The purpose of this project was to test a surgical navigation tool designed to help execute a surgical treatment plan. It consists of an electromagnetically tracked pencil that is used to mark bone intraoperatively. The device was tested on a precision block, an ex vivo pig mandible and during performance of six endoscopic vertical ramus osteotomies on pig cadavers. The difference between actual pencil position and that displayed by the computer was measured three times each at ten 2 mm holes on the block ( n = 30 observations) and on the ex vivo mandible ( n = 11 measurements). Errors between planned and actual osteotomy locations for the cadaver procedures were measured. The mean distance between known and displayed locations was 1.55 ± 0.72 mm on the precision block and 2.10 ± 0.88 mm on the pig mandible. The error measured marking the same point on the block multiple ( n = 5) times was 0.58 ± 0.37 mm. The mean error on the simulated osteotomies was 2.35 ± 1.35 mm. Osteomark was simple to use and permitted localisation of holes and osteotomies with acceptable accuracy. In the future, the device and algorithms will be revised to further decrease error and the system will be tested on live animals.
Computer-aided navigation was initially developed for neurosurgical operations. Surgical navigation systems allow visualisation of an operative site and surgical instruments simultaneously and relate them to the patient’s diagnostic images (e.g. computed tomographic (CT) scans and magnetic resonance imaging (MRI)). Surgical navigation is a powerful tool that has the potential to transfer a surgical plan accurately.
Intraoperative navigation is being used by a variety of surgical specialists such as otolaryngologists, craniofacial and orthopaedic surgeons . In recent years, oral and maxillofacial surgeons have become increasingly interested in minimally invasive surgical techniques for trauma (endoscopic treatment of subcondylar, zygomatic and orbital fractures), orthognathic surgery (e.g. endoscopic vertical ramus osteotomy, Le Fort I osteotomy, distraction osteogenesis), reconstructive surgery (endoscopic condylectomy with costochondral grafts), salivary gland diseases (sialoendoscopy for sialolithiasis and strictures) and cosmetic surgery . The complexity of these procedures requires precise preoperative planning and accurate transfer of the plan to the patient at the time of operation .
A number of surgical navigation systems are currently commercially available , but some of their specific features, developed for other surgical specialties, make them difficult to use for maxillofacial applications. For example, the need for a reference sensor head frame for neurosurgical procedures is cumbersome when doing maxillofacial surgical procedures. Sensors attached to the upper face do not allow tracking of structures on the mandible. The requirement for standard fiducial markers (registration points) to be placed on the patient before preoperative image acquisition is impractical in some situations (e.g. paediatric and trauma patients). The need for large and cumbersome targets on surgical instruments is not acceptable in the small operative fields in maxillofacial surgery and maintaining a line of sight between a tracker camera and the instruments may not be feasible. Currently existing navigation systems are expensive .
The specific aims of this study were to create and evaluate a surgical navigation system (Osteomark) that would be user friendly for the surgeon and operating room staff, that could be used on the mandible, midface and skull and that would not require additional imaging studies or cumbersome headframes and sensors.
Materials and methods
Osteomark consists of a transmitter, a marking tool, position and reference sensors, tracking electronics, and a computer ( Fig. 1 ). The transmitter emits precisely oriented electromagnetic fields that induce currents in receiving sensors. The sensors are 1.3 mm diameter cylinders attached to a cable. A position sensor is placed in the Osteomark pencil ( Fig. 2 ) and a reference sensor is attached rigidly to a tooth or any other convenient, fixed craniofacial point ( Fig. 3 ). The signals generated in these receiving sensors are detected by tracking electronics (3D Guidance™, Ascension Technology, Burlington, VT, USA), which compute the position and orientation of each sensor relative to anatomic regions or points of interest. The position information is transmitted to the computer for use in navigation.
For the purpose of this study, a sensor was placed in a ‘pencil’, used to mark osteotomies. The sensor can be attached to any surgical instrument. The marking pencil is made of high-impact plastic and has a 45° angled tip. The graphite pencil point is fixed in a nylon threaded insert so it can be replaced when it wears during use ( Fig. 2 ).
The first step is the registration process. This determines the geometric relationship between the anatomic structures of interest and the three-dimensional (3D) computer image constructed from the preoperative CT scan. High resolution maxillofacial non-contrast CT scans (GE LightSpeed, Milwaukee, WI, USA) consisting of 1.25 mm axial tomograms were used for the present study. Registration involves two steps. First, the reference sensor is secured to a non-mobile structure on the mandible such as a tooth ( Fig. 3 ). Then, the Osteomark pencil tip, prompted by the computer, is used sequentially to touch pre-selected registration points (fiducial markers) chosen by the surgeon ( Fig. 4 ). Registration points may be any anatomic structures that are recognisable on the preoperative image in relation to the reference sensor (e.g. teeth, skin, bone). An indicator on the screen shows each point. Each time a registration point is touched with the pencil, the computer records the location of the position sensor (pencil) and the reference sensor. Using at least three registration points, the computer calculates the physical position of the anatomic structure with respect to the sensors. The computer then uses this registration information to measure the position of the pencil relative to the preoperative CT scan. The patient’s head can be mobilized freely without the need to re-initialize the registration process because the reference sensor is rigidly attached to a tooth.