Left Lateral Bending Research Articles

The influence of skill and low back pain on trunk postures and low back loads of shearers

Shearing is a rural occupation developing considerable spinal loads and carrying a high risk of low back pain (LBP). Although the workforce has a skill structure, interaction between skill, spinal loads and LBP is unknown. We examined whether skill and LBP influenced trunk postures and loads within a sample of 80 shearers representing shear skill levels. A progression from junior to open class demonstrated a 100% increase in productivity, less time in severe flexion, more time in neutral lateral bend, and more time in axially twisted postures, with no increase in cumulative compressive and anterior shear forces. LBP prevalence increased linearly from 10% for junior through to 76% for open class shearers. Shearers with a history of LBP generated greater cumulative right medio-lateral shear forces, greater left lateral bend and left axial twist moments. Skill-based training that reduces asymmetric forces may help reduce such high prevalence levels of LBP. Statement of Relevance: Shearing is an important and physically demanding rural occupation. It requires sustained flexed postures that generate considerable spinal loads and a high risk of LBP. This research examines how skill and a history of LBP it carries interact to influence trunk postures and spinal loads within a sample of shearers.

The biomechanics research on the changes of pressures in adjacent inferior zygapophyseal joint because of different methods of surgery for the anterior cervical spine

Objective To compare the changes of pressures in adjacent inferior zygapophyseal joint after different anterior cervical spine surgery and to provide the biomechanics basis for their clinical application.Methods 11 fresh cervical spinal specimens (C3-T1) were obtained from young adult cadavers. The specimens were simulated as implementation of the C5/6 discectomy,intercalated disc replacement, interbody fusion and divided into the integrity group.the discectomy group,!he intercalated disc replacement group and the intervertebral fusion group.The changes of pressures in C6/7 left zygapophyseal joint were measured in the 4 groups unde 25N -150N loads of different grades and directions by using micro-resistance type transducer.And analyze the data by SPSS13.0.Results Pressure variety in C6/7 left zygapophyseal joint of replacement group and integrity group were similar.There was no statistically significant difference in pressures observed between them.(P>0.05).Significantly lower zygapophyseal joint pressures were demonstrated for discectomy group compared to integrity group and replacement group under left lateral bending,right lateral bending,flexion and extension (P<0.05).Significantly greater pressures were recorded in fusion group compared to integrity group and replacement group under axial direction, left lateral bending and extension (P<0.05).Conclusions The results have indicated that there was no significant difference on pressure in the adjacent inferior zygapophyseal joint between replacement group and normal cervical spine after intercalated disc replacement.And intercalated disc replacement can meet the requirements of the cervical biomechanics reconstruction.it has extensive application prospects. Key words: Cervical spine; Artificial cervical disc; Biomechanics; Zygapophyseal joint

A biomimetic artificial intervertebral disc system composed of a cubic three-dimensional fabric

Background context In the quest for clinically functional artificial intervertebral discs (AIDs), multidisciplinary technologies have been employed. Existing solid mobile AIDs essentially consist of the superposition of solid plates and core materials; however, it is thought that an ideal surgical AID technology has not yet been developed. To overcome the limitation of these existing AIDs, we developed a unique flexible AID disc system on the basis of our original biomimetic concept. The AID is composed of a cubic three-dimensional fabric (3DF) with a triaxial fiber alignment, which offers biomimetic long-term dynamic mechanical behavior along with durability. Purpose This article substantiates the potential clinical use of the 3DF disc system that quite differs from existing ones. Study design We designed the lumbar and cervical 3DF discs that improved the structural weaknesses caused by the collagenous fiber alignment of biological intervertebral disc. Bioresorbable hydroxyapatite particles were deposited on the surface layer of the 3DF disc to promote new bony ingrowth and to ensure secure binding at the interface of the contacting vertebral bodies. A stand-alone system was devised for surgical reliability in terms of both positioning and fixation, allowing tight press fitting with the vertebral bodies. Bioactive and bioresorbable pins were penetrated through the 3DF disc body and projected from the surface to allow ideal insertion and fixation to the disc space, preserving the precise position during dynamical movement. In vitro endurance of the 3DF disc was examined under long-term alternating stresses, and the in vivo animal tests were conducted in the intervertebral lumbar discs at L5–L6 excised from baboons and replaced with the lumbar 3DF disc. Methods The static mechanical endurance was assessed through a creep test. In vitro endurance of the 3DF disc under repetitive stresses including axial compressing, flexion-extension, torsional twisting, and lateral bending were applied to the 3DF disc for a long-term for up to 105 million stresses, which is roughly equivalent to exposure of natural biological movement for more than 50 years. In the animal test, eight baboons were euthanized 6 months postoperatively. To their extracted spines, six pure moments (flexion and extension, left and right lateral bending, and left and right torsion) were applied vertically to the superior end of the specimen and then values of range of motions (ROMs) were calculated. Histological analyses were conducted on 12 reticuloendothelial and systemic tissues. Results The 3DF disc retained its biomimetic “J-shaped” stress-strain behavior without generating wear debris for up to 105 million stresses. A 130-N loading for the creep test decreased the height of 0.3 mm during 80 to 1,000 hours. In the biomechanical test, ROM values of axial rotation and flexion-extension showed no significant difference from the intact excluding that of lateral bending because the location of each pin to stand alone certainly controlled the bending behavior only. The histological analysis indicated no significant pathologic changes induced by the 3DF disc. Conclusions The 3DF disc system is clinically suitable for human disc replacement arthroplasty based on the findings of long-term durability with dynamic motion in vitro and effective animal tests in vivo. This system surely overcomes the limitations of existing solid AIDs, and the clinical potential of the biomimetic 3DF discs has been verified. This new biomaterial technology delivers most of the functions and characteristics required by a clinically available AID if applied correctly by surgeons.

In vitro evaluation of a ball-and-socket cervical disc prosthesis with cranial geometric center

Few biomechanical in vitro studies have reported the effects of disc replacement on motion and kinematics of the cervical spine. The purpose of this study was to analyze motion through 3D load-displacement curves before and after implantation of a ball-and-socket cervical disc prosthesis with cranial geometric center; special focus was placed on coupled motion, which is a well-known aspect of normal cervical spine kinematics. Six human cervical spines were studied. There were 3 male and 3 female cadaveric specimens (mean age at death 68.5 +/- 5 years [range 54-74 years]). The specimens were evaluated sequentially in 2 different conditions: first they were tested intact; then the spinal specimens were tested after implantation of a ball-and-socket cervical disc prosthesis, the Discocerv, at the C5-6 level. Pure moment loading was applied in flexion/extension, left and right axial rotation, and left and right lateral bending. All tests were performed under load control with a 3D measurement system. No differences were found to be statistically significant after comparison of range of motion between intact and instrumented spines for all loading conditions. The mean range of motion for intact spines was 10.3 degrees in flexion/extension, 5.6 degrees in lateral bending, and 5.4 degrees in axial rotation; that for instrumented spines was 10.4, 5.2, and 4.8 degrees , respectively. No statistical difference was observed for the neutral zone nor stiffness between intact and instrumented spines. Finally, the coupled motions were also preserved during axial rotation and lateral bending, with no significant difference before and after implantation. This study demonstrated that, under specific testing conditions, a ball-and-socket joint with cranial geometrical center can restore motion in the 3 planes after discectomy in the cervical spine while maintaining physiological coupled motions during axial rotation and lateral bending.

Between-day reliability of electromechanical delay of selected neck muscles during performance of maximal isometric efforts

BackgroundThe purpose of this study was to assess the between-day reliability of the electromechanical delay (EMD) of selected neck muscles during the performance of maximal isometric contractions in five different directions.MethodsTwenty-one physically active males participated in two testing sessions separated by seven to eight days. Using a custom-made fixed frame dynamometer, cervical force and surface electromyography (EMG) were recorded bilaterally from the splenius capitis, upper trapezius and sternocleidomastoid muscles during the performance of efforts in extension, flexion, left and right lateral bending, and protraction. The EMD was extracted using the Teager-Kaiser Energy Operator. Reliability indices calculated for each muscle in each testing direction were: the difference in scores between the two testing sessions and corresponding 95% confidence intervals, the standard error of measurement (SEM) and intra-class correlation coefficients (ICC).ResultsEMD values showed no evidence of systematic difference between the two testing sessions across all muscles and testing directions. The SEM for extension, flexion and lateral bending efforts ranged between 2.5 ms to 4.8 ms, indicating a good level of measurement precision. For protraction, SEM values were higher and considered to be imprecise for research and clinical purposes. ICC values for all muscles across all testing directions ranged from 0.23 to 0.79.ConclusionEMD of selected neck muscles can be measured with sufficient precision for the assessment of neck muscle function in an athletic population in the majority of directions tested.

ATLANTOAXIAL ROTATORY SUBLUXATION WITH LIGAMENTOUS DISRUPTION

We evaluated the biomechanical effects of 4 instrumented configurations after induced atlantoaxial rotatory subluxation: transarticular screw fixation (T/A) and polyaxial C1 lateral mass and C2 pedicle screw and rod fixation (LC1-PC2) for atlantoaxial arthrodesis with unilateral and bilateral instrumentation. Three-dimensional intervertebral motion was tracked stereophotogrammetrically while 14 human cadaveric spine specimens underwent nonconstraining pure moment loading. Nondestructive loads were applied quasi-statistically in 0.25-Nm increments to a maximum load of 1.5 Nm during flexion-extension, right and left axial rotation, and right and left lateral bending. Hyperrotation injuries were created using torsional loads applied during left axial rotation until visible failure occurred. In the normal condition, the values for angular range of motion, lax zone (zone of ligamentous laxity), and stiff zone (zone of ligamentous stretching) were similar in both groups in all directions of loading, with no significant differences (P > 0.05) between groups at C0-C1 or C1-C2. Both instrumentation systems (bilateral configurations) substantially stabilized angular motion at C1-C2 (P < 0.05) during all loading modes for the T/A group, and during all but right lateral bending (P = 0.072) for the LC1-PC2 group. The mean failure load for both intact and instrumented specimens was slightly greater, but not significant for the LC1-PC2 group compared with the T/A group (P > 0.14). Both methods fixated atlantoaxial subluxation equally well. Compared with unilateral instrumentation, a bilateral configuration with the LC1-PC2 technique significantly increased stability during extension (P < 0.05). During axial rotation, bilateral T/A screws significantly increased stability compared with unilateral fixation (P < 0.02).

Deformation of the equine pelvis in response toin vitro3D sacroiliac joint loading

Sacroiliac joint injuries can cause poor performance; however, the interaction between pelvic mechanics and the sacroiliac joint is poorly understood. To measure pelvic displacement during 3D sacroiliac joint loading. Nine reflective triads were attached rigidly to bony prominences in sacropelvic specimens harvested from 14 horses for stereophotogrammetric analysis of triad displacements and joint kinematics. The sacrum was coupled to a load cell and mounted vertically within a material testing system (MTS). A pneumatic actuator was used to apply 90 Nm moments to the ischial arch to simulate nutation-counternutation and left and right lateral bending of the sacroiliac joints. Axial rotation of the sacrum was induced by torsion of the upper MTS fixture. Vectors of marker displacement within orthogonal planes of motion were measured during loading of the sacropelvic specimens. Comparisons in the magnitude and direction of triad displacements were made between paired left-right markers and paired loading conditions. Nutation-counternutation of the sacroiliac joint caused vertical displacement of the ischial tuberosities and cranial-caudal displacement of the wings of the ilium. Lateral bending induced rotational displacement within the horizontal plane of all pelvic landmarks, relative to the sacrum. Axial rotation of the sacrum caused elevation of the wing of the ilium ipsilateral to the direction of sacral rotation and depression of the contralateral ilial wing. Significant paired left-right differences occurred during most sacroiliac joint loading conditions. Comparable magnitudes of pelvic displacement were measured during nutation-counternutation, left and right lateral bending, and left and right axial rotation. The equine pelvis is not a rigid structure and asymmetric pelvic deformation occurs during most sacroiliac joint movements. Bony pelvic deformation should be considered a normal response to any sacroiliac joint movement.

Biomechanical Study of Lumbar Spinal Arthroplasty with a Semi-Constrained Artificial Disc (Activ L) in the Human Cadaveric Spine

The goal of this study was to evaluate the biomechanical features of human cadaveric spines implanted with the Activ L prosthesis. Five cadaveric human lumbosacral spines (L2-S2) were tested for different motion modes, i.e. extension and flexion, right and left lateral bending and rotation. Baseline measurements of the range of motion (ROM), disc pressure (DP), and facet strain (FS) were performed in six modes of motion by applying loads up to 8 Nm, with a loading rate of 0.3 Nm/second. A constant 400 N axial follower preload was applied throughout the loading. After the Activ L was implanted at the L4-L5 disc space, measurements were repeated in the same manner. The Activ L arthroplasty showed statistically significant decrease of ROM during rotation, increase of ROM during flexion and lateral bending at the operative segment and increase of ROM at the inferior segment during flexion. The DP of the superior disc of the operative site was comparable to those of intact spine and the DP of the inferior disc decreased in all motion modes, but these were not statistically significant. For FS, statistically significant decrease was detected at the operative facet during flexion and at the inferior facet during rotation. In vitro physiologic preload setting, the Activ L arthroplasty showed less restoration of ROM at the operative and adjacent levels as compared with intact spine. However, results of this study revealed that there are several possible theoretical useful results to reduce the incidence of adjacent segment disease.

Biomechanical analysis of multilevel discectomy and excision of posterior longitudinal ligament: An in vitro study in sheep

This experimental biomechanical study was performed to determine the effects of multilevel anterior cervical discectomy and excision of posterior longitudinal ligament (PLL) to stabilize the cervical spine using an in vitro animal model. Fifty fresh cadaveric C3-C6 sheep spine specimens were divided into five experimental groups: Group A was the control group; Group B, one-level discectomy; Group C, two- level discectomy, Group D, three-level discectomy, and Group E, three-level discectomy and excision of PLL, respectively. The specimens were subjected to non-destructive loads cycled from zero to five Newton-meter for flexion, extension, right and left lateral bending, and axial rotation on an electrohydraulic test machine. Load displacement curves were obtained via collected data using strain gauges. The values were obtained for all five groups, statistical differences were determined respectively (P<0.05, ANOVA). One-level discectomy was less stable than the control group, two-level discectomy was less stable than one-level discectomy and three-level discectomy was less stable than two-level discectomy, respectively (P<0.05). Excision of PLL did not seem to affect stability (P>0.05). Our data suggested that cervical discectomy decreases stability of sheep spine pieces.

Three-dimensional analysis of cervical spine motion: reliability of a computer assisted magnetic tracking device compared to inclinometer

We aimed to investigate the reliability and reproducibility of a magnetic tracking technique for the assessment of overall cervical spine motion (principal and coupled movements). Ten asymptomatic male volunteers with a mean age of 29.3 years (range 20-37 years) were included in the study. Flexion, extension, left and right lateral bending and left and right axial rotation were measured using a magnetic tracking device (MTD) mounted onto a custom head-piece. For rotational movements in the frontal and sagittal planes the results were compared with the measurements of two standard inclinometers. Intra-observer, inter-observer and intra-instrument reliability was assessed with the intraclass correlation coefficient method. There were no significant differences for all motion measurements between the MTD and the inclinometer. High inter-observer reliability was found in flexion, extension, axial rotation and lateral bending indicating that the testing routine is applicable for different examiners. The intra-observer variability was high in flexion and extension, whereas in lateral bending the reliability coefficients were lower and displayed a fair to good reliability for most of the measurements with the MTD. The results of the MTD were found to be highly comparable with the inclinometer results with an inter-instrument correlation coefficient ranging from 0.88 to 0.99. The MTD is a reliable, reproducible method for three-dimensional motion analysis of the cervical spine and therefore a valuable method both for the clinical assessment of various degenerative and traumatic disorders and as a supplement of different therapeutic procedures and rehabilitation.

Biomechanical comparison of two different concepts for stand alone anterior lumbar interbody fusion

Segmental instability in degenerative disc disease is often treated with anterior lumbar interbody fusion (ALIF). Current techniques require an additional posterior approach to achieve sufficient stability. The test device is an implant which consists of a PEEK-body and an integrated anterior titanium plate hosting four diverging locking screws. The test device avoids posterior fixation by enhancing stability via the locking screws. The test device was compared to an already established stand alone interbody implant in a human cadaveric three-dimensional stiffness test. In the biomechanical test, the L4/5 motion segment of 16 human cadaveric lumbar spines were isolated and divided into two test groups. Tests were performed in flexion, extension, right and left lateral bending, right and left axial rotation. Each specimen was tested in native state first, then a discectomy was performed and either of the test implants was applied. Finite element analysis (FE) was also performed to investigate load and stress distribution within the implant in several loading conditions. The FE models simulated two load cases. These were flexion and extension with a moment of 5 Nm. The biomechanical testing revealed a greater stiffness in lateral bending for the SynFix-LR compared to the established implant. Both implants showed a significantly higher stiffness in all loading directions compared to the native segment. In flexion loading, the PEEK component takes on most of the load, whereas the majority of the extension load is put on the screws and the screw-plate junction. Clinical investigation of the test device seems reasonable based on the good results reported here.

The use of surface strain data and a neural networks solution method to determine lumbar facet joint loads during in vitro spine testing

A new method for determining facet loads during in vitro spine loading using strain gauges and a neural networks solution method was investigated. A test showed that the new solution method was more robust than and as accurate as a previously presented graphical solution method for computing facet loads using surface strain. The technique was subsequently utilized to assess facet loads at L1–L2 during flexibility testing [7.5 N m pure moments in flexion (FL), extension (EX), right and left axial rotation (AR), and right and left lateral bending (LB)], and stiffness testing (FL-EX with 400 N compressive follower load) of six human lumbar spine segments (T12–L2). In contrast to other techniques, such as thin film sensors or pressure-sensitive film, the strain-gauge method leaves the facet joint capsule intact during data collection, presumably allowing more natural load transmission. During flexibility tests, the mean (±standard deviation) calculated facet loads (in N) were 46.1±41.3 (FL), 51.5±39.0 (EX), 70.3±43.2 (AR-contralateral side), 31.3±33.4 (AR-ipsilateral side), 30.6±29.1 (LB-contralateral side), and 32.0±44.4 (LB-ipsilateral side). During stiffness tests, the calculated facet loads were 45.5±40.4 (upright), 46.6±41.9 (full FL), and 75.4±39.0 (full EX), corresponding to an equivalent of 11.4%, 11.6%, and 18.8% of the compressive follower load (upright, full FL and EX, respectively). The error associated with this technique, which was below 11 N for loads up to 125 N, is comparable to that reported with other techniques. The new method shows promise for assessing facet load during in vitro spine testing, an important parameter when evaluating new implant systems and surgical techniques.

Validation of a Finite Element Model of the Young Normal Lower Cervical Spine

A Finite Element Model (FEM) of the young adult human cervical spine has been developed as a first step in studying the process of spondylotic degeneration. The model was developed using normal geometry and material properties for the lower cervical spine. The model used a three-zone composite disc annulus to reflect the different material properties of the anterior, posterior, and lateral regions of the annulus. Nonlinear ligaments were implemented with a toe region to help the model achieve greater flexibility at low loads. The model was validated against experimental data for normal, nondegenerated cervical spines tested in flexion and extension, right and left lateral bending, and right and left axial rotation at loads of 0.33, 0.5, 1.0, 1.5, and 2.0 Nm. The model was within in vitro experimental standard deviation corridors 100% of the load range for right and left lateral bending. The model was within 80% of the load response corridors for extension and flexion with a deviation <0.3 degrees from the SD corridors. For axial rotation, the model was within 70% of the SD corridors for left axial rotation within 83% of right axial rotation responses. The deviation from SD corridors for axial rotation was generally <0.2 degrees.

SU‐GG‐I‐60: Effective Dose to Patients Undergoing Radiostereometric Analysis of the Lumbar Spine

Purpose: To determine the effective dose of patients undergoing radiostereometric analysis (RSA) of the lumbar spine. Method and Materials: 12 patients with total disk replacement prosthetic devices in the lumbar spine participated in an IRB approved study to follow the sagittal and coronal range of motion using RSA. Image pairs were obtained at 6 weeks, and then 3, 6, 12, 18 and 24 months. Five image pairs were acquired at each visit (neutral, flexion, extension, and left and right lateral bend). Radiographic techniques were recorded for all acquired images. Patient mass was used to estimate abdominal AP thickness. Tube output and half value layers were measured at the appropriate tube accelerating potentials. This information was used with the image geometry to calculate the entrance kerma‐area product (KAP) incident on the patients. The KAP and patient thickness were then used to estimate the energy imparted to the patients and the resulting effective dose. Results: The average technique used was 141 kVp and 11.6 mAs. The mean effective dose per image pair was 0.304 mSv, and the mean per visit (5 image pairs) was 1.52 mSv with a standard deviation of 0.7 mSv. The average for the entire two year study was approximately 9.1 mSv. This compares very favorably with the value of 18 mSv reported in the 2000 UNSCEAR report for a single diagnostic lumbar spine study. Conclusion: Image quality requirements for RSA are not the same as for general diagnostic imaging. The use of high kV techniques that would result in unacceptable low contrast images for general diagnostic purposes are adequate for visualizing fiducial beads used in RSA. Low doses allow for the acquisition of multiple image sets to detect motion of prosthetic devices with total effective doses that are much lower than a typical diagnostic lumbar spine examination.

Assessing range of motion to evaluate the adverse effects of ill-fitting cervical orthoses

Background context Although previous studies have primarily focused on testing the effectiveness of cervical orthoses under properly fit conditions, this study focuses on analyzing the effects of an ill-fitted cervical orthosis (Miami J). This may have significance to health-care providers in understanding the effects of an improperly fitted neck brace. Purpose The aims of this study were threefold: first, to apply virtual reality (VR) feedback control to repeatedly measure orthoses effectiveness in the primary motions; second, to use this control methodology to test the orthoses ability to restrict flexion/extension (FE) as a function of axial rotation (AR); third, to test the effects of an ill-fitting Miami J on cervical motion. Study design/setting This study combines six degrees of freedom electromagnetic trackers and VR feedback to analyze the effectiveness of common cervical orthoses under less than optimal conditions. Patient sample Twelve healthy male subjects aged 21 to 35 (mean 29.44 years, SD 6.598) years with no previous spinal cord injuries or current neck pain participated in the study. Outcome measures Cervical range of motion (CRoM) measurements were used to determine the amount of motion restriction for each of the fitted (too small, correct size, and too big) Miami J orthoses. Methods One Nest of Birds (NOB) electromagnetic sensor (Ascension Technology) was placed on the head and another on the upper back to measure motion of the head relative to the torso. The VR goggles (i-O Display Systems) were worn so that real-time feedback was available to the subject for motion control. The subject executed the primary motions of FE, AR, and lateral bending (LB) in separate sets of five trials each. Next, in combined motion, the subject axially rotated to a set point and then FE to his maximums. This entire set of motions was repeated for each (soft collar, Miami J, Miami J with chest extension, Sternal Occipital Mandibular Immobilizer (AliMed, Inc.), (SOMI and Halo) as well as the Miami J (one size too small and one size too big); the fitting of each brace was done by a board certified orthotist. A repeated measures analysis of variance was used to determine differences between the tested states (∗p=.05). Results For the validation test, the primary motions recorded for subjects wearing each cervical brace, which demonstrated that the various orthoses all restricted CRoM. The soft collar restricted less motion than the other devices, whereas the Halo restricted the most motion throughout. For the ill-fitting cervical collar comparison, motion in the correct size collar was normalized to 1.0, and the correct size allowed less motion than either the too big or too small braces. In FE, the too big brace tended to allow more motion than the too small, but only the too big brace in extension was significantly different from the correct size. In AR, the too small brace seemed to allow more motion than the too big. Both the too big and too small braces were significantly different than the correct size in both left and right AR. In LB, the too big brace and too small brace were very similar in the amount of motion they were able to restrict. Both braces were significantly different than the correct size in right LB, whereas only the too small brace was significantly different from the correct size in left LB. In the combined motion data, both the too big and too small braces allowed more motion than the correct size. The too small brace seemed to allow more FE at all degrees of AR except for extreme right AR. Conclusions To our knowledge, the effects of improperly fitted cervical orthoses on CRoM are still unknown. Using the NOB electromagnetic tracking system combined with VR feedback, we were able to consider the motion restriction of ill-fitting Miami J orthoses for both primary and combined motions. For both motion types, increased motion was possible when the subject was improperly fitted with the Miami J. If not considered, these excessive motions could potentially have detrimental effects on patient satisfaction, clinical outcomes, or even lead to increased secondary injury.

Co-contraction of cervical muscles during sagittal and coronal neck motions at different movement speeds

Muscle co-contraction is important in stabilizing the spine. The effects of movement speed and direction on the cervical co-contraction were, however, not yet investigated. Surface electromyographies of three paired cervical muscles were measured in 17 young healthy subjects. The subjects performed voluntary neck movements in sagittal and coronal plane at fast, medium, and slow speeds. The co-contraction ratio was defined as the normalized integration of antagonistic electromyographic activities divided by that of total muscle activities. The results showed that the co-contraction ratio at fast speed (0.42 +/- 0.21) was smaller than that at medium (0.45 +/- 0.20) and slow (0.46 +/- 0.19) speeds (P </= 0.001). The co-contraction ratio was different between flexion and extension, but was similar between left and right lateral bending. The implications of co-contraction patterns include: (1) agonistic activities increase with speed but antagonistic activities do not increase accordingly; (2) neck extensors are highly activated during sagittal motions; and (3) muscular activations are symmetric during coronal motions. In conclusion, the co-contraction patterns from young healthy subjects demonstrate normal neuromuscular control in regulating the stiffness of cervical spine, and may further be references to assess the neck disorders in elderly or patients.

Biomechanical Characteristics of Hybrid Hook-Screw Constructs in Short-Segment Thoracic Fixation

Ex vivo biomechanical testing of human cadaveric thoracic spine segments. To determine whether a hybrid construct, using a combination of pedicle screws (PSs) and lamina hooks, was equivalent to a PS construct, in a short-segment thoracic spine fixation model. Comparisons have been made among PS, lamina hook, and hybrid screw-hook constructs, but these have generally been in long-segment scoliosis correction. In this study, we compared the hybrid and screw-only constructs in a short-segment thoracic fixation. For pullout testing, matched specimens were used for PS (n = 8) and hybrid (n = 8) constructs. Construct stiffness, and the force required for construct failure, were measured. Dynamic testing was carried out on specimens in the PS (n = 7) and hybrid (n = 7) groups in compression, flexion, extension, and left and right lateral bending. Each group was tested intact, after instrumentation, and after corpectomy. When compared with the hybrid group, a significantly greater force was required for construct failure in the PS group, and these PS constructs were significantly stiffer. No differences were found between groups in dynamic testing. A construct employing PSs is significantly stiffer and more resistant to pullout failure than a hook-screw hybrid construct.

Side Impact Causes Multiplanar Cervical Spine Injuries

Side impact may cause neck and upper extremity pain, paresthesias, and impaired neck motion. No studies have quantified the cervical spine mechanical instability and injury threshold acceleration due to side impact. The goals of the present study were to identify and quantify cervical spine soft tissue injury and the injury threshold acceleration for side impact, and to compare these results with previous findings. Six human cervical spine specimens (C0-T1) underwent 3.5, 5, 6.5, and 8 g impacts. Pre- and postimpact flexibility tests were performed. Soft tissue injury was defined as a significant increase (p < 0.05) in the average intervertebral flexibility above the baseline 2 g impact. The injury threshold was the lowest T1 horizontal peak acceleration that caused the injury. The injury threshold acceleration was 6.5 g, with injuries occurring at C4-C5 through C7-T1 in flexion, axial rotation, or left lateral bending. After 8 g, three-plane injury was observed at C4-C5 and C6-C7, whereas two-plane injury occurred at C3-C4 in flexion and left lateral bending and at C5-C6 and C7-T1 in axial rotation and left lateral bending. Side impact caused multiplanar injuries at C3-C4 through C7-T1 and significantly greater injury at C6-C7, as compared with head-forward rear impact.

In vitro biomechanical comparison of transpedicular versus translaminar C-2 screw fixation in C2–3 instrumentation

In instrumentation of the upper cervical spine, placement of pedicle screws into C-2 is generally safe, although there is the potential for injury to the vertebral arteries. Owing to this risk, translaminar screws into C-2 have been used. The aim of this study was to compare the stability of the in vitro cadaveric spine using C-2 laminar compared with C-2 pedicle screws in C2-3 instrumentation. Eight fresh frozen human cadaveric cervical spines (C1-6) were potted at C1-2 and C5-6. Pure moments in increments of 0.3 Nm to a maximum of 1.5 Nm were applied in flexion, extension, right and left lateral bending, and right and left axial rotation. Each specimen was tested sequentially in three modes: 1) intact; 2) C2 pedicle screw-C3 lateral mass fixation; and 3) C2 laminar screw-C3 lateral mass fixation. The sequence of fixation testing was randomized. Motion was tracked with reflective markers attached to C-2 and C-3. Spinal levels with instrumentation showed significantly less motion than the intact spine in all directions and with all loads greater than 0.3 Nm (p < 0.05). Although there was no significant difference between C2 pedicle screw-C3 lateral mass fixation and C2 laminar screw-C3 lateral mass fixation, generally the former type of fixation was associated with less motion than the latter. When pedicle screws in C-2 are contraindicated or inappropriate, laminar screws in C-2 offer a safe and acceptable option for posterior instrumentation.

Moment–rotation responses of the human lumbosacral spinal column

The objective of this study was to test the hypothesis that the human lumbosacral joint behaves differently from L1–L5 joints and provides primary moment–rotation responses under pure moment flexion and extension and left and right lateral bending on a level-by-level basis. In addition, range of motion (ROM) and stiffness data were extracted from the moment–rotation responses. Ten T12–S1 column specimens with ages ranging from 27 to 68 years (mean: 50.6±13.2) were tested at a load level of 4.0 N m. Nonlinear flexion and extension and left and right lateral bending moment–rotation responses at each spinal level are reported in the form of a logarithmic function. The mean ROM was the greatest at the L5–S1 level under flexion (7.37±3.69°) and extension (4.62±2.56°) and at the L3–L4 level under lateral bending (4.04±1.11°). The mean ROM was the least at the L1–L2 level under flexion (2.42±0.90°), L2–L3 level under extension (1.58±0.63°), and L1–L2 level under lateral bending (2.50±0.75°). The present study proved the hypothesis that L5–S1 motions are significantly greater than L1–L5 motions under flexion and extension loadings, but the hypothesis was found to be untrue under the lateral bending mode. These experimental data are useful in the improved validation of FE models, which will increase the confidence of stress analysis and other modeling applications.