Purpose: The purpose of this study was to create three-dimensional composite models of quad zygomatic implant-supported maxillary prostheses with a variety of alveolar bone defects around implant sites, and to investigate the stress distribution in the surrounding bone using the finite element analysis (FEA) method. Materials and Methods: Three-dimensional models of titanium zygomatic implants, maxillary prostheses, and human skulls were created and assembled using Mimics based on microcomputed tomography and cone beam computed tomography images. A variety of additional bone defects were created at the locations of four zygomatic implants to simulate multiple clinical scenarios. The volume meshes were created and exported into FEA software. Material properties were assigned respectively for all the structures, and von Mises stress data were collected and plotted in the postprocessing module. Results: The maximum stress in the surrounding bone was located in the crestal bone around zygomatic implants. The maximum stress in the prostheses was located at the angled area of the implant-abutment connection. The model with anterior defects had a higher peak stress value than the model with posterior defects. All the models with additional bone defects had higher maximum stress values than the control model without additional bone loss. Conclusion: Additional alveolar bone loss has a negative influence on the stress concentration in the surrounding bone of quad zygomatic implant-supported prostheses. More care should be taken if these additional bone defects are at the sites of anterior zygomatic implants. Keywords: finite element analysis, quad zygoma approach, stress distribution, zygomatic implant