Specimen
Sixteen female humerus specimens (average age of 73 ± 3, range of 69–83 years old) provided by Tianjin Hospital were used in this study. Those specimens had no congenital malformations, fractures, and tumors. All specimens were subjected to deep cryopreservation (− 100 °C).
The specimens were thawed at room temperature at 12 h before tests. Bone mineral density of humeral shaft was measured. The same regions of those humeral heads were delineated and the bone mineral density was assessed by Dual-energy X-ray absorptiometry (QDR-4500A ACCLAIM, HOLOGIC, United States). The samples were confirmed to meet the WHO standards of osteoporosis (BMD between 0.2 and 0.4 g/cm2) [12].
Construction of fracture model
Those specimens were randomized into control group (clover plate) and test group (external fixator) using random table method. Models of surgical neck two-part fracture of the humerus were constructed according to the previous reports [13, 14]. Osteotomy of the humerus surgical neck was performed in humeral shaft specimens using an electric oscillating saw with a 1 mm-thickness saw bit. A horizontal reference line was drawn at 3 cm below the apex of the greater tubercle and it crossed the base of the lesser tubercle and constituted 20° with the osteotomy line. All the fracture models were built by the same person.
Fixation procedures
The procedures in external fixator group were done under fluoroscopy as the following. The parts situated 2 cm above the lateral humeral epicondyle were vertically removed. Denture acrylic was embed vertically The surgical neck fracture was made in the proximal humerus with a handsaw. The intersection between the deltoid muscle’s insertion plane and lateral edge of the caput longum musculi bicipitis brachii was used as the entrance point of the first fixation pin (Schanz nail, 3.5 mm × 150 mm; Tianjin Xinzhong Medical Devices Co., Ltd. China). The insertion direction was backward tilted for 20–30° from anterolateral to posteromedial normal humeral head, whereby the humeral head center was located at posterior humeral shaft. For anterolateral insertion, the pin formed 45° in the coronal plane with the humeral shaft, and formed 30° in the sagittal plane with the humeral shaft. Thus, the insertion point was located below the humeral head center at 0.5-1 cm below the humeral head. The second pin was placed in front of the humeral shaft, and was inserted anteroposteriorly in the insertion plane of the deltoid muscle, where it formed 45° with the humeral shaft in the sagittal plane and formed 30° in the coronal plane. Therefore, the insertion point was located at the posterosuperior part of the humeral head. The third pin was located at the apex of the greater tuberosity of humerus, forming 30° with the sagittal plane of the humeral shaft. Considering that a driving screw reached the medial cortex of the humeral shaft beyond the fracture line, three driving screws (Tianjin Xinzhong Medical Devices Co. Ltd., Tianjin, China) were locked with fixed links and clamps (Figs. 1 and 2).
The surgery in the clover plate group was done through anterior approach. Trilobal plates (non-locking; Kanghui Ltd. Co., Changzhou, China) were placed at 1 cm below the lateral apex of the greater tubercle of the humerus. Clover plates (Kanghui Ltd. CO., Changzhou, China) were internally fixed. The end of the humeral shaft was fixed with 3 cortical screws (3.5 mm) and that of humeral head was fixed with 3 cancellous screws (4.0 mm) (Figs. 1 and 2). All above fixation procedures were done by the same person.
Measurement
The measurement was performed by reference to the previous reports. [15, 16] Ten strain gage pieces were attached along the two sides of the fracture line, with a vertical distance of 2 mm to the broken site. BE120-05AA-X30 foil gages (Hanzhong Zhonghang Electronic Measuring Instruments, China) were used at 120 ± 0.1 Ω resistance and a sensitivity coefficient of 1.94% ± 1.00%. Then, resistance fluctuation was assessed using YJ-33 static resistance strain indicator (Shanghai Automation Instrumentation Co., Ltd., Shanghai, China). Strain gage resistance might present a small variation (< 1 Ω) after the attachment (Fig. 3).
The fracture models were submitted to compression resistance test at 25 ± 1 °C and 30% humidity. To simulate the loading status of human shoulders, the specimens were vertically placed on the hydraulic servo dynamic biomechanical tester (Instron 8874, USA), with the distal ends completely fixed on the lower part of the tester. The upper clutch disk was adjusted to fully contact the distal end of specimens. A gradually-increasing load from 0 to 200 N was applied at a loading rate of 1.4 mm/min. The values of multiple strain gages at both sides of the broken ends were simultaneously acquired. Compression and torsion experiments were performed to determine the loads and torques by reference to the previous reports. [15, 16] Data acquisition time was 50s, and the specimens were rotated for 6 cycles or until damaged. Data acquisition for the fixations is shown in Figs. 4 and 5.
Statistical analysis
SPSS 21.0 software (SPSS, Chicago, IL, USA) was used. The average values, expressed as mean ± standard deviation, were used for statistical analysis. The comparison of the measures between the control group and the test group was performed using independent t-test. P-value < 0.05 was considered as statistical significance.