Experimental evaluation of relapse-risks in operated zygoma fractures

Objectives: Prevention of relapse, or postoperative dislocation, of the fixed zygoma is necessary to achieve optimal results in the treatment of zygoma fractures. Assuming that the occurrence of intensified stresses on mastication at the screw-bone interface (SBI) constitutes the essential cause of the relapse, we evaluated the stresses for three different fixation methods-fixation at the frontal process (FP), inferior orbital rim (IOR), and zygomatico-maxillary buttress (ZMB). Methods: We used 10 computer-aided design (CAD) models simulating zygoma fractures in the experiment. For each CAD model, we fixed the fractured zygoma with four screws and one mini-plate at the FP, IOR, or ZMB. After applying a 5.5 kg force simulating mastication, we calculated the intensity and distribution patterns of the stresses occurring at the SBIs of the fixation screws using the finite element method. Thereby, we evaluated dynamic stability of the fixed zygoma for each of the three fixation methods. Results: Greater stresses occur at the SBIs with IOR fixation than at those with FP and ZMB fixation. Although the stresses occurring at the SBIs on mastication demonstrated evenly distributed patterns with the FP and ZMB fixation, the stresses demonstrated concentration on one screw with the IOR fixation. Conclusions: The fixed zygoma is more likely to cause relapse with the IOR fixation than with the FP or ZMB fixation. Hence, in performing zygoma fixation at the IOR, care should be taken to minimize the likelihood of postoperative relapse that is caused by skewed distribution of the stresses on the fixation screws.