Resection-osteotomy guides are used to translate the virtual setting into the operation room, for accurate mimicking of the computer-assisted surgery. Accurate reconstruction is interconnected with the accuracy of the resection guide in transferring the three-dimensional position intraoperatively [7, 15]. With the lack of consensus regarding the ideal configuration of the resection guide, this study aimed to compare the accuracy of different resection guides in translating the three-dimensional virtual position intraoperatively.
A total of 52 osteotomies were conducted and analyzed in this study. The positional accuracy of the resection guide was evaluated in a linear deviation manner, by antero-posterior deviation measurement, and in an angular manner, by pitch rotation around the FHP and yaw rotation around the MSP difference calculation. Since the resection was implemented all the way from the buccal to the lingual, roll rotation analysis was of no value.
Regarding the liner antero-posterior deviation, the difference between the directional-guided and the edge-cutting guides was statistically significant (P < 0.001). The greatest reported mean deviation was 2.1 ± 1.2mm for the proximal plane in the control group, and the least reported mean deviation was 0.30 ± 0.19 mm for the distal plane in the study group. The resection guide accuracy evaluation is usually reported in the literature in a two-dimensional form analysis, with the presentation of the distance difference between the corresponding resection planes. Roser et al. conducted a retrospective study for the analysis of the accuracy of virtual planning in cases with mandibular resection and fibular graft reconstruction. Their study reported a total of 19 osteotomies, with a mean linear deviation of 2.00 ± 1.12 mm [9]. Shu et al. evaluated the accuracy of CAS in the reconstruction of mandibular defects with iliac crest graft. They documented a mean resection plane deviation of 2.3 ± 1.0 mm [17]. Both of the above-mentioned studies utilized a conventional edge-cutting resection plane, and their results fall in line with those obtained in the control group cases [9, 17]. Brouwer de Koning et al. utilized a resection guide with a directional guidance slot for the conduction of their mandibulectomy procedure[10]. They reported a mean deviation of 2.2 ± 0.9 mm and 1.2 ± 1.0 mm for the posterior and anterior osteotomies respectively [10]. In this study along with that conducted by Brouwer de Koning et al, a lesser value of deviation was reported in the distal plane than in the proximal one [10]. Zho et al compared the efficacy of two different resection guide forms in mandibular reconstruction using a vascularized iliac crest flap[18]. Their complicated guide contained a directional guidance slot, which reported a mean linear conjunction gap difference of 1.6 ± 0.7mm [18]. The favourable negatable millimetre deviation reported in this study is correlated with the favourable postoperative resection specimen histopathological analysis, where none of the cases showed a positive margin. Accordingly, one of the main leverages of the VSP in mandibular reconstruction is lowering the rate of positive bone margin.
Changes in the linear dimension may not provide a complete picture regarding the three-dimensional position of the resection plane. Regarding the angular antero-posterior deviation, the templet with a directional guidance slot reported a statistically significant difference in both the pitch and the yaw rotation deviations when compared to the conventional edge-cutting guide (P < 0.001). The greatest reported mean deviation was 9.1 ± 9o for the yaw rotation of the distal plane in the control group, and the least reported mean deviation was 1.1 ± 0.6o for the pitch rotation of the proximal plane in the study group. Brouwer de Koning et al reported a comparable plane-angular deviation, with a mean anterior osteotomy deviation of 2.6o pitch and 5.1o yaw, and a mean posterior osteotomy deviation of 4.2o pitch and 9.5o yaw [10].
The difference between both groups regarding the yaw angle indicates that the directional guidance slot was able to control the saw movement through the resection procedure, which was not achieved in the control group. Errors in angular deviation usually lead to deviation of the osteotomy toward the resected part, leading to a reduction of the safety margin, and less bone is removed. Furthermore, the impeccable angular deviation outcome in the cases where a guide with a directional slot was utilized demonstrates that the slot and the saw did not have any excess clearances that may allow any unwanted directional deviation. This was attained by preoperative calibration of the utilized oscillating saw blades and designing the slot in accordance with this size and an offset of 0.01. Additionally, Changes in the angulation of the resection plane may provide difficulties in the position of the reconstruction bone block, as the harvested blocks are usually fabricated to the shape of the defect in the virtual setting. Despite that, the reported angular deviation had a minimal effect on the graft-stump contact and implanted bone blocks did not need any manual adjustment, and a precise fit was obtained. However and according to our experience, the cases with the directional guidance slot showed a better fit in the reconstructive bone block insertion, especially when a customized reconstruction plate was utilized.
Despite being both conducted with satisfactory accuracy, the proximal plane showed a more accurate angular performance while the distal plane showed a more accurate linear performance. Brouwer de Koning et al conclude that seating the guide in the anterior region is an easier procedure since it is better exposed and accessed [10]. Positioning of the proximal osteotomy may be hampered by the soft tissue overlying the ramus and inadequate exposure, which increases the deviation of the cutting tool's 3D direction. This may show the importance of the creation of a directional guidance slot to contain the saw direction throughout the resection procedure. Which once seated, the direction of the cutting will not be affected
Insertion Accuracy of the harvested reconstruction bone block in the resected defect was analyzed to determine the accuracy of the preoperative VSP and the effect of the resection guide in maintaining the space for accurate reconstruction bone positioning. Both groups reported an excellent level of agreement between the virtual and actual defect extent (ICC = 1.00). Despite that, the guide with a directional guidance slot yielded an absolute mean difference of -0.09 ± 0.37 mm, in comparison to the 0.59 ± 1.27 mm in the conventional group.
The study was aimed at determining the accuracy of the resection template in transferring the three-dimensional resection plane position intraoperatively. Despite that, the study adhered to the guidelines proposed by van Baar et al and modified by El-Mahallawy et al in reporting CAS in mandibular reconstruction surgery [7, 15]. Standardization of the preoperative and postoperative MSCT machine and scanning parameters, reporting in Brown classification, overlapping the condylar segments, XYZ planes alignment, and finally statistical analysis with the agreement between virtual and postoperative actual measurements was performed in this study. These guidelines helped in a robust analysis of the resection plane positional accuracy, along with obtaining reproducible outcomes in an attempt for standardization.
In this study, primary reconstruction with implantation of bone graft/flap was conducted in 20 patients. Insertion Accuracy of the harvested reconstruction bone block is imperative in order to achieve an immanent graft-stump contact [9, 10]. The outcome of the resection guide with a directional slot was statistically significant when compared to the conventional edge-cutting guide in both lateral and vertical shifts, and in both the proximal and distal planes. The directional guidance template showed a millimetre accuracy with the least reported absolute mean deviation of 0.62 ± 0.24mm in the proximal plane vertical shift, and heights reported an absolute mean deviation of 0.74 ± 0.28mm in the proximal plane lateral shift. Ciocca et al analysed the accuracy of fibular graft insertion with the use of a 3D-printed reconstruction plate. They reported a lateral shift of 1.36 and 2.22, and vertical shifts of 2.93 and 2.90 mm for the anterior and posterior block points respectively [19]. The outcomes in this study are comparable and even more accurate, especially in the cases managed with a directional slot resection guide form. This satisfactory clinical performance in graft/flap insertion may be accredited to the utilization of a reconstruction-fixation guide, which utilized the same screw boreholes as the resection guide for three-dimensional spatial control of the area between the proximal and distal stumps during the period of plate fixation. In this study, we utilized the reconstruction-fixation guide in all of the enrolled cases and even in those managed with a patient-specific plate.
Custom-made resection guides are used to translate the virtual blueprint into the actual operation room. The main intent of any rehabilitative procedure is primarily the complete eradication of the diseased tissue with the appropriate safety margin in order to prepare the surgical bed for the reconstruction step [20]. The accuracy of the resection guide insertion and osteotomy performance is directly proportionate to the complete tumour resection and reconstructive procedure. The study is limited by the variability in the manner of inclusion of the mandibulectomy defect, however, it aimed to limit the confounding variables by excluding cases with condylar resection. Furthermore, the assessment of the resection plane and block position accuracy mandates proficiency in several computer programs with a steep learning curve.
Errors in the exact position of the osteotomy guide have an impeding effect on the fitting of the harvested graft, which increases operative time and lessens the accuracy of the procedure[21]. Chackartchi et al report that a more complex design and achieving more virtual work to produce a totally limiting implant placement guide consequences in a more precise implant positioning with few errors and a decrease in the procedure time [22]. The same goes for the maxillofacial reconstruction field. An improved design technique influences the procedure conveyance with the limitation of free-hand deviations. The use of virtual planning in mandibular reconstruction is of proven accuracy [7, 15]. Despite that, the literature lacks uniformity about a definitive sculpt for the guides as it is normally a surgeon/engineer experience-based with demand-based changes. The utilization of a directional guidance slot in the resection-osteotomy guide provided the surgery with impeccable accuracy and control over the three-dimensional position of the osteotomy and the reconstruction bone block.
Despite reporting a significant difference, both groups documented a satisfactory outcome which entails the well-proven accuracy of virtual surgical planning in maxillofacial surgery. At the same time, the conduction of the osteotomy was more proficient when a directional guidance slot was incorporated into the design. It allowed an accurate transfer of the three-dimensional position of the virtual resection plane intraoperatively with satisfactory overall clinical performance. The promising results may endorse the generalization of the resection guide design with a directional guidance slot in computer-assisted mandibular resection procedures.