Multiple muscle force simulation in axial rotation of the cervical spine

Effects of occipital-atlas stabilization in the upper cervical spine kinematics: an in vitro study

This study compares upper cervical spine range of motion (ROM) in the three cardinal planes before and after occiput-atlas (C0–C1) stabilization. After the dissection of the superficial structures to the alar ligament and the fixation of C2, ten cryopreserved upper cervical columns were manually mobilized in the three cardinal planes of movement without and with a screw stabilization of C0–C1. Upper cervical ROM and mobilization force were measured using the Vicon motion capture system and a load cell respectively. The ROM without C0–C1 stabilization was 19.8° ± 5.2° in flexion and 14.3° ± 7.7° in extension. With stabilization, the ROM was 11.5° ± 4.3° and 6.6° ± 3.5°, respectively. The ROM without C0–C1 stabilization was 4.7° ± 2.3° in right lateral flexion and 5.6° ± 3.2° in left lateral flexion. With stabilization, the ROM was 2.3° ± 1.4° and 2.3° ± 1.2°, respectively. The ROM without C0–C1 stabilization was 33.9° ± 6.7° in right rotation and 28.0° ± 6.9° in left rotation. With stabilization, the ROM was 28.5° ± 7.0° and 23.7° ± 8.5° respectively. Stabilization of C0–C1 reduced the upper cervical ROM by 46.9% in the sagittal plane, 55.3% in the frontal plane, and 15.6% in the transverse plane. Also, the resistance to movement during upper cervical mobilization increased following C0–C1 stabilization.

Spine biomechanical testing methodologies: The controversy of consensus vs scientific evidence

Biomechanical testing methodologies for the spine have developed over the past 50 years. During that time, there have been several paradigm shifts with respect to techniques. These techniques evolved by incorporating state-of-the-art engineering principles, in vivo measurements, anatomical structure-function relationships, and the scientific method. Multiple parametric studies have focused on the effects that the experimental technique has on outcomes. As a result, testing methodologies have evolved, but there are no standard testing protocols, which makes the comparison of findings between experiments difficult and conclusions about in vivo performance challenging. In 2019, the international spine research community was surveyed to determine the consensus on spine biomechanical testing and if the consensus opinion was consistent with the scientific evidence. More than 80 responses to the survey were received. The findings of this survey confirmed that while some methods have been commonly adopted, not all are consistent with the scientific evidence. This review summarizes the scientific literature, the current consensus, and the authors' recommendations on best practices based on the compendium of available evidence.

The effect of alar ligament transection on the rotation stress test: A cadaveric study

BACKGROUND

The rotation stress test is a pre-manipulative screening test used to examine upper cervical instability. This in vitro study simulates the clinical application of the rotation stress test before and after alar ligament transection.

METHODS

After the dissection of the superficial structures to the alar ligament and the fixation of C2, ten cryopreserved upper cervical columns were manually mobilized in right and left rotation without and with right alar ligament transection. Upper cervical rotation range of motion (RoM) and mobilization torque were recorded using the Vicon motion capture system and a load cell.

FINDINGS

Ligament transection resulted in a larger rotation range of motion in all specimens (contralateral rotation (3.6°, 12.9%) and ipsilateral rotation (4.6°, 13.7%)). The mobilization torque recorded during rotation varied among the different specimens, with a trend towards reduced torque throughout the test in contralateral rotation.

INTERPRETATION

This study simulated the rotation stress test before and after alar ligament transection. Unilateral transection of the alar ligament revealed a bilateral increase of the upper cervical rotation. Additional in vivo studies are necessary to validate the results of this study in patients with suspicion of upper cervical instability.

The MOVE-C Cervical Artificial Disc - Design, Materials, Mechanical Safety

Purpose

There are various cervical disc prostheses on the market today. They can be subdivided into implants with a ball-and-socket design and implants with a flexible core, which is captured between the implant endplates and sealed using various sheaths. Implants with an articulating surface are mostly metal-on-metal or metal-on-UHMWPE designs and, thus, do not allow for axial damping. The aim of this study is to provide mechanical safety and performance data of the MOVE-C cervical disc prosthesis which combines both an articulating surface and a flexible core.

Materials and Methods

MOVE-C consists of a cranial and caudal metal plate made of TiAl6V4. The cranial plate is TiNbN coated on its articulating surface. The caudal plate has a fixed polycarbonate-urethane (PCU) core. The TiNbN coating is meant to optimize the wear behavior of the titanium endplate, whereas the PCU core is meant to allow for a reversible axial deformation, a pre-defined neutral zone and a progressive load-deformation curve in all planes.

Results

Various standard testing procedures (for example, ISO 18192–1 and ASTM F2364) and non-standard mechanical tests were carried out to prove the implant’s mechanical safety. Due to the new implant design, wear and creep testing was deemed most important. The wear rate for the PCU was in maximum 1.54 mg per million cycles. This value was within the range of the UHMWPE wear rates reported for other cervical disc prostheses (0.53 to 2.59 mg/million cycles). Also in the creep-relaxation test, a qualitatively physiological behavior was shown with a certain amount of remaining deformation but no failure.

Conclusion

The mechanical safety of the MOVE-C cervical disc prosthesis was shown to be comparable to other cervical disc prostheses. Since PCU wear particles were elsewhere shown to be less bioactive than cross-linked UHMWPE particles, wear-related failure in vivo may be less frequent compared to other prostheses. This, however, will have to be shown in further studies.

Development and evaluation of RAMP II - a practitioner's tool for assessing musculoskeletal disorder risk factors in industrial manual handling

RAMP II is an observation-based tool developed for assessing a wide range of musculoskeletal disorder risk factors related to industrial manual handling. RAMP II, which is part of the RAMP tool, is based on research studies and expert judgments. The assessment relies mainly on direct or video observations of the work being assessed, but additionally on measured push/pull forces and weights of handled objects, and on perceived workload and discomfort. Over 80 practitioners participated in the development of the tool. According to the evaluations, 73% of the assessment items evaluated had acceptable reliability, and the majority of the potential end-users reported that RAMP II is usable for assessing risks and as a decision base. It is concluded that this study provides support that RAMP II is usable for risk assessment of musculoskeletal disorder risk factors in industrial manual handling. Practitioner summary: RAMP II is an observation-based assessment tool for screening and assessing major musculoskeletal exposures in industrial manual handling jobs. Over 80 practitioners participated in the development of the tool. This study provides support that RAMP II is usable for risk assessment of musculoskeletal disorder risk factors in industrial manual handling. Abbreviations: CTS: carpal tunnel syndrome; HARM: the Hand Arm Risk Assessment method; IMP: intramuscular pressure; κ_w: linearly weighted kappa; LBD: lower back disorders; LBP: lower back pain; MAWL: maximum acceptable weight of lift; MHO: manual handling operations; MSD: musculoskeletal disorder; MNSD: neck-shoulder disorder; NSP: neck-shoulder pain; OCRA: the Occupational Repetitive Action methods; OHS: occupational health and safety; PABAK: prevalence and bias adjusted kappa; p₀: proportion of agreement; RAMP: Risk Assessment and Management tool for manual handling Proactively; ROM: range of motion; RPL: risk and priority level; RSI: the Revised Strain Index; RULA: the Rapid Upper Limb Assessment; SWEA: Swedish Work Environment Authority; UEMSDs: upper-extremity work-related musculoskeletal disorders; WMSD: work-related musculoskeletal disorder; WRMSD: work-related musculoskeletal disorder; workday_8h: eight hours workday.

A Robotic Head Stabilization Device for Medical Transport

In this paper, the design and control of a robotic device intended to stabilize the head and neck of a trauma patient during transport are presented. When transporting a patient who has suffered a traumatic head injury, the first action performed by paramedics is typically to restrain and stabilize the head and cervical spine of a patient. The proposed device would drastically reduce the time required to perform this action while also freeing a first responder to perform other possibly lifesaving actions. The applications for robotic casualty extraction are additionally explored. The design and construction are described, followed by control simulations demonstrating the improved behavior of the chosen controller paradigm, linear active disturbance rejection control (LADRC). Finally, experimental validation is presented, followed by future work and directions for the research.

A subject-specific biomechanical control model for the prediction of cervical spine muscle forces

BACKGROUND

The aim of the present study is to propose a subject-specific biomechanical control model for the estimation of active cervical spine muscle forces.

METHODS

The proprioception-based regulation model developed by Pomero et al. (2004) for the lumbar spine was adapted to the cervical spine. The model assumption is that the control strategy drives muscular activation to maintain the spinal joint load below the physiological threshold, thus avoiding excessive intervertebral displacements. Model evaluation was based on the comparison with the results of two reference studies. The effect of the uncertainty on the main model input parameters on the predicted force pattern was assessed. The feasibility of building this subject-specific model was illustrated with a case study of one subject.

FINDINGS

The model muscle force predictions, although independent from EMG recordings, were consistent with the available literature, with mean differences of 20%. Spinal loads generally remained below the physiological thresholds. Moreover, the model behavior was found robust against the uncertainty on the muscle orientation, with a maximum coefficient of variation (CV) of 10%.

INTERPRETATION

After full validation, this model should offer a relevant and efficient tool for the biomechanical and clinical study of the cervical spine, which might improve the understanding of cervical spine disorders.

Trains of electrical stimulation of the trapezius muscles redistribute the frequencies of body oscillations during stance

We investigated the postural effects of trains of electrical stimulation (TES) applied unilaterally or bilaterally on the trapezius muscle in 20 healthy subjects (mean age: 23.1 ± 1.33 years; F/M: 8/12). The anterior-posterior (AP) displacements (AP axis), medio-lateral displacements (ML axis) and total travelled distances (TTW) of the centre of pressure (COP) remained unchanged with TES. However, detailed spectral analysis of COP oscillations revealed a marked decrease of the magnitudes of peak power spectral density (peak PSD) following application of TES. Peak PSD was highly correlated with the intensity of stimulation (P < 0.001 both the AP and ML axes). For the AP axis, the integrals of the sub-bands 0-0.4, 0.4-1.5, 1.5-3 Hz were significantly decreased (P < 0.001), the integrals of the sub-bands 3-5 and 5-8 Hz were not significantly affected (P>0.30) and the integrals of the sub-band 8-10 Hz were significantly increased (P < 0.001). The ratios of the integrals of sub-bands 8-10 Hz/0-3 Hz were markedly enhanced with bilateral TES (P < 0.001). For the ML axis, the effects were striking (P < 0.001) for the sub-bands 0-0.4, 0.4-1.5 and 8-10 Hz. For both the AP and ML axes, a significant inverse linear relationship was found between the intensity of TES and the average speed of COP. We show that TES applied over the trapezius muscles exerts significant and so far unrecognised effects upon oscillations of the COP, decreasing low-frequency oscillations and enhancing high-frequency oscillations. Our data unravel a novel property of the trapezius muscles upon postural control. We suggest that this muscle plays a role of a distributor of low-frequency versus high-frequency sub-bands of frequency during stance. Previous studies have shown that patients with supra-tentorial stroke show an increased peak PSD in low frequencies of body oscillations. Therefore, our findings provide a rationale to assess neurostimulation of the trapezius muscle in the rehabilitation of postural deficits in supra-tentorial stroke.

Altered Co-contraction of Cervical Muscles in Young Adults with Chronic Neck Pain during Voluntary Neck Motions

[Purpose] Muscle co-contraction is important in stabilizing the spine. The aim of this study was to compare cervical muscle co-contraction in adults with and without chronic neck pain during voluntary movements. [Subjects and Methods] Surface electromyography of three paired cervical muscles was measured in fifteen young healthy subjects and fifteen patients with chronic neck pain. The subjects performed voluntary neck movements in the sagittal and coronal plane at slow speed. The co-contraction ratio was defined as the normalized integration of the antagonistic electromyography activities divided by that of the total muscle activities. [Results] The results showed that the co-contraction ratio of patients was greater during flexion movement, lesser during extension movement, slightly greater during right lateral bending, and slightly lesser during left lateral bending compared with in the controls. [Conclusion] The results suggested that neck pain patients exhibit greater antagonistic muscle activity during flexion and dominate-side bending movements to augment spinal stability, while neuromuscular control provides relatively less protection in the opposite movements. This study helps to specify the changes of the stiffness of the cervical spine in neck pain patients and provides a useful tool and references for clinical assessment of neck disorders.

The prevalence of osteoarthritis of the atlanto-odontoid joint in adults using multidetector computed tomography

BACKGROUND

The prevalence of osteoarthritis of the atlanto-odontoid joint has been reported by radiology, autopsy, and conventional computed tomography (CT), but the prevalence has not yet been assessed by multidetector computed tomography (MDCT).

PURPOSE

To reveal the prevalence of osteoarthritis of the atlanto-odontoid joint and to analyze the inter-relationships among gender, age, and osteoarthritis with MDCT in adults.

MATERIAL AND METHODS

First, a series of 700 selected domestic patients aged >18 years undergoing an upper cervical MDCT scan were divided equally into seven age groups. Second, using the postprocessing technique of multiplanar reconstruction, osteoarthritis of the atlanto-odontoid joint was viewed from any direction and classified into four grades, which were normal, mild, moderate, and severe. Lastly, the incidence of the different grades of osteoarthritis was assessed, and the reproducibility was tested.

RESULTS

There was no significant difference between gender and osteoarthritis of atlanto-odontoid joint (P > 0.05). The rate of osteoarthritis was 16% in the age group 18-25 years, 23% in the age group 25-30 years, 33% in the age group 30-40 years, 54% in the age group 40-50 years, 70% in the age group 50-60 years, 87% in the age group 60-70 years, and 93% in the age group >70 years. Mild osteoarthritis appeared at the earliest at age 19.6 years, moderate osteoarthritis in at earliest at age 24.2 years, and severe osteoarthritis at the earliest at age 48.5 years. The inter-observer reliability was excellent (k = 0.86).

CONCLUSION

Osteoarthritis of the atlanto-odontoid joint could be detected by MDCT in a young adult. It increased rapidly with increasing age on MDCT.

Cervical spine rotation and range of motion: pilot measurements during driving

OBJECTIVE

Previous studies have evaluated the cervical range of axial rotation during simulated driving conditions. The goals of this pilot study were to describe cervical spine rotation during in-car driving and determine the percentage of time outside neutral neck rotation and peak cervical axial rotation angles that the subjects adopted during various driving conditions.

METHODS

Subjects drove around a specified route through the city of Guelph, Ontario, which included residential, thruway, and highway driving; additional minor driving tasks, such as lane changes, were also included. The cervical range of motion was measured continuously throughout the drive using an electromagnetic sensor; we also used videotape to document the specific driving tasks.

RESULTS

The subjects spent 87.0 percent (SD=8.8) of time with their cervical spine in the neutral axial rotation position (±15 degrees). The percentage of time that the subjects spent outside of the neutral range of cervical axial rotation depended upon the driving section (including residential, thruway, and highway), and driving task being performed (starts, stops, and lane changes). The subjects spent a significantly greater proportion of time with their necks rotated beyond neutral during residential driving compared to thruway and highway driving (19.1% SD=8.3 vs. 10.7% SD=9.5 and 9.3% SD=8.7, respectively; p<.001). During driving, the peak angles of cervical axial rotation were an average of 35.7 degrees (SD=14.2) left and 42.5 degrees (SD=18.0) right.

CONCLUSIONS

We observed a large degree of variability in cervical axial rotation during driving. We observed that most of the driving tasks related to stopping had increased proportion of time out of neutral rotation. Also, right-hand lane changes increased time out of neutral rotation more than left-hand lane changes. Drivers routinely adopt nonneutral head positions (on average 13% of the time); this is likely not enough to lead to injury.

Electromyography of superficial and deep neck muscles during isometric, voluntary, and reflex contractions

Increasingly complex models of the neck neuromusculature need detailed muscle and kinematic data for proper validation. The goal of this study was to measure the electromyographic activity of superficial and deep neck muscles during tasks involving isometric, voluntary, and reflexively evoked contractions of the neck muscles. Three male subjects (28-41 years) had electromyographic (EMG) fine wires inserted into the left sternocleidomastoid, levator scapulae, trapezius, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles. Surface electrodes were placed over the left sternohyoid muscle. Subjects then performed: (i) maximal voluntary contractions (MVCs) in the eight directions (45 deg intervals) from the neutral posture; (ii) 50 N isometric contractions with a slow sweep of the force direction through 720 deg; (iii) voluntary oscillatory head movements in flexion and extension; and (iv) initially relaxed reflex muscle activations to a forward acceleration while seated on a sled. Isometric contractions were performed against an overhead load cell and movement dynamics were measured using six-axis accelerometry on the head and torso. In all three subjects, the two anterior neck muscles had similar preferred activation directions and acted synergistically in both dynamic tasks. With the exception of splenius capitis, the posterior and posterolateral neck muscles also showed consistent activation directions and acted synergistically during the voluntary motions, but not during the sled perturbations. These findings suggest that the common numerical-modeling assumption that all anterior muscles act synergistically as flexors is reasonable, but that the related assumption that all posterior muscles act synergistically as extensors is not. Despite the small number of subjects, the data presented here can be used to inform and validate a neck model at three levels of increasing neuromuscular-kinematic complexity: muscles generating forces with no movement, muscles generating forces and causing movement, and muscles generating forces in response to induced movement. These increasingly complex data sets will allow researchers to incrementally tune their neck models' muscle geometry, physiology, and feedforward/feedback neuromechanics.

Differential control of the scapulothoracic muscles in humans

The control of the scapulothoracic muscles trapezius (Tr) and serratus anterior (SA) has been examined in normal human subjects. Electromyographic recordings were made from the SA and Tr muscles (upper trapezius UTr, lower trapezius LTr) using surface electrodes placed bilaterally. Magnetic stimulation of the motor cortex and electrical stimulation of peripheral nerves were used to examine their descending and reflex control. The average optimal site of cortical stimulation was found to be the same for SA, UTr and LTr (an approximate centre of gravity of -0.6 cm, 3.7 cm where the centre of gravity is expressed as the mean anterio-posterior position, the mean medio-lateral position). Some asymmetry in the cortical representation of UTr was found in each individual tested. Magnetic stimulation evoked bilateral MEPs in Tr (latency contralateral (c) UTr 8.5 +/- 1.6 ms, ipsilateral (i) UTr 19.0 +/- 2.7 ms) but only contralateral responses were evoked in SA (11.2 +/- 2.6 ms). Electrical stimulation of the long thoracic nerve at two sites was used to examine homonymous and heteronymous reflexes of SA, while electrical stimulation of cervical nerve of C3/4 was used to examine the heteronymous reflexes of Tr. Ipsilateral SA H reflexes were evoked at a latency of 9.9 +/- 0.8 ms (proximal site) and 10.8 +/- 1.2 ms (distal site). No group I reflexes were evoked from SA to its contralateral homologue. No group I reflexes were evoked between Tr and SA. Finally, cross-correlation of activity from the Tr muscle pairs and the SA muscle pair revealed that the motoneurones of the Tr muscles share some common presynaptic input whereas there was no detectable common presynaptic input to the SA muscle pair. This study extends and consolidates knowledge regarding the neural control of trapezius and for the first time explores the neural control of SA. The study demonstrates a contrasting bilateral control of Tr and SA. These patterns of connections are discussed in relation to the contrasting bilateral functional roles of these muscles.

Comparison of cervical disk implants and cervical disk fusion treatments in human cadaveric models

Articulating cervical disk implants have been proposed as an alternative to disk fusion in the treatment of cervical disk disease. To examine the mechanical effect of articulating cervical disk implants (ACDI) versus simulated cervical disk fusion, a mechanical test device was constructed and cadaveric tests were carried out. While results show little effect on the pressures above and below the treatment level, the percent hysteretic behavior of the specimens trended to be higher for the ACDI, indicating that these implants retain more of the natural energy absorption capability of the cervical spine.

Biofidelic whole cervical spine model with muscle force replication for whiplash simulation

Whiplash has been simulated using volunteers, whole cadavers, mathematical models, anthropometric test dummies, and whole cervical spines. Many previous in vitro whiplash models lack dynamic biofidelity. The goals of this study were to (1) develop a new dynamic whole cervical spine whiplash model that will incorporate anterior, lateral and posterior muscle force replication, (2) evaluate its performance experimentally and (3) compare the results with in vivo data. To evaluate the new model, rear-impact whiplash simulations were performed using the incremental trauma approach at maximum measured T1 horizontal accelerations of 3.6 g, 4.7 g, 6.6 g, and 7.9 g. The kinematic response of the new model, e.g., peak head-T1 extension and peak intervertebral rotations, were compared with the corresponding in vivo data. The average peak head-T1 extension was within the in vivo corridor during the 3.6 g whiplash simulation (9.1 kph delta V). The peak in vivo intervertebral rotations obtained during a 4.6 g whiplash simulation of a young volunteer were within, or only marginally in excess of, the 95% confidence limits of the average peak intervertebral rotations measured during the 4.7 g whiplash simulation of the present study. Thus, the new whole cervical spine model with muscle force replication produced biofidelic dynamic responses to simulated whiplash. The new model is capable of generating important biomechanical data that may help improve our understanding of whiplash injuries and injury mechanisms.

Soft tissue injury threshold during simulated whiplash: a biomechanical investigation

STUDY DESIGN

A newly developed biofidelic whole cervical spine (WCS) model with muscle force replication (MFR) was subjected to whiplash simulations of varying intensity, and the resulting injuries were evaluated through changes in the intervertebral flexibility.

OBJECTIVES

To identify the soft tissue injury threshold based on the peak T1 horizontal acceleration and the association between acceleration magnitude and injury severity resulting from simulated whiplash using the WCS + MFR model.

SUMMARY OF BACKGROUND DATA

Whiplash has been simulated using mathematical models, whole cadavers, volunteers, and WCSs. The measurement of injury (difference between prewhiplash and postwhiplash flexibilities) is possible only using the WCS model.

METHODS

Six WCS + MFR specimens (C0-T1) were incrementally rear-impacted at nominal T1 horizontal maximum accelerations of 3.5, 5, 6.5, and 8 g, and the changes in the intervertebral flexibility parameters of neutral zone and range of motion were determined. The injury threshold acceleration was the lowest T1 horizontal peak acceleration that caused a significant increase in the intervertebral flexibility.

RESULTS

The first significant increase (P <0.01) of 39.8% occurred in the C5-C6 extension neutral zone following the 5 g acceleration. At higher accelerations, the injuries spread among the surrounding levels (C4-C5 to C7-T1).

CONCLUSIONS

A rear-end collision is most likely to injure the lower cervical spine by intervertebral hyperextension at a peak T1 horizontal acceleration of 5 g and above. These results may aid in the design of injury prevention systems and more precise diagnoses of whiplash injuries.

Evaluation of axial and flexural stresses in the vertebral body cortex and trabecular bone in lordosis and two sagittal cervical translation configurations with an elliptical shell model

BACKGROUND

Osteoarthritis and spinal degeneration are factors in neck and back pain. Calculations of stress in clinically occurring configurations of the sagittal cervical spine are rare.

OBJECTIVE

To calculate and compare combined axial and flexural stresses in lordosis versus cervical configurations in anterior and vertical sagittal head translated positions.

DESIGN

Digitized measurements from lateral cervical radiographs of 3 different shapes were used to calculate axial loads and bending moments on the vertebral bodies of C2-C7.

METHODS

An elliptical shell model was used to model horizontal cross-sections of the vertebral bodies of C2 through T1. Axial and flexural stresses were calculated with short compression block equations. Elliptical shell modeling permitted separation of stresses into cortical and inner medullary regions. Digitized radiographic points were used to create polynomials representing the shape of the sagittal cervical curvatures from C1 to T1. To calculate bending moments at each vertebral segment, moment arms from a vertical line through C1 were determined from digitizing.

RESULTS

Compared with the normal lordosis, stresses on the anterior vertebral body cortical margins of C5-T1 in the sagittal translated postures are compression rather than tension. At the posterior vertebral bodies in the anteriorly translated position and vertically translated postures, the stresses change from compression to tension at C5 through T1. In absolute value (ABS) compared with values at the same segments in a normal lordosis, the magnitude of the combined anterior stresses in the sagittal postures are higher at C5-C7 (eg, ABS[sigma(straight)/sigma(normal)] approximately 1.25 to 4.25).

CONCLUSIONS

Vertebral body stresses are reversed in direction at C5-T1 in sagittal translated postures compared to a normal lordosis. Stress analysis, with implications for bone remodeling, indicates that both sagittal head translation postures, anterior head carriage, and vertical head translation, are undesirable configurations in the cervical spine.

Cervical SNAGs: a biomechanical analysis

A sustained natural apophyseal glide (SNAG) is a mobilization technique commonly used in the treatment of painful movement restrictions of the cervical spine. In the manual therapy literature, the biological basis and empirical efficacy of cervical SNAGs have received scant attention. In particular, an examination of their potential biological basis in order to stimulate informed discussion seems overdue. This paper discusses the likely biomechanical effects of both the accessory and physiological movement components of a unilateral cervical SNAG applied ipsilateral to the side of pain when treating painfully restricted cervical rotation. The use of flexion and extension SNAGS, and rotation SNAGS performed contralateral to the side of pain are not considered. Although a cervical SNAG may clinically be able to resolve painfully restricted cervical spine movement, it is difficult to explain biomechanically why a technique which first distracts (opens) and then compresses (closes) the zygapophyseal joint ipsilateral to the side of pain, and perhaps slightly distracts the uncovertebral cleft, would be superior to a technique which distracts the articular surfaces with both accessory and physiological movement components. Therefore, the reported clinical efficacy of cervical SNAGs cannot be explained purely on the basis of the resultant biomechanical effects in the cervical spine.

Mechanically simulated muscle forces strongly stabilize intact and injured upper cervical spine specimens

Although muscles are assumed to be capable of stabilizing the spinal column in vivo, they have only rarely been simulated in vitro. Their effect might be of particular importance in unstable segments. The present study therefore tests the hypothesis that mechanically simulated muscle forces stabilize intact and injured cervical spine specimens. In the first step, six human occipito-cervical spine specimens were loaded intact in a spine tester with pure moments in lateral bending (+/- 1.5 N m), flexion-extension (+/- 1.5 N m) and axial rotation (+/- 0.5 N m). In the second step, identical flexibility tests were carried out during constant traction of three mechanically simulated muscle pairs: splenius capitits (5 N), semispinalis capitis (5 N) and longus colli (15 N). Both steps were repeated after unilateral and bilateral transection of the alar ligaments. The muscle forces strongly stabilized C0-C2 in all loading and injury states. This was most obvious in axial rotation, where a reduction of range of motion (ROM) and neutral zone to <50% (without muscles=100%) was observed. With increasing injury the normalized ROM (intact condition=100%) increased with and without muscles approximately to the same extend. With bilateral injury this increase was 125-132% in lateral bending, 112%-119% in flexion-extension and 103-116% in axial rotation. Mechanically simulated cervical spine muscles strongly stabilized intact and injured cervical spine specimens. Nevertheless, it could be shown that in vitro flexibility tests without muscle force simulation do not necessarily lead to an overestimation of spinal instability if the results are normalized to the intact state.

A method to simulate in vivo cervical spine kinematics using in vitro compressive preload

STUDY DESIGN

An in vitro flexibility study of C2-T1 specimens under compressive preload.

OBJECTIVE

To determine three-dimensional flexibility test moments needed to obtain spinal kinematics representative of the in vivo spine studies.

SUMMARY OF BACKGROUND DATA

Most previous three-dimensional in vitro cervical spine studies have used equal moments in all three planes to evaluate spinal flexibilities. Recent advances have made it possible to apply physiologic compressive preload. It is unclear what moments should be applied to these preloaded spine segments to simulate in vivo kinematics.

METHODS

Six fresh human cadaveric cervical spine specimens (C2-T1) were used. The preload (100 N) was applied by flexible cables, which passed through guides attached to each vertebra. Flexibility tests of flexion-extension and bilateral axial torsion and lateral bending were performed. Two protocols were compared, 1:1:1 with equal pure moments of 1 Nm for each direction and 2:4:2 with pure moments of 2 Nm for flexion-extension and lateral bending and 4 Nm for axial torsion. Ranges of motion were calculated from the flexibility tests.

RESULTS

The 2:4:2 protocol resulted in significantly better agreement with in vivo data than did the 1:1:1 protocol. In flexion-extension, the 2 Nm value was within 17% of the average in vivo value. In axial torsion, the 4 Nm value was within 22% of the in vivo average. In lateral bending, the 2 Nm value was within 15% of the in vivo average.

CONCLUSIONS

To obtain human in vivo-like kinematics using 100 N preload, the 2:4:2 protocol is to be recommended.

Comparison of axial and flexural stresses in lordosis and three buckled configurations of the cervical spine

OBJECTIVE

To calculate and compare combined axial and flexural stresses in lordosis versus buckled configurations of the sagittal cervical curve.

DESIGN

Digitized measurements from lateral cervical radiographs of four different shapes were used to calculate axial loads and bending moments on the vertebral bodies of C2-C7.Background. Osteoarthritis and spinal degeneration are factors in neck and back pain. Calculations of stress in clinically occurring configurations of the sagittal cervical spine are rare.

METHODS

Center of gravity of the head (inferior-posterior sella turcica) and vertebral body margins were digitized on four different lateral cervical radiographs: lordosis, kyphosis, and two "S"-shapes. Polynomials (seventh degree) and stress concentrations on the concave and convex margins were derived for the shape of the sagittal cervical curvatures from C1 to T1. Moments of inertia were determined from digitizing and the use of an elliptical shell model of cross-section. Moment arms from a vertical line through the center of gravity of the head to the atlas and scaled neck extensor moment arms from the literature were used to compute the vertical component of extensor muscle effort. Segmental lever arms were calculated from a vertical line through C1 to each vertebra.

RESULTS

In lordosis, anterior and posterior stresses in the vertebral body are nearly uniform and minimal. In kyphotic areas, combined stresses changed from tension to compression at the anterior vertebral margins and were very large (6-10 times as large in magnitude) compared to lordosis. In kyphotic areas at the posterior vertebral body, the combined stresses changed from compression (in lordosis) to tension.

CONCLUSIONS

The stresses in kyphotic areas are very large and opposite in direction compared to a normal lordosis. This analysis provides the basis for the formation of osteophytes (Wolff's Law) on the anterior margins of vertebrae in kyphotic regions of the sagittal cervical curve. This indicates that any kyphosis is an undesirable configuration in the cervical spine. Relevance. Osteophytes and osteoarthritis are found at areas of altered stress and strain. Axial and flexural stresses at kyphotic areas in the sagittal cervical spine are abnormally high.

Use of surface electromyography to estimate neck muscle activity

This paper reviews the literature concerning the use of surface electromyography (sEMG) for the study of the neck musculature in response to work and workplace design during light work and semi-static tasks. The paper also draws upon basic research and biomechanical modeling in order to provide methodological recommendations for the use of surface electromyography in this region of the body and to identify areas which require further investigation. The paper includes review and discussion of electrode site location, methods of normalization, data reliability, and factors that can affect sEMG signals from this region, including noise, physiologic artifact, stress, visual deficiencies, and pain. General guidance for maximum exertions with the neck musculature, for sEMG normalization or other purposes, is also included.

Load-carrying capacity of the human cervical spine in compression is increased under a follower load

STUDY DESIGN

An experimental approach was used to test human cadaveric cervical spine specimens.

OBJECTIVE

To assess the response of the cervical spine to a compressive follower load applied along a path that approximates the tangent to the curve of the cervical spine.

SUMMARY OF BACKGROUND DATA

The compressive load on the human cervical spine is estimated to range from 120 to 1200 N during activities of daily living. Ex vivo experiments show it buckles at approximately 10 N. Differences between the estimated in vivo loads and the ex vivo load-carrying capacity have not been satisfactorily explained.

METHODS

A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 250 N. The experimental technique applied loads that minimized the internal shear forces and bending moments, loading the specimen in nearly pure compression.

RESULTS

A compressive vertical load applied in the neutral and forward-flexed postures caused large changes in cervical lordosis at small load magnitudes. The specimen collapsed in extension or flexion at a load of less than 40 N. In sharp contrast, the cervical spine supported a load of up to 250 N without damage or instability in both the sagittal and frontal planes when the load path was tangential to the spinal curve. The cervical spine was significantly less flexible under a compressive follower load compared with the hypermobility demonstrated under a compressive vertical load (P < 0.05).

CONCLUSION

The load-carrying capacity of the ligamentous cervical spine sharply increased under a compressive follower load. This experiment explains how a whole cervical spine can be lordotic and yet withstand the large compressive loads estimated in vivo without damage or instability.