Will The Numbness In My Knee From Surgery Ever Go Away?
Today I’m going to tackle one of the most frequently asked questions following knee surgery (and surgery in general): will the numbness around my knee ever go away? Unfortunately, the most common answer is “I don’t know”, and that has been deemed an acceptable answer because numbness is for the most part just an annoyance that doesn’t affect function. I think we can do better than that, so in today’s article I will discuss how often the numbness goes away and steps you can take to reduce the numbness.
How Common is Knee Numbness and What Causes it?
Numbness and tingling (aka paresthesia) following ACL surgery is due to damage to one of the sensory nerves in the knee region. The main nerve that is injured with both hamstring and patellar tendon grafts is called the saphenous nerve. The saphenous nerve is an extension of the femoral nerve that traverses over the medial side (inside) of the knee, down the inside of the lower leg, and ends on the inside of the foot; altered sensation over the inside of the knee occurs about 40-60% of the time after surgery (one study reported as high as 88%). It is also possible to damage a couple other nerves called the lateral sural cutaneous and lateral femoral cutaneous nerves, but this is less common (ironically, my saphenous nerve appears to be fine after my ACL surgery, but I’m missing sensation in the lateral femoral cutaneous nerve distribution; go figure). Numbness over the medial side of the knee is more common in individuals who receive a hamstring graft, likely because the saphenous nerve travels right near where the hamstring graft is taken from, which increases the risk of collateral damage.
The good news is that all 3 of the nerves I mentioned above are what we call sensory nerves, which means they only provide sensation and have no role is the function of muscles. This is good news, because even if you never recover the sensation in these nerves, you are not going to experience problems with muscle function as a result.
Unfortunately, if you experience numbness after surgery, it’s likely that it will stick around for quite a while. One study on the subject found that 74% of patients experienced numbness in the knee region following surgery. Within this group, only 20% reported an improvement within 6 months of surgery. The good news is that only 6.5% of the people with numbness reported that it significantly affected their lives, but if 80% of people with numbness have no improvement after 6 months, that’s pretty terrible!
Does Long-Term Numbness Impact Knee Function?
While numbness following surgery isn’t ideal, the good news is that it does not appear to lead to higher incidences of re-injury. In addition, there appear to be no differences in knee range of motion, pain scores, or self-reported function in individuals with and without numbness 24 months after ACL surgery. These findings are similar when looking at individuals following total knee replacements.
The one area where numbness does appear to affect function is in regard to kneeling, but even here the research isn’t great. What we do know is that difficulty kneeling appears to be more common in individuals who used a patellar graft when compared to a hamstring graft, most likely because kneeling also places pressure near the incision site on the bottom of the kneecap where the patellar tendon was taken out. One study found that 31% of individuals with a patellar graft experienced pain with kneeling following ACL surgery, whereas only 6% of individuals with a hamstring graft experienced pain; this finding has been backed up by at least one systematic review.
Can You Do Anything About the Numbness?
Short answer: maybe.
Long answer: there are several techniques that are used for a variety of conditions that help to restore sensation following nerve injuries, but none of them have been validated in individuals following ACL surgery. This is likely because the benefit of any said treatment likely doesn’t justify the costs of researching it in two ways. First, since this numbness does not cause many functional problems, it’s deemed unworthy of funding by agencies that give out research grants. Similarly, no company is going to do the research to prove it works if only a tiny fraction of individuals with numbness will invest their time and money in a product or intervention that claims to solve their problem. In order to provide some options that might work, I found a few low-risk, low-cost approaches that might help.
The first is the use of low-level light therapy (LLLT). The most commonly accepted mechanism for how LLLT works is by upregulating the activity of one of the enzymes (cytochrome C oxidase) that is responsible for cellular energy production (you can read more about this here). All of the research I found on this was in rat models, but one study found that the use of LLLT every day for the first 20 days after injury resulted in significant improvement in nerve function compared to a control. Of note, this study was done on a motor nerve, rather than a sensory nerve, so it’s possible the results don’t transfer. However, LLLT is a common device that a lot of clinics already have on hand, and it has a very good safely profile, so this would be a low-cost, low-risk therapy to try.
Another option is electrical stimulation (ES). When used in animal models following the surgical transection of a nerve, they found that the muscles innervated by the nerve showed innervation twice as fast as controls (3 weeks vs. 6 weeks). Most studies applied the ES for 1 hour each day, with a frequency of around 4-20 Hz. One review paper I read found that sensory nerves responded to ES, but they recovered less effectively than motor or mixed nerves; they believed this is due to decreased resilience in sensory nerves compared to motor nerves, which makes some sense: sensory nerves are less necessary for the survival of an organism than motor nerves (you’re more likely to escape a lion if you can’t feel your knee but are able to run than vice versa), so sensory nerves have evolved to be less resilient.
Another possible intervention is the use of therapeutic ultrasound. One study examined the use of ultrasound for nerve regeneration by creating a crush injury to the sciatic nerve in rats and then comparing the use of ultrasound to control (the ultrasound parameters were pulsed, at 1 MHz and an intensity of 0.4 W/cm2, for 2 minutes, which is comparable to what is typically used for other uses in humans). This study found a significant improvement in nerve regeneration in the ultrasound group compared to the control group. Similar to LLLT and ES, ultrasound is also a fairly safe modality, but should not be performed over open wounds, which may be a problem after surgery depending on where the numbness is.
The drawback to all of these strategies is that you probably need to use it in the first several weeks after surgery, because it will only work if the nerve is still viable; if you wait several months or years to try it, the damaged nerve will have died off. Of all of these options, ES is the most likely to produce results if used over a month after surgery.
Summary
Numbness after ACL surgery is very common and typically does not limit an individual’s function in the long term. Unfortunately, if you experience numbness in a certain area of the knee after surgery, it is unlikely to completely go away. The most common area for numbness is over the medial knee, which corresponds with the saphenous nerve, but it is also common to experience numbness over the lateral knee as well. While there are no interventions that have been studied to improve nerve regeneration after ACL surgery specifically, a couple low-cost, low-risk techniques you can try are low level light therapy, electrical stimulation, and therapeutic ultrasound.
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