Index to Chiropractic Literature
Index to Chiropractic Literature
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ID 18735
  Title The effect of line of drive on vertebral acceleration [platform presentation; the Association of Chiropractic Colleges' Thirteenth Annual Conference, 2006]
Journal J Chiropr Educ. 2006 Spring;20(1):26
Peer Review Yes
Publication Type Meeting Abstract
Abstract/Notes INTRODUCTION: Chiropractors have professed that the line of drive should be matched to the plane of the target vertebra’s facet joints. Although this approach has been taught and employed by chiropractors for decades, we are unaware of any data to support this rationale. This rationale has been challenged by a recent study that measured loads and displacements at the skin. Findings indicated that the skin-fascia interface is frictionless, impling that only normal forces are transmitted to the vertebra below. It was concluded that a manipulative thrust in any direction other than normal to the skin would result in a reduced force transmitted to the vertebra. This hypothesis was not confirmed by measuring the effect of non-normal forces on a vertebra directly. The purpose of this study was to determine the resulting acceleration response of a single vertebra to manipulative forces varying in their line of drive.

METHODS: A porcine thoracic spine was mounted to bookend supports by stripping the pedicles, ribs and all soft tissues from the terminal vertebrae to leave the remaining vertebrae suspended horizontally. A triaxial accelerometer was affixed to the middle vertebrae with the X axis oriented superior/inferior, the Y axis medial/lateral and the Z axis posterior/anterior. An Air Activator™ fitted with an in-series load cell was then used to apply ten thrusts of increasing force to the skin over the spinous process of interest. Sampling rate was 50,000Hz. From these data, a reference plot of input force vs. resulting Z axis acceleration was created. Ten maximum force applications were then applied to the same spinous at 23.5°, 45°, -23.5° and -45° in the X axis. For each different line of drive application, the theoretical Z-axis force was computed by multiplying the cosine of the angle by the recorded force. The experimental Z-Axis force was determined by taking the Z-axis acceleration resulting from each angulated force application and then computing the Z-axis input force from the reference plot.

RESULTS: For the lines of drive of 23.5°, 45°, -23.5° and -45°, the Z-axis and X-axis vertebral accelerations were always less than those obtained from a line of drive of 0°. For lines of drive of 23.5°, 45°, -23.5° and -45°, the mean percentage difference between the theoretical and experimental forces was -9% , -3%, -4% and -6% respectively.

DISCUSSION: Errors of less than 10% between the theoretical and observed forces support the hypothesis under consideration. Additional support comes from the observation that as the angle of applied force increased from the vertical, the acceleration of the vertebra in the Z and X-axes never increased beyond acceleration values obtained at 0°. Therefore, at angles other than 0°, there is no continuation of force along the line of drive when a frictionless system is interposed between the point of force application and the vertebral target.

CONCLUSION: When a frictionless tissue system underlies a point of force application, any change in the line of drive from one that is perpendicular to the skin results in a predictable decrease of force acting to accelerate the vertebra. The force lost in the Z-axis is not recovered in the direction of the line of drive. In other words, when there is soft tissue between the point of contact and the target vertebra, line of drive in a manipulation greater than 0° is irrelevant to vertebral motion and serves only to decrease the force applied by the practitioner.

This abstract is reproduced with the permission of the publisher.

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