This article presents a modification of intraoperative external fixation for mandibular reconstruction with free tissue flaps. This technique is indicated when preregistration of the reconstruction plate is not possible due to transmandibular tumor extension. Once standard external fixation has been carried out and prior to segmental mandibulectomy, additional pins are fixed to the connecting rod that delineate the mandibular contour in three-dimensional (3D) space. Following mandibulectomy, these pins allow accurate contouring of the reconstruction plate and improved restoration of mandibular contour, projection, and dental occlusion. A step-by-step description of the technique using models and intraoperative photos is presented. This method of mandibular reconstruction is a simple and time-effective alternative to intraoperative computer navigation and 3D modeling in select cases of oral carcinoma where tumor infiltration of the outer mandibular cortex precludes prebending of the reconstruction plates.
The introduction of mandibular reconstruction plates, and subsequently, vascularized bone-containing free flaps for mandibular reconstruction, has provided head and neck surgeons with the reconstructive tools to aggressively resect tumors involving the mandible. Reconstruction aims to recreate bone continuity and stabilize the remaining mandibular segments while maintaining preoperative occlusion, original projection, and contour.
In common situations, prior to tumor excision, a reconstruction plate is prebent over the mandibular cortex, to match the contour of the mandible. Bicortical screws are temporarily inserted and retrieved from the proximal and distal segments. Once the ablation is performed, the prefashioned plate permits accurate stabilization of the mandibular segments, and accurate bony reconstruction follows.
In particular situations where tumor infiltration of the outer mandibular cortex precludes prebending of the plates, other reconstruction strategies are required to maintain the preoperative three-dimensional (3D) bony anatomy. Mandibular position can rarely be maintained with intermaxillary fixation alone. In most cases, external fixation is necessary. The contour of the reconstructed segment may be improved with the use of 3D models or computer navigation.
Previous descriptions of external fixation used for stabilization of proximal mandibular segments during ablative surgery have demonstrated the efficacy of this technique.1, 2 Although the remaining mandibular segments are immobilized in their original position, the resected segment is reconstructed according to the surgeon's assessment, in the absence of guidance. This can result in inaccurate projection and bone contouring especially when the anterior segment is involved.
We present a simple modification of the standard intraoperative external fixation technique intended to help maintain the exact 3D anatomy of the reconstructed mandible.
The external fixator device (Synthes CMF, West Chester, PA) is placed at the time of the ablation prior to performing bone cuts. The titanium prebent rod does not require additional contouring in most cases. If needed, the rod is contoured using the rod bender with or without using the rod template. The rod should be positioned approximately 2 to 3 fingerbreadths away from the buccal cortex of the mandible. As opposed to the older system, new single-rod external fixation devices are far more rigid and minimize intraoperative movement of bone segments. Once the placement of the bone cuts has been determined, two Schanz screws will be placed on either side of the resected segment (Fig. 1). Screw placement must take into account and not interfere with the desired position of the reconstruction plate. Space for at least three holes of the mandibular reconstruction plate should be available from the Schanz screws to the cut edges. The four screws are attached to the rod with four adjustable clamps, and a rigid construct is obtained by tightening the clamps with the socket wrench. At least one or two additional Schanz screws are used as markers (guidance pins) to indicate the projection of the mandible to guide contouring of the plate. These pins must be placed with their tips closest to the lowest external mandibular cortex while avoiding violation of the tumor or involved soft tissue. To allow for the placement of the guiding pins, the rod must have been previously positioned at the appropriate distance from the mandible (2–4 cm). Bone cuts can be performed along with the tumor excision once the rigid system with guidance pins is set up. Accurate plate contouring is now performed following the guidance pins' position to avoid over- or underprojection of the jaw. The external fixator system can be removed once rigid internal plate fixation has been achieved. As a personal preference, the senior author uses single 2.0 or 2.4 locking reconstruction plates. This compensates for imperfect plate contouring and allows for easier fixation of the bone flap. Alternatively, nonlocking reconstruction plates may be used. The plate's thickness must be taken into account in the expected final projection. The technique was performed on a mandible model for illustration purposes (Fig. 1).
We report a modified use of external fixation that permits intraoperative registration of mandibular contour, replaces the need for expansive 3D models, and avoids inaccuracies that are common when using traditional external fixation. The use of the external fixator helps stabilize the proximal mandibular segments, and the use of guidance pins enables more accurate reconstruction of the diseased segment, which translates into better projection and contour.
Temporary external fixation alone is adequate for lateral defects when the mandibular angle and the ipsilateral parasymphysis can be preserved. The bridging plate only requires minimal bending, and the use of guidance pins does not offer significant benefit in this situation.
However, anterior defects or lateral defects that encompass the mandibular angle necessitate significant in-plane, out-plane bending and torquing of the reconstruction plate. These defects highlight the difficulty in recreating a curve with only two reference points (cut edges). Contouring the plate may result in over- or underprojection of the mandible, widening of the face and malocclusion.
The guidance pins provide multiple points of reference, permitting accurate restoration of mandibular contour. To serve as a template for plate bending, sufficient guidance pins should be placed along the inferior border of the mandibular segment to be resected. The utility of this technique is marginal in rare cases of pathologic fracture or massive expansion of the entire buccal cortex.
Although requiring familiarity with the external fixation device, this method is both time and cost effective. One significant risk is possible displacement of the guidance pins or loosening of the external fixation during the ablation of the tumor. The use of a single-rod device and proper placement of the Schanz screws, permitting unobstructed access for mandibulotomy, is crucial.
The combined use of 3D modeling with either intermaxillary fixation3 or external fixation4, 5 has been used to reconstruct anterior segments with external cortical involvement. A resin model is preoperatively manufactured based on computed tomography images, and the exophytic part (tumor) is burred down to the estimated premorbid contour. The model is then used as a template for plate bending and positioning of the screws. Although cumbersome and expensive, this method offers the possibility to recreate one half of the mandible as a mirror image of the other half. This is advantageous when pathological fractures occur prior to the reconstructive procedure.5 In other situations, however, the use of guiding pins offers an adequate substitute to the resin model.
We present a simple modification of the intraoperative external fixation applied to challenging cases of mandibular reconstruction when preregistration of the reconstruction plate is not possible due to transmandibular tumor extension. This should be considered as an alternative to 3D models and intraoperative navigation.
The authors thank Synthes (Canada) Ltd., Mississauga, Ontario, Canada, for material support.