Comparative biomechanical study of two configurations of cemented screws in a simulated proximal humerus fracture fixed with locking plate.

Introduction: Screw tip augmentation with bone cement for fixation of osteoporotic proximal humerus fractures seems to improve stability and to decrease the rate of complications related to implant failure. However, the optimal augmentation combinations are unknown. The aim of this study was to assess the relative stability of two augmentations combinations under axial compression load in a simulated proximal humerus fractures fixed with locking plate.

Material And Methods: A surgical neck osteotomy was created in five pairs of embalmed humeri with a mean age of 74 years (range 46-93 years), secured with a stainless-steel locking-compression plate. In each pair of humeri, on the right humerus were cemented the screws A and E, and in the contralateral side were cemented screws B and D of the locking plate. The specimens were first tested cyclically in axial compression for 6,000 cycles to evaluate interfragmentary motion (dynamic study). At the end of the cycling test, the specimens were loaded in compression force simulating varus bending with increasing load magnitude until failure of the construct (static study).

Results: There were no significant differences in interfragmentary motion between the two configurations of cemented screws in the dynamic study (p=0.463). When tested to failure, the configuration of cemented screws in lines B and D demonstrated higher compression load to failure (2218N vs. 2105, p=0.901) and higher stiffness (125N/mm vs. 106N/mm, p=0.672). However, no statistically significant differences were reported in any of these variables.

Conclusions: In simulated proximal humerus fractures, the configuration of the cemented screws does not influence the implant stability when a low-energy cyclical load is applied. Cementing the screws in rows B and D provides similar strength to the previously proposed cemented screws configuration and could avoid complications observed in clinical studies.