Transcutaneous Electrical Nerve Stimulation (TENS) is a non-invasive therapy that uses low-voltage electrical currents delivered through electrodes placed on the skin. A TENS unit provides temporary pain relief by stimulating sensory nerves, which helps block or modify the transmission of pain signals to the brain. This electrotherapy is used for various types of acute and chronic pain. A frequent question for individuals considering TENS therapy concerns the safety risks when a metal implant is present. The fundamental concern is how the electrical current might interact with foreign metal objects beneath the skin.
How TENS Interacts with Metal Implants
The primary concern regarding TENS use near metal implants stems from electrical conduction. Metal, such as titanium or stainless steel used in orthopedic implants, is a highly conductive material compared to surrounding biological tissues like fat, muscle, and bone. When a TENS current is applied to the skin, the presence of an underlying metal implant can potentially draw and concentrate the electrical field.
This concentration of electrical current around the metal can lead to two main risks. The first is a distortion of the current path, which may cause an unexpected or uncomfortable sensation for the patient. The second concern is the risk of localized heating in the tissue surrounding the implant. Concentrated electrical energy can be converted into thermal energy, potentially causing a temperature rise.
Scientific studies have investigated this thermal risk with common orthopedic metals. Research using therapeutic levels of TENS current on metal plates has shown that the resulting temperature changes are minimal, typically less than 3 degrees Celsius. This finding suggests that standard TENS, which uses a pulsed alternating current, does not generate sufficient energy to cause damaging heat in inert implants at clinical levels. Despite this, the potential for current distortion remains, necessitating careful consideration of electrode placement.
Safety Protocol for Inert Metal Implants
For patients with inert metal implants, such as joint replacements, surgical rods, screws, or plates, TENS use is generally acceptable with specific precautions. These implants are not powered and do not contain sensitive electronics that would be susceptible to electrical interference. The conditional safety of TENS hinges entirely on electrode placement relative to the implant site.
The fundamental safety protocol requires maintaining a significant distance between the TENS electrodes and the implant location. Clinicians often recommend a safe clearance of several inches from the implant site to prevent current concentration. This precaution minimizes the risk of current distortion and any resulting discomfort or possible localized heating.
The risk decreases if the metal implant is located deep within the body, far from the skin’s surface where the TENS electrodes are placed. Given that the therapeutic electrical current primarily travels through superficial tissue, deep implants are less likely to significantly alter the current path. However, the patient should never place the electrode pads directly over the implanted hardware.
Before starting any TENS therapy, a patient with a metal implant should consult a physician or physical therapist. These professionals can assess the specific location and type of implant and provide precise guidance on safe electrode placement. This individualized supervision ensures that the therapy can be safely integrated into a pain management plan.
When TENS Use Is Prohibited
TENS therapy is prohibited when the patient has electronic or active implants due to the risk of functional interference. These devices rely on precise electrical signaling to regulate bodily functions, and the external current from a TENS unit can disrupt that operation. Primary examples include cardiac pacemakers and implantable cardioverter-defibrillators (ICDs).
Applying TENS near these cardiac devices can cause electromagnetic interference, resulting in device malfunction or failure to deliver therapy, which can be life-threatening. Similar contraindications apply to other implanted neurostimulator devices, such as spinal cord stimulators, deep brain stimulators, and cochlear implants. The electrical impulses from the TENS machine could interfere with the programming or function of these sensitive electronics.
Beyond electronic implants, TENS should never be applied to specific areas of the body, regardless of the presence of metal. Placing electrodes over the front of the neck, near the carotid sinus, is prohibited because the stimulation could trigger a sudden drop in heart rate and blood pressure. TENS should also not be applied transcerebrally (across the head), as the effects of this stimulation on the brain are not fully understood and carry an unknown risk. These absolute contraindications must be respected to ensure the patient’s safety.