Optimizing Osteotomy Geometries in Posterolateral Mandibulectomies

Abstract

Importance: Reconstructive stability after mandibulectomy with osseous autogenous transplant is influenced by masticatory forces and the resulting stress on the titanium plate.

Objective: To determine an optimal geometry of mandibular osteotomy that minimizes undesirable loading of the reconstruction plate.

Design, setting, and participants: In this combined in silico and in vitro basic science study, segmented computed tomography images of an adult male human mandible downloaded from the Visible Human Project were analyzed. Data were collected from July to November 2023.

Exposures: Four posterolateral mandibular resections and bony transplants were modeled following (1) vertical, (2) angled, (3) step, and (4) sagittal osteotomies. Using SOLIDWORKS software, mastication was simulated under (1) incisal, (2) ipsilateral molar, and (3) contralateral molar loading. Mandible models were then 3-dimensionally printed, osteotomized, and plated. Masticatory loads were simulated using pulleys, and strains were measured using strain gauges.

Main outcomes and measures: On the reconstruction plate, von Mises stresses were measured in silico, and strains were measured using strain gauges in vitro. Stress and strain are reactions of a material to loading that can result in irreversible deformation or fracture.

Results: In silico, maximum plate stress was highest with the vertical osteotomy, followed by the angled osteotomy (median difference vs vertical: ipsilateral molar loading, 126 MPa; 95% CI, 18-172; incisal loading, -24 MPa; 95% CI, -89 to 31; contralateral molar loading, 91 MPa; 95% CI, 23-189), step osteotomy (median difference vs angled: ipsilateral molar loading, 168 MPa; 95% CI, 112-235; incisal loading, 80 MPa; 95% CI, 15-140; contralateral molar loading, -17; 95% CI, -115 to 83), and sagittal osteotomy (median difference vs step: ipsilateral molar loading, 122 MPa; 95% CI, 102-154; incisal loading, 197 MPa; 95% CI, 166-230; contralateral molar loading, 161 MPa; 95% CI, 21-232). An angled osteotomy had the lowest stress at 30° of angulation (median difference vs contralateral molar loading at 40° of angulation: 111 MPa; 95% CI, 4-186). In vitro, the vertical osteotomy had the highest maximum strain, followed by the angled osteotomy (mean difference vs vertical: incisal loading, 0.021 mV/V; 95% CI, 0.014-0.027; contralateral molar loading, 0 mV/V; 95% CI, -0.004 to 0.005), step osteotomy (mean difference vs angled: incisal loading, 0.015 mV/V; 95% CI, 0.003-0.028; contralateral molar loading, 0.021 mV/V; 95% CI, 0.016-0.027), and sagittal osteotomy (mean difference vs step: incisal loading, 0.006 mV/V; 95% CI, -0.006 to 0.018; contralateral molar loading, 0.020 mV/V; 95% CI, 0.015-0.026).

Conclusions and relevance: In this study, the traditional vertical osteotomy resulted in less favorable plate stresses in all loading scenarios compared with angled, step, or sagittal osteotomies, in silico and in vitro. Future clinical studies analyzing the impact of varying osteotomy geometries are warranted to translate these findings to the operating room.

Figure 1.. Three-Dimensionally Printed Mandible Model

The uncut mandible model 3-dimensionally printed in VeroDent PureWhite material (Stratasys) underwent a 5-cm vertical osteotomy, angled osteotomy, step osteotomy, and sagittal osteotomy followed by reconstruction with a 2.0-mm Stryker plate affixed with 3 strain gauges, secured with 8 screws in a constant position. Red lines were traced over the osteotomies.

Figure 2.. Configuration of Masticatory Loading Rig for Incisal and Contralateral Molar Loading Conditions

The rig was assembled with T-slotted framing on a heavy fixture plate, and pulleys on rods were attached to the frame with a custom 3-dimesionally printed part. One rod was positioned to contact either the incisors or contralateral molar, with the weights lowered depending on the loading condition.

Figure 3.. Finite-Element Analysis of Maximum von Mises Stress by Osteotomy Geometry

Within each loading condition, the median difference in maximum von Mises stress between different osteotomy geometries was estimated. The precision of the effect size was defined with 95% CIs. The midline indicates the median; the box, IQR; error bars, 1.5-fold the IQR; and the data points, individual simulated stress outputs.

Figure 4.. Finite-Element Analysis of Maximum von Mises Stress by Osteotomy Angle

Within each loading condition, the median difference in maximum von Mises stress between different osteotomy angles was estimated. The precision of the effect size was defined with 95% CIs. The midline indicates the median; the box, IQR; error bars, 1.5-fold the IQR; and the data points, individual simulated stress outputs.

Figure 5.. Comparison of Strain Gauge Readings by Osteotomy Geometry

Tests were repeated 10 times each to generate the box plots. Within each loading condition, the mean difference in maximum strain between each osteotomy geometry was estimated. The precision of the effect size was defined with 95% CIs. The midline indicates the median; the box, IQR; error bars, 1.5-fold the IQR; and the data points, individual simulated stress outputs.