CAS in mandibular reconstruction with fibular free flaps has become a new standard in medical treatment. A recent systematic review and meta-analysis showed various benefits, such as increased precision and accuracy in reconstruction, decreased operative time 6,13 and increased long-term stability 14.
In general, it was found that CAS for fibular free flaps can be divided into two parts. The first part is for surgical planning and creating cutting guides to assist in the surgery. The second part involves mandibular plate preparation for the fixation. According to published literatures, the subjects were separated into three categories by using the different plate fixation methods. The first group used CAD/CAM combined with patient-specific plate 14–16. The second group used CAD/CAM combined with manually bending reconstruction plate 7,8,17−25. The last group used CAD/CAM combined with miniplate 26,27.
The factors affecting the selection of fixation plate are the availability of equipment, urgency of the operation, surgical planning preparation time, 3D printing process, CAD/CAM equipment transportation time, and expenses. Moreover, CAD/CAM technology has limited availability, only special laboratories in some hospitals have this service. So, the patient-specific plate are not always available for use in mandibular reconstruction. For the alternative ways of the aforementioned issue, the authors aimed to determine which plate fixation technique is suitable and assists reconstruction of mandible with fibular free flaps.
Patient-specific Plate
Regarding patient-specific plate, we hypothesized that the cutting guide determines the accuracy of the mandibular reconstruction by affecting the positioning of screws. The condylar errors were greatest on the y-axis (superior-inferior direction). After we analysed the position of the screws on the bone surface, it was found that there was only a petite error (0.56 ± 0.38 mm) that was not proportionate to the condylar error (8.09±2.52 mm), which was 14 times greater. Therefore, an in-depth analysis of screws direction was conducted. Apart from screw position at the bone surface, the errors of screw direction inside the bone were high on the y-axis (superior-inferior direction) and directly resulted in the condylar error. The average errors of screw hole position at the bone surface were not significantly different regarding extent of the defect, which indicated the accuracy of the screw hole direction at the point of entrance. The screws that located near osteotomy sites affected the condylar errors for both direction and magnitude. Screw direction shifting was also found with the manually bending reconstruction plate. The underlying cause of direction shift in manually bending reconstruction plate was suspected from the relatively large screw size, which is more resistant the bone, resulting in a deviation of the screw from the predrilled axis. The finding supporting this hypothesis was that no significant error was found for fixation with miniplate, which uses smaller screws. In contrast, the smallest error on the x-axis (medial-lateral direction) could be explained by the supporting forces from surrounding soft tissue and TMJ on both sides of mandibular condyle.
Therefore, to reduce error on the y-axis (superior-inferior direction), the cutting guide must be meticulously assembled and applied, and the screws must be installed precisely in all axes through the predrilled tract.
According to the results of the current study, patient-specific plate do not have significant benefits over manually bending reconstruction plate in terms of accuracy in the limit defect. This may be due to the ability of surgeons to bend the commercially available manually bending reconstruction plate for short and uncomplicated defects using the hands and eyes. However, the surgeons’ hand skills cannot overcome the challenge of extensive defects in the mandible, as we will describe in the next section.
Manually Bending Reconstruction Plate
The error of the manually bending reconstruction plate was highest on the z-axis (antero-posterior direction). We believe that this error occurred from the difficulty of plate bending, which was impeded by plate thickness. Surgeons can use the bone model of mandible or the native mandible as a reference. The main bending points are usually the points that can be clearly seen, such as the seam between the anterior and lateral part of the mandible, by bending the plate at significant points, which normally have 1-3 angles. However, in reality, the mandible is gradually curving and changes direction for two or more axes along its part. The limitation of manually bending resulted in the inadequate alignment between plate and bone surface. Furthermore, tipping and ejection of plate on another side was occurred after fixation of plate on one side. The shifting of plate in antero-posterior axis (z-axis) might be occurred from fixation force that try to align the plate on mandible and there is room for the plate in this direction only (Figure 10).
The patient-specific plate has not only a great accuracy but also the durability to force load. There was an in vitro study by Kasper et al., which reported that patient-specific plate were seem to benefit over manually bending reconstruction plate in terms of durability against physiological force loading 16. These results suggested the benefit of using patient-specific plate over manually bending reconstruction plate.
Miniplate
Most of published literatures focused on stability of fixation and postoperative complications between using miniplate and manually bending reconstruction plate. The difference in complications between these two plates remained controversial 28–30. A finite element study showed better stability in manually bending reconstruction plate over miniplate 31. Despite this question was critical, the data regarding the accuracy of miniplate fixation were limited.
In our recent study, it was found that using of miniplate could create condylar error on any axis. The leading cause of this error might occur from the uncontrollable overall shape of neo-mandible upon miniplate fixation. The aforementioned error was solved by using a “bar” to connect between both sides of the cutting guide resulted in regulation of shape of the fibular segment and miniplate complex32. Nevertheless, there were factors involving errors of fixation, not only the bone, cutting guide, and plate complex, but also the forces from surrounding soft tissues, which distorted the plate and neo-mandible system postoperatively. Up to the present date, the studies on long-term outcomes of miniplate fixation are limited. Therefore, as the current evidence suggests, we do not recommend miniplate fixation as the first place. The summary of the errors in each plate type and recommendations is given in Table 1.
Table 1
The errors in each plate type and recommendations. The miniplate, manually bending reconstruction plate, and patient-specific plate had their own errors and limitations. However, there are several ways to minimize these errors, and surgeons should consider these issues to achieve favourable outcomes.
Type of plate | Findings | Management |
Miniplate | • Errors can occur in any direction. • The greatest error occurs along the x-axis. | • Use other types of plate instead of a miniplate. • Use connecting bar with a cutting guide 32. • Avoid widening gap between the fibula and plate lining. |
Manually bending reconstruction plate | • The manually bending reconstruction plate is not fit to the mandible. • Increasing error in extensive defects. • Errors of screw direction occur beyond the bone surface. | • Avoid using manually bending reconstruction plate for extensive defect reconstruction. • Avoid forceful drilling in patients with osteoporosis. • Direction of screws should follow the predrilled axis. • The screw should be gently inserted along the predrilled tract to avoid false tract creation. |
Patient-specific plate | • Errors of screw direction occur beyond the bone surface. (especially on the z-axis). | • Direction of screws should follow the predrilled axis. • Avoid forceful drilling in patients with osteoporosis. • The screw should be inserted gently along the predrilled tract to avoid false tract creation. |
Clinical Case Analysis
From the analysis of errors in the actual reconstructed mandible in vivo, there was lesser errors than the surgical simulation in bone model. Several hypotheses had been proposed for this manifestation. First, the bone model has a different internal supporting structure from human bone, which might cause different errors. In this regard, the surgeon may need to avoid forceful drilling when using relatively large screws, which are compatible with some types of plate, in patients who are fragile and/or have low bone density, such as elderly patients and those with osteoporosis. Furthermore, screws should be gently inserted along the predrilled axis to avoid false tract creation. Second, as mentioned above, is that in the actual patient, the surrounding soft tissues and both TMJs serve as support structures and guides for plate position in a functional position.
For the comparison of errors, the fibular error was greater than the mandibular error since the two errors in fixation occur first with the mandibular screw and second with the fibular screw. Hence, screw application and plate fixation should be performed meticulously to reduce this error.
Gravvanis et al. 33 recommended reattaching the masseter muscle to the neo-mandible to help reduce the patient's recovery time. On the other hand, this small error may not affect the patient's functional outcome, and one could question the value of the investment, in terms of cost and time, in CAD/CAM and patient-specific plate. For example, the issue of minimal clinically important differences (MCIDs) in mandibular reconstruction remains a question that requires further study 12.