The aim of the present research is to analyse the mechanical behaviour of the Cotrel-Dubousset spine internal fixator using numerical and experimental techniques. Simple experimental and numerical models of a spine section (four vertebrae of the thoraco-lumbar region) were developed in order to evaluate the response of the system, particularly the stress-strain state in the rods, by means of strain-gauge measurements. The system was subjected to combined compressive and bending static loading and the deformations of the system were measured on models simulating physiological and pathological conditions, with and without the fixator.