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,
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.
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.
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.