System for intra-operative monitoring of the cortical integrity of the pedicle during pedicle screw placement in low-back surgery: design and clinical results

The placement of pedicle screws may involve a damage to the pedicle wall as well as to the adjacent nerve roots. This is visible as a lowering of the trans-screw stimulation current threshold of evoked compound action potentials (CAP) of the leg muscles and as a decrease of the electrical impedance of the screw. The objective was to design a system for intra-operative monitoring of the integrity of the pedicle wall and motor nerve and to test it in clinical practice. Spontaneous EMGs and CAPs, evoked by electrostimulation of the nerve and via the screw, of 8 muscle groups in the leg were recorded and furthermore, the screw impedance was measured. 102 pedicle screws were monitored. It is concluded that during pedicle screw fixation, the current thresholds for CAPs and impedance measurements are useful for monitoring nerve root integrity. Impedance measurements provide an early warning of the cortex of the pedicle by screw placement.