A biologic model for assessment of osseous strain patterns and plating systems in the human maxilla.

PURPOSE This study was conducted to examine a biomechanical model and to help answer fundamental questions that relate to rigid plate fixation in the maxilla. Specifically, we sought to elucidate the principal strain patterns generated in the maxilla secondary to masticatory forces as well as the amount of permanent deformational changes incurred due to these loading forces. MATERIALS AND METHODS Cadaveric heads with the mandible removed were defleshed and placed in a 2-part testing rig to hold and position the skull for testing in a standard material testing system. Rosette strain gages were attached at predefined points on the skull, and an Instron machine was used to load the skull through the loading port on the tray. A Le Fort I osteotomy was then performed on the skull, and a Walter Lorenz Ultra-Micro plating system was applied by a surgeon to reconnect the upper jaw. A 2-mm gap was left at the line of the osteotomy, and a transducer was attached to measure closure of the gap. Again the skull was loaded with the Instron (Canton, MA) machine. RESULTS The results indicate a linear relationship exists with both maximum (tensile) and minimum (compressive) strain patterns relative to incremental load placement on the intact maxilla. The strain patterns after the Le Fort I osteotomy and plating were different and less linear. The differential variable reluctance transducer data showed a low rate of closure or transient increase in the gap at low loads (0 to 15 kilopond [kp] range) and a steeper slope of closure during high loads (15 to 60 kp range). It is also evident that axial loading forces cause permanent deformation and failure of osseous plating systems predominantly through bending. CONCLUSIONS This model provides a foundation of knowledge regarding biomechanical strains in the maxilla subjected to static compressive loads in the force range of mastication. In addition, it serves as a comparative reference to assess rigidity of various craniofacial plating systems and to validate proposed standardized synthetic models. With the advent of increasingly precise surgery and new plating systems, this model can be used to help guide placement and design of plating systems; thereby allowing for ideal stabilization and optimizing surgical outcome.