Image guided surgery has had extensive developments over the years and is now a very important tool in surgical procedures. Most of the developments have centered around imaging locations in the body where there is very little access, such as internal organs.
Oral surgery, which is defined herein as any surgery occurring within the oral cavity, can be just as difficult to conduct visually. The oral cavity is relatively small and difficult for a patient to maintain open for prolonged periods of time. Even if a surgical site is visible, once the drill penetrates, it becomes difficult to determine where the tip is at any given time.
Image guided surgery involves the use of a computed or computerized axial tomography scan, commonly referred to as CT or CAT scans, to create a digital image of the surgical site (typically in three dimensions). The surgeon then creates a plan for the surgery using the image. During surgery, the image generated from the prior CT scan is used in conjunction with a special instrument, to visually depict where the tip of the instrument is inside the patient.
In order to do so, the digital image from the scan must be accurately registered to the surgical site of the patient such that movement of the patient causes adjustment of the digital image. The exact location of the instrument tip relative to the patient must also be known.
For oral surgery, such as during dental implant placement, a doctor has to drill in free space while controlling the drill in six degrees of freedom with the patient potentially moving. This makes accurately drilling into good bone while avoiding roots and nerves very difficult. As such, image guided surgery has recently been used to facilitate the drilling process. CT scans of the patient's teeth are used by the doctors to accurately determine bone density, width and height, as well as understand relationships of other teeth and anatomical structures in order to plan a surgical event to provide the restorative solution that would likely be the most successful and least traumatic.
Planning software and fabrication systems exists today that uses the CT image to assist in translating a pre-surgical plan to a passive surgical guide, i.e., creating a virtual plan for the surgery and then prefabricating in the dental laboratory a surgical guide to implement the plan. These passive surgical guides help accurately direct the doctor to the proper location, angle and depth. Passive image guided surgery has limitations. They must be fabricated prior to surgery in a dental lab or by a guide manufacturer. This requires greater doctor and patient time and expense. If there is a change in a patients mouth or the doctor desires to change the plan, the guide is no longer useful. In many cases the patient is unable to open their mouth wide enough to accommodate the instruments needed and the guide.
Active image guided surgery solves many of the problems of passively guided systems, i.e., limited maximal mouth opening, the need to prefabricate a passive guide and the inability to change the plan during surgery can be overcome by actively guided systems. In order to provide active image guided surgery, the position of the patient's mouth, specifically the bone and teeth, must be accurately tracked and registered to the scanned image and the surgical tool. In order to do so, most conventional systems require the creation of a registration device that is attached to the patient's head or inserted into the mouth which includes fiducial markers and a sensor. Some registration devices are attached to the outside of the head, for example, a head mounted fixture. Others involve a fixture that is attached to the jawbone with the sensors located outside the mouth in order to limit the interference with the surgical zone and to permit optical sensors to track the movement of the fixture and surgical tool.
In order to create the oral fixture, an impression is taken, typically of both the upper and lower sets of teeth weeks in advance of the operation. The impression is then sent to a lab where a cast is made substantially duplicating the teeth. From the cast an oral fixture is made that either seats on the teeth or is designed to be drilled into the jawbone. The fixture includes at least the fiducial markers and also, if not fitted with a sensor, includes mounting locations for the optical sensors.
After the lab creates the fixture it is sent back to the dental surgeon. The patient is brought in, fitted with the fixture and a CT scan is taken. The patient is once again sent home. A digital image of the patient's oral cavity is created from the scan and the surgeon develops the surgical plan.
The patient is then brought in for the operation. The fixture is attached to the patient. Optical transmitters are located about the patient and emit signals that are detected by the sensor(s). The sensor(s) send a signal to the software as the patient's mouth moves and an adjustment is made to the digital image of the patient's oral cavity. The software also tracks the position of the instrument and depicts an image of the instrument in the proper location relative to the digital image of the teeth.
In addition to the inconvenience to the patient, existing systems tend to have some difficult accurately registering the patient to the digital scan. All present dental active image-guided surgery systems involve the use of optical tracking which requires that the fixture that is placed in the patient's mouth extends outside the mouth in order to be detected by the optical transmitter or receivers.