1. Field of the Invention
This invention relates to the use and display of a drill bit, probe, or other elongate instrument with a computer assisted surgery system.
2. Description of the Related Art
Computer assisted surgery systems have been developed to aid a surgeon in more accurately positioning instruments during a surgical procedure. These systems have been described for use with CT images, fluoroscopic images, and images derived from other modalities as well as without images or in conjunction with a surgical robot. When using images of a patient, they are often referred to as image guided surgery systems. These systems typically use a localizing device to track surgical instruments in real time. One commonly used localizing device is an optical localizer that employs a stereoscopic camera system to view infrared light emitters or reflectors that are placed on the surgical instruments. Other localizing devices may use electromagnetic radiation or a passive manipulator arm with position encoders at the joints. The computer assisted surgery system then uses the information regarding the instrument""s position to superimpose a representation of the instrument over the images or to generate some other graphic or alphanumeric display.
Commonly, surgical procedures performed with computer assisted surgery systems involve the insertion into the patient of an elongate instrument such as a drill bit or a probe. In these cases the computer-asssistance may consist of superimposing a line representing the trajectory of the drill or probe over images of the patient. Many systems additionally provide for a means for determining the location of the tip of the probe or drill bit. By comparing this tip location to the location of a target feature in the body, the software can calculate the distance remaining in the trajectory. This information can be displayed as a bar graph as taught by U.S. Pat. Nos. 5,251,127 and 6,006,127 or as a numeric value or other graphic as taught by U.S. Pat. No. 5,638,819. However, these approaches are not suitable for orthopaedic procedures, where the surgeon does not typically record a target point prior to the procedure.
Another problem with the approach of tracking the drill itself is that the drill bit may bend. This can cause the trajectory displayed by the computer assisted surgery system to be significantly different than the actual path of the drill bit. Bending of the drill bit is most likely to occur at a location between the drill and the body part, either as a result of deflection of the drill bit tip upon entering the body part or because of lateral forces placed on the drill by the surgeon. One way to alleviate this problem is to use a tool guide that allows the trajectory of the drill bit or probe to be measured as it enters the body part. Such tool guides are common in orthopaedic surgery and neurosurgery. Computer assisted versions of tool guides are described in U.S. Pat. Nos. 5,517,900 and 5,904,691, among others. With a tool guide placed flush against the bone the opportunity for drill bit bending is minimized and the trajectory displayed by the computer more closely matches the actual trajectory. However, tracking just the tool guide provides information regarding only the trajectory of the drill bit and not its progress into the body.
The present invention provides the surgeon with improved information for more accurately inserting a drill bit or other elongate object into the body, when used with a drill guide and a computer assisted surgery system.
Accordingly, one objective of the present invention is to provide a computer assisted surgery system for accurately positioning a drill bit into a body part.
Another objective of the invention is to provide a technique and apparatus for calculating and graphically displaying the pose of a drill bit attached to a drill and passed through a drill guide by determining the poses of the drill and drill guide. The invention has the advantage of accurately displaying the orientation and location of the drill bit.
Still another objective of the invention is to provide a technique and apparatus for reporting to the surgeon misalignment, malposition, or excessive bending of the drill bit relative to the drill guide.
Yet another objective of the invention is to provide a technique and apparatus for accurately calculating and graphically displaying the pose of a drill bit attached to a drill with only a single visible localizing emitter and passed through a drill guide.
Still another object of the invention is to provide a surgical drill, with three or more localizing emitters integrated into the drill housing, whose pose can readily be determined with the use of a localizer.
These and other objects of the present invention are achieved by the use of a computer assisted surgery system, including a drill and a drill guide, each outfitted with three or more localizing emitters, and an optical localizer for measuring the pose of the drill and drill guide. A drill bit is attached to the drill and the location of the tip of the drill determined during a calibration step in which the tip of the drill is placed against a dimple in the drill guide and the poses of the two instruments recorded. During drilling, the drill bit is inserted through the bore of the drill guide and the poses of the drill guide and drill are recorded in real time. The pose of the drill bit is calculated from the orientation of the drill guide and the location of the drill. A graphic representation of the drill bit is then displayed on the monitor screen in an appropriate position relative to the stored images or other instruments.
In an alternative embodiment of the drill, the three or more localizing emitters are integrated into the drill housing and are spaced widely apart to facilitate accurate pose determination.
In an alternative embodiment of the computer assisted surgery system, the drill may be outfitted with one or more localizing emitters. In the calibration step, the distance between the drill""s localizing emitter and the tip of the drill is determined. During drilling, the pose of the drill bit is calculated from the orientation of the drill guide and the location of the drill""s localizing emitters, prior to display of its representation on the monitor screen.