3-dimensional registration of micro-movements of vertebral implants by a specially adapted measuring system using magnetic transducers

Abstract

The aim of this study was to develop a biomechanical experimental set-up to quantify motion of ventrally inserted spinal implants at the implant/bone interface. The model we used was the vertebral column of the calf. Lumbar vertebrae L2 to L4 were "instrumented" with a screw-rod system. The adjacent vertebrae L1 and L5 were connected to a servohydraulic testing machine and axial compression applied. Shortening of the specimen and three-dimensional movement of the most cranial implant relative to the bone was recorded using 3 electromagnetic transducers. 100,000 cycles of axial loading varying between -0.5 kN and -1 kN were applied. Static shortening of the specimen of 8.5 mm and an elastic movement of 180 to 280 microns were measured. The greatest amplitude of single-plane motion was recorded in the sagittal plane in both static and elastic modes. Implant motion within each cycle was recorded accurately as load-displacement curves within a range of 1.35 to 30 microns. With this test set-up, primary stability of different spinal implant systems can be compared. The use of electromagnetic transducers permits three-dimensional implant motion analysis even when only a mono-axial servohydraulic testing machine is available.