Algorithm for planning a double-jaw orthognathic surgery using a computer-aided surgical simulation (CASS) protocol. Part 1: planning sequence

Abstract

The success of craniomaxillofacial (CMF) surgery depends not only on the surgical techniques, but also on an accurate surgical plan. The adoption of computer-aided surgical simulation (CASS) has created a paradigm shift in surgical planning. However, planning an orthognathic operation using CASS differs fundamentally from planning using traditional methods. With this in mind, the Surgical Planning Laboratory of Houston Methodist Research Institute has developed a CASS protocol designed specifically for orthognathic surgery. The purpose of this article is to present an algorithm using virtual tools for planning a double-jaw orthognathic operation. This paper will serve as an operation manual for surgeons wanting to incorporate CASS into their clinical practice.

The success of craniomaxillofacial (CMF) surgery depends not only on surgical techniques, but also on an accurate surgical plan. The adoption of computer-aided surgical simulation (CASS) is creating a paradigm shift in surgical planning for patients with CMF deformities. The Surgical Planning Laboratory of Houston Methodist Research Institute has developed a CASS protocol that is specifically designed for orthognathic surgery. In this protocol, a three-dimensional (3D) composite skull model of a patient is generated to accurately represent the CMF skeleton, the dentition, and the facial soft tissue. In addition, an anatomical reference frame is created for the 3D composite skull model. Virtual osteotomies are then performed and orthognathic surgery is simulated. Finally, surgical splints and templates are designed in the computer, fabricated by a rapid prototyping machine, and used during surgery to accurately position the bony segments. The protocol has been proven to be more accurate and efficient than the traditional planning methods.

Planning orthognathic surgery using CASS differs fundamentally from planning the same surgery using traditional methods. As a result, we have developed a new planning protocol for CASS. Over the years, this process has been improved to make it more efficient. The purpose of this paper is to present this streamlined CASS protocol ( Fig. 1 ).

Fig. 1
Algorithm for planning a double-jaw orthognathic surgery using the CASS protocol.

The CASS protocol

Before employing the CASS protocol, a surgeon must confirm that the patient is ready for surgery, i.e., the desired occlusion is achievable and the patient’s growth and medical condition are optimal for the surgery. In order to assess the occlusion, a surgeon hand-articulates progress dental models in class I occlusion, confirming that the patient is ready for surgery. This step is required because a computed tomography (CT) or cone-beam CT (CBCT) scan is needed for CASS, and patients should not be exposed to ionizing radiation if they are not going to have the surgery. For the purpose of this manuscript, the term ‘CT’ is used to refer to both CT and CBCT in the following text, unless specified otherwise.

The CASS protocol presented accomplishes the tasks of modelling, planning, and preparing for plan execution. The task of the modelling is to construct a virtual model of the head (cranium and face). This model has three distinct characteristics: (1) it renders the patient’s bones, teeth, and soft tissues with accuracy; (2) its mandible is in centric relation, an important reference position in orthognathic surgery; (3) it has an anatomical reference frame.

CT scans can be used to create 3D models of the facial skeleton, teeth, and soft tissues. However, the teeth of these 3D-CT models are not sufficiently accurate for surgical planning. The CASS protocol presented solves this problem by replacing the inaccurate teeth of the CT with accurate digital dental models that are created by scanning stone dental models. A facial model created by aligning and merging digital dental models into a maxillofacial CT is called a ‘composite model’. This aligning and merging process is called ‘registration’. The registration is done by aligning corresponding features (fiducial markers) that are presented in both images. The fiducial markers can be part of the anatomical structures being imaged, or easy to identify parts that are added in, on, or around the objects (or subjects) before the image acquisition.

We have developed and validated a fiducial registration system for making composite models. In addition to allowing for accurate registration, this system also assures the mandible is in centric relation during CT scanning. This system uses a two-part device consisting of a bite-jig and a fiducial face-bow ( Fig. 2 A) . The bite-jig serves two purposes: it anchors the fiducial face-bow to the patient, and it keeps the mandible in centric relation during CT acquisition.

Fig. 2
Creation of the composite model. (A) A face-bow with fiducial markers is attached to the bite-jig. (B) The patient bites on the bite-jig and face-bow during the CT scan. (C) Four separate but correlated computer models are reconstructed: a midface model, a mandibular model, a fiducial marker model, and a soft tissue model (not shown). (D) The bite-jig and face-bow is placed between the upper and lower plaster dental models during the scanning process. (E) Three separate but correlated digital dental models are also reconstructed: a maxillary dental model, a mandibular dental model, and a fiducial marker model. (F) By aligning the fiducial markers, the digital dental models are incorporated into the 3D-CT skull model. The computerized composite skull model is thus created. It simultaneously displays an accurate rendition of both the bony structures and the teeth.

The composite model used for planning must have an accurate anatomical frame of reference. This is a Cartesian frame that includes a midsagittal plane, an axial plane, and a coronal plane. This reference frame is the basis for most diagnostic and treatment decisions. Incorrectly defining the reference frame may cause postoperative deformity. The reference frame for a composite model is usually established using either anatomical landmarks or the neutral head posture (NHP).

Using anatomical landmarks to create a Cartesian frame seems simple. The midsagittal plane can be constructed using any three midline landmarks. The axial plane can be the Frankfort horizontal (FH) and can be constructed using three of four points: right orbitale, left orbitale, right porion, and left porion. The coronal plane can be constructed as the plane that passes through both porions, remaining perpendicular to the other two planes.

However, this method only works when the face is perfectly symmetrical. In facial asymmetry, various combinations of three midline landmarks produce different midsagittal planes. Various combinations of FH points also result in different axial planes. Moreover, FH is usually not perpendicular to the midsagittal plane in facial asymmetry—a fundamental requirement of a Cartesian system. Finally, a coronal plane cannot be constructed if the two other planes (midsagittal and axial) are not perpendicular. Since all faces have some degree of asymmetry, using anatomical landmarks to build a reference frame is complicated.

The principle behind using NHP is that a reference frame for the head can be derived from the patient’s head posture. When a patient stands upright looking straight forward, the cardinal directions of the face are orthogonal to gravity. The axial plane is perpendicular to the gravitational pull and the midsagittal and coronal planes are aligned with it. Thus, when the head is in the NHP, it is simple to construct a reference frame for the face. The axial plane is the horizontal plane that passes through both porions. The midsagittal plane is a vertical plane that best divides the face into right and left halves. The coronal plane is the vertical plane that is perpendicular to the other planes and is aligned with the coronal suture.

The remaining tasks of the CASS protocol are surgical planning and preparing for plan execution. Surgical planning uses a visualized treatment objective (VTO) approach, simulating the operation until the desired outcome is attained (visualized). Preparing for plan execution is getting ready for surgery. To complete this task, a surgeon collects clinical measurements and fabricates physical appliances such as occlusal splints, templates, and drill guides. These appliances are designed in the computer and fabricated using a rapid prototyping machine.

The CASS is implemented clinically in four steps: (1) collection of preoperative records, (2) data processing, (3) surgical planning, and (4) preparing for plan execution. The first and the third steps are completed by a surgeon, while the other two can be outsourced, either to an independent service provider or to a planning specialist within one’s clinic or institution.

Clinical implementation of the CASS protocol

Collection of preoperative records

In this step, preoperative data are gathered in an hour-long appointment. This appointment is scheduled 2–4 weeks prior to the surgery. The surgeon and an assistant accrue the records. These include dental impressions and their stone models, a bite-jig, clinical measurements, photographs, a NHP recording, a CT scan, and a bite registration of the final occlusion. This appointment has eight steps: (1) taking and pouring dental impressions; (2) fabricating a bite-jig; (3) taking clinical measurements; (4) clinical photographing the patient; (5) recording the patient’s NHP; (6) testing the fit of the bite-jig on the stone dental models; (7) acquiring a CT scan; and (8) establishing final occlusion.

The preoperative record appointment begins by taking and pouring upper and lower dental impressions. A surgeon takes the impressions which are then immediately poured by an assistant. The impressions are taken and poured at the beginning of the appointment to shorten its length. The fitting of the stone dental models on the bite-jig must be confirmed before the patient undergoes a CT scan. However, this check cannot be done until the stone models are set. Since it takes about 45 min for the stone to set, postponing the models will delay the CT. Thus pouring the impressions early allows the models to be ready on time.

The number of impressions needed for planning depends on the type of surgery. Obviously, the surgeon needs to take at least one upper and one lower impression. But, when one is planning to segmentalize a dental arch into two or more pieces, the clinician should take two impressions of the arch needing segmentation.

In the second step of the preoperative appointment, a patient-specific bite-jig is constructed by customizing a stock jig frame ( Fig. 3 ). The surgeon customizes the frame by adding a self-curing, dimensional-stable and rigid bite registration material (e.g. LuxaBite; DMG America, Englewood, NJ, USA) to the frame. The jig is then placed between the patient’s teeth until the material is cured. This bite registration should be taken in centric relation.

Fig. 3
Fabrication of the patient-specific bite-jig using a three-layer approach to prevent undercuts on the bite-jig and correctly record centric relation. The first layer of the registration material is only placed on the maxillary side of the bite-jig frame to capture the geometry of the maxillary dental occlusal surfaces (A). Before the material completely sets (e.g., about 2–3 min), the bite-jig should be gently and repeatedly taken off (B) and placed back (C) on the teeth a few times in order to get rid of any possible undercuts. Once the material is completely set (e.g., about 5 min), the first layer should be ground to thin it but remain thick enough to ‘lock’ onto the maxillary occlusal surface (D). The bite-jig is then replaced on the maxillary dental arch. The surgeon should make every attempt to position the patient’s mandible to centric relation. While in the centric relation, the second layer is added at the buccal and labial side of the bite-jig (E). Once the material is completely set, a third layer is added to capture the geometry of the mandibular dental occlusal surfaces (F). Guided by the second layer, the mandible should be positioned in centric relation for the third layer impression using the same method as indicated for the first layer.

In the third step of the preoperative appointment, key clinical measurements needed for planning are recorded. These include: (1) rest-incisal-show and smile-dentogingival-show, to determine the vertical position of the maxilla; (2) dental midpoint (midline) deviations, to determine transverse jaw position.

In the fourth step, clinical photographs of the patient are taken. The photographs should image the face and the teeth. The facial photographs should be taken with the patient in the NHP. In addition, a plumb line should be shown on the background, so the correct orientation of the face can be determined later. These photographs will be used to verify that the virtual head model is correctly oriented for planning.

In the fifth step of the preoperative appointment, the NHP is recorded. This measurement is needed to create an anatomical reference frame (midsagittal, axial, and coronal planes) for the virtual facial model. To record the NHP, the authors use an electronic orientation sensor (3DM; MicroStrain Inc., Williston, VA, USA). The sensor is first attached to the previously fabricated bite-jig ( Fig. 4 A, B ). The jig is then placed between the patient’s teeth. The patient is asked to stand upright with their head in NHP. Finally, in this posture, the pitch, roll, and yaw of the sensor are recorded.

Fig. 4
Orientation of the composite skull model to the neutral head posture (NHP) using the digital orientation sensor method. (A) A digital orientation sensor is attached to the bite-jig and face-bow. (B) The pitch, roll, and yaw of the sensor are recorded. (C) In the computer, a digital replica (CAD model) of the orientation sensor is registered to the composite skull model via the fiducial markers, and the two objects are attached to each other. (D) The recorded pitch, roll, and yaw are applied to the face-bow frame, reorienting the composite skull model to the NHP. (E) After the composite skull is orientated to the NHP, the CAD model of the orientation sensor is marked hidden.

The NHP can be attained by asking the patient to self-balance into this stance, or by manipulating their heads into this posture. The authors begin with a self-balance position, but feel free to manipulate it, if the patient’s posture is incorrect.

In the sixth step, a surgeon or an assistant tests the fit of the stone dental models on the bite-jig. This test is done to confirm that the models and the bite-jig are correct. If the models do not fit the bite registration, one can assume that either the dental models or the bite-jig are distorted. Common reasons for the lack of fit are air bubbles on the occlusal surfaces of the dental impressions and deep indentations in the bite registration. Air bubbles in an impression are filled with stone during pouring, resulting in spherical bumps that arise from the surfaces of teeth. These prevent the model from seating into the bite registration. Fortunately, they are easily spotted and removed with a sharp instrument to improve the fitting. Deep indentations in the bite registration, especially those that extend above the height of the contour of the teeth, also prevent the models from fully seating on the bite registration. This lack of seating occurs because the material used for bite registration is rigid. This problem can be solved by grinding the bite registration to reduce the indentations.

If the dental models and bite-jig still do not fit after the above manoeuvres, the bite-jig should be retested on the patient’s teeth. If the bite-jig fits the patient well, it must be assumed that the stone models are distorted. The dental impressions should be repeated. If the bite-jig does not fit well, the jig should be relined, and the NHP recording should be repeated, because relining of the bite-jig can affect the alignment of the orientation sensor.

In the seventh step of the preoperative appointment, a CT scan is obtained. Before scanning, the fiducial registration face-bow is attached to the bite-jig and the jig is affixed to the patient ( Fig. 2 B). The patient is instructed to keep his/her facial soft tissues relaxed during scanning. The scan can be completed using either a spiral multi-slice CT scanner (using the standard scanning algorithm: matrix of 512 × 512 at 0.625–1.25 mm slice thickness, 25 cm or lesser field-of-view, and 0° gantry tilt) or a CBCT scanner (with an isotropic voxel size of 0.4 mm).

In the final eighth step of the preoperative appointment, the surgeon establishes the final occlusion. This is usually done after the patient leaves the clinic. Currently, we establish the final occlusion on stone dental models because there is no reliable method to establish final occlusion in the computer, especially when one of the jaws is segmentalized.

When dental arch segmentation is not required, the maxillary and the mandibular stone dental models are mounted on a Galetti articulator and articulated into final occlusion. Final occlusion is then captured with a bite registration taken with a stable material like silicone or polyvinyl siloxane.

When arch segmentation is required (e.g., a three-piece Le Fort I osteotomy), a surgeon cuts the extra stone dental model into the required number of segments. Then, the surgeon hand-articulates each segment into final occlusion and makes a new plaster base for the segmented model. Finally, the surgeon articulates the upper and lower models into final occlusion (in a Galetti articulator), capturing the occlusion with a bite registration.

Clinical implementation of the CASS protocol

Collection of preoperative records

In this step, preoperative data are gathered in an hour-long appointment. This appointment is scheduled 2–4 weeks prior to the surgery. The surgeon and an assistant accrue the records. These include dental impressions and their stone models, a bite-jig, clinical measurements, photographs, a NHP recording, a CT scan, and a bite registration of the final occlusion. This appointment has eight steps: (1) taking and pouring dental impressions; (2) fabricating a bite-jig; (3) taking clinical measurements; (4) clinical photographing the patient; (5) recording the patient’s NHP; (6) testing the fit of the bite-jig on the stone dental models; (7) acquiring a CT scan; and (8) establishing final occlusion.

The preoperative record appointment begins by taking and pouring upper and lower dental impressions. A surgeon takes the impressions which are then immediately poured by an assistant. The impressions are taken and poured at the beginning of the appointment to shorten its length. The fitting of the stone dental models on the bite-jig must be confirmed before the patient undergoes a CT scan. However, this check cannot be done until the stone models are set. Since it takes about 45 min for the stone to set, postponing the models will delay the CT. Thus pouring the impressions early allows the models to be ready on time.

The number of impressions needed for planning depends on the type of surgery. Obviously, the surgeon needs to take at least one upper and one lower impression. But, when one is planning to segmentalize a dental arch into two or more pieces, the clinician should take two impressions of the arch needing segmentation.

In the second step of the preoperative appointment, a patient-specific bite-jig is constructed by customizing a stock jig frame ( Fig. 3 ). The surgeon customizes the frame by adding a self-curing, dimensional-stable and rigid bite registration material (e.g. LuxaBite; DMG America, Englewood, NJ, USA) to the frame. The jig is then placed between the patient’s teeth until the material is cured. This bite registration should be taken in centric relation.

Jan 17, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Algorithm for planning a double-jaw orthognathic surgery using a computer-aided surgical simulation (CASS) protocol. Part 1: planning sequence
Premium Wordpress Themes by UFO Themes