Bone conductors, signals and sores

Anecdotal reports associate long-term use of a headband bone-conductor with the potential formation of skin pressure sores. When a bone conductor applies skin pressure that exceeds blood capillary pressure, capillaries collapse and blood flow ceases. Pressure-sores can develop if blood flow is cut-off for extended periods of time. To test if bone conductor users were at risk of developing sores, eleven adults were fitted with headband-worn bone conductors (BC461 and B71). The skin contact-pressure was measured to see if it exceeded 3.7 kPa, the estimated pressure needed to collapse capilaries at the mastoid process. Contact pressure was found to be substantially greater than capillary pressure (17 kPa for a standard adult headband and B71, 11 kPa for a BC461). Pressure can be reduced by increasing contact area so a BC461 bone conductor was modified to attach larger footplates. The measured pressure for a 38 mm footplate diameter was found to produce a contact pressure close to the capillery closure pressure. However, increasing the contact area changes the device mechanical-coupling impedance and sensitivity. Threshold measurements for the larger footplate device showed slightly poorer results. Preliminary measurements indicated decreased skin sensitivity to vibration with larger footplates. To conclude, small footplate bone conductors (e.g., BC461) should be fitted with the least pressure possible commensurate with a bone conductor staying on the head. The skin contact position for the bone conductor should be moved regularly to avoid prolonged disruption of capillary blood supply to the skin. For future devices, it is recommended that bone conductors are designed with larger footplate areas. This reduces skin contact pressure with a small loss in measured thresholds, reduced skin vibration sensitivity and improved wearer comfort. Copyright