INTRODUCTION
Biomechanical modeling of lumbar spine motion responses to externally applied external forces has identified frequency-dependent characteristics related to age and pathological status, however little is known of their relationship to low back pain. Non-invasive quantification of spinal motion responses provides objective identification of mechanical factors used to help discriminate treatment targets, identify the most efficient frequencies to apply treatment, and gauge dosage. This biomechanical information, together with other clinical considerations may be further used to improve patient evaluation and management and outcome assessment tools aiming to minimize risk in non-surgical patient care. The aim of this prospective clinical study was to investigate the impedance-frequency response of the lumbar spine to posterior-anterior delivered forces using a driving point mechanical impedance technique.
METHODS
Twenty-two consecutive subjects (12 male & 10 female, mean age of 42.8 S.D. 17.5 years, range 15-73 years) underwent physical examination and completed outcome VAS, Oswestry, and SF-36 questionnaires. Driving point mechanical impedance was assessed for posteroanterior impulsive thrusts (»150 N, <5ms) delivered to over the L5, L4, L2, T12, T8 transverse (TP), spinous processes (SP), and sacroiliac joints (SIJ). From each impedance response curve, the first resonance (Min1) and anti-resonance peaks (Max1, Max2) were identified. Driving point mechanical impedance ½Z½ (Ns/m) was calculated as the ratio of FFT(force)/FFT(velocity). Across group analysis of the patient symptomatology and mechanical impedance parameters was performed using a Kruskal-Wallis one-way ANOVA by ranks followed by Scheffe’s test (p<0.05).
RESULTS
For the 22 subjects x 6 levels, the mean PA dynamic impedance was 63.7 (SD 4.87) and 73.9 Ns/m (SD 22.9) at 2.44 Hz and resonance (42.6 SD 5.6 Hz), respectively. This corresponds to a mean dynamic effective stiffness of 977 N/m (SD 75.8) and 19.6 kN/m (SD 5.97) at 2.44 Hz and Min1, respectively. Male subjects had a greater average thoraco-lumbar stiffness (2.44 Hz, Min1) compared to female subjects, but differences were significant for S1, L5 vertebrae only. The resonant frequency of the male subjects was significantly greater than females at all levels (except T8). Patients with frequent or constant LBP symptoms reported significantly higher VAS, Oswestry and perceived health status scores. For any given segment (T8, T12, L2, L4, L5, S1), the PA dynamic effective stiffness (2.44 Hz, Min1, Max1, Max2) and corresponding frequencies were, in general greater, but not significantly different for subjects with more chronic LBP history and more constant symptom frequency.
CONCLUSIONS
This in vivo PA dynamic impedance assessment procedure expands upon previous static and quasi-static stiffness analyses by providing a comprehensive description of the dynamic response of the PA spine. Although this preliminary study did not demonstrate significant changes in dynamic responses related to patient clinical status, the results are encouraging. Ultimately, impedance measurements may be used clinically to objectively assess the biomechanics of the spine.