading and (2) multiple freeze-thaw cycles, to aid in the planning and interpretation of in vitro studies. Two groups of thoracic spine motion segments from 6- to 8-week-old calves were tested in flexion/extension, right/left lateral bending and right/left axial rotation under moment control. Group A was tested with continuous repeated cyclic loading for 500 cycles with data recorded at cycles 3, 5, 10, 25, 50, 100, 200, 300, 400 and 500. Group (B) was tested after each of five freeze-thaw sequences, with data collected from the 10th load cycle in each sequence. Results of testing showed that for Group A: flexion/extension stiffness reduced significantly over the 500 load cycles (-22%; p = 0.001), but there was no significant change between the 5th and 200th load cycles. Lateral bending stiffness decreased significantly (-18%; p = 0.009) over the 500 load cycles, but there was no significant change in axial rotation stiffness (p = 0.137). Group B: there was no significant difference between mean stiffness over the five freeze-thaw sequences in flexion/extension (p = 0.813) and a near-significant reduction in mean stiffness in axial rotation (-6%; p = 0.07). However, there was a statistically significant increase in stiffness in lateral bending (+30%; p = 0.007). Study findings indicate that comparison of in vitro testing results for immature thoracic bovine spine segments between studies can be performed with up to 200 load cycles without significant changes in stiffness. However, when testing protocols require greater than 200 cycles, or when repeated freeze-thaw cycles are involved, it is important to account for the effect of cumulative load and freeze-thaw cycles on spine segment stiffness.

Microendoscopic Lateral Decompression for Lumbar Foraminal Stenosis

A biomechanical study. How much of the facet joint and the pars interarticularis (PI) can be removed in microendoscopic lateral decompression (MELD) for lumbar foraminal stenosis (LFS)? MELD is a surgical modality for patients with LFS. In severe degenerative cases, unilateral facet joint resection or unilateral removal of the lateral part of the PI are sometimes needed to decompress the nerve root adequately. Twelve human lumbar motion segments were tested according to the following loading protocol: axial compression, flexion, extension, lateral bending to the right and left, and axial rotation to the right and left. This loading protocol was applied to each motion segment after MELD in 2 experiments: (1) unilateral graded facetectomy was performed in stages of 25%, 50%, 75%, and 100% using 3 segments of L3/L4 and 3 segments of L5/S1; (2) unilateral removal of the lateral part of the PI was performed in stages of 25%, 50%, 75%, and 100% using 3 segments of L3/L4 and 3 segments of L5/S1. The relative stiffness of each motion segments was determined each time. (1) Unilateral facet joint resection of >75% can lead to a significant reduction in stiffness in axial rotation at both L3/L4 and L5/S1. (2) Unilateral removal of 75% of the lateral part of the PI can lead to significant reduction in stiffness in right and left rotation at L3/L4 and in left rotation at L5/S1. (3) Unilateral removal of 100% of the lateral part of the PI can lead to a significant reduction in stiffness in right axial rotation at L5/S1. It would seem judicious to remove no >50% of the facet joint or the lateral part of the PI in order to prevent postoperative instability when using MELD for LFS.

Mechanical behavior of a novel non-fusion scoliosis correction device

IntroductionWe developed an innovative non-fusion correction system (XS LATOR) consisting of two individual implants that are extendable and extremely flexible. One implant, the XS LAT, generates a lateral, bending moment and one implant, the XS TOR, generates a torsion moment. Two ‘inverse’ implants were developed for generating torsion and lateral bending in a porcine model was tested for force delivery. An in vitro experiment was set up to describe the mechanical behavior of both implants. Materials and methodsNarrow and wide (‘inverse’) versions of the XS TOR and XS LAT were mounted on an apparatus that was able to simulate different spinal geometries. The implants were anchored to three artificial vertebrae with integrated 6D force sensors, after which the vertebrae were rotated and translated towards the demanded position. The reaction forces and moments were recorded in all configurations. The maximal (lateral) bending moment, which occurred at the middle vertebra, was determined and, similarly, torque applied at the center of rotation of the middle vertebra was calculated. ResultsAs expected, the wide and the small versions of the XS TOR generate a torque that increases during the growth of the system. Similarly, the XS LAT generates a bending moment that slightly increases during the growth of the system. The produced moments approximate the theoretically predicted ones. The contribution to the spinal stiffness ranges between 0.01Nm/° and 0.04Nm/° in bending and between 0.03Nm/° and 0.08Nm/° in torsion. ConclusionsThe XS TOR and the XS LAT are able to generate a torque and a bending moment that remain (fairly) constant during spinal growth when a shape change due to the generated moment/torque is achieved. The stiffness of the implants is extremely low, being only a fraction of the stiffness of conventional, spinal fusion constructs. Current fusion systems, such as non-segmental spinal constructs generally, have 11 times higher stiffness in torsion and 6 times higher stiffness in lateral bending. Implantation of the XS LATOR adds 9% stiffness in axial rotation and 17% stiffness in lateral bending (to the original spinal stiffness). By preserving the flexibility of the spine after implantation, fusion of the vertebrae in the instrumented region is likely to be prevented.

Plate fixation in the cervical spine: traditional paramedian screw configuration compared with unique unilateral configuration

This study compared the fixing strength and stability achieved by a unilateral plate and screw configuration against a standard cervical fixation plate using a single-level corpectomy and allograft strut graft model. Multidirectional in vitro flexibility tests were performed using a robotic spine testing system. Human cadaveric spines were assessed for spinal stability after vertebral corpectomy and anterior instrumentation. Specimens were mounted cranially and caudally on custom jigs that were then attached to load cells on the robotic system's end effector and base pedestal. C2-T1 spine specimens (n = 6) were tested intact; then after C-5 corpectomy (the vertebral body was excised), allograft placement and anterior plate fixation were performed. The surgeons performed a uniform corpectomy and reconstruction of each specimen in a protocol fashion. Two plates were compared: a unilateral 4-hole cervical plate designed to obtain rigid fixation using 4 convergent fixation screws all placed unilateral to the vertebral midline, and a standard cervical plate with bilateral plate screw configuration. The plate testing sequence was selected at random to limit bias. Fixation screws were matched for length and diameter. Pure moments were applied under load control (maximum 1.8 Nm) in flexion, extension, left/right lateral bending, and left/right axial rotation. Vertebral motion was measured using an optoelectronic system. The mean relative range of motion between C-4 and C-6 was compared among groups using repeated-measures ANOVA (significance level of 0.05). In comparing the intact construct and 2 different plates in all planes of motion, only motion in extension (intact vs. unilateral plate, p = 0.003; intact vs. standard plate, p = 0.001) and left axial rotation (intact vs unilateral plate, p = 0.019) were significantly affected. In terms of immediate cervical stability after 1-level corpectomy and placement of an allograft reconstruction, the unilateral plate showed comparable stiffness to the standard plate in all 3 motion planes (flexion [p = 0.993], extension [p = 0.732], left lateral bending [p = 0.683], right lateral bending [p = 0.546], left axial rotation [p = 0.082], and right axial rotation [p = 0.489]). The unilateral plate showed a trend toward improved stiffness in axial rotation. In no direction did the unilateral configuration prove significantly less stiff than the traditional configuration. The unilateral plate design proposed here requires minimal dissection and retraction beyond the midline of tissues susceptible to scar, postoperative pain, and swelling. The authors' study suggests that a unilateral plate can be configured to provide comparable fixation strength and torsional stiffness compared with traditional, widely accepted plate designs.

In Vivo Passive Axial Rotational Stiffness of the Thoracolumbar Spine

The objective of this study was to report the passive axial rotational stiffness of the thoracolumbar spine in asymptomatic volunteers. A secondary objective was to investigate if an asymmetry exists between right and left axial rotational spine stiffness. Eighteen young asymptomatic volunteers, nine male and nine female, were tested using a previously validated axial rotational tester. Volunteers in this study were subjected to six moment magnitudes bilaterally, in a random order, and their passive axial spine rotation was measured. The results from this study showed that there is a statistically significant difference between the right (20.3 ± 5.8 Nm/rad) and left (17.4 ± 4.9 Nm/rad) axial rotational stiffness regardless of gender. This normative information regarding axial rotational spine stiffness can enable clinicians to assess the deviance in clinical samples and assist in treatment selection.

Parametric Control of the Stiffness of Lumbar Segments

Abstract: To clarify relations between kinematics and mechanics of lumbar spinal segments (L3/L4, L4/L5) versus thoracic T6/T7. Measurements of rotational angle–torque characteristics and migrations of the instantaneous helical axis (IHA) in cyclic segment motions; correlation of IHA-position and differential segment stiffness by applying theorem of Steiner. In neutral axial position of the thoracic segment (α = 0), IHA was found within central areas of the intervertebral disc. No change of axial IHA-positions by flexion/extension. In axial rotation IHA-migrations along wide arches from one joint to the other and S-shaped characteristics: strong dependence of IHA-migration and the differential stiffness on the degree of flexion/extension. In flexion/extension, J-shape of the characteristic and simultaneous anterior/posterior IHA-migration. Correlation of initial differential segment stiffness and IHA-position via theorem of Steiner. The following conclusions are obtained: 1. Non-linearity of the characteristics is hardly produced by the soft tissues but mainly by IHA-migration and IHA-tilting. 2. The segment stiffness in axial rotation is parametrically controlled via the degree of additional flexion/extension. 3. The autochthon muscles are able to control the segment stiffness in axial rotation via shifting the resulting force line in anterior/posterior direction without change of its amount about the factor 3–5.

Primary and coupled motions after cervical total disc replacement using a compressible six-degree-of-freedom prosthesis

This study tested the hypotheses that (1) cervical total disc replacement with a compressible, six-degree-of-freedom prosthesis would allow restoration of physiologic range and quality of motion, and (2) the kinematic response would not be adversely affected by variability in prosthesis position in the sagittal plane. Twelve human cadaveric cervical spines were tested. Prostheses were implanted at C5-C6. Range of motion (ROM) was measured in flexion-extension, lateral bending, and axial rotation under ± 1.5 Nm moments. Motion coupling between axial rotation and lateral bending was calculated. Stiffness in the high flexibility zone was evaluated in all three testing modes, while the center of rotation (COR) was calculated using digital video fluoroscopic images in flexion-extension. Implantation in the middle position increased ROM in flexion-extension from 13.5 ± 2.3 to 15.7 ± 3.0° (p < 0.05), decreased axial rotation from 9.9 ± 1.7 to 8.3 ± 1.6° (p < 0.05), and decreased lateral bending from 8.0 ± 2.1 to 4.5 ± 1.1° (p < 0.05). Coupled lateral bending decreased from 0.62 ± 0.16 to 0.39 ± 0.15° for each degree of axial rotation (p < 0.05). Flexion-extension stiffness of the reconstructed segment with the prosthesis in the middle position did not deviate significantly from intact controls, whereas the lateral bending and axial rotation stiffness values were significantly larger than intact. Implanting the prosthesis in the posterior position as compared to the middle position did not significantly affect the ROM, motion coupling, or stiffness of the reconstructed segment; however, the COR location better approximated intact controls with the prosthesis midline located within ± 1 mm of the disc-space midline. Overall, the kinematic response after reconstruction with the compressible, six-degree-of-freedom prosthesis within ± 1 mm of the disc-space midline approximated the intact response in flexion-extension. Clinical studies are needed to understand and interpret the effects of limited restoration of lateral bending and axial rotation motions and motion coupling on clinical outcome.

The biomechanical contribution of varying posterior constructs following anterior thoracolumbar corpectomy and reconstruction

Thoracolumbar corpectomy is a procedure commonly required for the treatment of various pathologies involving the vertebral body. Although the biomechanical stability of anterior reconstruction with plating has been studied, the biomechanical contribution of posterior instrumentation to anterior constructs remains unknown. The purpose of this study was to evaluate biomechanical stability after anterior thoracolumbar corpectomy and reconstruction with varying posterior constructs by measuring bending stiffness for the axes of flexion/extension, lateral bending, and axial rotation. Seven fresh human cadaveric thoracolumbar spine specimens were tested intact and after L-1 corpectomy and strut grafting with 4 different fixation techniques: anterior plating with bilateral, ipsilateral, contralateral, or no posterior pedicle screw fixation. Bending stiffness was measured under pure moments of +/- 5 Nm in flexion/extension, lateral bending, and axial rotation, while maintaining an axial preload of 100 N with a follower load. Results for each configuration were normalized to the intact condition and were compared using ANOVA. Spinal constructs with anterior-posterior spinal reconstruction and bilateral posterior pedicle screws were significantly stiffer in flexion/extension than intact spines or spines with anterior plating alone. Anterior plating without pedicle screw fixation was no different from the intact spine in flexion/extension and lateral bending. All constructs had reduced stiffness in axial rotation compared with intact spines. The addition of bilateral posterior instrumentation provided significantly greater stability at the thoracolumbar junction after total corpectomy than anterior plating and should be considered in cases in which anterior column reconstruction alone may be insufficient. In cases precluding bilateral posterior fixation, unilateral posterior instrumentation may provide some additional stability.

Biomechanical Evaluation of Short-Segment Posterior Instrumentation With and Without Crosslinks in a Human Cadaveric Unstable Thoracolumbar Burst Fracture Model

This study evaluates the biomechanical characteristics of spinal instrumentation constructs in a human unstable thoracolumbar burst fracture model simulated by corpectomy. To compare the biomechanical characteristics of short-segment posterior instrumentation, with and without crosslinks, in a human unstable burst fracture model simulated by corpectomy. Unstable thoracolumbar burst fractures are serious injuries, and their management remains controversial. Some authors advocate the use of short-segment posterior instrumentation for certain burst fractures. Whether crosslinks contribute additional stability has not been determined. Six fresh frozen human spines (T10-L2) were potted to isolate the T11-L1 segments, and biomechanically tested in axial rotation, lateral bending, flexion, and extension. A custom spine testing system was used that allows motion with 6 degrees of freedom. After testing was completed on intact specimens, a corpectomy was performed at T12 to simulate an unstable burst fracture with loss of anterior and middle column support. Short-segment transpedicular instrumentation was then performed from T11 to L1. Each specimen was retested with 1, 2, or no crosslinks. Construct stiffness and motion data were analyzed with each intact specimen serving as its own internal control. Torsional stiffness in axial rotation was significantly increased (P < 0.05) in short-segment fixation constructs with 1 and 2 crosslinks, but none was restored to the preinjury baseline level. Significant reductions in standardized motion were also achieved with 1 and 2 crosslinks compared to no crosslinks (P < 0.05), but they remained greater than baseline. Crosslinks significantly increased stiffness and decreased motion in lateral bending, beyond the baseline level (P < 0.05). In flexion, all constructs had significantly decreased stiffness and increased motion compared to the intact specimen (P < 0.05), with crosslinks providing no additional benefit. Conversely, none of the constructs demonstrated a significant change in extension compared to baseline (P > 0.05). When attempting to load the constructs to failure, screw pullout was seen in all specimens. Crosslinks, when added to short-segment posterior fixation, improve stiffness and decrease motion in axial rotation, but do not restore baseline stability in this corpectomy model. Short-segment posterior fixation is also inadequate in restoring stability in flexion with injuries of this severity. Short-segment posterior instrumentation alone can achieve baseline stability in lateral bending, and crosslinks provide even greater stiffness.