Written by Dr. Robert Donaldson in Total Knee Replacement
As a physical therapist, having used TENS units to help all kinds of patient’s pains on almost every body part, I was introduced to how powerful a TENS unit can be about 40 years ago, early in my career.
A senior citizen presented to the physical therapy clinic at the hospital where I was the Chief of Physical Therapy. She was referred by a local orthopedic physician for knee pain control. She was very limited in ambulation endurance because of knee pain and was using a cane to decrease the weight-bearing on the knee because of the pain.
After my in-clinic treatment session, the patient had pain relief, but the pain levels remained significantly high.
I decided to incorporate the use of a home TENS unit on this painful knee as part of the patient’s home program. She was to wear the TENS unit as much as she needed it to control the pain.
Two days later she returned to the clinic with a very swollen and painful knee.
Upon questioning, the patient told me she was wearing the TENS unit continuously and the TENS unit had completely eliminated her knee pain after our first physical therapy session, allowing her to walk without a cane or any ambulation assistive device, and without pain.
She felt so good she did a lot of walking, even some gardening, and was very active all day the day after she started wearing the TENS unit. She wore the TENS unit all day, even when she was walking and gardening.
I did not expect that result. I had no idea that a TENS unit could be that effective so I had failed to give the patient adequate instructions on activity restrictions if the TENS unit eliminated the pain.
I learned a valuable lesson that day and gained a new respect for the power of a TENS unit to reduce or eliminate pain.
Using a TENS unit for pain control after total knee replacement surgery is a common protocol for many orthopedic surgeons.
The use of a TENS unit after a total knee replacement can decrease the patient’s pain and opioid consumption.
A study, Transcutaneous electrical nerve stimulation for postoperative pain control after total knee arthroplasty published in the US National Library of Medicine confirms that patients receiving a TENS unit after total knee surgery experience less pain and a decreased need of opioid medications.
Just the reduction in opioid medication is well worth the investment in time and money.
Opioid medications come in various strengths.
Many of the total knee replacement patients I see in their own homes a day or two after surgery have been issued two different strengths.
The two most common opioids I see issued my total knee patients on discharge from the hospital are:
I will use the Tramadol and Norco in the following explanation, however, your surgeon may prescribe different-named opioids, but the principle remains the same.
The more powerful opioid medication issued my patients on discharge from the hospital, such as Norco, are intended to be used as “break-through” medications and are to be used when the lesser strength opioid, Tramadol, fails to control the pain adequately. That’s usually when the pain reaches 7/10 or more while taking the lesser strength opioid Tramadol.
Break-through opioid medications like Norco are taken on a pain-contingent basis, meaning the patient is to take the medication only when the pain is not controlled with the lesser strength opioid, and is to be taken on an “as needed” basis within guidelines on the prescription label.
It would say something like “Take 1-2 tables every 4-6 hours as needed”.
Tramadol is used on a time-continent basis, meaning the patient is to take the medication on a regular time interval. Frequently the prescription will direct the patient to take the Tramadol every 4 hours. This is because the doctor wants the patient to have a consistent level of pain medication in their bloodstream.
Norco, the more powerful of the opioids, is to be used intermittently to “break” a severe pain episode. Norco is used on a pain-contingent basis, meaning the patient takes the Norco in a severe pain episode but as soon as the patient’s pain is under control using the Tramadol, the patient is to discontinue taking the Norco and rely on the Tramadol to once again keep the pain at tolerable levels, usually at about 6/10 or less.
Unfortunately, a lot of my total knee replacement patients do not have a clear idea of how they should be using these medications. Often the instructions are given to the patient before discharge from the hospital but after the total knee surgery. The patient is already impaired in concentration due to the medications and anesthesia they are already receiving and the ability to understand and remember the medication instructions.
Not exactly the best time for the patient to receive simple instructions, let alone complicated instructions about new medication.
It is not uncommon to find my patients taking the “break-through” pain medication on a “time-contingent” basis instead of the “pain-contingent” basis.
It is not uncommon for me to see these patients taking the “break-through” medication every 4 hours even when their pain is as low as 3/10.
The usual instructions from opioid manufacturers recommend opioids not be taken longer than two weeks. Some studies suggest taking these heavy opioids for as little as two weeks can cause the patient to become addicted to the opioid.
An article on WebMD states: Opioid Dependence Can Start in Just a Few Days.
Transcutaneous Nerve Stimulators decrease pain two ways:
There is a third way TENS can reduce pain. This technique uses the “burst mode” and stimulates acupuncture points.
Recent studies show TENS causes physiological changes that help decrease pain. These physiological changes take place at the furthest reach of the nervous system, the many kinds of receptors embedded in the skin.
The TENS electrode is placed on the skin and the areas under and between the electrodes changes the physiology of the nerve-ending receptors.
The more pain-stimulus signals these nerve-ending receptors are sending to the bain, the more pain a patient is feeling.
The more pain-inhibiting signals these nerve-ending receptors are sending to the brain, the less pain the patient is feeling.
TENS changes the signal going to the brain.
Physiological Pain Reduction Mechanism
The human body has its’ own mechanism of dampening pain input from the pain receptors through nerve pathways called “descending inhibitory pathways”
TENS targets the opioid and α-2 noradrenergic receptors under and around the area of the TENS electrodes causing the receptors to send pain reduction signals to the brain mimicking the same blocking mechanism as opioid medication.
This electrical stimulation of the nerve-ending receptors pass the signal up the nerve, into the spinal cord, and ends up in the brain stem.
Along the way the TENS signal interacts with receptors in the spinal cord and brain stem that activate opioid, serotonin, and muscarinic receptors, thereby mimicking the effects of opioid medications.
Studies such as Effectiveness of Transcutaneous Electrical Nerve Stimulator for Treatment of Hyperalgesia and Pain have shown that TENS stimulation produces physiological changes in these descending inhibitory pathways making them more effective at pain control.
Other studies show an increase in circulation and a decrease of inflammation in areas stimulated by TENS.
Gate Control Theory
Before the research demonstrated pain reduction via physiological mechanisms, the only theory to explain how TENS works was to speculate that TENS stimulation filled the same pain pathway to the brain as the activated pain receptors, therefore there too much traffic on the nerve pathway for the brain to receive all the signals from the activated pain receptors.
More recent research, The Gate Theory of Pain Revisited, has proven this 1965 theory to have merit.
The use of a TENS unit may promote wound healing of a total knee surgical site by causing the release of substance P and calcitonin gene-related peptide.
These two substances present at the surgical site would increase blood flow to the surgical site and speed up the healing of the surgical wound.
The potential to produce this healing effect on a new surgical wound after a total knee replacement surgery is supported by a study in the National Institute of Health: The effects of transcutaneous electrical nerve stimulation on tissue repair.
This, in my estimation, is just a beneficial side-effect to using it to help control pain.
I have seen many total knee replacement patients that have needed help to control pain, with or without the use of opioids, but I do not see total knee patients that have trouble with healing unless there is an infection involved.
That makes me wonder if the speed-up of the healing process would have any impact on whether or not the infected total knee would have become infected if it had healed faster.
That would be an interesting study … compare new total knee infection rates and healing time between patients using TENS and patients that did not use TENS.
Infection is a rare occurrence with my patients as I’m lucky enough to be working with cutting-edge doctors operating out of university hospitals. The referring doctor that makes up the bulk of the total knees I see has the lowest infection rate in California.
How a TENS unit is used after total knee replacement surgery depends on 4 factors:
One Or Two Channel TENS Unit:
The one-channel TENS units are available but rarely preferred over the two-channel units.
The TENS units we will be discussing are the industry standard two-channel TENS units identified as a TENS 3900 or a Comfy TENS / 7000. Both allow the patient full control of all functions and both are easy to set their controls.
All instructions below also apply to one-channel TENS units.
There are certainly more fancy TENS units that come with “pre-set” functions, usually identified by pictures on the unit showing different body parts.
These pre-sets are fine if the patient responds favorably to the pre-set, but if the settings need some individual tuning to get that particular patient’s pain under control, well that’s another matter.
I would always recommend a TENS unit with 2 channels, my personal experience is that 2 channel units are just more effective in the hands of an informed user.
The TENS unit settings: Mode, pulse width, pulse frequency, timer:
Settings on the TENS 3900 and Comfy TENS / 7000 are applied to each unit entirely differently, however the individual settings specifics are the same.
The TENS 3900 settings are adjusted with 2 twist knobs and 2 toggle switches.
The TENS Comfy TENS / 7000 settings are adjusted through an LED screen.
Mode:
Both the TENS 3900 and COMFY TENS / 7000 have 3 modes in common:
The Comfy TENS /7000 has one additional mode, SD2, that is a variation on the modulated mode.
Modulated Mode:
Modulated mode is what I choose when setting up a TENS for pain control after a total knee.
Modulated mode accepts the setting for pulse width and and pulse frequency, but then modulates them by increasing and decreasing those settings by 50% in a 1 second cycle.
The reason for the modulation has to do with the body’s ability to adapt to a stimulus by a mechanism called “accommodation” An easy example is wearing a ring or a watch. After the body gets “accommodated” to the sensation, the brain no longer pays any attention to the input it receives from the ring or watch and you don’t even remember that you’re wearing them.
We do not want accommodation to the TENS electrical impulses. Modulating the pulse width and pulse frequency will prevent much of the brain’s ability to ignore the impulse.
And after all, using the “gate control” theory, we want the brain to pay attention to the electrical impulses and ignore the pain receptor impulses it is receiving from the total knee surgery.
Burst Mode:
Burst mode, in my opinion, is designed to stimulate acupuncture points in an attempt to encourage the body to produce more endorphins and enkephalins. These are the body’s own anti-inflammatory and painkillers.
This is the mode I would choose for my patient if all the modulated parameters did not control the patient’s pain.
This is not using the “gate control” theory any longer, the burst mode is trying to elicit a physiological response to decrease the pain.
Normal Mode:
I rarely ever choose “Normal Mode” for my patients.
Normal mode allows the parameters of pulse width and pulse frequency to be set, just like in the modulated mode, but the TENS unit stimulates at that setting continuously without increasing and decreasing those set parameters. This allows for the brain to accommodate to the stimulation and ignore it.
Normal mode, in my opinion, is designed to be used with patients that are so sensitive they can’t tolerate the feel of the electrical impulses.
If my total knee patient is intolerant of feeling the electrical impulses, I would choose this mode to make the patient comfortable, but realize that pain reduction effect may be greatly reduced.
Pulse Width and Pulse Frequency:
Here is where the benefit of having a TENS unit allowing the patient to set these metrics pays off big time for patients that do not respond to the pre-set stimulation programs on the more expensive TENS units.
I start all my patients off with the same setting for both the pulse width and pulse frequency at 120 and 120.
Pulse width sets how long the electrical spike remains at the top amount of stimulation and pulse frequency tells the TENS unit how many of these spikes to push into a 1 second time frame.
If the patient tells me it feels “scratchy” I adjust the pulse width up or down a little until the scratchiness goes away.
Most of the time these pulse width and frequency settings do the job right from the start and never need to be adjusted again unless the TENS unit becomes less effective at controlling the pain. Then we start to try different settings and strategies.
Timer:
The timer on the TENS 3900 has 4 settings. Three are timed to shut off after a specific amount of time. The 4th setting, “C” is continuous and only stops stimulating when the patient turns the unit off.
The TEMS 7000 is set using an LED screen so the timer can be set at any time interval before shutting off. It too has a “C” setting in the timer that allows continuous stimulation until the patient turns the unit off.
I always set the timer to continue. I want my patient to be in charge of how long the TENS unit operates.
How long at a time do I use the TENS unit is a different question.
I recommend my patients to stimulate one hour then turn the TENS off.
I ask them to rate their pain from 0 to 10 at the start of the session and then again at the end of an hour of stimulation.
Assuming there was pain reduction, I then ask my patient to tell me how long it took after shutting off the TENS unit, for the pain to return to the pre-stimulation pain or how long it to reach the leveling off point if the pain did not return all the way back to the pain level recorded at the start of the session.
My experience coincides with studies I have read in the past that state a person can have the pain return immediately upon turning off the TENS unit, but the pain reduction can produce pain reduction up to 10 times the amount of time the patient was stimulated by the TENS unit.
In other words, people that wear the TENS unit for 1 hour and the pain returns immediately may have to wear the TENS unit all the time to get the desired pain reduction.
Some people will have a residual pain reduction that could last for 10 hours after 1 hour of stimulation.
And for those that have no pain reduction it’s time to start experimenting with pulse frequency, pulse width, and electrode placement. Maybe even changing modes.
Intensity:
I always instruct my patient to increase the intensity high enough to feel it reasonably strong but below the threshold of causing muscular contraction.
I want my patients to use their TENS unit for as much time as needed, but I like for them to use the unit for at least 1 hour or more when they do use the TENS unit.
Muscular contraction for an hour can cause the muscle to fatigue and build up lactic acid in the muscle, a pain most runners are familiar with. We don’t want that!
But we do want the stimulus to have enough intensity to distract the brain from the body’s pain receptors.
It is an individual metric that is easily found once the patient begins to increase the intensity for the first time.
Electrode Placement
Modulated Mode:
If the patient has a one channel TENS unit, I would recommend my total knee replacement patient to place the electrodes across from each other at the knee. One electrode on the lateral aspect of the knee and the other electrode on the medial aspect of the knee.
If the patient has a two channel TENS unit, I would place one channel’s electrodes above and below the knee pain on the lateral aspect of the knee and the other channel on the medial aspect of the knee. AKA as the “bracketing” method because it brackets the pain.
Most recommendations found online, from very reputable websites, recommend using the “criss-cross” method of electrode placement. I have some concerns that crossing the electrical current can decrease the effectiveness of the stimulation.
The criss-cross method is what I use when using an IFC (interferential current) machine, trying to get the electrical stimulation deep within the joint to decrease swelling, but not with the TENS unit.
Burst Mode:
If I’m trying to elicit a physiological response to reduce pain instead of trying to block the pain through brain distraction, I will choose burst mode and apply the electrodes to the major acupuncture points associated with the knee.
Final Thoughts:
Wishing you a pain-tolerant recovery.
Dr. Donaldson is dually licensed; physical therapy in 1975 and doctor of chiropractic in 1995. He held credentials of Orthopedic Clinical Specialist in physical therapy for 20 years, QME in California, and taught at USC. He owns and operates an orthopedic physical therapy practice. See “About Me” page.
