A new method of accurate pedicle screw navigation.

One of the most established approaches to navigate pedicle screws is the planning and alignment (PA) method. Thereby a trajectory and associated entry point (EP) is planned and navigated after referencing to patient anatomy. However, deviations from the planned EP potentially lead to an altered screw position. The aim of this study was to investigate the influence of these EP deviations and to examine possible alternative methods. The merits of two new points of reference (screw tip point STP and midpoint MP) were therefore analyzed. STP represents the point on the optimal screw tip, MP the point at the center/midportion of the pedicle at its narrowest portion. The adapted screw trajectory was defined as the directional vector from any chosen EP to the STP or MP. First, computer simulations were used to evaluate the performance of these new approaches. Subsequently, the navigation technique yielding more acceptable screws in case of an EP deviation was analyzed on phantom-sawbone models. Both new methods showed a significantly larger number of possible screw trajectories in the simulations (p < 0.01). Even with a deliberate deviation of 4.5 mm (IQR 3.3) from the optimal EP, a perforation-free screw diameter of 4.9 mm (IQR 5.7 mm) could be achieved using the new navigation techniques. The simulated perforations were mainly located laterally with a median of 8.45 mm (IQR 3.95) distance to the medial pedicle wall. The PA method seems to be susceptible to EP deviations. The STP and MP methods are possible improvement mechanisms to overcome this disadvantage.