Lateralization has minimal biomechanical impact on tuberosity fixation with the use of a stem-based repair and a 135° humeral implant in reverse shoulder arthroplasty for 4-part proximal humerus fracture.

BACKGROUND

Lateralization in reverse shoulder arthroplasty (RSA) has many proven and potential benefits. Concern over the increase in tension on the tuberosities imparted by glenoid lateralization and the subsequent effect on tuberosity healing may limit its use in the setting of RSA performed for fracture. This study evaluated whether glenoid lateralization increased tuberosity fracture gapping in a biomechanical model of a 4-part proximal humerus fracture with a stem-based tuberosity repair. We hypothesized that increased lateralization would increase fracture gapping.

METHODS

Eight cadaveric shoulders (mean age, 62 +/- 2.4 years; range 52-70 years) were tested with a custom testing system that permits loading of rotator cuff muscles and humerothoracic muscles. A 4-part proximal humerus fracture was simulated and then repaired with a stem-based tuberosity repair. The repaired tuberosities were cycled in internal and external rotation with 1.1-Nm torque at 2 mm and 6 mm of glenoid lateralization. For the 6-mm lateralization RSA, the torque was then increased to reach the range of motion (ROM) values achieved with the 2-mm lateralized RSA and cycled 10 times, followed by doubling the torque values for 10 cycles. ROM, muscle length and fracture gapping were assessed at 2 mm and 6 mm of glenoid lateralization.

RESULTS

Internal rotation and total ROM demonstrated a significant decrease in the 6-mm RSA when compared to the 2-mm RSA ( < .05). The 6-mm lateralized RSA significantly increased rotator cuff muscle lengths when compared to the 2-mm lateralized RSA condition except for infraspinatus by an average of 2.7 ± 1.9 mm ( < .05). There was no significant gapping of the proximal fracture for any condition. There was a significant increase in the gapping of the distal fracture gap in the 6-mm lateralized component condition only after 10 cycles of doubled rotational torque, which measured 1.9 ± 1.5 mm ( = .031).

DISCUSSION

We hypothesized that lateralization would increase fracture gapping. Fracture gapping did not occur proximally and only occurred at a slight amount distally after 10 cycles of doubled rotational torque. This may have implications on the choice of glenoid components when performing a RSA for fracture.