Measurement and analysis of the in vivo posteroanterior impulse response of the human thoracolumbar spine: a feasibility study.

OBJECTIVES: To (i) measure lumbar intervertebral motion patterns produced during low force, high frequency posteroanterior (PA) thrusts applied to adjacent thoracolumbar spinal segments; (ii) determine the dependence of PA stiffness and impedance characteristics of the thoracolumbar spine on loading frequency; and (iii) ascertain the feasibility of using PA stiffness or impedance to characterize the in vivo mechanical response of the spine during spinal manipulation.
SETTING: Hospital in Gothenburg, Sweden.
SUBJECTS: Three subjects--one normal (male), one patient diagnosed with L4-5 degenerative disk disease (female), and one patient diagnosed with L5 retrospondylolisthesis (male).
INTERVENTIONS: Intervertebral motion device (IMD) attached to pins inserted into the L3-4 or L4-5 spinous processes. Four repeated PA impulses were delivered to each of the spinous processes (T11-L3) using an Activator Adjusting Instrument with a force-acceleration measurement system. OUTCOME MEASURES: Peak-to-peak intervertebral axial displacement, PA shear displacement and flexion-extension (FE) rotation were obtained using the IMD. Thoracolumbar PA impedance (force/velocity) vs. frequency histories and peak PA dynamic stiffness (impedance x frequency) were determined from the force-acceleration measurements. Averages and standard deviations of these measures were calculated from the repeated interventions performed at each level. MAIN
RESULTS: For the normal subject, the AAI PA impulses applied to the L2 spinous process (72 +/- 9 N) produced a 1.62 +/- 1.06 mm peak-to-peak intervertebral axial displacement, 0.48 +/- 0.1 mm PA shear displacement, and 0.89 +/- 0.49 degrees FE rotation at the L3-4 spinal segment. The amplitude of the lumbar intervertebral motion in the normal subject's spine decreased approximately sixfold when the AAI impulses were delivered further from the IMD measurement site. In both patients the axial, PA shear and FE lumbar intervertebral motions were of the same magnitude, but showed less variability than the normal subject as the AAI impulses were delivered closer to the IMD measurement site. The normal thoracolumbar spine exhibited a maximum dynamic PA impedance at a frequency of approximately 100-150 Hz, resulting in a peak PA stiffness ranging from 62 KN/m (L2 segment) to 124 KN/m (T11 segment). Thoracolumbar PA stiffness values tended to be higher for the patient with a severely degenerated disk (85-362 KN/m), whereas the patient with retrospondylolisthesis had a lower PA stiffness (32-96 KN/m).
CONCLUSIONS: In vivo kinematic measurements of the normal and pathologic human lumbar spine indicate that low force, PA impulses produce measurable segmental motions and reinforce the notion that mechanical processes play an important role in spinal manipulation and mobilization. Calculations of the peak dynamic stiffness derived from impedance vs. frequency measurements indicate that the dynamic stiffness of the thoracolumbar spine is considerably greater than previously reported stiffness values obtained using static and quasistatic manipulation and mobilization procedures. Computations of spinal input impedance are relatively simple to perform, can provide a noninvasive measure of the dynamic mechanical behavior of the spine, appear to have potential to discriminate pathologic changes to the spine, and warrant further study on a larger sample of normals and patients. Ultimately, chiropractic clinicians may be able to use low force, impact type spinal manipulation, together with dynamic impedance analysis procedures, to quantify the mechanical response of the normal and abnormal spine, to perform spinal diagnosis and subsequently to prescribe therapeutic treatment to patients.