Research Review By Dr. Shawn Thistle©

Date Posted:

February 2010

Study Title:

The relation between the application angle of spinal manipulative therapy (SMT) and resultant vertebral accelerations in an in situ porcine model.

Authors:

Kawchuck GN & Perle SM

Author's Affiliations:

University of Alberta – Faculty of Rehabilitation Medicine, Alberta, Canada; University of Bridgeport College of Chiropractic, Bridgeport CT, USA.

Publication Information:

Manual Therapy 2009; 14: 480-483.

Background Information:

Spinal manipulative therapy (SMT) can be beneficial for spinal pain. As we know, SMT involves the application of a high-velocity, low-amplitude force to spinal tissues, with the goals of moving vertebrae, cavitating joints, influencing soft tissue function, modulating neurological input/output, and so on.

Typically, SMT is applied at a specific angle under the assumption that the underlying vertebra moves in that same specific direction. Further, the application angle is most often described as being parallel to the articular space of the zygapophyseal joints so that vertebral displacement can be maximized.

Recently, the long-held belief that SMT can create vertebral acceleration in specific directions, determined by the practitioner, has come under scrutiny. This is in large part because of recent evidence suggesting that only the normal component of applied forces (that is, the portion applied perpendicular to the skin surface) is transmitted to spinal tissues, due to the frictionless interface between posterior spinal tissues and the underlying spinal structures (Bereznick, Ross and McGill 2002 – definitely recommended reading for those who have not yet read it). If this is the case, any force not applied at 90° to the skin surface would not contribute to vertebral acceleration, and hence could be considered wasted.

Since 2002, no studies have been able to support or refute this concept. Therefore, the goal of this study was to further investigate this by utilizing a porcine (pig) model. Of interest was the relation between the angle of SMT application and the acceleration response of the underlying vertebra. Often, the basis of medical techniques comes from research on animal models, and chiropractic is no different.

The authors acknowledge that there are differences between pig and human posterior spinal tissues. However, the intent of this study was not to extrapolate directly to humans, but rather to test the previously published hypothesis and establish directions for future research in this area.

Study Methods & Pertinent Results:

Pigs were utilized in this study because they are inexpensive, readily available, and also have the potential for use as a future in vivo model for studying SMT. The spines were excised and mounted in a neutral position. SMT was applied with standardized levels of increasing force with an Activator at 60°, 90° and 120° to the skin surface.

Resultant vertebral acceleration was measured using an accelerometer that was mounted to the specimen (the x, y, and z axes corresponded to the superior-inferior, medial-lateral and posterior-anterior axes, respectively).

Pertinent Results Include:
  • 6 vertebrae were tested in total (two vertebrae from 3 different animals)
  • Force applications at 60° and 120° resulted in reduced vertebral accelerations in all axes when compared to forces applied at 90°
  • At any angle of SMT application, the forces in the z axis (posterior-anterior) were always greater than those recorded simultaneously in the x or y axes

Clinical Application & Conclusions:

The results of this study support previous literature (1), further strengthening the argument that forces applied to the spine at non-normal angles do not influence vertebral acceleration, and that the interface between the vertebrae and posterior spinal tissues is frictionless. In fact, when force is applied at non-normal angles, the resultant acceleration diminishes in all planes (yet notably the highest values are still maintained in the posterior-anterior axis). These results were further strengthened by the authors’ ability to predict the reduction in acceleration at varying angles using simple trigonometry.

A physicist may raise the question about why acceleration occurs in all axes when only the normal portion of the force is transmitted to the vertebra. An anatomist would remind us that vertebrae do not float freely in space, but rather are governed in their movement by adjacent soft tissues, articular boundaries, and so on.

As chiropractors, we are adept at integrating concepts, and this is no different. We impart the force to the spine, and essentially the vertebra moves where it moves. The point of this research is to remind us that applying the force at a 90° angle maximizes efficiency, and (at least in theory) minimizes unnecessary joint stress on those delivering the manipulation. The next step is to perform similar studies on human cadaveric specimens…stay tuned.

Study Strengths / Weaknesses:

This was a well executed study with clear results. Of course, it is inherently limited because an animal model was utilized, but for this investigation, this model was appropriate and necessary. The authors mention a few points of contention that may be raised when discussing their results:
  • An Activator, despite providing a consistent and adjustable force application, may not be similar to other adjustive techniques commonly employed clinically. Therefore, the findings of this study cannot be globally applied to all forms of SMT.
  • Preloading or compressing the tissue, as is commonly done in practice, may increase tissue friction to a point where non-normal forces may influence vertebral motion. This scenario has been studied (1), and the results indicate that this is not the case.
  • Skin tension, taken prior to or during the manipulation, may affect the frictionless interface. There is no evidence at this point to support this, and some evidence to suggest that skin tension increases further during manipulation (2), suggesting that maximal skin tension is not achieved in pre-load.

Additional References:

  1. Bereznick DE, Ross JK & McGill SM. The frictional properties at the thoracic skin-fascia interface: Implications in spine manipulation. Clinical Biomechanics 2002; 17(4): 297-303.
  2. Herzog W, Kats M, Symons B. The effective forces transmitted by high-speed, low-amplitude thoracic manipulation. Spine 2001; 26(19): 2105-2110.