Vibration arthrometry in assessment of knee disorders: the problem of angular velocity

A knee joint that has sustained a painful injury will typically require skillful examination, by an orthopaedic surgeon, for signs of internal damage. These signs include characteristic sounds and vibrations, which are produced by the knee when it is stressed. The technique of vibration arthrometry is being developed to assist the clinical examiner in identifying these vibrations and to improve diagnostic accuracy. To detect and record the knee vibrations, small lightweight accelerometers are positioned on various bony prominences around the knee. These produce electronic signals which permit objective analysis of the vibration characteristics. It has been found that varying the investigative procedure can affect the magnitude of some parameters of the vibration signal. If these parameters are to be used in evidence of knee pathology, the effect of the investigative procedure must be normalized. The effect of speed of joint movement has been quantified in a pilot study involving 24 patients with internal knee damage. Custom-designed hardware was used to measure joint speed as the rate of change of joint angle, which was measured by an electrogoniometer. It was found that the energy content of the vibration, reflected by the peak amplitude and root mean square value was strongly affected by joint speed. However, the characteristic shape of the vibration, reflected by the peak frequency in the harmonic spectrum of the signal, remained similar for the range of joint speed in the investigation.