Secondary Perforation Risk in Plate Osteosynthesis of Unstable Proximal Humerus Fractures: A Biomechanical Investigation of the Effect of Screw Length

Abstract

Secondary perforation of screws into the joint surface is a commonly reported mechanical fixation failure mode in locked plating of proximal humerus fractures (PHF). This study investigated the influence that screws tip to joint distance (TJD) has on the biomechanical risk of secondary screw perforation and the stability of PHF. Ten pairs of cadaveric proximal humeri with a wide range of bone mineral density were used. Each specimen was osteotomized and instrumented with the PHILOS plate, simulating a highly unstable 3-part fracture. Bones were randomized into a long screw group (LSG) with 4 mm TJD, or a short screw group (SSG) with 8 mm TJD. A custom biomechanical setup was used to test the samples to failure cyclically with a constant valley load and an increasing ramp. The number of cycles to the initial screw loosening event was significantly higher for the LSG (mean ± standard deviation: 17,532 ± 6,458) compared with the SSG (11,102 ± 5,440) (p < 0.01). The mode of failure during testing was lateral-inferior displacement combined with varus collapse, with calcar screws perforating first. The number of cycles to failure event for LSG (27,849 ± 5,648) was not significantly different compared with SSG (28,782 ± 7,307) (p = 0.50). Screws that purchase closer to the joint had better initial stability and resistance against loosening. Placing longer screws, within limits dictated by the surgical guide, is expected to decrease the risk of secondary perforation failures in unstable PHF. These findings require clinical corroboration.

Morphology and novel classification of proximal humeral fractures

“…These investigations demonstrate that longer screws provide a stronger fixation. It is generally accepted that the distance between the tip of the screw and the articular surface should be 5–8 mm ( Ciric et al, 2019 ). Using a longer screw within this range can prevent the screw from loosening without increasing the risk of screw penetration.…”

Predictive Indicators for Complications of Proximal Humerus Fractures Treated with Locking Plate or Intramedullary Nail Fixation

“…A recent series made by Ciric et al . reported that placing calcar screws closely to the subchondral bone of humerus head did not increase the risk of screw loosening 34 . In contrast, placing short calcar screws could lead to screw loosening and secondary varus deformity.…”

“…A recent series made by Ciric et al reported that placing calcar screws closely to the subchondral bone of humerus head did not increase the risk of screw loosening. 34 In contrast, placing short calcar screws could lead to screw loosening and secondary varus deformity. In a finite element analysis of Yang et al, it pointed out that with the absence of medial support, the intense pressure near the calcar screws of the locking plate contributed to the high rate of plate breakage and iatrogenic fractures.…”