Results
Concentrated Stresses of Screw, Plate, and Bone
The stress distributions of the four plate systems and the maximal stresses of the screw, plate, and bone are shown in Figures 4 and 5, respectively. In general, the medial opening highly stresses the implants, thus causing some screws and plate holes to become potential locations for failure by yielding and cracking. The screw stress of the T construct was the highest, followed by the T+I and TomoFix, and the π construct was the least stressed. Compared with the TomoFix construct, the screw stresses of the T+I and π constructs respectively increased 13.5% and decreased 9.5%. For the T+I construct, the addition of the I plate can significantly decrease screw stress by 34.4%.
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Figure 4.
The stress distributions of the (a) four plate systems and (b) plate inner surfaces.
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Figure 5.
Maximal von Mises stresses of the screw, plate, and bone for the four plate systems.
For the plate stresses, the T construct was still the most stressed and was followed by the TomoFix. The plate stresses of the other constructs were comparable and 67.5% less than that of the TomoFix construct. Similar to the screw stress, the use of the I plate can decrease 56.6% of the plate stress. For the T-shaped design, the use of locking screws and titanium alloy made the plate stress of the TomoFix system 31.9% less than that of the T plate. The difference in bone stress between the four plate-bone constructs was similar to that of the differences between the stresses in the plates. The surrounding bone of the T plate was the highest stressed and the stress values were 55.3%, 68.0%, and 128.7% higher than the TomoFix, T+I, and π constructs, respectively. For the one-leg systems (i.e. TomoFix and T), the screw, plate, and bone stresses concentrated at the interfaces, and the most proximal screw below the opening, was the most stressed. The posterior leg of the two-leg system (i.e. T+I and π) was more loaded and also stressed highest at the same region as the one-leg design.
Height Change and Load Ratio
Along the three edges, the changes in the height of the medial opening are shown in Figure 6. For all plate systems, the opening wedge at the edge cc was compressed and the micromotion was the highest than the other edges. Nearest to the wedge tip, the micromotion at the edge aa was the least. At the edge cc, the micromotion of the T+I and π constructs were respectively reduced by 50.7% and 70.2% compared with the TomoFix. The hybrid use of the T and I plates can decrease micromotion by 76.9% compared to that of the T construct. Compared with the TomoFix and T plates, the two-leg design can stabilize the opening wedge at the edge aa and eliminate the undesired tension at the wedge tip. On average, this can reduce wedge-tip micromotion by 92.3% in the one-leg design. The load ratio was respectively 4.7 and 3.9 between the posterior and anterior legs of the T+I and π plates.
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Figure 6.
The wedge-edge micromotion of the four plate systems.