Virtual surgical planning can aid both precise resection and accurate reconstruction.
Surgical navigation is a technique developed initially for neurosurgery but has been adapted for head and neck resections, particularly involving the midface and base of skull.
Surgical navigation in conjunction with virtual surgical planning is useful for reconstructing traumatic defects.
Surgery of the head and neck requires the surgeon to meet both functional and esthetic goals when reconstructing traumatic or ablative defects. Reconstruction is complicated by the high esthetic demands of the region; the face plays an important role in a person’s identity and deformity can have a negative psychological effect. To the extent possible, the goal is to achieve symmetry and adequate projection. Functionally, speech, deglutition, and vision should be addressed as well. Rigid internal fixation allows the precise placement of bone and the advent of microvascular reconstructive techniques greatly expands the reconstructive surgeon’s ability to restore defects. Computer-aided surgery (CAS) has become a mainstay of maxillofacial reconstruction as technological advances in imaging, computer-aided design, and manufacturing techniques enable the surgeon to precisely plan and execute the case. Improvements in accuracy and operating time have been demonstrated. , CAS can also be used intraoperatively in conjunction with a navigation system to guide surgical resections, particularly at the skull base, avoid critical structures, assess fracture reduction, and aid with margin identification. , Advances in imaging and computer technology led to the development of frameless navigation systems, which have been incorporated into the current systems used today. , We review the work flow involved in planning and carrying out a case using CAS and navigation.
Steps in computer-aided surgery and navigation
The first step is data acquisition. This typically requires a fine-cut computed tomography (CT) scan of the face without contrast, with cuts 1 mm or less for adequate resolution. If osseous reconstruction is planned, imaging of the donor site may be indicated, such as a CT angiogram of the lower extremities in the case of a free fibula flap or a CT angiogram of the chest if a scapula flap is to be used. If the maxilla or mandible is involved in the surgical plan, dental records are taken either in the form of stone casts, which are subsequently scanned, or digital impressions, which are combined with the CT face to create a composite 3-dimensional (3D) image with an accurate occlusion. Similarly, soft tissue can be modeled using a 3D camera, merging the soft tissue profile with the underlying skeletal information from the CT. Other modalities, such as MRI, can be merged to create a composite 3D image.
The next step is the process of segmentation, which is the delineation of structures of interest (eg, bone, nerve, vessels). In the past, this was done manually, selecting bone and soft tissue based on Hounsfield units. Today, this step is largely done by the planning software and can be fine-tuned by the engineer assisting in the treatment planning.
A meeting is then set up with the commercial vendor and takes place online with an engineer. This portion will of course be tailored to the specific case. If it is pathology, the lesion can be outlined and a margin established. If it is a trauma case, the fracture is identified. Bony cuts can then be simulated on the 3D model; the orientation of the cuts is evaluated in all 3 dimensions to ensure that structures, such as nerves and tooth roots, are avoided. If applicable, the unaffected contralateral side can be mirrored to demonstrate the proper reduction or bony contour. This is helpful in the context of trauma or in a secondary reduction, where the normal anatomy has been lost. Cutting guides are designed and can be tailored to the specific approach used and bone exposure anticipated. They should include a feature to register the guide to the bone. In the dentate portion of the maxilla or mandible, this may be an occlusal guide. Elsewhere, it may be a lip or tab that limits the degrees of freedom of the guide on the bone. Cutting guides can also incorporate predictive holes to predrill the screw holes used in the reconstruction plate. Predictive holes can obviate the need for intermaxillary fixation. If a reconstruction is planned using an osseous free flap, the graft is planned as well to fit the defect. If needed, osteotomies can be planned to contour the graft precisely to the space, which will be incorporated into the cutting guides. Dental rehabilitation can also be planned at this time. Endosseous implants can be incorporated into a fibula, for example, and their placement can be determined at this time. A temporary dental prosthesis can be made off of a stereolithic model. Hardware is then designed and is customized to the patient. In comparison to using stock plates, custom hardware offers great flexibility in terms of shape and screw hole placement. As with the cutting guides, nerves and tooth roots are avoided when designing the plate. The plans are finalized.
Fabrication of models, plates, and guides
Stereolithic models, cutting guides, and custom plates are manufactured. A number of benefits are realized by pre-manufacturing the hardware. For one, operative time can be reduced because the plates do not have be shaped intraoperatively. The hardware enjoys an excellent fit that is hard to replicate with hand bending. Furthermore, the strength of a reconstruction plate is higher than a bent plate because the custom milled plate does not incur the stress of bending. Everything is shipped to the hardware representative and is sterilized before surgery.
If navigation is to be used, registration of the patient is performed before prepping and draping. This process will be unique to each navigation system, but generally involves the accurate orientation of the patient’s skull to the reference imaging in the navigation system using markers called fiducials. A variety of fiducials have been used, from implanted titanium markers, to dental splints, to light-emitting diode (LED)-containing masks. In some systems, a CT is obtained with the fiducial marker in place; for some systems this is not necessary. Intraoperatively, a headset is either anchored to the skull or attached to a mask that is in view of the navigation tower called the locator. An instrument is equipped with either reflectors or LED markers that can be visualized by the locator. The locator is used to identify points on the headset and other fiducials using the handheld instrument. The navigation system uses these points of reference to correlate the patient’s skull with the imaging.
Surgery is then performed, using the appropriate approach for the surgical resection or to expose the fracture. If bone is to be resected, the cutting guides are fitted and secured with monocortical screws. Care must be taken to ensure that the cutting guide fits flush with the bone, and is not impeded by soft tissue. Predictive holes are then drilled, which will help maintain the occlusal relationship. The resection is performed. If an osseous flap is being harvested simultaneously, cutting guides are used for the bony flap as well. Implants are placed if applicable. The reconstruction plate can be fitted either to the facial defect, or if applicable, to the osseous free flap before transfer. With the pre-drilled holes, affixing the plate to bone is effortless. For a free flap, the pedicle is then divided and the flap inset.
Intraoperatively, navigation will be used in a manner specific to the surgery. If it is an orbital floor fracture, for example, navigation may be used to confirm the seating of the reconstruction plate on the posterior ledge. For an orbital resection, navigation can be used to navigate the base of skull, or to guide osteotomies. Navigation is often used in conjunction with intraoperative CT or cone-beam CT to confirm an adequate reduction or appropriate reconstruction.
Periorbital trauma often results in an enlargement in the orbital volume. This can cause dystopia, enophthalmos, resulting in diplopia. It has historically been difficult to reestablish the pretraumatic orbital volume, complicated as it often is with posttraumatic swelling, limited access, and loss of bony landmarks. CAS provides a predictable approach to reconstructing the orbit. , If the contralateral orbit is unaffected, it can be mirrored and used to simulate the appropriate contours for the reconstructed orbit. A stereolithic model can be created from that simulation and an orbital reconstruction plate adapted to the defect. Custom plates can even be made, largely removing the guesswork of adapting the hardware. Intraoperative navigation can be used to both verify the correct positioning of the plate as well as to avoid critical structures at the orbital apex such as the optic nerve. Alternatively, intraoperative CT is useful to confirm the orbital repair.
CAS has transformed the work flow for planning orthognathic cases. Traditional model surgery is labor intensive and involves multiple steps; small errors made early on are magnified in the subsequent steps and can cause significant inaccuracy. Furthermore, complex asymmetries are challenging to model using traditional methods. With CAS, a fine-cut CT maxillofacial without contrast is obtained, as are dental models, either digital or digitized from plaster models. These are sent to a third-party vendor and melded together to create an accurate digital representation of the bony skeleton and dentition. Traditional orthognathic measurements and photos are taken as well and used in the analysis. A treatment planning session is arranged and the desired bony cuts are made and the movements simulated. A significant advantage to performing the surgery “virtually” is that bony interferences can be anticipated that can save time in the operating room. Occlusal splints are fabricated and custom cutting guides and plates can even be designed if desired.
Temporomandibular joint surgery
Severe temporomandibular joint disease is often treated with alloplastic reconstruction. Currently, reconstructive options for total joint replacement include stock and patient-specific prostheses. For a custom prosthesis, similar to an orthognathic surgery workup, a fine-cut CT maxillofacial without contrast and digitized dental models are merged. A prosthesis is then fabricated off of a stereolithic model by a third-party vendor. CAS allows complex planning; bilateral joint replacement and simultaneous orthognathic surgery can be reliably executed.
CAS has applications in the resection of benign and malignant tumors in the head and neck. For odontogenic lesions, or lesions requiring a bony resection, the tumor can be delineated during the process of segmentation, and resection margins are determined during the planning session. Structures such as nerves, vessels, and teeth are identified and avoided if possible or included in the resection.
Navigation also can be useful when resecting head and neck tumors. Some investigators have described the use of navigation to label resection margins. Frozen margins can be identified and marked using navigation. This can orient the pathologist and can aid in identifying areas that need to be re-resected. The labeled imaging also can be useful in communicating margin status with the radiation oncologist.