The effect of applied spinal manipulation force vector on resultant vertebral accelerations: an in vivo animal model

¹Department of Kinesiology, Arizona State University, Tempe, Ariz.; ²Bone and Joint Research Center, Department of Orthopaedic Surgery, University of North Texas Health Science Center, Fort Worth, Texas; Department of Orthopaedic Surgery, John Peter Smith Hospital, Fort Worth, Texas; ³Department of Orthopaedic Surgery, Eeuwfeestkliniek Hospital, Antwerpen, Belgium; ⁴The Adelaide Centre for Spinal Research, Institute of Medical and Veterinary Science, Adelaide, South Australia

Abstract

Introduction: In the performance of spinal manipulative therapy (SMT) and/or chiropractic adjustment it is theorized that the application of force should be in a specific angle to maximize efficiency of the therapeutic intervention. Recent research in an in situ porcine model found that SMT force vectors that were perpendicular to the spine maximized vertebral accelerations as opposed to directing forces vectored at 30 degrees cephalad or caudal.

The purpose of the current study was to determine the effect of varying SMT applied angle on vertebral accelerations in an in vivo ovine model.

Methods:  Twelve healthy, young Merino sheep (mean 24 kilograms, s/d/ 3 kilograms) were examined using a protocol approved by the Institutional Animal Ethics Committee of the Institute for Medical and Veterinary Science in Adelaide, Australia. Prior to testing, anesthesia was induced with an intravenous injection of 1gram thiopentone and was maintained after endotracheal intubation by 2.5 percent halothane.

Throughout testing the animals were ventilated and the respiration rate was linked to the tidal volume keeping the monitored C02 between 40 to 60 mmHg.

To quantify the dynamic, vibration response of the spine 10-g piezoelectric, tri-axial accelerometers were attached to intraosseous pins rigidly fixed to the L2 and L4 spinous processes under fluoroscopic guidance (Figure 1). The X-, Y- and Z-axes of the accelerometers were oriented with respect to the medial-lateral (ML), dorsoventral (DV) and cranial-caudal or axial (AX) axes of the vertebrae. Only AX acceleration responses are reported herein.

Figure 1. High frequency tri-axial accelerometers were rigidly attached to stainless steel pins inserted into the L2 and L4 spinous processes to quantify the vertebral accelerations during the chiropractic thrusts.

SMT was applied to the L3 spinous process using a hand-held Impulse Adjusting Instrument equipped with an impedance head (load cell and accelerometer). In each trial, thirteen SMTs (~200 N peak force) were applied over 2.5 seconds (~6 Hz) at input force vectors of 90 degrees (z-axis), 60 degrees (cephalad), and 120 degrees (caudal) in a randomly determined order (Figure 2). A protractor held over the right sided transverse processes was used to determine the input SMT force vectors.

Figure 2.  Three input force vectors were examined (A) 60 degrees (cephalad), (B) 90 degrees (posteroanterior), and (C) 120 degrees (caudal) to investigate the effect of chiropractic line of drive on spinal motion responses.

Three trials were conducted for each applied force vector in each animal using repeated measures design. During the SMTs, vertebral accelerations at L2 and L4 were recorded at a sampling frequency of 5000 Hz using a 16-bit data acquisition system. Peak-peak acceleration responses at L2, L4, and L2-L4 were computed for each trial. A one-way analysis of variance (ANOVA) compared the mean acceleration responses between the three applied force vectors. Statistical significance was p.05.

Figure 3.  L3 vertebral accelerations (m/s2) for 90 degree PA, 60 degree cephalad, and 120 degree caudal vectored SMTs (*p.01).

A significant increase in mean L2, L4, and L2-L4 vertebral accelerations were also observed for all SMTs directed at 120 degrees (p.01). For SMTs directed at a cephalad force vector (60 degrees), mean L2 vertebral accelerations were significantly greater than L4 vertebral accelerations (p.01), and conversely, for caudally directed thrusts (120 degrees) L4 vertebral accelerations were significantly greater than accelerations observed at L2 (p.01). SMTs applied perpendicularly to the spine at 90 degrees caused larger vertebral accelerations at L2 compared to SMTs directed cephalad (60 degrees), however L4 accelerations were significantly greater for SMTs directed cephalad compared to those delivered at 90 degrees (p.01) (Figure 4).

Results: Significantly increased peak-to-peak accelerations were observed for caudal (120 degree) vectored thrusts at the segmental contact point (L3) (Figure 3).

[BOLD]Figure 3.[/BOLD] L3 vertebral accelerations (m/s2) for 90 degree PA, 60 degree cephalad, and 120 degree caudal vectored SMTs (*p.01).

A significant increase in mean L2, L4, and L2-L4 vertebral accelerations were also observed for all SMTs directed at 120 degrees (p.01). For SMTs directed at a cephalad force vector (60 degrees), mean L2 vertebral accelerations were significantly greater than L4 vertebral accelerations (p.01), and conversely, for caudally directed thrusts (120 degrees) L4 vertebral accelerations were significantly greater than accelerations observed at L2 (p.01). SMTs applied perpendicularly to the spine at 90 degrees caused larger vertebral accelerations at L2 compared to SMTs directed cephalad (60 degrees), however L4 accelerations were significantly greater for SMTs directed cephalad compared to those delivered at 90 degrees (p.01) (Figure 4).

Figure 4.  L2 and L4 adjacent segment vertebral accelerations (m/s2) for 90 degree PA, 60 degree cephalad, and 120 degree caudal vectored SMTs (*p.01).

Conclusions:  The applied force vector at which SMT is delivered has a significant effect upon the segmental contact point and adjacent segment vertebral accelerations. Enhancing vertebral motions by selecting an appropriate force vector is an important biomechanical consideration for clinicians whom are attempting to improve intersegmental mobility during spinal manipulative treatments.

Discussion:  Chiropractic technique classes throughout the world’s chiropractic colleges emphasize the importance of doctor line of drive. In the current study, we examined this issue using a validated gold-standard technique for quantifying vertebral motions.

We hypothesized, that superior vectored thrusts (cephalad) would result in larger magnitude vertebral accelerations when compared to those thrusts delivered straight PA as we have previously observed in humans. However, the facet angulations of the ovine spine (figure 5) may have had an influence in our findings that 120 degree (caudal) directed thrusts produced the largest magnitude vertebral accelerations.

Figure 5.  Facet angulation of the ovine spine.

Because improving or restoring spinal mobility is a fundamental goal of spinal manipulation and chiropractic adjustments, research into techniques best suited to maximize vertebral motions are of interest to clinicians and form the basis for this and future work.

*This study was presented, in part, at the 2009 European Chiropractors’ Union Convention, Alghero, Sardinia, Italy, May 21-23, 2009.

This research was provided by Neuromechanical Innovations.

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