Design of Bioabsorbable Polymeric Humeral Fracture Fixation Device

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Approximately 55,500 proximal humeral fractures require surgical fixation annually. The current standard for internal humeral fracture fixation involves implantation of rigid metallic devices to prevent dislocation of bone fragments. However, these devices have high stiffness characteristics which can cause stress shielding in bone. A second method of fixation, called biological fixation, decreases stiffness which reduces stress shielding by utilizing more flexible devices. This approach tends leads to increased incidences of delayed healing and nonunion of fracture fragments. Therefore, this device design implements two bioabsorbable polymers in two distinct layers that degrade at different rates. The purpose of this design is to provide rigid fixation during the initial fracture healing phase followed by a period of biological fixation, allowing for functional healing along with a reduction in stress shielding over time compared to current devices. The bioabsorbable property permits the device to remain in situ, thus eliminating the need for removal surgery and reducing the risk of surgical site infection. Using finite element analysis, the design has been demonstrated to exhibit varying axial, torsional, and flexural stiffness over time. The final device was fabricated by injection molding, and tested for flexural stiffness. In addition, the polymers were tested for stiffness at specific time intervals over the course of the degradation period. All stiffness tests were performed under simple three point loads. A Nikon 3200 camera (Nikon Inc., Melville, NY) was used to sequentially image the material samples and plate throughout each load application. The flexural stiffness of the device was determined by utilizing Digital Image Correlation analysis in Matlab (MathWorks, Inc.) to analyze surface displacements between image frames. The success of the device was determined by comparing the observed difference in stiffness to standard stiffness values for humeral fixation devices currently available on the market. A substantial decrease in stiffness combines the benefits of rigid and biological fixation devices as well as eliminates the complications associated with each, providing an improved solution for proximal humeral fractures.