Stiffness properties of the human lumbar spine: A lumped parameter model

Objective. To characterise with a mechanical model, the force-displacement response of the human lumbar spine to posteroanterior loading. Design. Single case with repetition, Background Previous attempts to charac terise the spine's force-displacement response have been simplistic and onl y considered the loading curve. These approaches ignored valuable informati on such as viscosity, non-linear elasticity and inertia of the lumbar spine . Methods. The Spinal Assessment Machine applied a postero-anterior load to t he spines of 23 asymptomatic subjects and measured the force-displacement r esponse. The data was analysed by two methods; by a traditional linear regr ession of part of the loading curve and by a new method where an equation i ncluding non-linear stiffness and damping was used to characterise the whol e force-displacement relationship. Results. The equation developed was found to account for virtually all of t he variance in the raw data (R-2 > 0.993). Four elements derived by the equ ation determine the contributions of linear elasticity, non-linear elastici ty, linear viscosity and non-linear viscosity to the overall stiffness. Conclusions. Considering the excellent fit of the new equation to the raw d ata and its poor correlation with existing measures, it is proposed that th e traditional measures provide an incomplete description of the force-displ acement response.