Mechano-electric coupling in bone tissue. A confounding factor in the bone adaptation process

Bone remodeling activity is regulated by the mechanical strains that arise in bone tissue during functional activity. Reduction in activity leads to bone resorption, while increased activity will initiate new bone formation. While it may be possible that mechanical strains directly perturb the bone cells through physical deformation, and thereby affect cellular activity, the fact that strains of less than 0.01% are capable of affecting remodeling activity suggests that a secondary signal may mediate the remodeling process. A prime candidate for this signal is the electrokinetic current produced within bone during loading. We have undertaken in vivo studies in an effort to determine whether such mechano-electric coupling is important in the remodeling process. The mechanical loading experiments have shown that bone demonstrates a response to strain that increases monotonically with frequency, up to at least 30 Hz. Electric field induction experiments indicate that bone has a peak sensitivity to electric fields near 20 Hz, and can respond to fields as low as 1-10 microvolts/cm. The response of bone to mechanical loading frequency clearly supports a transduction mechanism dependent on the derivative of the mechanical load, sufh as elecltrokinetic currents. But the reported electrokinetic field measurements in bone suggest that these currents are both too large and have too low a cutoff frequency to explain the in vivo mechanical and electrical exposure data. These observations lead to the suggestion that an alternative source of electric fields (i.e. piezoelectric processes) may be important to the remodeling process. Alternatively, it may be that the electrokinetic currents arising in the larger channels of the bone, which are not easily recorded because they are smaller in magnitude and have faster relaxation times, may dominate the remodeling signal transduction mechanism.