In spinal surgery, skiving refers to the unintended sliding or directional deviation of a drill, awl, or screw from the planned trajectory when it encounters a bony surface. The phenomenon is of particular clinical relevance when using navigation systems and robot-assisted surgery[^1].
Background
In navigated and, especially, robot-assisted spinal surgery, the surgeon relies on a trajectory planned preoperatively or intraoperatively. Because robotic arms and guide sleeves exhibit a certain degree of mechanical elasticity (“compliance”), lateral deflection may occur when they encounter oblique or very hard areas of bone, even before the instrument penetrates the cortex.
Mechanism
Skiving results from an interaction of physical factors:
Angle of incidence: The more acute the angle between the instrument and the bony surface, the higher the likelihood that the shear force will exceed the normal force and the instrument will slip.
Bone density: Sclerotic areas of bone increase resistance. The instrument follows the path of least resistance, which often runs parallel to the cortex.
Lever arm and flexibility: Long, thin drills or guidewires (K-wires) are more prone to bending (deflection) than rigid instruments.
Robotic tolerance: Every robotic system has a design-related mechanical tolerance. Under excessive pressure, the arm may yield minimally.
Clinical relevance
Skiving is one of the main causes of malpositioning of Pedicle screws despite a technically correct setup.
Prevention strategies
Various techniques can be used to minimize skiving effects:
Preparation of the entry site
The most effective method is to create a flat “entry zone.” A high-speed burr is used to flatten the oblique cortex at the planned entry site so that the subsequent instrument encounters a planar surface perpendicularly.
Trajectory adjustment
Adjustment of the entry point in the planning software to an area with less steep anatomy.
Stabilization
The robot’s guide sleeve or navigation reference frame should be brought as close to the bone as possible to minimize the drill’s free lever arm.
It may help to retract the drill a few millimeters into the drill sleeve and bring the drill to the bone at maximum speed, rather than beginning to drill once the drill is already on the bone.