Axial vibration of threaded external fixation pins: detection of pin loosening

The hypothesis of this study was that a nondestructive vibrational method could detect bone lysis at the external fixation pin-bone interface prior to current clinical and radiographic methods. In vitro models were used to simulate changes observed during pin loosening in vivo. Fixation pin axial natural frequency decreased with decreasing tensile modulus of the material into which it was implanted. In a live animal study the right tibia of 12 dogs was fractured and stabilized with a four-pin unilateral external fixation frame. The axial natural frequency of each pin was measured and radiographs were taken at 0, 2, 4, 6, 8, and 10 weeks after surgery. The natural frequency did not change when the first radiographic changes around the interface were observed. Pins were palpably stable at this point. As loosening progressed, the natural frequency did decrease. Frequency and quasistatic tests of dissected pin-bone structures revealed a good correlation between natural frequency and pin-bone interface stiffness. In addition, the measurement of natural frequency was more sensitive to bone structure changes at the pin-bone interface than low-load quasi-static stiffness. Therefore, a nondestructive vibration technique could be used instead of low-load quasistatic tests for assessing the pin-bone interface ex. vivo.