Axial preload in external fixator half-pins: a preliminary mechanical study of an experimental bone anchorage system

Objective. To demonstrate the effect of axial preload achieved with an experimental external fixator pin prototype on the initial stability of the pin-bone interface. Design. An in vitro mechanical study comparing pin stability of an axially-preloaded experimental pin and a radially-preloaded conventional external fixation pin. Background. The most common problem in clinical practice of external fixation is pin site failure. An experimental external fixator half-pin has been designed introducing axial preload as an alternative method of achieving a stable pin-bone interface. Methods. Standardized measurements of pin insertion and removal torque comparing the experimental pin prototype and a conventional external fixator pin in a synthetic composite material and ovine tibial cortical bone. Results. There is a positive correlation between axial preload and torque resistance of the coaxial experimental half-pin as well as significantly increased torque resistance of the axially-preloaded experimental pin over the conventional pin tested. In addition, significantly lower insertion torque of the experimental pin was observed with the applied insertion technique. Conclusions. ‘Axial preload’ achieved with appropriately-designed half pins might represent an effective biology-sparing method of increasing the mechanical stability of the pin-bone interface in unilateral external fixator frames.