When attaching a drill guide to a patient’s skull, surgeons must rely on feel to determine whether the bone-implanted anchor is correctly seated. But, according to Vanderbilt University researchers, “anchor placement is complicated by the variation in bone density and by obscuring tissue and bleeding that make visual confirmation of seating very difficult if not impossible.” In an effort to prevent misplacement, which increases the potential for guide failure, the team has developed a device called the PosiSeat.
 

The PosiSeat is a depth-release rotary driver that stops rotating when a prescribed seating depth is reached. The scientists say that the device consists of a sleeve that features an end with an internal hexagonal pattern (see Figure 1 to view the different parts of the device). This end engages with the anchor and allows for relative axial motion, but prohibits rotation between the sleeve and the anchor. The other end of the sleeve has an external hexagonal pattern that allows for relative axial motion between the sleeve and stem base, but prohibits relative rotation.
 

Researchers say, “the height of the hexagonal portion on the sleeve is equivalent to the depth to which the fastener is to be driven.” Additionally, there is a spring between the sleeve and stem, which places downward pressure on the sleeve to keep it firmly seated against the bone. The upper end of the stem engages with a rotationally powered tool. External threads on the stem attach to threads on the stem base and permanently join the two parts. There is also an extrusion on the stem, which transmits axial force to the anchor, according to the Vanderbilt scientists.
 

During operation, a tool such as an electric driver is attached to the stem to provide torque while it is pushed toward the anchor. The anchor threads go deeper into the bone and the anchor moves down axially relative to the sleeve. The stem and stem base also move down, and less of the hexagonal interface between the sleeve and stem base is engaged.
 

“Once the seating depth has been reached (goes to zero),” researchers say, “the hexagonal surfaces are no longer in contact and the sleeve is no longer rotationally constrained to the stem base. As such, the sleeve will now be stationary along with the anchor while the stem and stem base are still rotating.” At this point, it is clear to the operator that that anchor is properly seated.
 

J. Michael Fitzpatrick, a professor in the Department of Electrical Engineering and Computer Science at Vanderbilt, says that the device can be customized to accommodate different shapes and sizes of anchors and screws. “The customization is required so that (a) the head of the anchor or screw can be driven by the device and (b) torque will be released when, and only when, the head is seated.”
 

Currently, surgeons are testing the PosiSeat at beta sites outside Vanderbilt. Fitzpatrick says their comments could result in minor ergonomic changes.
 

In addition to attaching drill guides, the research team says the device could have application to the insertion of plating screws, which are used in maxillofacial, orthopedic, and spinal surgery to attach metal plates. “As with an anchor, ideal attachment requires that the screw be driven completely into the bone so that its head contacts the plate,” Fitzpatrick says.
The team plans to test the plating application at Vanderbilt later this year. It says it may need to make changes to the design during this alpha testing stage.
 

Other Vanderbilt contributors include Robert F. Labadie, associate professor in the department of otolaryngology; Jason E. Mitchell, an engineer in the department of mechanical engineering; and John E. Fitzpatrick, a mechanical engineering student.