Time-dependent behaviour of bone accentuates loosening in the fixation of fractures using bone-screw systems

Abstract

Aims: Loosening is a well-known complication in the fixation of fractures using devices such as locking plates or unilateral fixators. It is believed that high strains in the bone at the bone-screw interface can initiate loosening, which can result in infection, and further loosening. Here, we present a new theory of loosening of implants. The time-dependent response of bone subjected to loads results in interfacial deformations in the bone which accumulate with cyclical loading and thus accentuates loosening.

Methods: We used an 'ideal' bone-screw system, in which the screw is subjected to cyclical lateral loads and trabecular bone is modelled as non-linear viscoelastic and non-linear viscoelastic-viscoplastic material, based on recent experiments, which we conducted.

Results: We found that the interfacial deformation in the bone increases with the number of cycles, and the use of a non-linear viscoelastic-viscoplastic model results in larger deformations, some of which are irrecoverable. There is an apparent trend in which interfacial deformations increase with increasing porosity of bone.

Conclusion: The developed time-dependent model of the mechanical behaviour of bone permits prediction of loosening due to cyclical loads, which has not been possible previously. Application of this model shows that implant loosening will be accentuated by cyclical loading due to physiological activities, and the risks of loosening are greater in osteoporotic patients.Cite this article: S. Xie, K. Manda, P. Pankaj. Time-dependent behaviour of bone accentuates loosening in the fixation of fractures using bone-screw systems. Bone Joint Res 2018;7:580-586. DOI: 10.1302/2046-3758.710.BJR-2018-0085.R1.

Keywords: Bone volume ratio; Cyclic loading; Irrecoverable strain; Viscoelastic-viscoplastic.

The geometry of the bone-screw system showing a symmetry surface with location of the application of load (a); section A-A (b); load application - each model was subjected to 500 cycles of triangular load of 300 N amplitude followed by 1000 s of recovery (c).

The displacement (µm) contours at the symmetry surface and section A-A for seven representative cycles when load at its peak (300 N) (a); when load is zero (b) and recovery after 1000 seconds (c). They are magnified ×150, and the section plots are superimposed with undeformed geometry for better comparison with its original shape.

Peak displacement of trabecular bone at section A-A for both VE and VEP models when load is at its peak (300 N) (a); when load is zero and after 1000 seconds of recovery (b)

Maximum (a, c, e) and minimum (b, d, f) principal strain contours at the symmetry surface and section A-A for seven representative cycles when load at its peak (300 N) (a, b); when load is zero (c, d) and recovery after 1000 seconds (e, f). Strain is expressed as a percentage.