Biomechanical Evaluation of Humeral Prosthesis Fixation

Synbone prosthetic humeri (No. 5010, humeral length 361 ± 4 mm, humeral head diameter 53.0 ± 0.6 mm, SYNBONE-AG, Switzerland) were used in this study. A total of 12 prosthetic humeri were randomized into four groups, each consisting of three prosthetic humeri: the cortex contact + screw fixation group (Fig. 1a), the cortex contact group (Fig. 1b), the screw fixation group (Fig. 1c), and the control group (Fig. 1d). An oscillating saw was used to develop a wedge osteotomy model at the proximal humerus, during which all procedures were conducted manually. At the same time, all the specimens were fixed with the 10-well slotted bone locking plate PHILP (PHILP, Jiangsu Ideal Medical Science & Technology Co., Ltd., China) that was made of titanium, and those in the cortex contact + screw fixation and screw fixation groups were fixed with calcar screws as well and were placed in the position according to the standard rotation and fixation for the surgery (Fig. 1).Fig. 1

Examples of the Synbone prosthetic humeri as used in each experimental model. The cortex contact + screw fixation group (a), the cortex contact group (b), the screw fixation group (c), and the control group (d)

All specimens were consistent in height, structure, load, and fixation method and underwent the same mechanical test method, to ensure experimental accuracy. Both ends of the Synbone prosthetic humerus were embedded with denture base resin (Shanghai Medical Instrument Co., Ltd., China) and fixed, and only the bone plate fixation area at the proximal end was exposed. Then, the fixtures at both ends were fixed in parallel to the Zwick/Roell dynamic mechanics tester (Amsler HFD 5100B, Germany). Via the center on the osteotomy section, the relative displacement was measured using a high-precision digital grating displacement sensor (Shanghai University of Science and Technology Instrument Factory, China), with an accuracy of 5 um. After each specimen was carefully mounted, cyclic compression was applied with a loading of 500 N (10 H_Z) in the axial direction to test the dynamic fatigue properties. The finite fatigue life of each group was measured [21 ].
For quasi-static mechanical testing, each specimen underwent axial compression testing, three-point bending testing, and torsion testing sequentially. The load and loading rate in the axial compression test were respectively 500 N and 1.50 mm/min, and the load and the maximum bending moment in the three-point bending test were respectively 250 N/min and 7.5 N m; in the torsion test, the load was gradually increased stepwise by 0.6 N m/min to a maximum load of 3 N m. During the loading process, the stress and displacement of the humerus were measured and automatically recorded using the YD-14 dynamic digital resistance strain gauge indicator (Huadong Electronic Instrument, China). In each experiment, the specimen was first pre-loaded with 100 N to eliminate bone relaxation, creep, and other rheological effects. For each loading, the measurement was repeated three times, and each measurement result was used to calculate the mechanical parameters of each group [21 ].