A Review of the Biomechanical Role of a Unilateral External Fixator in the Fracture Repair Process

Introduction A unilateral external fixator comprises of a collection of pins, clamps and sidebars to form a construct on one side of the limb that can be used to stabilise bone fractures and promote healing. An external fixator can allow the operator to alter the flexibility of the fixation. In the presence of flexible fixation, fracture healing happens through the creation of callus, which mechanically joins the bone together. There are 4 stages to this secondary bone fracture healing to include: inflammation, soft callus, hard callus, and remodelling stages. Excessive movement at the fracture site can predispose to non-union and creation of fibrocartilage. However, movement at the fracture site is necessary to stimulate the repair process in the early stages and the magnitude of strain at the site of healing has a role in predicting the tissue type created e.g. fibrocartilage or bone. The strain (εStrain) at the fracture gap is given by the relationship: ε Strain = ∆g/g0