A large number of bone fractures are treated with stabilisation devices that utilise metal
wires or screws, which traverse the bone and are connected to an external frame or
internal plate. Clinically, fixation devices are required to be able to: sustain loads;
minimise patient discomfort and possible implant loosening; and promote healing. In
the recent years locking plates have become increasingly popular for osteoporotic or
complex fractures, which can be difficult to manage. It, however, remains unclear as to
how these devices need to be configured for optimum clinical performance. This thesis
investigates the mechanics of locking plates, factors that influence their performance
and provides guidance to optimise the placement of screws. Finite element simulation
and analytical models were developed and validated using lab-based experimental
models.
The local behaviour around the screw-bone interface is considered and the implications
of different modelling assumptions assessed. A novel method of simulating the effect of
radial interference due to pilot-hole size is proposed. Different screw types are evaluated:
osteoporotic bone is found to be particularly susceptible to the screw tightening preload
used in compression screws; far-cortical locking screws are found to slightly reduce
device stiffness but substantially increase strain levels around screw holes. Finite
element simulations show that many of the local effects, such as preloads and contact
modelling, can profoundly influence the prediction of strains around screws but do not
generally influence the global load-displacement behaviour; the screw-plate connection
and bone/plate material and geometric properties are found to have an influence on
global stiffness predictions. The key determinants of load-displacement behaviour
evaluated through models are the loading and restraint conditions, which explain the
huge range of stiffness predictions in the literature (three orders of magnitude). An
analytical model based on 7 bone-plate construct parameters is developed. Despite its
simplicity, the model is found to be able to predict the axial stiffness for experimental tests conducted and for 16 other cases from five previous studies with an average error
of 20%. The manner of load application, not considered in the literature, is shown to
dramatically alter predictions of plate stress, strains within the bone and conclusions
regarding screw placement. Even with the inclusion of muscles forces, the choice of
restraint condition dominates the mechanical behaviour. Using the models, the influence
of screw position is systematically evaluated in varying bone qualities under axial
loading and torsion and guidance for optimising fixation is developed