Vertebral artery strains during high-speed, low amplitude cervical spinal manipulation

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Abstract

Spinal manipulative therapy (SMT) has been recognized as an effective treatment modality for many back, neck and musculoskeletal problems. One of the major issues of the use of SMT is its safety, especially with regards to neck manipulation and the risk of stroke. The vast majority of these accidents involve the vertebro-basilar system, specifically the vertebral artery (VA) between C2/C1. However, the mechanics of this region of the VA during SMT are unexplored. Here, we present first ever data on the mechanics of this region during cervical SMT performed by clinicians. VA strains obtained during SMT are significantly smaller than those obtained during diagnostic and range of motion testing, and are much smaller than failure strains. We conclude from this work that cervical SMT performed by trained clinicians does not appear to place undue strain on VA, and thus does not seem to be a factor in vertebro-basilar injuries.

Introduction

Spinal manipulative therapy (SMT) has been recognized as an effective treatment modality for many back, neck and musculoskeletal problems (Haldeman, 1986, Herzog, 2000a). It has received wide-spread acceptance in a variety of disciplines including chiropractic, physiotherapy, nursing, and mainstream health care. However, despite its surge in popularity as a cost-effective treatment modality, there is little basic research as to the beneficial mechanisms underlying SMT (Herzog, 2010).

Spinal manipulative therapy typically consists of a high-speed, low-amplitude thrust delivered by a practitioner to a specific landmark on a patient’s body. We and others, have quantified the force–time histories of SMTs in a variety of settings and for a variety of clinical problems (Conway et al., 1993, Downie et al., 2010, Forand et al., 2004, Herzog, 2000a, Hessel et al., 1990, Triano, 2000). Several results have emerged from these studies: (i) the forces applied by a given clinician are fairly consistent, but they vary dramatically across clinicians and location of application with peak forces ranging from approximately 200 N (44 lbs) to values of up to 1400 N (308 lbs); (ii) the speed of the treatment thrust is consistent (within about 100–200 ms for experienced clinicians) (Herzog, 2000a); and (iii) the forces applied by male and female clinicians are similar (Forand et al., 2004), as are the forces applied to patients or non-patients subjects in a laboratory setting (Symons et al., in Press), but the forces applied on cadaveric specimens (which have been used to study the potential injurious effects of SMTs) are significantly greater and are applied significantly faster than the forces applied to patients and non-patient control subjects (Symons et al., in Press).

Forces applied during SMT have been shown to elicit a variety of mechanical, biological and physiological responses including relative movements of the intervertebral joints (Gál et al., 1997a, Gál et al., 1997b), muscle reflex responses (Dishman and Bulbulian, 2000, Dishman and Burke, 2003, Floman et al., 1997, Gibbons et al., 2000, Herzog, 2000b, Herzog, 2010, Herzog et al., 1999, Lehman et al., 2001, Suter et al., 2005, Suter et al., 2009), changes to the blood biochemistry (Brennan et al., 1991, Triano et al., 1991), and cavitation of joints (Cascioli et al., 2003, Conway et al., 1993).

Although the total forces during SMTs can be very high, these forces are typically distributed across a large contact area which increases with increasing forces because of the soft tissue contacts between patient and clinician (Herzog et al., 2001), thereby reducing the local forces to 5–10 N (1–2 lbs) for typical thoracic manipulations. Nevertheless, there has been increased interest over the past three decades to elucidate the possible damaging effects of SMT forces on internal tissues (Cote et al., 1994, Haldeman et al., 2001, Haldeman and Rubinstein, 1992a, Haldeman and Rubinstein, 1992b, Ianuzzi and Khalsa, 2005, Paciaroni and Bogousslavsky, 2009, Powell et al., 1993, Rubinstein, 2008, Rubinstein and Haldeman, 2001, Terrett and Kleynhans, 1980). One of the major issues of the use of SMT is its safety, especially with regards to neck manipulation and the risk of stroke (Lee et al., 1995, Paciaroni and Bogousslavsky, 2009, Rubinstein, 2008, Wuest et al., 2010). Although the estimates of stroke associated (but not necessarily caused) by SMT is small – about one in a million (Hurwitz et al., 1996) – the severity and irreversible nature of such accidents makes this a material risk (Herzog and Symons, 2002, Symons et al., 2002).

The vast majority of these accidents involves the vertebro-basilar system, specifically the vertebral artery (VA) between C2/C1 and the cephalad/ distal loop as the VA exits the C1 foramen transversarium and travels to the foramen magnum (Haldeman et al., 1999). Because of the specific anatomy of the VA in that region, it has been assumed that the VA experiences considerable stretch during extension and rotation of the neck, which may lead to hemodynamic occlusions and damage to the VA, predisposing the patient to stroke (Herzog and Symons, 2002, Symons et al., 2002). However, recent evidence suggests that such damage appears unlikely in the distal extra-cranial loop of the VA (between C1 and the foramen magnum) and the proximal/caudal loop between C6 and VA’s origin from the subclavian artery (Austin et al., 2010, Herzog and Symons, 2002, Symons et al., 2002), but the regions between C1 and C6 remain unexplored, except for some preliminary data (Wuest et al., 2010).

Here, we review the results of existing studies on human VA strains during high-speed, low-amplitude SMTs administered by qualified clinicians and compare them to the strains encountered during full range of motion (ROM) tests, and furthermore, add the summarized results of unpublished works from strains measured from all sections of 8 VAs using data from 3 clinicians, resulting in a total of 3034 segment strains obtained during SMTs and 2380 segment strains obtained during full ROM testing.

Section snippets

Subjects

Tests were performed on a total of 12 human cadavers. Two embalmed cadavers were initially used for pilot testing and evaluation of all measurements (not included in the results), and 10 fresh, unembalmed human cadavers were used for measurements of strains in 16 VAs. Five of these specimens and 6 VAs were used for measurements of the extra-cranial loop of the VA (from C1 to the foramen magnum and the proximal/caudal loop (from C6 to the subclavian artery) (Herzog and Symons, 2002, Symons et

Results

Mean strains (and minimal and maximal strains) for the V1, and V4 segments for the ROM testing in our initial study (Symons et al., 2002) were 3.2% (2.0–4.9%), and 5.9% (1.2–12.5%), respectively, while the corresponding values for the SMT testing were 6.2% (4.5–8.0%) and 2.1% (1.4–2.7%) (Table 2). Note that the ipsilateral values for the SMT testing were not used in this analysis as they had been shown to be affected by direct contact of the clinician with the sonomicrometry markers, and were

Discussion

Vertebro basilar accidents associated with high-speed, low-amplitude spinal manipulative treatments have been a concern for clinicians and patients alike. The primary argument for such accidents has been that SMTs stretch vertebral artery segments to such a degree that there is an occlusion of blood flow, and possible stretch-induced damage to the VA. Since the thrust phase of neck SMTs lasts 100–150 ms (Herzog and Symons, 2001, Triano, 2000), any occlusion would likely have little to no effect

Conclusion

The results from this study demonstrate that average and maximal VA strains during high-speed low-amplitude cervical spinal manipulation are substantially less than the strains that can be achieved during ROM testing for all vertebral artery segments. Furthermore, VA strains obtained during SMT and ROM testing are substantially smaller than average failure strains. Therefore, we conclude that cervical spinal manipulations, as tested here, are safe from a mechanical point of view for normal,

Acknowledgements

The Canadian Chiropractic Protective Agency, the Canadian Chiropractic Research Foundation, and the Alberta College and Association of Chiropractors.

Walter Herzog received his BSc in Physical Education from the Federal Technical Institute in Zurich (1978), and his MSc/Ph.D. from the University of Iowa in Bio- mechanics (1985). He then pursued post-doctoral training in Neuroscience at the University of Calgary where presently he is a full professor in Kinesiology, Engineering, Medicine and Veterinary Medicine. He holds the Killam Memorial Chair and the Canada Research Chair in Cellular and Molecular Biomechanics. His research interests are

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    Walter Herzog received his BSc in Physical Education from the Federal Technical Institute in Zurich (1978), and his MSc/Ph.D. from the University of Iowa in Bio- mechanics (1985). He then pursued post-doctoral training in Neuroscience at the University of Calgary where presently he is a full professor in Kinesiology, Engineering, Medicine and Veterinary Medicine. He holds the Killam Memorial Chair and the Canada Research Chair in Cellular and Molecular Biomechanics. His research interests are in mechanisms of muscle contraction, joint injuries and diseases, and applied clinical and sport biomechanics.

    Tim Leonard received his B.Sc. in Biological Sciences in 1982 from the University of Calgary. He received his Ph.D. in Kinesiology in 2010 from the University of Calgary. He is currently a research associate with Dr. Walter Herzog in the Human Performance Laboratory in the Faculty of Kinesiology at the University of Calgary. He has several research interests including the biomechanics of spinal manipulative therapy, sub-cellular muscle mechanics and the adaptive responses of skeletal muscle to exercise.

    Bruce Symons is a research associate at the Human Performance Laboratory in the Faculty of Kinesiology at the University of Calgary and a chiropractor in private practice. He received his Bachelors in Zoology from the University of British Columbia in 1986, a Diploma in Bioengineering Technology from the Southern Alberta Institute of Technology in 1988, a Masters in Medical Science from the University of Calgary in 1992, and his Doctor of Chiropractic from Canadian Memorial Chiropractic College in 1997. His current research interests include the forces evolved during spinal manipulation and their biomechanical effects on anatomic structures such as the vertebral artery.

    Dr. Conrad Tang received a BSc. in health and exercise physiology from the University of Calgary in 2003 and a Doctor of Chiropractic degree from the Canadian Memorial Chiropractic College in 2009. He is currently completing a Sport Sciences residency and is a Master’s of Science candidate in biomechanics under the supervision of Dr. Walter Herzog at the University of Calgary. His areas of interest in research range from manual therapies for sport related injuries to corrective rehabilitative exercises.

    Sarah Wuest received a BA in psychology from the University of Alberta in 1995 and a Doctor of Chiropractic degree from the Canadian Memorial Chiropractic College in 2006. She is currently completing a Sport Sciences residency and practicing in a collaborative medical clinic in Calgary AB. Her areas of interest in research range from spinal manipulative therapy and vertebro basilar stroke to sport-related biomechanics and injury.

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