Effect of clamp type on type II external fixator stiffness

In order to investigate the effects of two clamp types on type II external fixator stiffness, models using wooden dowels were constructed and mechanic ally tested. Kirschner-Ehmer clamps were compared to Secures clamps in four -pin type II external fixator configurations. Increasing axial loads were a pplied and displacement at a gap in the centre of the model measured. Linea r regression and comparison of the slope of the regression lines before yie ld, after yield, and throughout 700N assessed model performance. The result ing load displacement curves were linear. There was no difference in model stiffness before yield, but was greater with models constructed with Secure s clamps after yield and overall. The mean relative stiffness of the models constructed with the Kirschner-Ehmer type clamps were 320 +/- 53.8 Newtons /mm, and with models constructed with Secures type clamps were 425 +/- 74.8 Newtons/mm. Utilizing Secures clamps resulted in significantly (p<0.05) gr eater frame stiffness that is attributable to higher load at yield and grea ter stiffness after yield. Increasing resistance to rotational moments at t he clamp/connecting bar junction should result in less stress imparted to t he bone/pin interface, decreasing strain and decreasing pin loosening.