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. RELEVANCE: Several aspects of the current external fixator pin design and insertion technique have been implicated for the significant incidence of failure at the pin-bone interface manifested as pin loosening, pin track infection or even osteomyelitis. An axially-preloaded bone fastener system may be mechanically and biologically superior and therefore clinically advantageous.