The force/time relationship of clinically used sensory testing instruments.

The stimuli of commonly used sensibility measurement instruments tested in this study demonstrate unequivocally that "hand-held instruments" produce variations in application force from one stimulation to another, one instrument to another, and from one examiner to another. These application force variations cannot be compensated for by care in technique and need to be controlled for measurement reliability. Only the Semmes-Weinstein monofilaments provide some control of force during application and can be considered force controlled if calibrated and applied correctly. The monofilaments, too, become less controllable if applied too quickly and bounced against the skin. Two-point discrimination instruments, in particular, lack control of application force, with the force of one point significantly different from two points. A difference in applied force makes it possible for a patient to solve the two-point discrimination test by discerning the difference between heavier and lighter forces, rather than one or two-point recognition. Tuning fork instruments for vibration testing have even larger variations in application force amplitude rendering their stimulus highly uncontrolled and masking the actual vibration of the tuning fork. Spectral analysis of the force frequency signal produced by hand held sensibility measurement instruments shows they all produce both high and low frequency signals sufficient in strength to stimulate both slowly adapting and quickly adapting end organs, and are not capable of stimulating one particular group. These dynamic properties of testing stimuli explain why our tests are not as repeatable and sensitive as desired. The understanding of these dynamic properties in sensibility measurement is a key for improved instruments and more repeatable findings in clinical testing.