| External fixation consists of external bars attached by clamps to pins which are inserted into the bone. These devices are popular in orthopedics, although pin loosening is one of the most frequent complications with these applications. This failure can result from biological processes or when shear stress at the pin-bone interface is greater than the pin's holding power. Reduced holding power may be a result of a local load greater than the yield strength of bone, or bone resorption due to high stresses at the pin-bone interface.; A 1-D beam-element finite element model (FEM) and beam on elastic foundation model (BEF) were used to calculate the approximate normal pin-bone interface stress under ideal conditions. Three stress components, axial, normal, and shear, at the pin-bone interfaces were calculated using 2-D and the 3-D linear FEMs.; Nonlinear contact elements were also used at the pin-bone interfaces of 2-D, 3-D elements with one pin and 3-D elements with full pin FEMs. The mechanism of stress dependent pin-bone loosening behavior was investigated through these nonlinear FEMs. The interface superior near cortex of pin 1, inferior near cortex of pin 1 and inferior near cortex of pin 2 had maximum gap separation, interface pressure, and sliding distance respectively. These nonlinear FEMs were used to investigate other mechanical parameters, such as force-fit ration (FFR) and friction coefficient. High FFR could reduce the sliding at the interfaces. However the configuration still might not hold when friction coefficient is smaller than 0.4%. Increased friction coefficient does not affect the gap distance and pressure at the interfaces. The results showed 1% of FFR can close all the gaps at the interfaces and eliminate all the sliding at the interfaces, with only a limited increase in the normal pressure.; With this information pin profile, pin configuration, and/or predrilling can be modified to provide the strength and long term stability required of an external skeletal fixator. |
