External ring fixators: an overview

External fixation is widely used in the fixation of fractures and limb defo rmities. The mechanical characteristics of a specific external fixator are major factors in determining the biomechanical environment at a fracture/os teotomy site and, hence, affect the healing process. Although the optimal b iomechanical environment for healing of a fracture or an osteotomy is unkno wn, a specific range of interfragmental-gamma motion exists which promotes healing. It is therefore desirable that the mechanics of an external fixato r can be manipulated to enable the surgeon to control the range of interfra gmentary motion. The characteristics of an external fixator are defined by a large number of variables. Therefore, to gain control over the degree of interfragmentary motion, an understanding of the effect of each variable an d how it interacts with the others to determine the overall characteristics of the device is required. For the past two decades, individual components and whole-frame configurations have boon studied in depth. This article pr ovides a summary of previous work concerning the mechanics of external ring fixators and how they affect the biomechanical environment at the fracture /osteotomy site.