[Finite element analysis of influence of humerus torque screw tip distance on stability of proximal humeral fracture plate fixation]

Objective: To explore changes of humerus torque screw tip distance on stability of proximal humeral internal locking system (PHILOS) by finite element analysis, in order to provide reference for selection of intraoperative plant size.

Methods: The proximal humerus 3D model was constructed based on Synbone artificial bone model in 3D engineering drawing software, and the corresponding 3D model was constructed based on PHILOS bone plate contour. The model was modified to simulate comminuted proximal humerus fracture, and the operation model was simulated after fracture, and the fixed operation model was assembled, the apex distance of humerus moment screw was set as 4, 8, 12 and 16 mm respectively. The axial and torsional loads were applied under the conditions of vertical downward and 20° abduction respectively, and the maximum displacement of humeral head and the peak stress of cortical bone and cancellous bone of humeral head were calculated under the corresponding working conditions, respectively, to evaluate influence of screw tip distance change on the risk of humeral head varus after PHILOS plate fixation.

Results: The peak displacement of humerus head did not generally increase with the increase of torque screw tip distance. On the contrary, the peak displacement of humerus head decreased slightly in all working conditions with the gradual increase of torque screw tip distance. At 20° abduction, the maximum displacement of humerus head was significantly higher in all models than that under vertical loading. At the same time, the lengthening of tip distance of torque screw did not lead to significant stress concentration of the humerus head after operation. The peak stress of cancellous bone decreased gradually with the extension of the peak distance of humerus moment screw, while the peak stress of cortical bone increased first and then decreased. When the peak distance was 12 mm, the peak stress of cortical bone was the largest under all working conditions, and then decreased with the further extension of the peak distance.

Conclusion: In the case of a small humeral head interlocking screw tip distance(less than 4 mm), increasing the humeral torque screw tip distance does not lead to a significant loss of stability after PHILOS plate fixation and a significant increase in the risk of postoperative complications.