5Across Intact Skin: Passive Transcutaneous Bone Conduction

FClosing the skin, keeping the bone route

The percutaneous abutment delivers superb sound but never lets the skin fully heal, and that open interface needs lifelong care. The passive transcutaneous design answers that problem by keeping the skin completely closed. An implant carrying a magnet (or a magnetic plate) is fixed to the skull under the skin, and the external unit, which contains both the microphone-processor and the vibrating transducer, holds itself in place by magnetic attraction across the intact skin.

The word passive is the key. In this design the vibrating part stays outside the body; nothing electronic or mechanical is implanted beyond the magnet and its housing. The transducer shakes the external assembly, and that vibration is pressed through the skin and into the magnet and skull beneath. The pay-off is obvious: no hole, far fewer soft-tissue reactions, and a tidier cosmetic result. The cost is that the vibration must now cross living tissue again.[2015]

TThe skin tax and the magnet itself

Forcing vibration through skin and subcutaneous fat costs output. The soft tissue behaves like a compliant spring that absorbs energy, and the loss grows with frequency: measurements of a passive transcutaneous path show attenuation rising from only a few dB around 1 kHz to roughly 20 to 25 dB above 6 kHz. The thicker the overlying tissue, the worse the toll, so candidacy is more sensitive to the sensorineural component of the loss than it is for a percutaneous device with the same processor.

The magnet is also the source of the design’s characteristic problems. Holding the external unit on requires enough magnetic force to overcome both gravity and the springiness of the skin, but too much force ischaemises the tissue caught between the magnets, causing pain, redness, and in some cases pressure necrosis or skin breakdown directly over the implant. Clinicians manage this by adjusting magnet strength with spacer pads and by counselling on wear time. The same internal magnet creates imaging difficulties discussed below.[2015][2016]

CThe systems and who they suit

Cochlear markets the passive transcutaneous design as Baha Attract: a BI300 fixture carries an implanted magnet plate, and a matching external magnet holds the standard Baha processor and its transducer against the skin. The Sophono device (later marketed by Medtronic as the Alpha system) uses a pair of implanted magnets in a sealed bone-bed plate rather than an osseointegrated screw, again coupling to an external processor across intact skin. Both serve conductive and mixed losses and single-sided deafness, with the same broad principle and the same skin-attenuation limit.

These systems suit patients who want to avoid an open abutment and its hygiene, who have had or fear skin complications, or for whom cosmesis is paramount, including many children. The trade-off must be explained candidly: head-to-head pediatric data show lower aided gain and somewhat poorer speech outcomes than the percutaneous Connect system, balanced by markedly fewer skin and wound complications. Where the sensorineural loss is more than mild, the skin tax may push the result below what the patient needs.[2020][2019]

CMagnets, skin care, and the MRI question

Day-to-day, the dominant issue is the magnet-skin interface. Too-strong coupling causes pain and pressure injury; too-weak coupling lets the processor fall off. Spacer discs of increasing thickness let the audiologist titrate retention against comfort, and patients are taught to inspect the skin and limit continuous wear. Genuine skin breakdown over the magnet, though less common than peri-abutment reactions, can require explantation.

The implanted magnet also dictates MRI behaviour. Passive transcutaneous systems with a fixed internal magnet are typically conditional at 1.5 Tesla but produce a large image artifact that obscures the temporal bone and brain on the implanted side, and they can experience torque, pain, or even demagnetisation, particularly at higher field strengths or with repeated scans. The external processor must always be removed, and for higher-field imaging the internal magnet may need to be surgically removed and later replaced. This contrasts sharply with active transcutaneous designs that use a non-magnetic piezoelectric transducer and so avoid much of the MRI penalty.[2016][2018]