Stability of external circular fixation: A multi-variable biomechanical analysis

Objective. To determine how the manipulation of the parameters of fixation and components of the circular external frame could improve and maintain optimal stability of bone fragments. Design. We performed a multi-parametric biomechanical analysis of the extrinsic parameters effecting bone fragment stabilization. Results of testing are presented as a percent change in stiffness due to the manipulation of frame components and their interaction with other fixation parameters. Background. Although there have been investigations of the biomechanical characteristics of circular external fixation, they have been limited to either individual frame components or full frame comparisons. Therefore, these studies did not provide a comprehensive understanding of how the manipulation of circular fixator components influences bone fragment stability. Methods. Mechanical testing was performed in three phases examining the effect of numerous components including ring diameter, wire angle, ring separation, etc. on axial, torsional and bending stiffness. Results. For phase I (single ring) and phase II (double-ring block), ring diameter was the most significant factor affecting axial and torsional stiffness, while wire angle, ring separation, and their interaction had the most influence on bending stiffness. Phase III (two double-ring blocks) showed that ring positioning with respect to the osteotomy site had the most affect on bending and torsional stiffness while axial stiffness was non-linear and dependent upon the applied load. Conclusions. The stability of bone fragments within a circular external fixator is affected by manipulation of the parameters of fixation or individual components of the frame. The contribution of each component to overall bone fragment stability is dependent upon the mode of loading. The changes in overall stability of bone fragments are dependent not only on the individual frame components but also upon their interaction with other parameters of fixation.