Electrical Stimulation and Bone Healing (Direct/Capacitive/Inductive Coupling)

Bone is piezoelectric. Bone generates electrical potentials when it is mechanically loaded, the stress-generated potentials. The concave (compression) surface becomes electronegative, which is associated with bone formation, while the convex (tension) surface is electropositive and associated with resorption. It is an electrical correlate of Wolff's law.

Two origins. Classical piezoelectricity is a property of the oriented collagen-apatite lattice: deforming the crystal generates charge, and it is demonstrable even in dry bone. Streaming potentials are the dominant mechanism in wet, living bone. Load drives interstitial fluid through the canalicular network, and the moving charged ions stream past the negatively charged bone matrix, generating a potential.

What they share. Both make the loaded, concave, compression surface electronegative and osteogenic. The same fluid shear is sensed by osteocytes, and the cellular mechanotransduction is developed in our Osteocytes & Mechanotransduction topic.

What a stimulator does. Electrical and electromagnetic stimulation aim to mimic this signal and create an osteogenic environment. At the cellular level they modulate calcium signalling and upregulate osteogenic growth factors (BMP, TGF-beta), osteoblast activity and angiogenesis, promoting bone formation across a fracture or fusion. Only devices with specific waveforms and adequate daily exposure achieve this.

View larger

Direct current is the invasive one. An electrode is implanted with the cathode at the fracture or fusion site and delivers a constant current. The cathode creates an electronegative, relatively hypoxic, osteogenic microenvironment. It is used for established non-unions and as a spinal-fusion adjunct.

Why the cathode forms bone. At the cathode, electrochemical reduction consumes local oxygen, lowering oxygen tension, generates hydroxyl ions that raise the pH toward alkaline, and produces hydrogen peroxide. The resulting low-oxygen, alkaline, electronegative microenvironment favours osteoblastic bone formation, and peroxide-driven VEGF and angiogenesis bring new vessels.

Capacitive coupling. Two external skin electrodes, acting as capacitor plates, are placed on opposite sides of the limb and connected to a generator. They create an electric field across the fracture without anything being implanted.

Inductive coupling (PEMF). An external coil placed around the limb produces a time-varying magnetic field that, by Faraday's law, induces an electric field and current within the bone. The combined magnetic field (CMF) device is a related form.

Compliance. The non-invasive devices are worn for several hours daily over weeks to months, so compliance is essential.

A nudge, not a substitute. Stimulation does not replace adequate reduction, mechanical stability and biology. It suits the non-union whose mechanics are already adequate and whose biology just needs a nudge, in a patient who can comply with hours-per-day wear for months.

The right candidate. The non-union to stimulate is aligned and stably fixed, with no gross motion and a small gap or none. It has no infection, no established synovial (true) pseudarthrosis and no interposed soft tissue.

Operate instead when any of these is present, because each needs surgery to fix the deficient element:

• An atrophic non-union with poor biology needing graft
• Gross instability or inadequate fixation
• A large gap or bone loss
• A deformity needing correction
• Infection, which needs debridement
• An established synovial pseudarthrosis
• Interposed soft tissue

The diamond concept. Healing needs mechanical stability, an osteoconductive scaffold, osteogenic cells and osteoinductive growth factors, with host vascularity. Electrical stimulation, like LIPUS, addresses mainly the biological, osteoinductive vertex. It cannot substitute for a missing mechanical vertex or for absent scaffold or cells, so correct the deficient vertex (revision fixation, grafting) rather than relying on a stimulator alone. Non-union classification and management are developed in our Nonunion Management topic, and the ultrasound adjunct in our LIPUS topic.

Where it is used. Stimulation is an adjunct for problem healing:

• Established non-unions, the principal indication
• Delayed unions
• Fractures at high non-union risk
• Adjunct to spinal fusion, especially revision or high-risk cases
• Stress fractures
• Early osteonecrosis

What the evidence supports. Non-union series report union rates of around 80%, and the non-invasive devices avoid the risks and costs of surgery. The literature is mixed, though, and the effect is device-specific, since only certain waveforms work, and depends on sustained patient compliance. The series figure is uncontrolled and cannot be set beside the sham-controlled result in the Evidence section, which is the one to quote.

Cautions. Take care in patients with pacemakers or other implanted electronic devices and in pregnancy. Ensure the correct device is selected and properly applied.