Research Review By Dr. Michael Haneline©


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Date Posted:

May 2023

Study Title:

Vertebral arteries do not experience tensile force during manual cervical spine manipulation applied to human cadavers


Gorrell L, Sawatsky A, Edwards W & Herzog W

Author's Affiliations:

Integrative Spinal Research Group, Department of Chiropractic Medicine, University Hospital Balgrist and University of Zürich, Switzerland; Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Canada

Publication Information:

Journal of Manual & Manipulative Therapy 2022 Nov 15; 1-9. doi: 10.1080/10669817.2022.2148048

Background Information:

Neck pain affects 30% to 50% of the adult population globally, resulting in a large proportion of musculoskeletal dysfunction worldwide (1). Cervical spine manipulation (CSM) has been shown to be an effective treatment for neck pain (2) and is recommended in several clinical treatment guidelines (3). Neck pain is the second most common reason patients seek chiropractic care and CSM is the treatment modality most used during a chiropractic visit for this condition.

CSM frequently involves extension and rotation of the patient’s head and neck which has led some to suggest that the vertebral artery (VA) becomes stretched during the procedure, resulting in potential tissue damage, including dissection. However, this theory is mainly supported in the literature by case studies which point to a temporal link between CSM and serious adverse events, such as VA dissection and stroke. On the other hand, population-based studies advocate that any temporal link is not likely causal (4). Instead of a causal association, it is likely that this temporal association between CSM and VA dissection is due to patients with early symptoms of cervical arterial dissection (neck pain and headache) seeking care from a chiropractic or medical practitioner and the dissection progressing to neurological involvement whether CSM is provided or not.

Previous cadaveric studies have shown that any changes in VA length during CSM are relatively small, much smaller than what is required to cause failure of the VA. Based on these findings, Symons et al. reported that a typical manipulative thrust was unlikely to mechanically disrupt the VA (5). However, another study reported VA failure following lower percentage of length changes and suggested that the VA may be susceptible to tissue damage with longitudinal elongation like what occurs during CSM (6).

No studies have tried to reproduce the results of the previous studies which quantified failure length changes along the full length of the VA. Therefore, the purpose of this study was to measure and compare VA lengths under four conditions:
  1. in situ, with the head and neck in arbitrary resting positions;
  2. at peak elongation (length change) during spinal manipulation applied bilaterally to all cervical vertebrae;
  3. at the occurrence of first force with elongation (i.e. when slack was removed); and
  4. at mechanical failure.

Pertinent Results:

518 CSM trials and 14 mechanical failure tests were performed. Vertebral artery (VA) length changes during CSM were generally greater in the contralateral artery than ipsilateral for both rotation and lateral flexion manipulations. Ipsilateral VA length changes were typically lower in lateral flexion compared to rotation techniques, but no differences between these two techniques were noted for the contralateral VA.

The highest average peak length changes were 5.1% (range 1.1 to 15.1%), which occurred in the contralateral VA when lateral flexion manipulations were applied to C5 (this is not where most VA dissections occur – they are typically in the upper cervical region). The lowest average peak length changes were 1.1% (range 0.0 to 2.9%) for the ipsilateral VA when lateral flexion manipulations were applied to C1.

During mechanical failure testing, the VA had to be elongated by an average of 33.5% (range 4.6 to 84.6%) before slack was taken up and first force could be measured. Thus, the arteries were never stretched during CSMs, they were only elongated (taking slack out of the arteries versus stretching them, essentially). An average length change of 51.3% (range 16.3 to 105.1%) was required before VA failure occurred at an average force of 3.4 Newtons (range 1.4 to 9.7 N).

Clinical Application & Conclusions:

As measured from the original positions of the cadavers’ head and neck, the vertebral arteries (VAs) were in a state of slack. During cervical extension and rotational CSM, VA length changes were below slack length, and they were never stretched during the intervention. The authors suggested, however, that further testing that employs a defined head/neck reference position for all measurements will be necessary to determine whether CSM is safe from a mechanical perspective, and ultimately VA length changes should be measured on living human subjects.

The findings of this study provide further evidence that it is unlikely that a healthy VA can become injured during CSM. Patients who develop signs/symptoms of VA dissection and/or stroke following CSM remain likely to have had a dissection in progress prior to CSM or had some type of predisposition (i.e. an unhealthy or pathological artery). Remember, neck pain and headache are two common symptoms of cervical artery dissection, which may prompt a patient to seek chiropractic care.

Study Methods:

Seven un-embalmed, post-rigor human cadavers were obtained for this study. The cadavers’ anterior cervical regions were dissected to expose the 14 vertebral arteries (VAs) by removing the soft tissues and the mandibles. The minimum amount of tissue was removed to preserve the contributions of the lateral and posterior soft tissues to the head and neck kinematics. Piezoelectric ultrasound crystals were attached along the VAs bilaterally to measure length changes.

Three experienced chiropractic clinicians performed CSM, with 2 of the clinicians having previously delivered cervical CSM to cadavers. To control for inter-practitioner variability in the delivery of CSM, data were collected from 2 clinicians for each cadaver. Diversified style (high-velocity, low-amplitude) manipulations were applied to each cervical vertebra bilaterally with the force applied to the articular process of the involved spinal level. The cadaver’s head and neck were placed in flexion, ipsilateral lateral flexion, and contralateral rotation prior to thrusting, followed by a quick, low-amplitude thrust in a posterior-anterior or medial and slightly inferior direction. The peak force of the manipulative thrust was recorded using a thin, flexible pressure pad which was placed between the clinician’s contact and the cadaver’s neck.

All arteries were collected using blunt dissection after CSM testing was completed and stored in a saline solution prior to mechanical failure testing. The dissected arteries were elongated to the length of first force (i.e. longitudinal force measured in the VA, not the first force noticed by the clinician during CSM) and then stretched to mechanical failure at 8.33 mm/sec.

Study Strengths / Weaknesses:

This was a well-done study in which the researchers were careful to ensure the accuracy of measurements that were taken and that extraneous factors were limited. However, cadavers were used rather than living subjects; therefore, the findings are not directly applicable to the clinical setting, arguably. Also, implanting piezoelectric ultrasound crystals caused some minimal damage to the VAs, which may have reduced the forces involved in total length change measurements. Implanting ultrasound crystals may also have affected the mechanical failure tests due to a weakened vessel.

The authors listed several other study limitations, as follows:
  • Only passive longitudinal force was measured, while other stresses or strains that are present in vivo were not considered in this study. Likewise, information about circumferential and radial length changes were not considered.
  • Properties of the different layers of the VA wall (intima, media and externa) were not investigated, so it is unknown if the recorded elongations/forces were uniform across all layers. Nevertheless, since there was no longitudinal force applied during CSM, it is unlikely that any longitudinal force would have occurred in any VA layer.
  • Data were collected on human cadaveric donors of advanced age (average 85 years) with pre-existing health conditions. Thus, generalizing the results to the population receiving CSM is limited (particularly the younger population in which VAD has occurred in temporal relation to CSM in the literature).
  • Arteries exhibit viscoelastic properties, becoming stiffer with increasing loading rate, so it is possible that the VA may have failed at higher forces if stretch to failure had been applied at faster speeds. It is also possible that the reported VA length changes may be different from those that occur in the clinical setting.
  • Unlike a clinical scenario, CSM was applied repeatedly to the cadavers which may have weakened the VAs, making them more disposed to failure during testing. However, none of the VAs were ever stretched during CSM, only some of the slack of the VA was taken up, so weakening likely had little if any effect on the results of failure testing.

Additional References:

  1. Martin B, Deyo R, Mirza S, et al. Expenditures and health status among adults with back and neck problems. J Am Med Assoc 2008; 299(6): 656–64.
  2. Bronfort G, Haas M, Evans R, et al. Effectiveness of manual therapies: the UK evidence report. Chiropr Osteopat 2010; 18(1): 3.
  3. Bussieres A, Stewart G, Al-Zoubi F, et al. The treatment of neck pain-associated disorders and whiplash-associated disorders: a clinical practice guideline. J Manipulative Physiol Ther 2016 Oct; 39(8): 523–64.
  4. Cassidy J, Boyle E, Côté P, et al. Risk of vertebrobasilar stroke and chiropractic care: results of a population-based case-control and case-crossover study. Eur Spine J 2008; 17(1): 176–183.
  5. Symons B, Leonard T, Herzog W. Internal forces sustained by the vertebral artery during spinal manipulative therapy. J Manipulative Physiol Ther 2002 Oct; 25 (8): 504–510.
  6. Johnson C, How T, Scraggs M, et al. A biomechanical study of the human vertebral artery with implications for fatal arterial injury. Forensic Sci Int 2000 Apr 10; 109(3): 169–82.

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