Biomechanical in vitro assessment of fixed angle plating using a new concept of locking for the treatment of osteoporotic proximal humerus fractures

Abstract

Locked plating attempts to improve mechanical stability via better anchorage of the screws in the bone. In 22 paired osteoporotic humeri an AO/ASIF 11-B 1 fracture was created. Locked and conventional plating using the same device of the latest generation was performed. Torsional loading around three axes (x = varus/valgus, y = flexion/extension, z = axial rotation) with an increasing moment (2, 3.5, 5 and 7.5 N·m) was applied. Interfragmentary motion within the locked group was lower for all three axes with higher cumulative survival rates (p < 0.05). The typical mode of failure was loss of fixation in the humeral head occurring earlier in the conventional group. The locking mechanism investigated provides more ultimate strength in an osteoporotic proximal humerus fracture model. Correlation with BMD suggests that this device may especially be suitable for use in osteoporotic bone.

Fig. 1

Simulation of an AO/ASIF 11-B1 fracture (dotted red lines). The standardised position of the moving point is shown (a = 2 cm, b = 1 cm, c = 9 cm), as well as the three parallel bone mineral density (BMD) sections (*)

Fig. 2

a Non-contact-bridging (NCB^®) plate for the proximal humerus (PH) (Zimmer, Inc.). b Secondary locking of screws used as a locking cap (Courtesy of Images © Zimmer, Inc.)

Fig. 3

Experimental set-up. A plastic saw bone was mounted to the spinal simulator. The ultrasound-based motion analysis system was fixed to the bone. The stepper motors apply pure moments around three axes (x = varus/valgus, y = flexion/extension, z = axial rotation)

Fig. 4

a–c Calculation of the survival rate according to Kaplan–Meier for the three respective axes (i.e. x, y and z)

Fig. 5

Regression analysis of the bone mineral density (BMD) vs. the total sum of cycles applied around the x axis

Fig. 6

a Typical mode of failure of locked plating with loss of reduction in the humeral head (specimen no. 3, left humerus, anterior view). b Backing up of screws and loss of fixation in the humeral head in a case of conventional plating (specimen no. 6, right humerus, anterior view)