Background. Open reduction and internal fixation using plate osteosynthesis for midshaft clavicle fractures is often associated with hardware prominance. Although clinical studies have suggested a role for the use of thinner 2.7mm plates as a means of increasing cosmetic acceptability this still remains an area of controversy. We investigated the effect of plate size (2.7mm vs. 3.5mm), plate treatment (annealed vs. cold worked) and number of screws on the stiffness and yield point. Methods. Twenty-four synthetic clavicles were randomly divided into four treatment groups - Synthes (Synthes, Paoli, PA) 2.7mm cold-worked calcaneal reconstruction plate with six or eight bicortical screws; 3.5mm LCP reconstruction plate (RP) and 3.5mm LCP pre-contoured superior-anterior clavicle plate (PCSA). After measuring the baseline stiffness of the intact specimens, all clavicles were plated, a wedge-shaped inferior defect was created and testing performed using a cantilever-bending model. Statistical analysis was performed using one-way ANOVA with Tukey's multiple comparison test with significance set at a P value <0.05. Results. The 3.5mm RP construct was significantly stiffer than both of the 2.7mm CR constructs (P < 0.0001). The yield point for the 3.5mm PCSA construct was greater than the other three constructs (P < 0.0001), while the yield point for the 2.7mm CR plate with 6 screws and with 8 screws was higher than the 3.5mm RP construct (P = 0.0002 and P = 0.0023 respectively). The amount of displacement required to reach the yield point was highest for the 2.7mm CR plate with six screws and this was significantly higher than the values for the other three constructs. Conclusion. The 3.5mm plates demonstrated increased bending stiffness compared to the 2.7mm plates. Despite the lower resistance to bending forces, the cold worked 2.7mm plate exhibited a significantly higher yield point and required significantly more superior to inferior displacement to deform