Introduction:

Lumbar spinal disorders including radial  tears, disc degeneration, segmental instability and segmental  dysfunction have been considered common causes of persistent back pain  and sciatica. Such disorders may be characterized as exhibiting  alterations in the mechanical behavior to loading, notably, changes in  spinal stiffness. Studies investigating posteroanterior (PA) forces in  spinal stiffness assessment have shown relationships to spinal level,  body type, and lumbar extensor muscle activity. Such measures may be  important determinants to discriminate between patients with low back  pain and asymptomatic subjects. However, little objective evidence is  available discerning variations in PA stiffness, a more complete  assessment based upon dynamic stiffness measurements (driving-point  impedance) and concomitant neuromuscular response may offer more  information concerning mechanical properties of the low back, Thus, the  aim of the current study was to determine the stiffness and  neuromuscular characteristics of the asymptomatic and symptomatic low  back,

Methods:

This study is a prospective clinical study  investigating the mechanical and muscular behavior of lumbar spinal  segments to high loading rate PA forces, 22 subjects (12 male & 10  female, mean age of 42.8+ or – 17.5 years, range 15-73 years) underwent a  comprehensive physical examination consisting of history,  orthopedic/neurologic examination, lumbar range of motion, pressure  algometry and plain film radiographic exanimation of the lumbar spine. A  visual analog score (VAS), Oswestry Low Back Disability Index, and  Health Status Questionnaire (SF-36) were obtained for all subjects and  categorization was made on the basis of symptom frequency, as well as  positive vs. negative orthopedic exam, acute vs. chronic (>12 weeks)  low back pain (LBP) history and electromyography (EMG) response to PA  mechanical stimulation. Each subject was placed in the prone position by  use of a motorized vertical/horizontal table. Surface, linear  enveloped, EMG recordings were obtained from electrodes (8 lead s)  located over the L3 and L5 paraspinal musculature to monitor the  bilateral neuromuscular activity of the erector spinae group during the  PA stiffness measurement protocol, Prior to and immediately following  the PA mechanical stimulation, each subject performed three consecutive  maximal effort isometric trunk extensions to normalize EMG data. A  hand-held Activator II Adjusting Instrument equipped with a load cell  and accelerometer was used to deliver high rate (<0.1 msec ) PA  mechanical stimulation (450 N) to several common spinal landmarks  including the PSIS, sacral base and L5, L4, L2, T12, T8 spinous and  transverse processes. Driving point impedance (Z, Ns/m) was calculated  for each of the thrusts, from which the effective dynamic stiffness (Z x  2(3.21)f) was determined.

Results:

Two of the subjects were asymptomatic (no prior history of LBP), 6 had occasional LBP symptoms, 4 intermittent, and 10 had chronic symptoms of LBP. Subjects with chronic symptoms were characterized by higher effective dynamic stiffness at all levels and had a 2.5-fold higher Oswestry index and VAS score in comparison to the other subjects. Ten of the subjects had an abnormal orthopedic examination and were characterized by a significantly higher dynamic stiffness at all levels. These ten subjects also had over a 2.5-fold greater Oswestry index and VAS score in comparison to the subjects with a normal exam. LBP chronicity was also associated with a 2.5-fold and 3~fold greater Oswestry and VAS score, respectively, in comparison to acute pain sufferers. no differences in dynamic stiffness were observed between these subject groups, however. Of interest was our finding that 16 of the subjects exhibited a hyper-neuromuscular response in response to the PA mechanical stimulation. A hyper-neuromuscular response was characterized as a prominent EMG response (≥ 10% of the isometric extension EMG response) in 10% or more of the EMG recordings (80 total/subject). In this group of subjects the Oswestry index and VAS score were nearly 3-fold and 6-fold greater, respectively, in comparison to subjects which showed little or no mechanically-activated EMG response. Also noteworthy, was the finding that, while lumbar level PA stiffness measurements were similar for these two groups, the thoracic level PA stiffness values were significantly greater in the hyper-neuromuscular group.

Discussion:

The results of this preliminary study provide additional support for clinical assessment strategies that utilize a non-invasive dynamic stiffness measurement system to probe and quantify the mechanical characteristics of the spine. It was noted that subjects with hyper-neuromuscular responses presented with more severe disability outcome scores and a positive orthopedic exam. Further measurements of the dynamic stiffness and neuromuscular characteristics of the symptomatic and asymptomatic LBP population are required to clarify the significance of this observation. Such diagnostic measurements, when combined with conservative manipulative care of the back may prove to be a particularly effective means to diagnostically probe and treat lower back disorders.

Reference: Christopher J. Colloca, D.C., Tony S. Keller,  Ph.D. , Arlan W. Fuhr, D.C.; Muscular And Mechanical Behavior Of The  Lumbar Spine In Response To Dynamic Posteroanterior Forces; Proceedings  of the 26th Annual Meeting of the International Society for the Study of  the Lumbar Spine, Kona, Hawaii. Toronto: ISSLS, 1999: 136A.