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Effect of humerus rotation on the initial stability of locking plate fixation for proximal humerus fracture: A
Min-Yao Chuang1, Lin-I Hsu2, Cheng-Yu Wu3
1Department of Orthopaedic Surgery, MacKay Memorial Hospital, Taipei City, Taiwan; Department of Physical Therapy and Assistive Technology, National Yang Ming Chiao Tung University, Taipei City, Taiwan; Department of Medical Research, Biomechanics Research Laboratory, MacKay Memorial Hospital, New Taipei City, Taiwan.
Background:
Limb rotations of the upper arms frequently happen during daily activities, and these actions can produce significant torsional forces which aggravate patients' conditions. However, there is limited research concerning the impact of humerus rotation on the initial stability of locking plate fixation after proximal humerus fracture reconstruction. The current study conducted a biomechanics analysis to investigate this important issue.
Methods:
Dynamic mechanical tests with repetitive torsional loads were executed in six fracture-model specimens. The mechanical behaviors of resistance to torque, maximum torque, and energy dissipation were recorded. A new surveillance system of digital image correlation (DIC) technology was used to observe the continuous strains of the fixation system in real time.
Results:
Mean resistance to torque was significantly reduced when the rotation exceeded 3.2°. Beyond 3.2° of rotation, the mean energy dissipation increased by approximately fourfold. DIC observations showed large strains concentrated around the screw holes at the proximal shaft and waist of the plate.
Conclusions:
Under our experimental conditions, we observed a marked rise in non-elastic energy dissipation beyond 3.2° of rotation, which indicated the onset of permanent construct deformation in vitro. Surgeons should remind patients that daily upper extremity torques may affect the initial rotational stability of fixation system after surgery.
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