Understanding Pain
Knee Replacement is Highly Effective
It’s good to know that knee joint replacement surgery is highly effective and leads to great improvements in people’s quality of life. Knee replacement surgery is done when pain from arthritis or an injury stops people from doing their day-to-day activities. The aim of knee surgery is to improve people’s independence and quality of life. 9 out of 10 people who have a knee joint replacement experience greater mobility in their knee after the procedure, and very few need further revision procedures. For most people, a successful knee replacement leads to a higher quality of life, less pain, and better mobility. Complications such as infections, blood clots, dislocations or loosening of the knee replacement are rare and only occur in up to 2 in 100 people, and these are managed by your surgeon. The sort of activities you can expect to get back to include walking, climbing stairs, gardening, playing golf, swimming, driving or dancing.
Although complications are rare, it is common to experience swelling, stiffness and pain after surgery. These symptoms do not mean that there is a serious complication or that the surgery has not been successful. It is also common to feel or hear clicking or popping sounds in the knee when bending or walking. These sounds usually decrease or go away after several months and are not a sign that there is anything wrong.
When people experience pain in the days or weeks after surgery, they can be worried, but we know that people who understand the reasons for having some pain after surgery have an easier recovery in the long run. The information here is designed to help you understand the reasons not to be alarmed, and to provide information to assist your recovery.
Why do People have Pain after Surgery?
Lesson One: Hurt does not equal Harm
First it’s important to understand how pain works. People often think that pain is a sign of damage in the body, because we have all had many experiences where we injure ourselves and it hurts, right? But pain is actually more complicated than that.
There are times when people have a LOT of damage and NO pain, and times when people have little or NO damage and LOTS of pain. Think about a papercut, or stepping on a piece of LEGO with bare feet. You will probably agree that there is no severe injury, but these things can be very painful! Many people have also had severe injuries without feeling them at the time. For example, soldiers in battle often report not noticing severe injuries until they get to safety.
In 2003, a surfer called Bethany Hamilton was attacked by a 15 metre shark, and lost her left arm. In an interview afterwards she said “I was just in shock, I didn’t feel any pain, which I’m really lucky because if I felt pain, things might not have gone so well”.
Lesson Two: The Powerful Brain & Spinal Cord
To feel pain is more complicated than you might realise. Your body contains a nervous system, which is made up of the brain, the spinal cord, and nerves that go from the spinal cord to almost all parts of the body.
When nerves detect too much pressure, heat, or particular chemicals, they send ‘danger’ or ‘alarm’ signals to the spinal cord. Chemicals then transfer the danger signal to another nerve, which sends the signal up to the brain.
The brain then uses information about the whole situation to determine whether these ‘danger alarm’ signals should be interpreted as painful or not.
The brain can then send signals down to the spinal cord, telling it to release either a) your body’s natural pain relieving chemicals such as endorphins (the body’s own morphine), which can block more ‘danger’ or b) other chemicals, which can increase the danger signals.
For example, if you touch a hot stove, your brain will receive danger signals - and instantly produce pain to make you take your hand away from the heat. If the brain decides that pain isn’t useful right now (such as in some life-threatening situations) it can release powerful chemicals to block the ‘danger’ signals. These chemicals are no weaklings: they can block out the pain of a shark bite!
This means that pain is not really a good measure of how much damage has occurred in the body. Instead pain depends on whether the brain tries to protect the body by a) producing pain or b) dampening the danger signals.
One way to imagine how the brain and spinal cord works is to picture the brain as a ‘tap’ which can release a medicine-cabinet full of pain relieving chemicals down the spinal cord. When the tap is turned on and the chemicals flow, you won’t experience a lot of pain, when the tap is turned off, you will have much more pain.
The 'Wet' Brain
Normally, the brain is able to trigger the spinal cord to release your own natural pain relieving chemicals such as endorphins and serotonin to relieve pain.
In the picture the tap is open and the brain is flushing chemicals down the spinal cord to dampen upcoming danger messages.
For example, if you stub your toe, you may experience pain for a short time, but the pain quickly reduces as the brain uses chemicals to switch off pain.
The 'Dry' Brain
After a person has had pain for a long time, the brain begins to produce fewer pain relieving chemicals.
In this picture the tap is closed and the danger signals from the body are not dampened by the brain, so the person will experience more pain.
This is probably a survival strategy of the brain as it becomes more worried about the painful body part over time. It tries to get you to protect or treat the painful body part by closing the tap and increasing pain.
Lesson Three: Knee Osteoarthritis (OA) ‘Sensitises’ the Nerves & Spinal Cord
People with knee arthritis usually have pain for many years before they receive a knee replacement, and you may have had your pain for a long time. The pain in knee arthritis starts with wear and tear in the joint. When this happens, the nerves detect pressure or chemical changes (due to inflammation), and send ‘danger/alarm’ signals up to the brain. We know that ordinary painful activities for people with knee arthritis (such as walking across the room) are not actually dangerous (in fact movement and exercise are good for knee arthritis!) so you would think the brain might activate it’s medicine cabinet full of pain relieving chemicals and block out those signals! Unfortunately this is not always the case.
Rather than getting used to the pain and reducing it over time, the brain and spinal cord often become MORE sensitive to pain over time. This is true of many long-lasting painful conditions. Over time, the nerves can become easily stirred up and more likely to send danger signals, and instead of switching off quickly, they keep sending signals for a long time. They even ‘wake the neighbours’, and nerves in the surrounding area can start sending danger signals too. This is how pain can spread to other parts of your body. Also, over time the brain becomes more likely to interpret danger signals as ‘pain’, and less likely to send signals back to the spinal cord to reduce pain.
It’s as though there an easy-to-follow, well-worn path from the knee, up the nerves, to the brain, sending danger signals so that over time the body gets better at sending danger signals and producing pain. Although it makes sense for the brain to dampen these signals with powerful pain-relieving chemicals, over time the brain can go into ‘alarm mode’, and the ‘tap’ releasing the pain relieving chemicals runs dry. It actually turns the pain dial up. The nervous system can become “wound-up” or “sensitised”. It’s like a faulty alarm system that keeps going off for no reason, even when the knee is not in any danger. This is a normal response of our nervous systems to long-term pain, and scientists have found that it even happens in animals too! The longer a person has pain, the more sensitised their nervous system can become. Scientists call this “central sensitisation”.
After knee replacement surgery it can take some time for people’s nerves to calm down and become less sensitised. Why are some people’s nerves slow to calm down? For some people, there might be so many issues surrounding the knee problem that the brain decides it’s best to keep the alarm system activated. Some things that can keep the alarm system activated are:
A long time in pain: People who have experienced pain for a long time have nerves that may be slower to calm down, even with a brand new joint.
Unhelpful thoughts or expectations: After a long time in pain, people can feel as though pain will never end and have negative expectations or less hope that treatments (such as surgery) can help. These negative thoughts can actually affect the way that the brain responds to danger messages. It’s important to remind yourself that after your knee replacement, you can expect a big improvement in the way your knee functions and in the activities you can do. You should be optimistic about what you hope to achieve after surgery.
Sleep problems: Your sleep influences your pain alarm system a lot and so although not everyone can be a perfect sleeper, it’s important to get the best sleep you can.
Stress: Knee problems can cause stress for people’s family relationships, work, finances, and can stop us from doing some of the relaxing things we enjoy. Health problems, appointments, and receiving treatments can also be stressful, and of course there can be other sources of stress in our lives. All this stress affects the nervous system and can mean the brain is less likely to turn the alarm system down.
Lack of movement: Movement and exercise are important for nerve health. The nerves need blood and oxygen to function well, and the way nerves get this is through movement and exercise. It’s important to do exercise, even with arthritis, as this helps to calm nerves down and turn on the “tap” of pain relieving chemicals in the brain.
Fear: It’s normal for people to worry about surgery, and staying in hospital. Some people with pain are also worried that exercise or movement could cause damage, cause an injury or re-injury, or create more pain. But this fear of exercise is likely to keep the alarm system on (which increases pain) rather than turning it off.
Lesson Four: Surgery Can Stir Up Sensitised Nerves
Surgery is highly effective as it takes away the original source of the pain - the damage in the knee joint. The new surfaces of your knee are designed to glide smoothly and function well. However, if you’ve had knee arthritis for a while, chances are your pain is a mix of mechanical pain (from the joint itself), and sensitised nerves (from the brain and spinal cord). The surgery itself involves a lot of activity which will cause the nerves around your knee to send a lot more danger signals than usual. Because these nerves are sensitised, it can take time for them to calm down and stop sending their danger signals. Also, having surgery and staying in hospital can be stressful, which can make it more difficult for your nerves to calm down after your surgery.
Sensitised nerves vary a lot in the length of time they take to settle down, so if you have pain in the weeks after surgery don’t be alarmed. They can take time to switch off, and it can take time for the “tap” of pain relieving chemicals in the brain to start flowing. If we imagine that your pain system is a bit like an alarm system, then once you’ve had your surgery, it’s as though the burglars have left the building, but the alarm might still be ringing. It can take a while to slowly turn off.
Now open your “My Knee Surgery Plan” document and complete the checklist on Page 2, 'Things that influence your pain alarm system’.
