Quantifying Strain in the Vertebral Artery With
Simultaneous Motion Analysis of the Head and Neck:
A Preliminary Investigation

This section was compiled by Frank M. Painter, D.C.
Send all comments or additions to:   Frankp@chiro.org

FROM:   Clin Biomech (Bristol, Avon) 2014 (Dec); 29 (10): 1099–1107 ~ FULL TEXT

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Steven L Piper, Samuel J Howarth, John Triano, Walter Herzog

Canadian Memorial Chiropractic College,
Toronto, Ontario
M2H 3J1, Canada.

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      Chiropractic & Manual Therapies 2022

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Background:   Spontaneous vertebral artery dissection has significant mortality and morbidity among young adults. Unfortunately, causal mechanisms remain unclear. The purpose of this study was to quantify mechanical strain in the vertebral artery while simultaneously capturing motion analysis data during passive movements of the head and neck relative to the trunk during spinal manipulation and cardinal planes of motion.

Methods:   Eight piezoelectric crystals (four per vertebral artery) were sutured into the lumen of the left and right vertebral arteries of 3 cadaveric specimens. Strain was then calculated as changes in length between neighboring crystals from a neutral head/neck reference position using ultrasound pulses. Simultaneously, passive motion of the head and neck on the trunk was captured using eight infrared cameras. The instantaneous strain arising in the vertebral artery was correlated with the relative changes in head position.

Findings:   Strain in the contralateral vertebral artery during passive flexion-rotation compared to that of extension-rotation is variable ([df=32]: -0.61
Interpretation:   The results of this study suggest that vertebral artery strains during head movements including spinal manipulation, do not exceed published failure strains. This study provides new evidence that peak strain in the vertebral artery may not occur at the end range of motion, but rather at some intermediate point during the head and neck motion.

Keywords:   Cervical spine; Kinematics; Spinal manipulation; Spontaneous vertebral artery dissection; Strain.

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From the FULL TEXT Article:

Introduction

Spontaneous vertebral artery dissection (sVAD), although low in prevalence rates (2.5–5 per 100,000 population), has significant mortality (23%) and morbidity, such as motor, sensory and cognitive deficits reported as high as 28% among young adults (Hassan et al., 2011). In addition, sVAD accounts for up to 25% of ischemic stroke in young adults b45 years old (Hassan et al., 2011). Given that stroke has an estimated annual cost of $73.1 billion (US) and that ischemic stroke accounts for 87% of all strokes, it follows that the approximate economic burden of sVAD may be up to $15.9 billion (US) (Lloyd-Jones et al., 2010). Unfortunately, causal mechanisms associated with sVAD remain unclear.

Case reports have described sVAD that occurred from movement of the head and neck during activities of daily living such as yoga, sleeping awkwardly, receiving dental work and cervical spinal manipulation (Haldeman et al., 2002; Haneline and Lewkovich, 2005; Kawchuk et al., 2008). These case reports, however, cannot be used to establish mechanisms of injury linking head and neck motion with sVAD. Previous clinical guidelines suggested provocative testing of the vertebral artery, including sustained rotation of the head on the neck, may help to elicit symptoms of sVAD (Magarey et al., 2004), while other studies report using positional testing, such as the extension rotation test lacks validity (Cote et al., 1995). In addition, reports on cervical spinal manipulation as it relates to ischemic stroke are unclear about the type of manipulation delivered (Wynd et al., 2013). There is also variability that exists with application of spinal manipulation techniques between clinicians, and repeatability of consistent parameters, that makes interpretation of forces imparted difficult to assess (Descarreaux et al., 2012).

To date, we know that once a tear has occurred in the intimal lining of the vertebral artery (VA), blood may accumulate within the arterial layers; obstructing blood flow and leading to stroke (Kim and Schulman, 2009). Previous mechanistic studies have focused on hemodynamics and formation of embolisms, yet none of these explain the development of strain in the intimal lining that may lead to initial tearing of the VA (Callaghan et al., 2011). It is important to realize that although trivial, certain motions of the head and neck may create more intimal strain than others.

A previous case–control study addressed the arterial stress placed on the internal carotid artery (ICA) during head movements in subjects who previously experienced spontaneous ICA dissection (Callaghan et al., 2011). The study made use of finite element modeling (FEA). The results suggested that stress placed on the intimal layer of the ICA is comparable in spontaneous ICA dissection patients versus healthy controls (Callaghan et al., 2011). FEA is a valuable tool, but relies on assumptions of constitutive equations, tissue dynamics and local deformations (Duan et al., 2009). FEA is typically employed when physical measurements cannot be determined and, therefore, independent validation of direct observation of the tissue stress and strain is lacking.

Mechanical failure of the ex vivo VA begins with a force of approximately 8.2 N to 8.8 N, and strain between 53% and 62% of its original resting length (Symons et al., 2002). Previous in vitro studies have reported peak VA strains up to 12.2% at the end range of head and neck motion in flexion/extension, lateral bending and axial rotation, with the largest strains being produced in axial rotation (Symons et al., 2002). Rapid neck movements, such as those produced during high-velocity, low-amplitude spinal manipulation (SM), have generated peak strains up to 12.9%, which is within the tolerable limits of VA (Herzog et al., 2012; Wuest et al., 2010). Previous in vitro studies have reported VA strains between 3.3% and 12.2% during motions of the head and neck in flexion/extension, lateral bending and axial rotation with the largest strains being produced in axial rotation (Symons et al., 2002). Rapid neck movements, such as those produced during high velocity low-amplitude spinal manipulation (SM), generated strains ranging between 0.9% and 3.8%, which were considerably lower than strains observed during cardinal planes of motion (Herzog et al., 2012; Wuest et al., 2010). Measurements from previous work suggest that strain does appear to fluctuate based on head and neck movements, such as head rotation, although a degree of motion has yet to be determined (Wuest et al., 2010).

The complex anatomy and coupled motions of the cervical spine complicate interpretation of previous results on VA strain (Herzog et al., 2012). The VA branches off the subclavian artery (V1 segment), before traveling vertically through the transverse foramen of the cervical vertebrae from C6 to C2 (V2 segment) (Netter, 2006). As the VA exits through C1 transverse foramen, it makes a radical shift in its path (V3 segment). The V3 segment of the VA shifts from a vertical orientation into a horizontal as it travels posteriorly, before entering the occiput (Netter, 2006). The V3 segment of the VA is the most common reported site of an ischemic event and it also is noted for having the greatest amount of motion (Haldeman et al., 2002). One may hypothesize that due to the increased motion occurring in the V3 segment, there are large strains in the VA that may explain dissection of the artery at the V3 location. Ultrasonography has been used to estimate strain in the V3 segment during motions of the head. However, these strains have not been linked to either the segmental or global kinematics of the head and neck (Callaghan et al., 2011). Global kinematics of the head and neck are quantitatively reliable (Lansade et al., 2009) and are assumed to be bound biomechanically to segmental coupled motions to create arterial strain. Vertebral joint geometry and intersegmental tissue properties govern segmental motion coupling that sums to a global movement of the neck, with respect to the thorax.

The purpose of this study was to quantify the segmental mechanical strain in the VA, while simultaneously capturing kinematic data during passive global motion of the head and neck. In particular, flexion-rotation and extension-rotation, on the same side in cadaveric specimens, were quantified. Secondary aims include quantitatively describing the variability in strain between multiple cadaveric specimen segment levels during passive range of motion (RoM) tests, in addition to observing strain and global motion of the head and neck during SM with varying clinicians. Our null hypothesis was that the relative strain (elongation from neutral) in the VA at the V3 segment during maximum flexion-rotation passive RoM, was not associated with the strain in maximum extension-rotation passive RoM.

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Methods

This study was approved by both the Canadian Memorial Chiropractic College Research Ethics Board and the University of Calgary Conjoint Health Research Ethics Board.

      Study design

This study used a cross-sectional sample to observe a potential relationship between the measured VA strain and passive global motion of the head and neck relative to the torso during extreme combined displacements in the cardinal planes.

      Sample specification

A convenience sample of three fresh, unembalmed, post-rigor cadavers was used. A priori sample size estimate indicated that six vertebral arteries (three cadaveric specimens) were required to achieve sufficient statistical power (see Justification of sample size and data analysis). Any excessive superficial plaque formation observed on the outside layer of the VA as a result of calcification of the VA was also noted, however not excluded. To ensure that relatively normal ranges of motions were observed, cadavers were excluded if, upon visual inspection after dissection, they were noted to have obvious osteophytes or other indications of bilateral osteoarthritis which could impede motion of the cervical spine.

The experienced anatomist and experimenter came to agreement, on visual inspection, that ranges of motion would not be impeded. A RoM pre-screen was performed by an examiner, using a goniometer placed on the head of the cadaver. Side to side asymmetry, with a tolerance of 5° of variance, was allowed. If the cadaver did not achieve previously listed values, it was excluded from testing. Kinematic data obtained from the goniometer allowed for exclusion on the basis of previous RoM guidelines, determined in healthy living subjects (Lansade et al., 2009). Other exclusion criteria included visually observed damage to the VA at any level.

      Description of experimental maneuvers

Figure 1

Following the RoM pre-screen evaluation, an experienced (10 years) anatomist exposed the anterolateral aspect of the VA, between each vertebral body segment, bilaterally using blunt dissection. A small incision was made above C1 at the V3 segment and also at each VA segment between the cervical vertebrae from C2 to C4 (V2). Piezoelectric ultrasonic crystals with a diameter of 0.5 mm (Sonometrics Corp., London, Ontario, Canada), were then sutured into the vertebral artery wall at each level (Figure 1). A total of 4 crystals were inserted into each vertebral artery. Range of motion and SMT trials were performed in random order by a licensed chiropractor with one year of experience, and educated in RoM testing. Additional SM trials were conducted by three additional other licensed chiropractors, with varying years of experience (1–5 years). During each data collection trial, the head was moved in the chosen direction before being returned to the neutral position.

Figure 2

All RoMdatawere collected using a data acquisition system (Motion Analysis Corp., Santa Rosa, California, US) using eight infrared cameras, recording at 200 Hz, to monitor the positions of spherical 1.0 cm retro reflective markers. Three markers were affixed to a rigid plate over a centered anatomical landmark of the head (Figure 2, marker I). The Frankfort plane was used to construct the head's local coordinate system. The Y-axis, in the coronal plane (positive to the left) was calculated using right and left tragions, in the notch just above the tragus. A vector connecting the lateral canthus of the eye, perpendicular to the Y-axis, determined the X-axis in the sagittal plane (forward is positive) (Fig. 2, marker II).

The Z-axis was then positive superiorly. The position for the head center of mass (COM) was approximately 15 mm forward along the X-axis of the Frankfort plane (COM) (Fig. 2, X). The COM of the head was designated as the location for the coordinate system in relation to the rigid body marker set. The trunk of the specimen was anchored to the examining table using a strap. Markers for the trunk were mounted on a second plate, fixed to the sternum, and a second local coordinate system was constructed parallel to the head system, to define a neutral relative starting position. Prior to the examiner moving the head and neck, the VA was perfused with ultrasonographic gel to amplify the soundwave and to approximate the fluid filled state of the VA.

Figure 3

During all RoM trials, passive head movements were controlled by manual contacts on the base of the occiput and zygomatic arches, avoiding contact with the exposed vertebral artery regions in the neck (Figure 3). Three RoM trials were performed for each cardinal plane and combined movements of either flexion-rotation or extension-rotation were performed. The order of movement direction was randomly determined prior to testing. After each group of three RoM trials, the head was returned to neutral for a period of 2 min, to limit the effects of stress relaxation in subsequent trials.

During all SM trials, similar to the RoM trials, each clinician was instructed to use the base of the occiput as a contact point and complete a diversified technique, upper rotary cervical chiropractic adjustment (Fig. 3) (Bergmann and Peterson, 2010). The clinicians were made aware of the placement of the ultrasonic crystals in vertebral segments so as not to attempt contact directly on the vertebral segment. Three SM trials were performed by each clinician, in each direction (right rotary upper adjustment versus left). After each SM trial, the head was returned to neutral for a period of 2 min, to limit the effect of stress relaxation in subsequent trials.

      Outcome measures

Ultrasonography

Strain measurements were quantified using sonomicrometry sampled at 200.8 Hz. The piezoelectric crystals oscillate at high frequencies, to produce an ultrasonic pulse that travels at the relative speed of 1440 m/s. The distance between crystals at any instant during head motion, was quantified by multiplying the speed of the ultrasonic pulse by the time for the wave to travel between a pair of crystals. This method has been previously demonstrated to have a spatial resolution of 16 µm (Symons et al., 2002). The experimental procedure for previous spatial resolution was repeated for this study in an aqueous gel medium with similar outcomes. The distance between pairs of crystals, with the head in the neutral position represented the initial length and was used as a reference for calculating VA strain ([L–L0] / L0) during the subsequent RoM and SM trials.

Kinematic analysis

The average peak displacement angles achieved during RoM and SM trials were collected at 200 Hz, using a joint coordinate system. A joint coordinate system was used to calculate the relative 3D angular movements (flexion–extension, lateral bend and axial rotation) between the head and the trunk (Kintrak, University of Calgary Human Performance Laboratory, Calgary, AB, Canada). The Euclidean norms of the component angular kinematic data were then used to calculate the overall angular displacement between the head and trunk.

Maximum peak strain and the strain at maximum angular displacement, were measured during all RoM testing (cardinal planes of movement and combined movements) and during the diversified rotary SM trials. Displacements on opposite sides of the cardinal planes were considered independent from each other (flexion and extension). In combined motions involving more than one plane, descriptions were classified as being ipsilateral or contralateral with respect to the direction of head movement. For example, during rotation to the left, the left VA was considered to be ipsilateral and the right VA was considered to be contralateral. The average peak angles of displacement were then synchronized with strain in the VA.

Data capture for kinematics and ultrasound were initiated by assistants who triggered collection based on a verbal countdown from the examiner. Recording was stopped upon return of the head to the neutral start position and cueing from the examiner. Since the ultrasonography was collecting at 200.8 Hz and the kinematic analysis at 200 Hz, a post process digital synchronization was performed to line up signals in time.

Data filtering

The kinematic data were not filtered. The ultrasonographic crystal data were filtered with a 2nd order dual pass Butterworth filter with a cutoff frequency of 1 Hz. The cutoff frequency was derived from residual analyses performed on the displacements derived from the crystals.

      Justification of sample size and data analysis

Descriptive statistics (mean and standard deviation) were used to characterize the peak strain, and the strain at maximum angular displacement was measured during all RoM testing (cardinal planes of movement and combined movements) as well as during the diversified rotary SM trials. A Pearson's product correlation coefficient quantified any association between the strains on the VA during combined motion of flexion-rotation versus extension-rotation. The sample size estimate was based on Cohen's “r” for a proposed correlation coefficient of 0.9 with a Type I error of 0.05 and a Type II error of 0.2. A correlation as high as 0.9 as the boundary for clinical relevance, accounting for 81% of variability, was chosen since the potential impact of small relational changes in motion to sVAD leading to stroke is unknown. A total of 3 cadavers involving data from both arteries were required (Cohen, 1977).

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Results

Table 1 Table 2 Table 3 Figure 4 Table 4 Table 5 Figure 5 Figure 6 Figure 7

The demographic information obtained for each specimen, including gender, height, weight, cause of death and pre-existing medical conditions, suggested that all causes of death and pre-existing medical conditions were unrelated to sVAD (Table 1). Only Specimen 3 demonstrated notable anatomic features of the vertebral bodies and arteries on inspection of the spine after dissection. Significant osteophytes on the vertebral bodies were observed that did not appear to encroach on the vertebral artery itself. The VA appeared to bifurcate above the subclavian artery with one branch traversing through the left transverse process of C6 and a second branch that rejoined the V2 segment at the C5–C6 levels. Furthermore, anomalies were observed in the V3 segment of the VA in Specimen 3. The artery, with its transition from vertical to horizontal orientation typically noted above C1 was instead, observed inferior to the C1 transverse processes bilaterally. None of the observed anomalies in Specimen 3 appeared to affect the pre-screen RoM. In addition post-testing observation of kinematic data and strain data collected was comparable to previous specimen.

      Range of motion trials

The mean angular displacements in the cardinal planes of motion, achieved in all three subjects, were comparable to previous results in healthy age-matched populations. (Lansade et al., 2009) The strains observed in all trials were comparable to previous results measuring strain in the VA (Herzog et al., 2012). The correlation between strain developed in the contralateral VA during flexion-rotation and extension rotation was variable and did not achieve a positive correlation greater than 0.55 (Pearson's r ranges [df = 32]: –0.61 to 0.55) (Table 2). The VA strain at maximum angular displacement of the head and neck, relative to the trunk, varied remarkably across the artery's segments and between different movements (Table 2). Axial rotation of the head on the trunk produced the largest amount of strain in the contralateral VA (Table 3). In general, the maximum VA strain, that occurred during either combined or cardinal planar movements, did not coincide with the maximum angular displacement (Figure 4).

      Spinal manipulation trials

VA strain that occurred at the maximum angular displacement during SM varied across VA segments and between clinicians (Table 4). The maximal amount of strain values produced in the VA during diversified rotary adjustments, from various clinicians, did not exceed the maximum strain achieved during the RoM trials (Tables 2, 4 and 5). The maximum angular displacement achieved during manipulation also varied between the four clinicians (Table 5). Interestingly, the two clinicians who produced larger angular displacements during manipulation also produced larger strains in the contralateral VA at the maximum angular displacement (Table 5). The clinician with the largest observed angular displacement had similar patterns of displacement in flexion and lateral bend compared to other clinicians in the study, however the axial rotation was much greater (Table 5, Figure 5). Peak strain on the contralateral VA also appeared higher in the upper segments of the neck during spinal manipulation done by clinicians 1 and 3 (Table 5). Similar to the RoM trials, the maximum VA strain that occurred during SM did not coincide with maximum angular head displacement (Figure 6). Although the VA strain patterns shown in Fig. 6 appear to precede the maximum angular displacement, a statistical trend was not observed.

Overall, the maximum strain value in the VA achieved during any trial of this study, either SM and/or RoM trials, did not exceed previous values with respect to global failure limits of the VA reported by Symons et al. (Symons et al., 2002) (Figure 7).

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Discussion

Lansade et al. reported a three-dimensional (3D) kinematic analysis on a large sample of living asymptomatic subjects to compare the impact of age and gender of subjects on active neck movements (Lansade et al., 2009). The results suggested that aging is associated with a decrease in cervical active RoM in the cardinal planes, but does not affect coupled motions, and gender does not appear to affect cervical RoM (Lansade et al., 2009). All specimens in our study were female and, therefore, no observations on gender specific differences with respect to motion and strain could be made. Additionally, it was observed that specimens in our study had similar neck and head RoM to healthy age-matched populations, although overall range of motion was lower than that observed in the majority of the population measured by Lansade et al. (2009). Additionally, it was observed that specimens in our study had neck and head RoM similar to the healthy age-matched populations; although the results should be interpreted with caution, as the Lansade et al. (2009) study measured active range of motion and it should be assumed that passive motion in these healthy subjects should be greater than the subjects used in this study. It should also be noted that overall range of motion was lower than the majority of the population measured by Lansade et al. (2009). Dvorak et al. (1992) have previously measured cervical passive range of motion in a large sample of healthy subjects. The results of this study are comparable to the Dvorak et al. (1992) study of females over the age of 60 years except flexion/extension measures. As previous literature suggests, range of motion decreases as age increases. Since the values of the current study are similar to the previous results, one could assume strain values may also be similar to some extent, although future detailed studies of segmental motion are required.

VA strain was not linearly related to passive global motion of the head and neck.

There appears to be a relationship between strain and motion, specifically rotational motion, although segmental movements (not measured) of the vertebral bodies may be better correlated to VA strain than global head and neck movements. Rotational motions produced the greatest VA strains. Considering the contralateral VA during rotation, one would expect an increase in strain while the ipsilateral VA would decrease in strain. This was not a consistent finding with this study although mean values did appear to be slightly higher overall in the contralateral VA when compared to the ipsilateral VA, during opposing direction of movement. It should also be noted that translational motion (long axis distraction of the head and neck on the torso) was not measured in this study and, therefore, it is difficult to determine how much slack in the VA is taken up during rotation.

An important clinical finding of this study is that all peak VA strains during SM and RoM testing were considerably smaller than the smallest VA failure strain. (Symons et al., 2002) Previous studies reported similar results of VA strains during RoM and SM testing, but here, we add the important component of head and neck motion during these testing procedures. To date, this is the first study attempting to quantify an amount of angular displacement of the head and neck with respect to observed strain in the VA. Although our results are preliminary, future work may provide important quantitative relationships between head and neck motion, corresponding to segmental VA strains. Also, of clinical relevance is that maximum VA strains precede maximum angular displacements during passive global motion of the head and neck and SM (Fig. 6). The greatest VA strains occurred at different time points prior to maximum angular displacement (Figs. 4 and 6, Tables 2–5). The reliability of these results is preliminary in nature and will need to be interpreted with caution until future studies can reproduce similar results. The authors speculate that tissue viscoelasticity and segmental motion may play an important role in determining at what point the slack of the VA is taken up.

A previous report suggests that arterial tissue has non-linear viscoelastic behavior when stressed in a longitudinal direction (Garcia et al., 2012). Theoretically then, the stress relaxation behavior of the VA may also not behave in a linear fashion. A narrative review on segmental cervical spine motion, provided by Bogduk and Mercer (2000), suggests that vertebrae may move independently of each other. Furthermore, a cervical segment may achieve its own end range of motion and proceed slightly into an opposite range, prior to the global neck reaching full end range of motion. Theoretically then, and based on the findings from the current study, a vertebral segment may reach end range, and therefore have the greatest strain on the VA, prior to reaching global end range of motion.

During SM, the rotational component of head and neck motion contributes the greatest amount to the VA strain. It is worth noting that clinicians 1 and 3, with the least number of years of experience, used the largest amount of rotation to accomplish the SM's. In addition, although all clinicians in our study reported using a diversified technique for the upper rotary cervical adjustment, clinicians 1 and 3 graduated from the same college in the same year and, therefore, may have been taught by the same instructors. The clinicians were not instructed to contact a reported segment of the cervical spine nor were they told to take the head to preload range typically performed in clinical practice.

Therefore, a rotary thrust greater than what is typically seen in clinic may have been achieved. This would suggest that there may be less rotation during adjustments in clinical practice. Future studies will need to address the degree of angular displacements achieved by more experienced clinicians.

Previous work by Symons et al. (2012) suggests, regardless of clinician experience or training, that forces delivered during SM on cadavers are greater than forces exerted on live human subjects. The authors speculate that based on the Symons et al. (2012) results, angular displacements may also be greater during SM in cadaveric studies when compared to live human subjects.

      There are several limitations of the current study.

A convenience sample of 3 female cadaverswas used for this study. Therefore, a limitation of gender bias in global motion of the head and neck may exist.

Previous studies suggest that there may be gender differences in cervical range of motion (Dvorak et al., 1992); however, it should be noted that gender differences in the Dvorak et al. (1992) study were not denoted in ages greater than 80 years of age. Lansade et al. (2009) also suggested that women may have greater mobility than men, in primary movements of the cervical spine, although statistical significance was only achieved for females compared to males aged 70–79 years.

Based on these previous studies, the 3 females used in this study may have had greater motion compared to age matched males and a younger sample set. This might also suggest that the strain on the vertebral artery in our subjects may have increased variability, as their motion may have been greater.

A second limitation is the calcification of arterial tissue in the VA. The authors speculate that calcification of the intimal layer of VA would cause stiffening and therefore decrease the amount of strain observed. Although a superficial observation of the VA was completed post-dissection and none of our subjects were excluded due to excessive plaque formation, a more detailed observation of the intimal lining was not completed. A previous study suggests calcifications of the VA are common in older adults who have had ischemic stroke (96.3%) however the authors could not conclude whether the plaque formation and subsequent calcification caused the stroke (Pikija et al., 2013).

A third limitation to the study is the reproducibility of motion patterns in the RoM and SM trials. The results of this study were highly variable. Since the movement of the head and neck does not occur in a linear fashion when repeated, perhaps this will effect small undetectable changes in the VA segment. The use of a single technique of upper rotary cervical chiropractic adjustments with an occiput contact applied by four chiropractors of similar skill and experience may also limit generalizability to the wide variety of chiropractic adjustment techniques used in general practice.

A fourth limitation is that maximal VA failure strain is likely greater than strains where micro-tearing of the intimal layer may occur. To date, only histological studies can show micro-tearing of the VA, thus assessing micro-damage is beyond the scope of current in vivo technology.

The fifth limitation is the dependence of samples, which also limits our conclusions, as both VA's from the same cadaver were assessed. The anatomy, biomechanics and segmental movements in one VA may not be independent of the opposite VA in the same specimen and future studies will need to assess segmental movements.

The sixth limitation is that although blunt dissection and movement during testing may decrease tissue integrity, any compromise of tissue may result in an under-estimation of how much local strain a typical VA can handle.

Finally, the measurement of strain used in this study differs slightly from the typical engineering strain and is worth mentioning. The strain measurements in this study calculated change in distance from an initial resting position in which the VA V3 segment is slack. Engineering strain would suggest that measurement begins when the VA begins to deform and takes up a measureable amount of stress (visually observed at typically 12% strain) (Chaffin and Andersson, 1984; Symons et al., 2002). Currently, there is no feasible method to directly measure segmental motion of the cervical vertebrae combined with engineering strain in the VA V3 segment.

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Conclusions

The results of this study suggest that the strains on the VA during global head and neck movements, including SM, do not exceed published failure strains.

Furthermore, this study supports previous evidence that the greatest amount of strain in the VA occurs during rotation (Herzog et al., 2012).

This study provides new evidence that peak strain in the VA may not actually occur at the end range of motion, but rather at some point when the end range has not been reached yet.

This finding leads to the conclusion that sVAD, due to global head and neck movements, is not predictable unless achieving large amounts of strain causing global failure, which was never observed in the current study.

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Acknowledgements

The authors would like to thank Dr. Conrad Tang (chiropractor), Dr. Evan Durnin (chiropractor), Dr. Tim Leonard (technical assistance), Ms. GlendaMcNeil (technical assistance), Mr. Calvin Cockerline (anatomist) and Mr. Hoa Nguyen (technical assistance) for their time and expertise. This study was funded by the Canadian Chiropractic Research Foundation and the Alberta College and Association of Chiropractors (RT691362). This study was awarded with the Ontario Chiropractic Association/CanadianMemorial Chiropractic College Student Research Award, 2012.

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