Intramedullary cortical bone strut improves the cyclic stability of osteoporotic proximal humeral fractures

Abstract

BackgroundProximal humeral fractures treated with locking plate can fail due to varus collapse, especially in osteoporotic bone with medial cortex comminution. The use of an intramedullary strut together with locking plate fixation may strengthen fixation and provide additional medial support to prevent the varus malalignment. This study biomechanically investigates the influence of an intramedullary cortical bone strut on the cyclic stability of proximal humeral fractures stabilized by locking plate fixation in a cadaver model.MethodsTen cadaveric humeri were divided into two groups statistically matched for bone density. Each specimen was osteotomized with 10 mm gap at the surgical neck. The non-augmented group stabilized with locking plate alone; in the augmented group, a locking plate was used combined with an intramedullary cortical bone strut. The strut was retrograded into the subchondral bone, and three humeral head screws were inserted into the strut to form a plate-screw-strut mechanism. The cyclic axial load was performed to 450 N for 6000 cycles and then loaded to failure. Construct stiffness, cyclic loading behavior and failure strength were analyzed to identify differences between groups.ResultsThe augmented constructs were significantly stiffer than the non-augmented constructs during cycling. On average, the maximum displacements at 6000 cycles for non-augmented and augmented groups were 3.10 ± 0.75 mm and 1.7 ± 0.65 mm (p = 0.01), respectively. The mean peak-to-peak (inter cycle) displacement at 6000 cycles was about 2 times lower for the augmented group (1.36 ± 0.68 mm vs. 2.86 ± 0.51 mm). All specimens showed varus collapse combined with loss of screw fixation of the humeral head. The failure load of the augmented group was increased by 2.0 (SD = 0.41) times compared with the non-augmented group (p < 0.001).ConclusionsThe stability and strength of the locking plate augmented with an intramedullary strut were significantly increased. For bone with poor quality, the subsidence of the locked screws led larger displacement, decreased the stability of the constructs, however, the plate-screw-strut mechanism provided more rigidity to stabilize the fixation. This study emphasized the importance of intramedullary support for the proximal humeral fractures fixed with a locked plate under cyclic loading, especially in bone with poor quality. This work is based on the results of cadaver model, further in vivo analysis is necessary to determine if the clinical results can be extrapolated from this data.

Biomechanical analysis of plate systems for proximal humerus fractures: a systematic literature review

“…The earliest study was by Koval et al [ 7 ] in 1996 and the latest being by Hsiao et al [ 24 ] in 2017. From the nature of literature, one could categorise the included studies on a variety of bases such as the type of plates tested, types of parameters determined and even chronologically.…”

“…The most common set-up was to fix the humeral shaft and load the humeral head, which was often potted in a polymer holder. In static tests, displacement-control loading at a rate of 5 mm/min have been most frequently employed [ 24 , 26 , 27 , 30 , 49 , 66 , 67 , 70 ] while displacement rate of 0.1 mm/s [ 68 , 69 ] and 20 mm/min [ 25 ] and load rates of 1 N/s [ 28 ] and 20 N/s [ 51 ] have also been used. In terms of the loading order, seven [ 24 , 25 , 27 , 28 , 51 , 68 , 69 ] of the eleven studies involving both static and cyclic axial loading, performed a static loading-to-failure step at the end to characterise constructs’ load to failure behaviour.…”

“…Failure was most often defined as the complete (or irreversible) closure of fracture gap [ 24 , 30 , 50 – 52 , 68 , 69 ] and as the clear deviation in linearity of the load–displacement curve [ 26 , 27 , 50 , 52 ]. Based on the load–displacement curve plots, failure was also defined as a point of a major drop in the load [ 24 , 51 ] and this was elaborated by Zettl et al [ 28 ] to be a greater than 30% drop in the pressure. Another criterion described failure as humeral displacement greater than 20 [ 29 ] or 30 mm [ 28 ] on the load–displacement curve.…”

“…For cyclic loading, number of cycles to failure [ 26 , 51 ], plastic deformation after a certain number of cycles [ 27 , 28 , 50 ] and maximum [ 39 ] and final [ 51 ] plastic deformation have been calculated. Hsiao et al [ 24 ] determined peak-to-peak (inter- cyclic) displacement and cumulated deformation at specific cycles.…”

“…Similarly, Katthagen et al [ 69 ] showed superior performance with the use of femoral head allografts. Hsiao et al [ 24 ] report significantly stiffer constructs with intramedullary cortical bone strut augmentation than the non-augmented constructs during cyclic compression tests.…”