Session: 07-08-04: Computational Modeling in Biomedical Applications

Paper Number: 191643

A 3D Modelling Approach to Evaluate the Impact of Reconstruction Planning Decisions on Predicted Bone Union Propensity in Mandibular Reconstruction Surgery 

Mandibular reconstruction, following tumor-related resection, is essential for restoring form and function of the jaw by replacing the removed segment with a donor bone. Achieving bony union is critical; however, mechanisms affecting union rates remain poorly understood. There are several preoperative planning and intraoperative decisions made during reconstruction surgery such as fixation plate type, plate placement and osteotomy angle, that likely affect bone union. Recent computational work has demonstrated that bone cut angle significantly affects predicted bone union propensity (BUP) when using full-length reconstruction plates. BUP is a metric which quantifies the proportion of a cut surface exposed to stresses known to simulate bone growth during the chewing cycle.  

Our group is currently developing an alternative navigated surgical procedure that uses mini-plates to secure adjacent segments, in part to reduce issues of infection, plate exposure and stress shielding associated with full-length plates. Mini-plate-based fixation offers a promising alternative; however, little is currently known about the effect of mini-plate-based fixation strategies on BUP. The purpose of this study was to investigate the influence of key surgical decisions on predicted BUP when using mini-plates and determine whether these results are consistent across different defect sites within the mandible.  

Using a physics-based simulation and finite-element modelling, we investigated the effect of plate type, plate placement and bone cut angle across 6 defect types involving combinations of the Ramus, Body and Symphysis. The study involved two key steps; developing a virtual surgical plan and simulating the resulting stresses across the bone cut interfaces over the chewing cycle. Mandible and donor bone computed tomography (CT) scan segmentations were imported into Artisynth’s biomechanical modelling package to virtually reconstruct the mandible. Bone cut angle was varied systematically to represent a range of physiologically feasible angle variations relative to baseline cut angles that represent conventional surgical cut planes. Additionally, we tested several mini-plate designs representing common geometric variations in mini-plate fixation. Plate placement was also varied across low, middle, and high configurations to represent different feasible placements relative to the donor bone segments.  

We modelled the planned reconstruction by treating the maxilla, hyoid, and screws as rigid bodies and the temporomandibular joint (TMJ), donor bone, and mini-plates as finite-element models. The model includes 24 Hill-type muscles, ligaments and a food bolus with 110 N resistance. 7 mm locking screws fixed the plates to the donor and mandible. The chewing simulation was run for a single cycle with muscle activations set to pre-reconstruction levels representing early stages of reconstruction rehabilitation. 

To assess bone healing, the resulting normalized strain energy density (SED), was computed for a single element layer on the left- and right-side mandible-donor interfaces over time. Bone healing is likely to occur when SED exceeds a certain threshold, based on remodeling principles. The apposition percentage was determined as the ratio of elements at each interface exceeding this threshold, to the total number of elements in the layer. We therefore define a predicted bone union propensity (BUP) as the average apposition percentage for the left- and right-sides respectively over the chewing cycle simulation. 

Our results confirm previous reports on the effect of cut angle on predicted BUP in full-length reconstruction plates, extending these findings to mini-plate fixation. When using certain mini-plates, osteotomy angle has a significant effect on BUP increasing predicted BUP from a baseline value of ~23% to as much as 42%. Plate placement also showed a ~6% difference in BUP across the 3 configurations tested. Preliminary results further suggest that some mini-plate designs increase predicted BUP by ~10% compared to traditional full-length reconstruction plates.  

While BUP is a metric based on established bone remodeling principles, limited clinical validation of the model has been performed, warranting further validation using patient-specific clinical data. Overall, our results suggest that the key surgical planning decisions investigated influence predicted BUP, and that the potential for optimizing these decisions should be further investigated and clinically validated.  

Presenting Author: Cassidy Tizzard University of British Columbia

Presenting Author Biography: Cassidy Tizzard is a Master’s student in Biomedical Engineering at the University of British Columbia. Her research focuses on computational modeling, investigating the effects of surgical planning decisions on predicted bone healing outcomes for mandibular reconstruction surgery.

Authors:

Cassidy Tizzard University of British Columbia
Hamidreza Aftabi University of British Columbia
John E. Lloyd University of British Columbia
Eitan Prisman University of British Columbia
Sidney Fels University of British Columbia
Antony J. Hodgson University of British Columbia