Shear strength of the human lumbar spine

BackgroundShear loading is recognised as a risk factor for lower back pain. Previous studies of shear loading have either not addressed the influence of age, bone mineral density, axial height loss due to creep or were performed on animal specimens.MethodsIntact human lumbar motion segments (L2–3) were tested in shear using a modified materials testing machine, while immersed in a Ringer bath at 37 °C. Vertebrae were rigidly embedded in neutral posture (0° flexion) and subjected to a constant axial compression load of 500 N. Shear was applied to three groups: ‘Young-No-Creep’ (20–42 years), ‘Young-Creep’ (22–38 years, creep 1000 N for 1 h) and ‘Old-No-Creep’ (44–64 years). Failure was induced by up to 15 mm of anterior shear displacement at a rate of 0.5 mm/s. The trabecular and apophyseal joint bone mineral densities were evaluated from computed tomography images of the intact lumbar spines.FindingsPeak shear force correlated positively with trabecular bone mineral density for specimens tested without axial creep. No significant differences were observed with respect to age. During shear overload specimens increased in height in the axial direction.InterpretationTrabecular bone mineral density can be used to predict the peak force of lumbar spine in shear in neutral posture.