![]() Therefore maximising interference at the beginning serves to promote bone hold for longer. 2) traditionally it is at its maximum at the time of pin insertion and may decrease gradually as the fixator is loaded. Interference is a measure of the ‘grip’ the pin has of bone (Fig. Hence general guidelines for pin diameter have evolved and both 5- and 6-mm diameter pins have a place in the tibia and femur. In practice it is advisable to keep pin sizes to within a third of the diameter of the bone to reduce the risk of fracture on removal of the half-pin. The limit to increasing pin size is set by the diameter of the bone in which the pin is inserted - a hole exceeding 20% of the diameter of the bone will reduce torsional strength by 34%, and if the hole size is greater than 50% the reduction is 62%. This in turn can reduce the stresses at the bone-pin interface. Larger diameter pins have a higher resistance to bending forces (the cross-sectional moment of inertia of any rod or bar structure increases with the fourth power of its radius). Two important parameters that influence interface stresses and bone hold are pin diameter and interference. This is the crux of stability - starting with a good hold and keeping a good hold of bone. The fixator configuration (how it is assembled on the inserted bone pins). Three variables which directly influence the contribution to stability by the external fixator are: This important concept of shared stability introduces a dimension of external fixator adjustments, based on biomechanics, which a clinician is able to harness in order to utilise the device effectively - it can be a far cry from the practice of using a ‘standard’ anteromedial unilateral fixator for all types of tibial fracture. The fracture pattern too has important bearings on stability stresses at the bone-pin interface and ultimately the ability of the fixator to maintain its hold are dependent on the amount of contact and shape of the fracture line. Consequently stability can be conceived as the sum of contributions from both endo- and exoskeletons, and it is here the surgeon is able to adjust the relative contribution from the external fixator towards the total. Manipulations through the exoskeleton produce parallel effects in the endoskeleton. One analogy for the external fixator is that it is an exoskeleton applied in order to support the endoskeleton - the bone which is fractured, deficient or deformed. This review of variables that influence fixator stability will assist those wishing to expand their knowledge and experience. The argument that a provisional spanning external fixator can be applied without due attention to detail is false if the device is applied for a reason (be it temporary stabilisation for provisional care of the patient or for soft tissue care), then the objective is best met when the fixator is applied correctly. Consequently orthopaedic trainees are trained towards proficiency with internal fixation methods, and have only a superficial knowledge of external fixation. Many instructional courses on fracture management focus on internal stabilisation, leaving external fixation and non-operative strategies as minor inclusions.
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