Assessments of spinal stiffness are becoming more  popular in recent years as a objective biomechanical means to evaluate  the human frame. 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 and its clinical  significance. No study has investigated the relationships of invivo PA  spinal stiffness to radiographic images. L5-S1 disc to body height  ratios were calculated from digitized plane film lateral radiographs of  eighteen symptomatic LBP patients (8 females and 10 males, 15-69 years,  mean 44.3 SD 15.4 years). Posterior disc height ratio (PDR) and anterior  disc height ratio (ADR) were compared to the L5 posterior-anterior  dynamic effective stiffness determined using a validated in vivo  mechanical impedance assessment procedure [1]. Dynamic effective  stiffness (N/m) was calculated from the impedance-frequency response  spectrum as the dynamic mechanical impedance (Force/Velocity, Ns/m) x  circular frequency (rad/sec). Dynamic effective stiffness (minl) at the  first resonance frequency (fminl)is reported. No correlation was noted  between minl and ADR at L5. However, minl was positively correlated to  the PDR at L5 {minl=232 x PDR + 32 (R=0.76)}. Dynamic spinal stiffness  assessments may provide additional biomechanical data that may be prove  to be of use to clinicians in the diagnosis of lumbar spinal disorders.

Reference: Christopher J. Colloca,DC; Tony S. Keller,PhD; Terry  K. Peterson,DC; Daryn E. Seltzer,DC; Arlan W. Fuhr,DC. Proceedings of the International  Conference on Spinal Manipulation , Sept 21-23,2000.