The use of resonant frequency measurements for the noninvasive assessment of mechanical stiffness of the healing tibia.

A study was performed to assess the usefulness of measuring the resonant frequency of vibration of the tibia as a determinant of its flexural rigidity in fracture healing. The measurement system consisted of an electronic tapper that struck the anteromedial distal surface of the tibia and an accelerometer, held against the proximal surface, that received the response. Custom software displayed the frequency spectrum from which the first mode bending frequency of the tibia could be pinpointed. We have found that resonant frequency was highly correlated to the log of tibial flexural rigidity, with an exponent of 0.582 (r2 = 0.815). In human volunteers, a series of four tests at monthly intervals showed a reproducibility of approximately 3% (95% confidence interval) with no significant variability between right and left legs. In 14 patients with healing tibial fractures, resonant frequency ratio (healing/injured leg) was correlated to time ratio (time of test/time healed) by a second-order polynomial fit (r2 = 0.58).