Bone segment navigation
Bone segment navigation is a surgical method used in the field to find the anatomical position of displaced bone fragments in fractures, allowing a good fixation by osteosynthesis. It has been developed for the first time in oral and maxillofacial surgery.
After an accident or injury, a fracture can be produced and the resulting bony fragments can be displaced. In the oral and maxillofacial area, such a discplacement could have a major effect both on facial aesthetics and organ function: a fracture occurring in a bone that delimits the orbit can lead to diplopia; a mandibular fracture can induce significant modifications of the dental occlusion; in the same manner, a skull (neurocranium) fracture can produce an increased intracranial pressure.
Surgical planning and surgical simulation
An osteotomy is a surgical intervention that consists of cutting through bone and repositioning the resulting fragments in the correct anatomical place. To insure optimal repositioning of the bony structures by osteotomy, the intervention can be planned in advance and simulated. The surgical simulation is a key factor in reducing the actual operating time. Often, during this kind of operation, the surgical access to the bone segments is very limited by the presence of the soft tissues: muscles, fat tissue and skin - thus, the correct anatomical repositioning is very difficult to assess, or even impossible. This led to the necessity of a preoperative planning and simulation on models of the bare bony structures.
Materials and devices needed for preoperative planning and simulation
The osteotomies performed in orthognathic surgery are classically planned on cast models of the tooth-bearing jaws, fixed in an articulator. For edentulous patients, the surgical planning is made by using stereolithographic models. These tridimensional models are then cut along the planned osteothomy line, slid and fixed in the new position. Since the 1990s, modern techniques of presurgical planning were developed – allowing the surgeon to plan and simulate the osteotomy in a virtual environment, based on a preoperative CT or MRI; this procedure reduces the costs and the duration of creating, positioning, cutting, repositioning and refixing the cast models for each patient. The first system that allowed such a surgical simulation environment is the Laboratory Unit for Computer Assisted Surgery (LUCAS), that was developed in 1998 at the University of Regensburg, Germany, with the support of the Carl Zeiss Company.
Transferring the preoperative planning to the operating theatre
The usefulness of the preoperative planning, no matter how accurate, depends on the accuracy of the reproduction of the simulated osteotomy in the surgical field. The transfer of the planning was mainly based on the surgeon's visual skills. Different guiding headframes were further developed to mechanically guide bone fragment repositioning. Such a headframe is attached to the patient's head, during CT or MRI, and surgery. There are certain difficulties in using this device. First, exact reproducibility of the headframe position on the patient's head is needed, both during CT or MRI registration, and during surgery. The headframe is relatively uncomfortable to wear, and very difficult or even impossible to use on small children, who can be uncooperative during medical procedures.
The first system that allowed a seamless bone segment navigation for preoperative planning was the Surgical Segment Navigator (SSN), developed in 1997 at the University of Regensburg, Germany, with the support of the Carl Zeiss Company. This new system does not need any mechanical surgical guides (such as a headframe). It is based on an infrared (IR) camera and IR transmitters attached to the skull. At least three IR transmitters are attached in the neurocranium area to compensate the movements of the patient's head. There are three or more IR transmitters are attached to the bones where the osteotomy and bone repositioning is about to be performed onto. The 3D position of each transmitter is measured by the IR camera, using the same principle as in satellite navigation. The workstation of the Surgical Segment Navigator (SSN) is constantly visualizing the actual position of the bone fragments, compared with the predetermined position, and also makes real-time spatial determinations of the free-moving bony segments resulting from the osteotomy. Thus, fragments can be very accurately positioned into the target position, predetermined by surgical simulation.
The hard tissue segment navigation is more and more frequently used in orthognatic surgery (correction of the anomalies of the jaws and skull), in temporo-mandibular joint (TMJ) surgery, or in the reconstruction of the mid-face and orbit.
- Marmulla R, Niederdellmann H: Computer-assisted Bone Segment Navigation, J Craniomaxillofac Surg 26: 347-359, 1998