The pin-bone interface in external fixator: a standardized analysis in a sheep osteotomy model

The Influence of Sagittal Pin Angulation on the Stiffness and Pull-Out Strength of a Monolateral Fixator Construct

Monolateral pin-to-bar-clamp fixators are commonly used to stabilize acute extremity injuries. Certain rules regarding frame geometry have been established that affect construct stability. The influence of sagittal pin angulation on construct stiffness and strength has not been investigated. The purpose of this biomechanical study was to demonstrate the effect of a pin angulation in the monolateral fixator using a composite cylinder model. Three groups of composite cylinder models with a fracture gap were loaded with different mounting variants of monolateral pin-to-bar-clamp fixators. In the first group, the pins were set parallel to each other and perpendicular to the specimen. In the second group, both pins were set convergent each in an angle of 15° to the specimen. In the third group, the pins were set each 15° divergent. The strength of the constructions was tested using a mechanical testing machine. This was followed by a cyclic loading test to produce pin loosening. A pull-out test was then performed to evaluate the strength of each construct at the pin-bone interface. Initial stiffness analyses showed that the converging configuration was the stiffest, while the diverging configuration was the least stiff. The parallel mounting showed an intermediate stiffness. There was a significantly higher resistance to pull-out force in the diverging pin configuration compared to the converging pin configuration. There was no significant difference in the pull-out strength of the parallel pins compared to the angled pin pairs. Convergent mounting of pin pairs increases the stiffness of a monolateral fixator, whereas a divergent mounting weakens it. Regarding the strength of the pin-bone interface, the divergent pin configuration appears to provide greater resistance to pull-out force than the convergent one. The results of this pilot study should be important for the doctrine of fixator mounting as well as for fixator component design.

Finite element analysis of the equivalent stress distribution in Schanz screws during the use of a femoral fracture distractor

To evaluate the mechanical stress and elastic deformation exercised in the thread/shaft transition of Schanz screws in assemblies with different screw anchorage distances in the entrance to the bone cortex, through the distribution and location of tension in the samples. An analysis of 3D finite elements was performed to evaluate the distribution of the equivalent stress (triple stress state) in a Schanz screw fixed bicortically and orthogonally to a tubular bone, using two mounting patterns: (1) thread/shaft transition located 20 mm from the anchorage of the Schanz screws in the entrance to the bone cortex and (2) thread/shaft transition located 3 mm from the anchorage of the Schanz screws in entrance to the bone cortex. The simulations were performed maintaining the same direction of loading and the same distance from the force vector in relation to the center of the hypothetical bone. The load applied, its direction, and the distance to the center of the bone were constant during the simulations in order to maintain the moment of flexion equally constant. The present calculations demonstrated linear behavior during the experiment. It was found that the model with a distance of 20 mm between the Schanz screws anchorage in the entrance to the bone cortex and the thread/shaft transition reduces the risk of breakage or fatigue of the material during the application of constant static loads; in this model, the maximum forces observed were higher (350 MPa). The distance between the Schanz screws anchorage at the entrance to the bone cortex and the smooth thread/shaft transition of the screws used in a femoral distractor during acute distraction of a fracture must be farther from the entrance to the bone cortex, allowing greater degree of elastic deformation of the material, lower mechanical stress in the thread/shaft transition, and minimized breakage or fatigue. The suggested distance is 20 mm.