Humerus Head

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 ].

Both elliptical and spherical prosthetic humeral heads were custom made (Arthrex Inc., Naples, FL, USA). The designs, including equations for dimension width, radius of curvature, and height, were chosen according to previously published studies [7 (link), 8 (link)]. A small hole in the undersurface allowed for securely placing the humeral head prosthesis on the protruding spike of the trunnion, avoiding rotation of the head prosthesis during testing and allowing for easily switching heads between testing conditions.

During testing, an axial compression load of 40 N was constantly applied via the lever arm of the X-Y table to center the joint [13 (link)]. Each specimen underwent the three following conditions: (1) native, TSA with a (2) matched-fit elliptical head and (3) matched-fit spherical head. According to Jun et al., [13 (link)] 50° of internal and 50° of external axial rotation were alternatingly applied to the humerus at 0°, 30°, 45° and 60° of glenohumeral abduction in the scapular plane.
According to Harryman et al., [1 (link)] who stated that glenohumeral translation is considered obligate in that it cannot be prevented by application of an oppositely directed force of 30 N, an anterior directed force was applied to the humeral head during external rotation due to the posterior translation of the humeral head. Conversely, a posterior directed force was applied to the humeral head during internal rotation due to the anterior translation of the humeral head [1 (link)]. As the humerus was fixed in the testing rig, the force of 30 N was consequently had to be applied to the X-Y table (glenoid) in the posterior direction during external rotation and in the anterior direction during internal rotation. The force was applied via a friction-less cable, which was attached to a servohydraulic testing system (Mini Bionix 858; MTS) or 30 N hanging weight, depending on the direction of force (Fig. 1A).
By means of a 3-dimensional digitizer (MicroScribe G2; Immersion) with a position accuracy of 0.23 mm, the position of the X-Y table was measured by carefully digitizing the center of a defined groove on the X-Y table without relevant influence by touching off with the digitizer. The position of the groove was determined at the beginning (start position) and the end (end position) of each application of internal and external rotation. Changes in the position represented the glenohumeral translation and were given in anteroposterior (x-axis) and superoinferior (y-axis) directions. In addition, overall compound motion during internal and external rotation was calculated as the square root of the sum of the squared anteroposterior (x-axis) and squared superoinferior (y-axis) translation.
Internal and external rotation were alternatingly applied five times for every condition. Values of each specimen were then averaged and presented as the final values. Throughout entire testing, specimens were not removed from the testing rig, nor was the testing rig disassembled. The protruding spike of the trunnion allowed for easily switching between the elliptical and spherical head design during testing. To avoid selection bias, the order of glenohumeral abduction positions (0°, 30°, 45°, 60°) and head designs (elliptical or spherical) were randomly assigned.