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Back to Journal »Clinical, Aesthetic and Research Dentistry» Volume 13

The role of navigation in oral and maxillofacial surgery: a surgeon's perspective

Author Anand M, Panwar S

Published on April 15, 2021, Volume 2021: 13 pages, pages 127-139

DOI https://doi.org/10.2147/CCIDE.S299249

Single anonymous peer review

Editor approved for publication: Professor Christopher E. Okunseri

Manish Anand, Shreya Panwar Department of Oral and Maxillofacial Surgery, Meenakshi Ammal School of Dentistry, Chennai, Tamil Nadu 91 8056246581, India Email [email protected] Abstract: The maxillofacial surgery involves a complex network of anatomical structures. Due to the complexity of important structures, the surgical team is required to respect each anatomical boundary. In the past, the number of cases of surgical errors was unusually high. These errors are not due to defects in the skills or techniques of the surgeon, but due to lack of resources. Visualization is one of the key factors that determine the accuracy of any surgical result. Advances in surgical planning have led to the introduction of "navigation" systems that can help surgeons see more, learn more, and ultimately do more for patients. The usefulness of the navigation system in oral surgery has been proven through its surgical application in cranio-maxillofacial trauma, orthognathic surgery, head and neck pathological resection, complex skull base surgery, and surgery involving the temporomandibular joint. The vast majority of research literature shows that under the guidance of 3D planning and navigation, surgical results have been significantly improved. However, with such disorderly progress, financial costs and a gradual learning curve have always been restrictive factors for surgical navigation. This article outlines the indications for navigation in craniofacial surgery, focusing on the application, planning, and solution of future problems. Keywords: navigation, computer-assisted surgery, cranio-maxillofacial, orthognathic, trauma

In the past few years, technological advances have significantly affected surgical outcomes. From knife to robotic surgery, from 2D imaging to 3D imaging, technology has become an indispensable part of any surgery. Navigation-assisted surgery is one of the technical advantages applied in medicine. Simply put, navigation refers to equipment that can accurately locate key anatomical structures, the safest way to reach the target, and the direction in which safe and reliable operations can be performed. It can help surgeons unlock "unreachable" areas that traditional imaging techniques cannot access. Over time, this masterpiece has evolved into a powerful technique that enables surgeons to perform more challenging operations that were once considered impractical and impractical.

Neurosurgery is the first field to integrate navigation in its surgery. The brain is the most vulnerable organ of the human body. Since ancient times, surgeons have been trying to develop new techniques to perform minimally invasive surgery around this area. This led to the discovery of navigation. Stereotactic is the first neurosurgery performed under the guidance of navigation. 1 With further improvements, this technology is further integrated into other surgical fields and specialties. Recently, with a better understanding, oral and maxillofacial surgeons have begun to use this technology to plan critical operations. It first started with the correction of secondary deformities and soon cooperated with other branches of maxillofacial surgery.

Some evidence in the literature confirms that the use of navigation systems is superior to other imaging methods. Dubois et al. studied the final implant positions of 10 cadaver models secondary to orbital trauma in 2015 and concluded that navigation-assisted surgery had a positive effect on the expected results. Their research shows that the use of navigation technology in maxillofacial pathology surgery can improve predictability by promoting accurate safety margins and protecting important anatomical boundaries. 2,3 Wu et al. conducted a systematic review in 2019 to ensure real-time navigation surgery during the second cheekbone implantation. 4 Although the research literature has always supported the positive results of the navigation system, there is still a lack of clinical research on a large number of patients. , It is unwise to give more control over the navigation. Test the system.

There has been a lot of discussion about 3D planning and navigation surgery. Understanding "how does navigation work?" requires basic understanding. This article focuses more on planning and designing workflows to use the evidence in the existing literature to understand this complex mechanism on a simple basis.

The surgical navigation system works entirely on the principle of the Global Positioning System (GPS), which synchronizes position and time data from point A to point B. The surgical navigation system consists of three main components-a locator, similar to satellite space; computer tomography (CT) scan data, similar to ground control or route maps; and a surgical probe in the navigation system, similar to user equipment. 5

In GPS, the satellite is used as a fixed reference system, and the signal it sends is read and interpreted by the GPS device fixed in the user equipment. Once the sensor probe (connected to a smart phone or smart watch) receives the signal, it will be converted into a microwave signal to determine the location of the fixed reference point. In navigation-guided surgery, the satellite or locator is fixed on the patient's forehead and sends out a signal, which is picked up by the surgical probe and then converted into a digital image (Figure 1). The digital image is then picked up by a monitor that determines the patient's current spatial position on a pre-registered CT or magnetic resonance imaging (MRI) file. 5,6 Figure 1 Locator (top) and surgical probe or tracker (bottom))-the technical part of the navigation system. The positioner is fixed on the patient's forehead, and the surgical probe or tracker guides the surgeon to perform point registration.

Figure 1 Positioner (top) and surgical probe or tracker (bottom)-the technical part of the navigation system. The positioner is fixed on the patient's forehead, and the surgical probe or tracker guides the surgeon to perform point registration.

Navigation-assisted surgery has to go through the following six main steps-

The acquired CT data is first converted into medical digital imaging and communication.

(DICOM) image, and then import it into the planning software. The obtained image is segmented based on Hounsfield unit to distinguish soft tissue from hard tissue.

Once the data is uploaded to the system, the next step is to mirror the defective side to the normal side on the opposite side. Therefore, this step allows the surgeon to measure the critical defect volume and then 3D print the reconstructed implant based on the known defect volume. In resection surgery, virtual surgery is initially performed before mirroring, where software-guided control allows the surgeon to remove pathological masses after 3D calculation of anatomical safety margins. After removing the pathological mass, the formed defect is mirrored to the opposite side, and the final template is made, and on this basis, 3D printing is performed to reconstruct the implant.

The preoperative planning software shares data presentation control from radiologists to surgeons. He/she can freely modulate and analyze the virtual model on the 3-D axis. This provides accurate predictions of depth, width, and projection. The surgeon can evaluate bone defects or deformities that are difficult to see in the two-dimensional image. 7

As mentioned above, the first three steps (1-3) are performed before the operation, and the next three steps (4-6) are performed in the operating room. Point registration is the most critical step in navigation surgery, which is performed by the sensor probe or tracker connected to the navigation system [5,6]. Generally speaking, these points are bony landmarks, which serve as guide points for predicting key anatomical landmarks, and determine the direction of the surgical plane based on these references (Figures 2 and 3). Although these reference points can vary according to the surgical requirements, the commonly used reference points are: the fronto-nasal junction, the medial and lateral orbital rim, the superior and inferior orbital rim, the protruding cheekbone, the tip of the nose, the angle of the mandible and the chin point. . These points are marked before the incision, and then registered in the navigation software, and the registered points are reflected on the display. 8 Figure 2 Positioning device and surgical probe during operation. The tip of the surgical probe is used to mark anatomical landmarks. Figure 3 Intraoperative verification of axial, coronal and sagittal plane points. The red arrow indicates the supraorbital bony landmarks verified by the convergence of the lines when the navigation probe is used for point registration.

Figure 2 Positioning device and surgical probe during operation. The tip of the surgical probe is used to mark anatomical landmarks.

Figure 3 Intraoperative verification of axial, coronal and sagittal plane points. The red arrow indicates the supraorbital bony landmarks verified by the convergence of the lines when the navigation probe is used for point registration.

Surgical exposure proceeds as planned, and the sensor probe is matched to the area of ​​interest. Then, by matching the preoperative landmarks with the mid-operative points, the confirmation points are obtained on the screen. The surgeon can now see two lines that coincide in all three planes (sagittal, coronal, and axial). 8,9 This verifies the correct anatomical plane. In the case of reconstructive surgery or implant placement, the final inspection is performed after implant placement and before fixation. For example, in orbital reconstruction, the mesh must fall on the posterior O lateral wall at the junction of the inner orbital wall and the infraorbital wall. 10 After the implant is placed, check the three planes at the junction where all the implants are located, usually represented by two overlapping lines. Once this is verified, it is then fixed and closed.

Although neurosurgeons were the first to introduce navigation technology in their practice, it was not until 2002 that a surgical team led by Alexander Schramm performed navigation-guided surgery of the temporomandibular joint to remove ankylosing lumps. 11 During the operation, they reported that the success was very precise in identifying anatomical structures, which marked the beginning of complex maxillofacial surgery navigation. Over time and a better understanding, the surgeon will be able to understand navigation more deeply and start experimenting with other procedures. The system quickly became one of the most reliable inventions for surgery, and it continued to excel in surgical intervention, making the operation safer and less invasive. 1 At present, navigation-guided surgery has become one of the most critical components of any complex surgery on the head and neck. With current research and further software modifications, this technological advantage will soon become the backbone of cranio-maxillofacial and plastic surgery.

One of the areas where navigation is widely used is trauma involving the upper third and middle third of the face. The cheekbones are the cornerstones that support the face. Any trauma that causes this area will cause disproportion and disfigurement of the face. 12 In many cases, the anatomical structure of the zygomatic-maxillary complex is so complicated that even after adequate treatment, it may lead to secondary deformities and cause problems for the patient's aesthetics. A study by Ellis et al. found that the difference of 2 mm bilateral lateral difference is considered to be visually symmetrical. Interestingly, Xi Gong et al. concluded in one of his studies that the bilateral lateral difference in the navigation-assisted surgery group was in the range of 1.24-1.36 mm, while the bilateral lateral difference in the manual reduction group was significantly higher (2.24mm-3.60 mm), as the lateral width of the face increases, it is visually perceivable. 14 Navigation allows us to accurately copy the model on the uninjured side and then mirror it to the injured side. Then calculate the difference in defect volume, and print the virtual implant based on these differences. In theory, since the injured side is mirrored to the uninjured side, it helps to achieve close to similar symmetry. Once this symmetrical bone framework is achieved, it can be corrected by autologous fat grafts or silicone mesh in the later stage.

Another very challenging operation that is very difficult to correct is trauma involving the nasal orbit-ethmoid sinus complex. Trauma in this area may lead to aesthetic weakness, such as late distal eye, sunken eyeball, protruding eyeball, dystopia, etc. 15 Orbital injury significantly causes the fracture line to pass through the inferior orbital wall (because it is the weakest area of ​​the orbit), causing the orbital floor to shift and cause the eyeball to sink. Retraction of the eyeball greater than 2mm is the main indicator of orbital reconstruction, resulting in a significant increase in orbital volume. The 3-4 mm difference between the positions of the eyeballs is clinically visible because the position of the eyeballs receded is obvious. To correct this problem, surgeons usually place a titanium mesh to reconstruct the orbital floor to support the earth. The implant must be located exactly at the confluence zone, the anatomical junction where the inner and posterior walls of the orbit meet, approximately 22 mm from the inferior orbital rim. 16 Early surgeons used to reconstruct the defect. Based on clinical operations and CT findings, the implant placement was inaccurate and the expected results were never achieved. The confluence area is inaccessible and almost impossible to see with the naked eye; but with the advent of navigation systems, the surgical surgeon can first superimpose these defects and print out the implant for the patient. Therefore, he/she can visualize the confluence area on the monitor and determine the final prosthesis (Figure 4). Figure 4 Preoperative verification of the orbital grid (left) using navigation. Under the guidance of the navigation, the plan during the operation is reproduced (right).

Figure 4 Preoperative verification of the orbital grid (left) using navigation. Under the guidance of the navigation, the plan during the operation is reproduced (right).

By consulting the literature, we found a study conducted by Yu et al. in 2013. They recruited 34 patients with zygomatic-orbital complex fractures who planned to undergo surgery under navigation guidance. 17 They concluded that navigation-assisted surgery is very helpful for precise anatomical reduction, asymmetry correction, and safe operation along fine anatomical structures. Another study conducted by Jia-Ruei Yang et al. in 2019. They retrospectively analyzed 17 patients with orbital complex fractures treated under navigation guidance from 2015 to 2017. They concluded that the inverted eyeballs averaged from 2.99 mm is reduced to 0.68 mm. mm, no postoperative complications; a major study conducted by 18 He et al. between 2008 and 2010 involved 64 patients with delayed orbital zygomatic fractures. They concluded that navigation-assisted surgery is compatible with 3D models and titanium mesh It is the best way to treat delayed orbital zygomatic fractures of large invaginated eyeballs. 19 A similar study by Yu et al. further confirmed the accuracy of navigation and concluded that intraoperative anatomical structure registration and pre-operative CT scan. He further pointed out that when comparing the postoperative CT scan with the preoperative treatment plan, the maximum deviation is less than 2 mm. 20

The use of navigation in orthognathic surgery provides us with the benefit of clear visualization of key anatomical structures and allows the surgeon to accurately enter under guidance. Once access is obtained, the surgeon can easily position his/her surgical instruments accordingly. The final result of any orthognathic surgery mainly depends on how the surgeon analyzes the relevant anatomy of the area. One of the complications of these procedures is that the relevant structures are located deep, and operating around these areas requires extremely high dexterity. Therefore, an in-depth understanding of each patient's anatomical characteristics and positional relationship is very important. 5 In addition, due to the accurate monitoring of key structures during the operation, the operation can be performed within a safe range.

The most common surgery performed on the mandible to correct the difference before and after is sagittal split osteotomy, followed by branch osteotomy. In these two operations, the route of the inferior alveolar nerve must be accurately identified21, because neurosensory disorders secondary to these osteotomies are very common. Under the guidance of the navigation system, any surgical instrument can monitor the exact position of its tip during use. 5 Through navigation, the surgeon can accurately locate the position of the swing saw relative to the lingual nerve and the sigmoid notch during the operation. Vertical branch osteotomy-thereby reducing the chance of neurovascular bundle damage. 5,22

In the maxilla, the most common operation is the Le fort 1 osteotomy, which requires the correct position of the pterygoschis to prevent any unnecessary complications related to the downward fracture of the maxilla. The most terrible complication of Le Fort 1 osteotomy is the bleeding secondary to the injury of the internal maxillary artery and pterygoid venous plexus. 20 Past reports have also shown cases of blindness after a downward fracture of Le fort 1, because the excessive force generated during downward fracture movement reaches the optic nerve through the cleft pterygopalatine. Through the navigation, the surgeon can accurately position the pterygoschis at the joint of the pterygoid bone plate, so that the maxilla and the wing bone plate are neatly separated, and at the same time, the trauma to the surrounding structure is minimized. 13 The accuracy provided by navigation technology reduces the chance of fractures and accidental injuries during these processes.

Another notable feature of navigation-guided surgery is the use of virtual 3D planning software, which allows the surgeon to reproduce the pre-operative plan during the operation. You can plan to use the final splint to manipulate and fix the osteotomy segment before surgery. This allows the surgeon to place the upper and lower jaw segments in more ideal and advantageous positions. 5,9 An error of 2 mm when placing the intermediate splint and the final splint may significantly cause the midline shift and misalignment.

In Distraction Osteogenesis, it allows the growth of incremental bones. The use of the navigation system here allows the surgeon to accurately point out the osteotomy site, guide the placement of the screw hole to the planned retractor position, and align the retractor vector with the planned position. 23 In addition, navigation surgery can also be used to predict the vector motion of the segment before the planned distraction osteogenesis. Accessing this information and using navigation to assess these locations can verify the expected bone changes intraoperatively in more detail. 5

When reviewing the past literature, we found that Badiali et al. conducted a retrospective study on 15 patients who underwent treatment for asymmetric dental deformities between 2010 and 2012, and concluded that simulated guided navigation had no postoperative results. Error, smaller average deviation. When using the navigation system for maxillary repositioning, the error based on point and 3d surface analysis before and after surgery is more than 1mm. 24 Another study conducted by Shiba et al. on 46 patients who underwent orthognathic surgery to correct dental and maxillofacial deformities found that 25 Based on the author’s experience and past research, we can conclude that navigation-guided surgery is in planning orthognathic surgery It has a clear role and also helps to verify the movement after surgery. -Operately

Skull-based surgery is one of the most challenging procedures because of their extremely low accessibility and complex anatomical network. Many times, surgeons must combine these operations with endoscopes, but this requires extreme precision and agility. Due to the lack of clear vision, surgeons often leave residual tumor cells, which may lead to recurrence in the future. The location, invasion and margins of the tumor are the decisive factors in the choice of surgical technique. In the past, malignant tumors located outside the palpable area such as the middle of the skull base or the inferior temporal fossa were considered inoperable due to the difficulty of touching the negative marginal tumor cells, and it was difficult to stop bleeding when a large number of tumors occurred. Bleeding. 26

However, with the introduction of navigation, it is now safer and faster to enter the skull base. In addition, the scope and direction of the bone drilling can be planned before the operation so that the surgeon can work under the "safety net" during the operation. Skull base surgery has a universal requirement for accuracy. In order to operate safely, it requires an error of less than 0.5 mm. More than 27 studies have proven the accuracy of sub-millimeter navigation in clinical and laboratory applications. 28,29 In tumor surgery, the use of navigation systems reduces the need to remove important structures and provides clear images of the anatomical relationship with skull base lesions. Tumors involving the posterior and lateral skull base may require additional endoscopic visualization because these areas are almost impossible to reach. 11 Combination of navigation and endoscopy to deal with sellar/parasellar disease, benign pituitary disease and tumors invading the skull base, such as inverted papilloma, dilated fibroids, etc. Skull base area navigation is an upcoming technology, and the few clinics equipped with this technology have to rely on endoscopic visualization. Reviewing the past literature, we can conclude that navigation is beneficial, especially when operating on benign skull base tumors, because it has a clear advantage over endoscopy in delineating the edges of bone and soft tissue. 30-32

In the biopsy of the head and neck area, Navigation first set foot, and later established its position in the field of frameless stereotaxic. Because the lesions in and around the skull base and pharyngeal cavity are difficult to access, it may be difficult to obtain enough tissue for histopathological sampling. 5,30 Usually, these lesions are very deep and require general anesthesia. In addition, because the location of the lesion is difficult to access, there is a risk of damage to important structures. 33 Ultrasound-guided hollow needle biopsy is simple, safe and minimally invasive. However, due to bone intervention and reduced visibility, it is not suitable for skull base areas and deep lesions.

With the advent of navigation, the surgical team can first determine the extent of the lesion in a 3-dimensional view based on the CT data included in the navigation software. Then plan the trajectory of the biopsy needle. It provides surgeons with two-way advantages. First, the needle trajectory can be toward the path of the least important anatomical barrier, and second, a safety margin can be delineated so that the surgeon can clearly understand the scope of the tumor when performing the final resection. 34 In addition, like CT or MRI-guided biopsy, there is no potential risk of radiation hazards. In addition, navigation is very useful in previously operated areas, which lack a clear surgical plane because they were damaged during the first intervention. In this case, navigation provides the surgeon with a clear boundary between the vascular plane and the avascular plane. For further understanding, readers are advised to refer to the study by Yang et al. They reported 90% of the diagnosis accuracy of skull base and parapharyngeal pathological lesions, spanning five years. 35

The principle of reconstruction guides us-reconstruction with the same lost materials. The reconstruction of lost structures secondary to tumor resection determines the outcome of any tumor surgery-both aesthetically and functionally. In the case of limited early technology, the main focus of surgeons is to give back more functional components to the patient while sacrificing aesthetic requirements. With the introduction of complex tools, this trend is changing. Today, aesthetics and the functional needs of patients are the most concerned issues for surgeons. Through navigation, the surgical margin can be identified and the defect volume of all three planes can be calculated before the operation. Based on these calculations, the surgeon can design the graft more accurately. 36,37 Although autologous grafts work well in reconstruction, the feasibility of the graft and the absorption of the graft after surgery-there are always problems. 38 In order to overcome this problem, surgeons now prefer allograft reconstruction methods-mainly using titanium mesh or steel plate. As mentioned earlier, the navigation can accurately determine the defect volume. Using it, surgeons can print patient-specific and defect-specific titanium implants that will accurately fit the void created after resection. In order to obtain perfect symmetry, the surgeon can choose soft tissue grafts, mainly fat grafts, by calculating the difference in the defect of the contralateral normal side.

Removal of foreign bodies from the maxillofacial region is a rather difficult task because of their close proximity to important structures and insufficient visual access. In high-impact traumas, such as gunshots or explosions, foreign objects (cassette shells or nuclei) are likely to be embedded in deeper structures. 5 The abnormal anatomy of this area makes it more difficult to remove it safely. Although preoperative CT scans with image intensifiers can correctly locate these foreign bodies, it can be challenging for surgeons to explore the correct plane during the operation. In addition, when conducting reconnaissance on an incorrect plane, the risk of damaging important structures is relatively high. 5,8 This in turn can lead to catastrophic postoperative morbidity. In this case, navigation provides the surgeon with two benefits—not only can it help the surgeon locate and remove the foreign body, but it also opens up space for immediate reconstruction—thus simultaneously reducing treatment time and cost.

The technology of removing foreign objects with the help of navigation follows the same principle as other navigation-assisted surgery. Once the foreign body is located by the CT image before the operation, the navigation will be used to evaluate the specific point through the sensor probe. The exact position cannot be determined until the lines of the two views (CT and navigation) coincide. 39,40 Once the point is determined, the surgeon will position their instruments along the created plane and perform a blunt dissection until the foreign body is removed. encounter. Surgeons have begun to use navigation to remove foreign bodies from the maxillofacial region, which can be proven from the data in the existing literature. A study conducted by Sießegger et al. 41 on patients with head and neck foreign body complications concluded that with the help of navigation, the foreign body was successfully removed with minimal intervention. In addition, compared with similar interventions using traditional techniques, it reduces the operation time by more than 40%. 41 A review of past case series and case reports in the literature shows that intraoperative navigation is significantly better than other positioning and mapping methods. Surgical access. Navigation can also reduce intraoperative time and promote postoperative recovery. 39-45

In many cases, due to limited access and complicated neurovascular anatomy, surgery in and around the oral cavity becomes laborious for the surgeon. The most routinely performed procedures that can use navigation are removing unemerged teeth and performing implant surgery. Determining the exact location of the unemerged tooth that needs to be extracted is the most critical step in planning the operation. By combining navigation, the surgeon can first clearly see the anatomical boundaries. This allows easy removal of teeth along the path of least resistance. For example, the removal of the impacted maxillary third molar always has the risk of maxillary sinus root displacement, maxillary tubercle fracture and the whole tooth displacement in the potential space. 46 Similarly, the mandibular third molars also have the following risks: lingual and mandibular nerve damage, tooth migration into the lingual space, and iatrogenic mandibular fractures. 47 The navigation system allows the surgeon to track important boundaries within 6 degrees and continuously calculate the safe distance to these boundaries. 48 ,49 Once these points are tracked, the surgeon can orient the surgical handpiece to the desired path to complete the remaining surgical steps. These unemerged teeth are usually associated with pathological changes, such as odontogenic cysts or odontogenic keratotic cysts, which require extra care to protect important anatomical surgical planes that lack the usual structural obvious planes. In these cases, navigation-assisted surgery is very helpful because it allows the operator to delineate and distinguish soft tissue masses and hard tissues, and preserve the neurovascular bundles.

Another commonly used alveolar surgery is implant surgery for missing teeth or tooth replacement. The most critical steps in implant surgery that determine the outcome of the surgery are the placement of the initial osteotomy drill, the angle of the implant corresponding to the angle of the surrounding natural teeth, the best implant-bone-soft tissue interface, and a good gap with the tooth. Related anatomy. 48,50,51 Navigation-guided surgery allows surgeons to perform implants under more ideal conditions than traditional free-hand implant surgery. A study by Aydemir and Arisan et al. pointed out that the maximum acceptable deviation is less than 9 mm linear and 20.42 degree axial angle is generated by freehand execution. In contrast, the maximum deviation provided by navigation-guided implant surgery is only 0.7 mm linear and 5 degree axial angle. 52 This technology allows the surgeon to precisely position the implant in the bone in all 3 dimensions without even opening the flap. After connecting the tracker, the surgeon will record the relevant anatomical area and guide the operator to drill the initial osteotomy incision. The color-coded depth indicator (attached to the system) helps the operator navigate and reach the target site. The color change from green to yellow means that the drill bit is exactly 0.5 mm away from the ideal position. 48 On the navigation screen, the operator can virtually plan the angle between the implant and the corresponding dentition, and further guide the surgeon to drill the hole along the required plane. One of the main advantages of this system is that the surgeon can continuously monitor the operation process during the operation. All anatomical landmarks with ideal orientations are visualized on the screen so that the implant can be placed in a more biologically and mechanically compatible environment to ensure minimal errors and better postoperative results.

The study by Elian et al. can further verify the accuracy of navigation-guided implant surgery. They reported that the average linear accuracy of the implant neck area is between 1.1 mm and 1.45 mm, and the average angle deviation is 2 degrees. 53 Another clinical study by Siessegge et al. showed that they placed 18 dental implants on the damaged site and concluded that it has been proved that image-guided navigation surgery is superior to traditional dental implant surgery in all practical aspects. 54 Although there has been a lot of discussion about the accuracy of navigation. For conventional surgery, a recent study by Wu et al. concluded that there is no significant statistical difference between navigation-assisted surgery and traditional surgery in terms of accuracy and postoperative results. 55

Reviewing the best possible evidence in the literature, it can be concluded that although navigation guidance is a useful aid in complex situations, it is not necessary for experienced surgeons. In other words, navigation can help surgeons perfect the operation, but experienced surgeons can usually provide similarly perfect results—even without navigation.

Navigation technology has become one of the most promising tools in the surgical field, and it has greatly changed the dynamics of intraoperative surgical access and visibility. However, the current navigation technology has its limitations, which hinder its application in specific scenarios. A key disadvantage of navigation technology is that it cannot continuously monitor dynamic motion and provide a single static frame of reference for the surgeon. For example, surgery involving the upper third and middle third of the face is relatively easy to perform because these structures are located in a fixed frame of reference about the base of the skull. If the lower third also involves the upper or middle face, it is difficult to synchronize with the preoperative CT scan because the mandible can move freely in all three planes. 5,17,48,49,56 To obtain in these situations, a single frame of reference becomes very challenging for surgeons, and they have to rely on the dexterity of their hands. Few authors recommend the use of special sensors fixed on the mandible, which will guide the surgeon to continuously track changes in the position of the mandible during the operation. 48

Another limitation of navigation technology can be seen in situations where bilateral operations are required. This is because the working principle of navigation is to map the normal side to the faulty side. In these cases, it is almost impossible to determine the normal side. 5,56 However, scientists have proposed to develop a skull map that can be used as a template for bilateral reconstruction. In theory, an ideal craniofacial model can also be constructed based on the CT database of the population, and can be used as a standard reference for reconstruction. 49 A study by Badiali et al. demonstrated that the method of constructing an average virtual 3-D skull model can be used as a template to plan maxillofacial surgery in advance. In addition, the template can also be combined with wearable augmented reality, which will help the surgeon to virtually orient the 3-D model to the patient's anatomy in real time. 57

The soft tissue deformation and volume difference between preoperative and intraoperative images are another problem that limits the use of navigation in soft tissue correction surgery. Structural image drift is a phenomenon caused by changes in intraoperative soft tissue topographical landmarks, which is opposite to the registered preoperative image. This is because the soft tissue manipulation during the operation will cause the intraoperative fixed volume data to change. 17,48

Other possible factors that have nothing to do with technology but may limit navigation are the high cost of the equipment and the steep learning curve. For clinics in developing countries, the cost of setting up navigation equipment can be very high. However, considering the advantages of the system, it is worthwhile to equip our operating room with such a technological gift. 5,49

In recent years, continuous improvements in modern medical care have opened up new possibilities for improving navigation-assisted surgery to meet continued patient-centric results. So far, the latest developments of the top five in navigation technology have been reported in the literature, which are briefly introduced in the following subsections.

In a traditional navigation setting, the display is located outside the surgical area. Therefore, the surgeon must accurately coordinate the movement of his/her hands on the patient without actually looking at the screen. 58 On the other hand, when viewing the screen during the operation, the surgeon must arbitrarily place the instrument on the patient because his/her eyes are fixed on the screen and the screen is not on the line of the surgical field. An augmented reality system tracker was introduced to overcome this problem, which allows the surgeon to visualize the surgical process without moving his/head away from the surgical area. In AR, virtual images are converted into real images with the help of a tracker connected to the surgeon’s wearable headset. This provides a potential advantage for the surgeon to perform surgery without having to move his/her head away from the surgical site. AR has been further modified through automatic unmarked registration and stereo tracking to minimize the intrusiveness of registration marks, just like traditional navigation systems. 59,60

The EM micro sensor is a digital chip connected to the navigation instrument, and the EM field generator is located under the patient's head. The electromagnetic wave generated by the EM field generator is picked up by the digital chip and guides the surgeon to register the anatomical landmarks more accurately. The advantage of this instrument is that its entire length is flexible. This unique feature of 60 EM has been potentially used in sinus surgery because it provides surgeons with a route map to visualize paranasal sinuses and can perform mouth dilation with minimal patient morbidity. Although EM sensors can accurately record landmarks with a minimum error range of 0.26-0.67 mm, they may be affected by metal deformation. 61

In ultrasound imaging, the registration point is not marked by CT scan, but the point is marked intraoperatively under ultrasound guidance. Generally, in traditional navigation systems, registration is based on CT images for marking. However, in most cases, due to the delay of the patient's visit to the surgical team, the underlying tissue may become fibrotic and lose direction due to the body's natural healing mechanism. This will cause dot registration errors. US imaging allows the surgeon to register the points according to the patient's current tissue state during the operation, which can minimize the chance of registration failure. 62

FAM is an improvement of virtual reality (VR) that allows the surgeon to continuously visualize the 3D reconstruction of the surgical space. This technique is widely used in cardiac catheters for minimally invasive cardiac surgery, and has recently been combined with navigation in tumor ablation surgery (ie, skull-based surgery). 60 FAM helps the surgical team to visually represent the surgical process performed during tumor surgery by delineating the boundary of the tumor mass and the key anatomical landmarks nearby.

It is an alternative to the navigation system, which combines three-dimensional visualization with six degrees of freedom. Mainly used for complex maxillofacial trauma cases. It has high visual spatial perception and high tactile intuitive feedback for the alignment of bone fragments during fracture reduction. This technology allows the surgeon to view the patient's specific anatomy during the planning stage, and can choose to touch, move, and rotate objects (such as bone fragments) and grafts (such as moving real objects). The wearable device connected to the surgeon's head enables him/her to visualize the entire working space from different angles by rotating the head in the desired line of sight. 63,64 The tactile system allows the surgeon to perceive extremely difficult landmarks, such as the fit between condyles after reduction of occlusal fractures. For example, in the case of a mandibular fracture, the reduction of the tongue is evaluated, which is impossible to see with the naked eye and can be easily visualized for fixation. In addition to complex trauma cases, tactile technology has also been successfully applied in the field of oncology surgery, especially the reconstruction of fibular bone flaps. 64

Although neurosurgery is the first discipline to introduce navigation technology in its surgical practice, maxillofacial surgeons have fully realized its potential to improve the final surgical outcome. In short, navigation-assisted surgery can be divided into: trauma or craniofacial deformity secondary deformity correction, tumor resection and reconstruction, foreign body positioning, implant implantation and secondary deformity correction. With a better understanding, this system now has access to most of the procedural areas of the head and neck area. Further research and technological progress will correct its limitations and provide a tool that is not only a supplement to surgery, but also indispensable.

CT, computer tomography; GPS, global positioning equipment; MRI, magnetic resonance imaging; DICOM, medical digital imaging and communication; AR, augmented reality; ER, electromagnetic micro-sensor; the United States, ultrasound guidance; FAM, rapid anatomy.

Thanks to the surgical team of New Hope Hospital for introducing the navigation software to us.

The authors report no conflicts of interest in this work.

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