Nerve or Joint? Pain Speaks Two Languages | The Pained Brain, Chapter 20

Nerve pain after knee replacement

Nerve Pain After Knee Replacement: What Testing Can and Cannot Show

Nerve pain after knee replacement is common: 53 to 74 percent of people with troublesome pain three months after surgery, depending on the questionnaire, screened positive for neuropathic features. A physician’s exam along the nerve finds it; sudomotor and autonomic testing show whether small fibers are affected more widely, but cannot pinpoint one injured nerve.

A knee that looks perfect on X-ray can still burn. When the pain follows a nerve instead of the joint, the useful questions change, and so do the measurements.

A knee replacement can be perfect on the X-ray and still hurt more than the arthritis did. When the pain burns along the shin, flares under the weight of a bedsheet and wakes you at night whether you walked or rested, nerve pain after knee replacement belongs on the list of explanations, and the joint may not be the problem at all. That argument runs through chapter 20 of The Pained Brain by Dr. Gurpreet Singh Padda, MD, MBA, MHP, and Dr. KrisJay Fucanan, MD, presented in the video above, Nerve or Joint? Pain Speaks Two Languages. The research behind each figure, and its limits, is in the book companion for chapter 20.

Is it nerve pain or joint pain?

Think of a car’s check-engine light. Sometimes the light is right and the engine is damaged; that is nociceptive pain, from injured tissue such as an arthritic joint, carried by healthy nerves. Sometimes the wire to the light is frayed and it glows on its own; that is neuropathic pain, from injury or disease of the nerves themselves. Fixing the engine does nothing for a frayed wire. Many people carry both at once, a mixture the field calls mixed pain and has never formally defined.

The two respond to different treatment. Anti-inflammatory tablets have been studied for nerve pain in only two trials with 251 participants, with no sign of meaningful relief. That is not a reason to stop anything on your own. It is a reason to ask which kind of pain each item on your medication list is meant to treat.

How common is nerve pain after knee replacement?

Among 363 people with troublesome pain three months after knee replacement, 53 percent screened positive for neuropathic features on one questionnaire and 74 percent on another, and in 56 percent of those followed it was still there a year later. Before surgery, knee arthritis itself screened possible neuropathic in 40 percent of patients and probable in 20 percent across 39 studies. In two cohorts totaling 524 people, a neuropathic-like score before the operation carried 2.83 times the odds of moderate-to-severe pain a year afterward. A smaller Korean cohort of 148 people using a different questionnaire found no such difference, so the signal is real but not settled.

Small skin nerves cross the incision line of a knee replacement, and a nerve cut during a successful operation can start firing on its own. The chart then says the knee failed, and the offer on the table becomes a revision.

How do doctors diagnose nerve pain after knee replacement?

Neuropathic pain is graded rather than detected by one test. A fitting history and a pain pattern that follows nerve anatomy earn possible. Sensory findings on examination in that same area, especially numbness or skin that hurts to light touch, earn probable, which is usually enough to start treatment. A questionnaire can point the way: the one used in the book’s composite case has a sensitivity of 85 percent and a specificity of 80 percent, so it misses some cases and flags some that are not. A physician pressing along the nerve, and sometimes a small numbing injection at that spot, is what separates a nerve generator from a joint generator.

Measura [Cardiometabolic and Autonomic Health Analysis] cannot do that job, and it would be dishonest to suggest otherwise. It is a testing service; it measures, it does not treat, and no Measura test can diagnose a painful joint or pinpoint one injured nerve beside a scar.

What can sudomotor and autonomic testing show?

What testing can show is whether the small nerve fibers are affected more widely than one spot. Sudomotor testing measures sweat-gland function, which depends on the small nerve fibers of the hands and feet. Autonomic nervous system testing looks at the automatic nerves that regulate heart rate, blood pressure and sweating. A result that suggests reduced small-fiber function goes to your physician as one piece of a larger picture, described on sudomotor dysfunction. The test that confirms small-fiber neuropathy directly, a three-millimeter skin punch biopsy with no serious side effects reported in about 35,000 procedures, is a different test done elsewhere.

Blood sugar and the firing nerve

Metabolism is the second biological driver. In recordings from people with diabetes, 79 percent of the pain fibers sampled were pathologically altered, and the fibers in painful neuropathy were far more likely to fire without any stimulus than in painless neuropathy. Among 13,592 people screened in Malaysia, 40.1 percent of those with diabetes screened positive for probable neuropathic pain. The composite patient in the book had an A1c of 6.4 on top of the injured shin nerve. Laboratory panels put numbers on that terrain, and the pattern of burning feet it can produce is described in burning feet at night.

The social driver is the schedule. The examination that sorts nerve from joint takes about ninety seconds, yet a rushed follow-up rewards reordering the scan and renewing the prescription. Dr. Padda has said plainly that he has watched good physicians treat a nerve as a joint, and that he has done it himself.

Questions to bring to your surgeon or pain physician

Is my pain nociceptive, neuropathic, or a mix?
Has anyone examined the skin around the incision for numbness or pain to light touch?
Would a diagnostic nerve block answer the question before a revision is discussed?
Is there a reason to check small-fiber or autonomic function, and my A1c?

Choosing a clinic that asks these questions is its own problem, covered in questions to ask a pain management doctor, with more prompts on questions worth asking your doctor.

Frequently asked questions

Why does my knee burn after a successful replacement?

A well-placed implant can sit beside pain that comes from somewhere else. When pain was still troublesome three months after surgery, 53 to 74 percent of people screened positive for neuropathic features, and small skin nerves near the incision are a common suspect. An examination along the nerve and sometimes a numbing injection help sort it out. Typical symptoms are described on numbness, burning and tingling.

Can a sudomotor test show whether my knee pain comes from a nerve?

Not directly. Sudomotor testing measures sweat-gland function driven by the small nerve fibers of the hands and feet, so it can suggest wider small-fiber involvement. It cannot locate one injured nerve at the knee or rule a joint in or out. Your physician uses it alongside the examination and history. What the test involves is explained on sudomotor testing.

What is the difference between small-fiber and large-fiber nerve damage?

Small fibers carry pain and temperature and help control sweating; large fibers carry vibration, position and fine touch. Burning pain and pain to light touch point toward small fibers, and standard nerve conduction studies mostly assess large ones. That is why a normal nerve study does not end the question. The distinction is explained in small-fiber versus large-fiber neuropathy.

Does blood sugar matter for nerve pain?

It can. In recordings from people with diabetes, most sampled pain fibers were pathologically altered, and spontaneous firing was much more common in painful neuropathy. In one large screening study, 40.1 percent of people with diabetes screened positive for probable neuropathic pain. Numbers below the diabetes line can still matter, as explained in insulin resistance and metabolic health.

Who reads my Measura results?

Your physician does. Measura sends every result to the physician who ordered testing, and that physician interprets it alongside your examination, imaging and history before deciding whether anything changes. Measura does not diagnose disease on its own or recommend treatment, and it does not replace a diagnostic block or a skin biopsy. More on reading a report is in understanding your results.

How long does nerve pain last after knee replacement?

Often longer than people are told to expect. Among people with troublesome pain three months after knee replacement, 53 to 74 percent screened positive for neuropathic features, and in 56 percent of those followed it was still there a year later. Pain that has not settled by then is a reason to have the nerve examined, not simply to wait.

What are the signs of nerve damage after knee replacement?

Nerve pain tends to burn along the shin, flare under the weight of a bedsheet and wake you at night whether you walked that day or rested. On examination, numbness or skin that hurts to light touch in the area of a nerve points toward a nerve source. A knee can look perfect on X-ray and still hurt this way.

Should I have a revision if my new knee still hurts?

Ask first whether the pain is coming from the joint at all. A nerve cut during a successful operation can start firing on its own, and the chart may then say the knee failed. A physician pressing along the nerve, and sometimes a small numbing injection at that spot, can separate a nerve source from a joint source before a revision is discussed.

See the nerve picture around the pain

Ask about sudomotor, autonomic and metabolic testing if burning or tingling pain has outlasted a joint repair. Measura sends every result to your physician.

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References

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