AXIAL VIBRATION OF THREADED EXTERNAL FIXATION PINS - DETECTION OF PINLOOSENING

The hypothesis of this study was that a nondestructive vibrational met hod could detect bone lysis at the external fixation pin-bone interfac e prior to current clinical and radiographic methods. In vitro models were used to simulate changes observed during pin loosening in vivo. F ixation 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 f our-pin unilateral external fixation frame. The axial natural frequenc y 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 whe n the first radiographic changes around the interface were observed. P ins were palpably stable at this point. As loosening progressed, the n atural frequency did decrease. Frequency and quasistatic tests of diss ected pin-bone structures revealed a good correlation between natural frequency and pin-bone interface stiffness. In addition, the measureme nt of natural frequency was more sensitive to bone structure changes a t the pin-bone interface than low-load quasi-static stiffness. Therefo re, a nondestructive vibration technique could be used instead of low- load quasistatic tests for assessing the pin-bone interface ex vivo. ( C) 1998 Biomedical Engineering Society.