An ODT article from 2019 discussed various devices using electrical stimulation of pain-modulating nerves for orthopedic issues. While new products continue to emerge, consistent patient response remains elusive. New research published in the past few years sheds light on the mechanistic reasons why electro-stim doesn’t always deliver as promised. Fortunately, several new energy therapies targeting specific pain mechanisms, including focal mechanical oscillation, pulsed electromagnetic field therapy, and low laser stimulation with heat, can bridge the gap between pain control and hospital discharge. As government task forces confirm pain relief is a process, not a silver bullet, other additive therapies targeting the same mechanoreceptors are a promising piece of the pain management arsenal in the new, short-stay world.
To understand the new modalities of pain relief, it’s helpful to understand how and why physical interventions block pain. Think of sharp pain from a needle, injury, or surgery as an irritating message, like the shrill chirp of a fire alarm requiring a battery change. After the recognition of the message along fast-but-tiny a-delta nerves, the body is tuned to prioritize other nerve signals that facilitate action to escape the pain. These nerves needed for escape are primarily mechanoreceptors—nerves that feel mechanical touching and force, and transmit a variety of information more quickly than pain. While analgesics turn down the volume of the shrill signal, making other escape sensations “louder” can also render pain harder to perceive.
There are four mechanoreceptors the body prioritizes over the chirp of pain.
- Meissner corpuscles to detect light touch
- Ruffini that feel stretch
- Merkel disks register prolonged pressure
- Pacinian respond to position sense
Slower nerves—called “c-fibers”—coat the underside of the skin in the same way the a-delta fast pain nerves do, but with a different effect. These nerves transmit slowly building noxious sensations that can add up to danger, like ice or unpleasant electrical sensations. Initially, they build up and the part of the brain that interprets danger (the anterior cingulate cortex) says, “Wait. This is annoying, but not dangerous. Let’s shut down all perception here.” Some texts call this Descending Noxious Inhibitory control, some call it Conditioned Pain Response, but the idea is like starting a brush fire to stave off a wildfire.
These nerves are employed to relieve pain in everyday situations that are taken for granted. Rubbing a bumped elbow overwhelms pain with Merkel and Pacinian messages. Placing a burned finger under running water activates c-fibers and Meissner, as well as Pacinian. Massage and yoga stretch Ruffini corpuscles. One orthopedic intervention taking advantage of these techniques is Mechanical Tape.
Rather than a static support from kinesiotherapy tapes, Mechanical Tape adds a stretch that can stimulate Ruffini pain relief, in addition to remodeling pathologic gaits or motion.1 Likewise, rollers can give a Merkel-generated, pressure-mediated pain relief that can persist after use.
The way external e-stim is supposed to work is to trigger mechanoreceptors to instigate a-beta pain modulation.2 Electrical charge can stimulate a nerve to fire when applied transcutaneously (through the skin) from a positive pole to a negatively charged lead. Iontophoresis uses the charged flow to take hydrophilic lidocaine across the skin barrier, while transcutaneous electrical nerve stimulators (TENS) and neuromuscular electrical stimulation (NEMS) are relying on direct stimulation or muscle twitching to indirectly stimulate an adjacent nerve for pain relief.
Slow electrical frequencies with tiny amplitudes can tingle the superficial Meissner corpuscles to relieve minor pain, using the same mechanism as menthol in products such as Tiger Balm or Icy Hot. These signals are low frequency (2-5 Hz) and low amplitude. The benefit is the electrical signal is fairly easy to ignore while the constant counter-irritant reduces pain. For some of the menthol creams, the scent may be more noticeable than stealth pads and electricity. While menthol will wear off, the light electrical tingle can last as long as the battery permits it.
In 2015, Harper found the strongest pain relief came from stimulating the Pacinian nerve, the deepest of the four mechanoreceptors.3 In 2012, research proved these nerve bodies respond when triggered with a 180-250 Hz stimulus.4 These two studies combined shine light on when and why electrostim lets clinicians down. First, to trigger the Pacinian, electricity must twitch a muscle to twitch the nerve, or must penetrate deeply. This means increasing the amplitude, which 50 percent of patients find uncomfortable enough to lead to noncompliance.5 Second, most TENS units have their high amplitude strength in the 120-150 Hz range. Even at strong current, the frequency is wrong to stimulate the Pacinian gating response.
Before the specifics of the Pacinian importance and frequencies were published, vibratory pain devices had a decade of support for IV access and injections. The first on the market—Buzzy from Pain Care Labs—used a proprietary 200 Hz mechanical vibration combined with ice to more effectively reduce pain by combining a-beta and c-fiber inhibition simultaneously.6 Over a 10-year period, physicians and patients used the device for blood draws, injecting biologics to treat rheumatoid arthritis, and steroid injections. Since the same a-delta nerve transmits the pain alarm from a needle as well as a hip or knee injury, patients used the device for the pain of their underlying conditions as well. Currently the palm-sized devices (reconfigured as VibraCool) are used specifically to reduce pain and opioid use after extremity surgeries, either by proximal application or as part of a brace or sling.
At the recent American Academy of Physical Medicine and Rehabilitation annual assembly, a randomized crossover trial was presented that evaluated the efficacy of a 20-minute session of 180-200 Hz mechanical oscillatory vibration (0.1 m/s2 amplitude with VibraCool) compared to a generic model-TENS 3000 applied to pain. TENS units used 150 Hz frequency with a pulse width of 200 ms, asymmetrical biphasic square pulse waveform, and amplitude as high as comfortable on a 0-80 mA using a 500 ohm load per channel. Mean pain relief with the VibraCool was greater than with the TENS [-2.2 ±1.34 (95 percent CI -2.85 to -1.55, P<0.0001)], with the greatest difference noted for spine, joint injury, and post-surgical pain (5-6) and least for osteoarthritis pain (2-3).7 Logically, the impact was greatest where muscle and tendon Pacinian joint-position nerves have the highest density.
The benefits of mechanical vibration compared to electrical stimulation are specifically relevant for rehabilitation. For pain, since the mechanoreceptors are designed to be triggered by mechanical force, a vibration stimulus is both more efficient and more comfortable than NMES.8 The angle of torque penetrates beyond the Pacinians, where vibration also sympathetically stimulates increased blood flow.9 The increase in blood flow noted may balance concerns of vasoconstriction when ice is used simultaneously. With the advent of fast, inexpensive light eccentric motors, the commensurate research publications accurately reporting frequencies and physiologic outcomes numbers over 200 in the past decade.
For training, the mechanical oscillatory force in various frequencies has been evaluated for separation of muscle fibers to reduce delayed onset muscle soreness.10 Post-operatively, vibratory frequencies of 150 Hz have been found to reduce quadriceps hypotrophy after ACL repair,11 improve function, and speed bone repair. Human bodies are designed to grow with appropriate stress (think of lifting weights), so multiple forms of energy can be focused to accomplish this goal. From massage to low-amplitude/high-frequency mechanical oscillation to ultrasound to pulsed electromagnetic frequency to light, as energy spreads through tissues, the physical energy impacts cellular growth.
While small intense vibration with or without ice can improve acute and post-operative pain more intensely, lower amplitude energy is also a cellular stimulator of growth and pain relief. Two therapies with expanding research support are pulsed electromagnetic field stimulation (PEMF) and low-level laser therapy. The former uses swinging magnets in the same way an MRI variably stimulates cells of different density to stimulate growth or relieve pain. The laser therapies use various frequencies to accomplish the same objective.
As with early vibration research, the field is littered with contradictory findings based on aspects of energy transmission not yet understood. Just as a 110 V product won’t work in a 220 V outlet, the frequencies and transmission modalities are critical. Laser penetration is different through different thicknesses of skin; PEMF may work on soft tissues, but showed no efficacy for osteoarthritis. Both therapies increase expression of protein kinases, and show promise for improving healing.12 As research refines the physics of energy and tissues needed for reliable clinical effect, both modalities will likely be part of the pain and post-op armamentarium.
An up-and-coming therapy that may be incorporated with orthopedic bracing and post-operative treatments is low level laser light therapy. At a minimum, chronic pain may benefit from heat. The underlying mechanism may relate more to the central than peripheral pain reception. After prolonged pain, the brain becomes sensitized to even mild sensations like cloth or touch. By substituting an expected pain experience with reduced cramping or soothing warmth, over time the brain may begin to properly interpret signals, reducing pain. One device with clinical support—Willow Curve—incorporates laser, infrared, and LED light emission in the 292-4,000 Hz range. While too large to incorporate into a brace, one clinical review found “our experience with the WC device was quite rewarding, in that we were not expecting such strong treatment effects.”13
As research progresses on new energy modalities, perhaps the most important lesson is that patients want options. The current climate’s anti-opioid focus supports experimentation of safe and physiologically plausible interventions. Pre-surgical opioid use is associated with poorer outcomes. Catastrophizing, or the tendency to feel out of control or easily overwhelmed by thoughts of the worst case scenario, predisposes to prolonged pain. By incorporating multiple modalities, patients are empowered to take control of their pain. From energy to ice to heat, a new evidence-based non-pharmacologic era is beginning.
[Author Note: While Amy Baxter, M.D., F.A.A.P., F.A.C.E.P., is chief medical officer of Pain Care Labs, any studies reported related to Pain Care Labs products were independent and unfunded.]
- Robinson NA, Spratford W, et al. Does Dynamic Tape change the walking biomechanics of women with greater trochanteric pain syndrome? A blinded randomised controlled crossover trial. Gait Posture. 2019 May;70:275-283.
- Vance CG, et al. Using TENS for pain control: the state of the evidence. Pain management. 2014 May;4(3):197-209.
- Hollins M, Corsi C, Sloan P. Pacinian Signals Determine the Direction and Magnitude of the Effect of Vibration on Pain. Perception. 2017 Aug;46(8):987-999.
- Manfredi LR, et al. The effect of surface wave propagation on neural responses to vibration in primate glabrous skin. PloS one. 2012;7(2):e31203.
- Serrano-Munoz D et al. Intensity matters: Therapist-dependent dose of spinal transcutaneous electrical nerve stimulation. PloS one. 2017;12(12):e0189734.
- Ueki S, Yamagami Y, Makimoto K. Effectiveness of vibratory stimulation on needle-related procedural pain in children: a systematic review. JBI Database System Rev Implement Rep 2019 Jul;17(7):1428-1463.
- Tiziano M, Baxter A. Crossover trial of novel mechanical oscillatory vibration frequency device versus TENS for musculoskeletal pain. AAPM&R, November 2019 Poster 7212
- Mineto M et al. Contralateral effect of short-duration unilateral NMES and focal vibration in healthy subjects. Eur Jour Physical and Rehab Med 2018 December;54(6):911-20.
- Ichioka S, Yokogawa H, Nakagami G, Sekiya N, Sanada H. In vivo analysis of skin microcirculation and the role of nitric oxide during vibration. Ostomy Wound Manage. 2011 Sep;57(9):40-7.
- Lu X, Wang Y, et al. Does Vibration benefit delayed-onset muscle soreness?: a meta-analysis and systematic review. J Int Med Res. 2019 Jan;47(1):3-18
- Benedetti MG1, Boccia G et al. Localized muscle vibration reverses quadriceps muscle hypotrophy and improves physical function: a clinical and electrophysiological study. Int J Rehabil Res. 2017 Dec;40(4):339-346
- El-Makakey AM, El-Sharaby RM, et al. Comparative study of the efficacy of pulsed electromagnetic field and low level laser therapy on mitogen-activated protein kinases. Biochem Biophys Rep. 2017 Jan 25;9:316-321
- Tiziano M, Majewski M. Pain /therapy Options for Home: a patient-based outcome review of at-home pain managment devices, including willow Curve, Quell, and VibraCool. Practical Pain Management 19(1):56-59.
Amy Baxter M.D. is CEO and chief medical officer of Pain Care Labs, founded in 2006 to eliminate unnecessary pain. After Yale and Emory Medical School, she founded the Emergency Research program at Scottish Rite, CHOA. Federally funded for low back pain relief and opioid reduction research, she publishes and lectures nationally and internationally on pain management. Inventions include VibraCool Vibrational Cryotherapy for tendinitis and as a post-operative opioid alternative; her Buzzy device has blocked pain for over 35 million needle procedures. Other contributions include a hepatic enzyme algorithm timing child abuse, creating and validating the BARF nausea scale for pediatric cancer patients, identifying the cause of the needle phobia increase, and eight patents on the pain blocking frequency. Speaking venues include Exponential Medicine, Bloomberg, Converge, TEDx Peachtree, and TEDMED. Awards include Healthcare Transformer, Wall Street Journal “Idea Person,” Innovative GA Bio CEO of the Year, and a Top 10 Disruptor in Medical Tech.