Axial vibration of threaded external fixation pins in vivo: detection of pin instability and pin tract lysis

The objective of this study was to investigate the ability of a non-destructive vibrational method to detect bone lysis at the external fixator pin-bone interface. The right hind leg of 12 dogs was fractured and stabilized with a 4 pin unilateral frame. The resonant frequency of each pin was measured and radiographs were taken at 0, 2, 4, 6, 8, and 10 weeks post-op. At 11 weeks dogs were sacrificed and quasi-static stiffness was measured for each pin. There was good correlation between frequency and quasi-static stiffness. The vibrational method was unsuccessful 28% of the time in detecting the changes in bone quality around a pin that had been determined using standard radiographs. Resonant frequency was not sensitive to small changes in pin tract bone quality that had not resulted in a significant decrease in pin stability. It was concluded that the vibrational method used in this investigation was not an improvement over standard radiographs for diagnosing pin tract bone lysis. However, the vibrational method was more sensitive to changes at the pinbone interface than low-load quasi-static tests.