SA1 - Can We Trust Predicted Screw Lengths of Virtual Surgical Planning?

Abstract: Computer assisted surgery (CAS)-guided reconstruction confers a statistically significant improvement in volumetric surgical accuracy, however, there is little evidence to show the accuracy of the pre-determined CAS screw lengths. In head and neck reconstructive surgery, individualized CAS can facilitate the precise preoperative planning of screw lengths for ideal bi-cortical placement. Therefore, the purpose of this study was to determine the accuracy of pre-determined CAS screw lengths.

Inclusion criteria consisted of patients who had received head and neck reconstructive surgery with individualized preoperative CAS completion. The study was completed by comparing patients' postoperative CBCT scans to the preoperative CAS. CBCT scans showcased the actual length and placement of screws in native bone from 3D printed drill guides intraoperatively. Clinical outcomes were measured in terms of millimeters of difference from ideal positioning; screws that were past the posterior cortex were given positive value and screws that did not reach the posterior cortex were given negative value.

The study analyzed clinical and surgical predictor variables that could influence these outcome measures. These variables include benign vs malignant pathology, post height on 3D printed drilled guides, intraoral vs extraoral approach to reconstruction, and screw placement within the anterior or posterior segment of native bone. Statistical analysis involved paired t-test analysis to study the impact of predictor variables on the outcome measures.

Data was collected from 32 patients who fulfilled the inclusion criteria. Of the 32 patients, 17 males, 15 females, and 46.8 was the average age. Thirty patients had mandibular resections, and 2 patients had maxillary resections. Seventeen reconstructions had both anterior and posterior screw placements in native bone and 15 had anterior screw placements in native bone. Of the 32 reconstructions, 9 reconstructions occurred via extra-oral approach and 23 reconstructions were intra-oral. Four patients had malignant pathology and 28 patients had benign pathology. With 32 drill guides used, 2 mm post heights were used in 9 drill guides, 4 mm post height was used in 1 drill guide, and 8 mm post heights were used in 22 drill guides.

In total, 201 screw placements were analyzed across 32 head and neck reconstructive surgeries. Noted sources of error include the 2 mm error in CBCT measurements, and recognition that screw lengths do not come in every size. Therefore, we considered ± 2 mm to be within the range of accuracy, for an overall 98.5% accuracy. Screws lengths from preoperative CAS planning were more accurate in anterior native bone placements (59.5%) vs posterior bone placements (57.14%) but showed no statistically significant difference (P > .05). Screws length accuracy from preoperative CAS planning via reconstruction from intra-oral (58.6%) or extra-oral approach (57.6%) showed no statistically significant difference (P > .05). Screws lengths from preoperative CAS planning were more accurate in benign (60%) vs malignant pathology (50%) but showed no statistically significant difference (P > .05). Screws lengths from preoperative CAS planning showcased 62% accuracy with 8 mm post heights, 38% accuracy with 4 mm post heights, and 54% accuracy with 2 mm post heights, with no statistically significant difference (P > .05). Therefore, the current study demonstrates that pre-determined CAS screw lengths are accurate for ideal placement in head and neck reconstructive surgery with no clinical and surgical predictor variables affecting outcomes.


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