Chronic knee pain in runners: how much is in the mind?

Pain and maladaptive pain

It’s well-known fact that pain can and usually does occur as a result of an injury. However, what is less-well known is that chronic, long-term pain that follows the initial injury often occurs even though the injury is fully healed. This is often dubbed ‘maladaptive pain’. How is this possible? Sports physiologists and pain specialists now believe that this occurs when the central nervous system becomes over sensitized to the initial pain following the injury – so-called ‘central sensitization’^(6-8). To understand this process, sports physiotherapist, SPB contributor and pain specialist Steve Robson provides a useful analogy (see this article):

“Imagine a line of dominoes, where the first domino in line represents tissue injury, (disc, nerve, ligament, muscle etc), the second represents the local pain receptors surrounding the injured tissue, and the further dominoes represent all of the subsequent nerve connections right up into the brain. If the first domino is toppled (signifying tissue injury), this causes each domino in turn to topple all of the way down the line, just like a sequence of nerve signals from the tissue going up to the brain.

Hopefully, you can see that even if you stand the first domino back up again (the equivalent of healing the offending tissue) the other toppled dominoes may still be lying flat. Therefore these pain signals can continue to reverberate within the central nervous system long after tissues have healed. This is part of the reason why many people including athletes still continue to experience pain even after they have undergone surgery to remove injured tissues like discs, or cartilage, or after they have received treatments to help injured tissues heal.

We now understand that most persistent, ongoing pain associated with an injury is driven and maintained by mechanisms and changes that occur within the ‘circuitry’ of the central nervous system as a result of injury - and that this is often not in fact a sign of any active ongoing tissue injury. In simple terms, it’s quite possible to have ongoing pain without any actual tissue damage whatsoever. This also helps to explain why the treatment of chronic pain resulting from injury is so frequently unsuccessful; the original injury may already have healed and it’s the circuitry of the central nervous system that really needs attention! However, this does NOT mean that the pain isn’t real. The pain is very real, but now originates in the central nervous tissue, not the injured tissue”

Maladaptive pain in runners

Since we know that many runners who develop PFP are afflicted with pain for long periods of time (often years) following the initial onset, is it reasonable to ask whether this pain is maladaptive – ie has the initial knee tissue healed but left the nervous system sensitized?

This is a complex question to answer; we know that individuals with maladaptive pain resulting from chronic lower extremity conditions (knee, foot injuries etc) exhibit impaired lower extremity mechanics (eg altered running and walking gait), which are thought to be related to overactive sensory input⁽⁹⁾. It’s also thought that these changed patterns of movement, which persist beyond the original protective phase of the injury, may lead to longer-term changes in function, and that this can further affect recovery⁽¹⁰⁾.

To further complicate matters, pain is highly subjective and is influenced by individual differences in pain perception⁽¹¹⁾. Fortunately, objective assessment of nervous system sensitivity can be carried out using a process called ‘quantitative sensory testing’ (QST) via thermal (heat, cold) or mechanical (pressure) testing. Using QST, it is possible to objectively measure the central and peripheral components of pain, and detect the facilitation and inhibition of pain⁽¹²⁾. And now new research has used this technique to investigate the degree to which nervous sensitization contributes to long-term pain in runners with PFP.

New research

In a brand new study published in the International Journal of Sports Physical Therapy, US researchers tested and measure pain sensitivity in runners with and without chronic PFP to see what degree of central sensitization was present⁽¹³⁾ Twenty healthy, pain-free female runners and 20 female runners with chronic PFP symptoms were enrolled for the study from local running clubs and via social media.

To be included, all runners had to be between 18 and 45 years of age (no older, to rule out pain from conditions of aging such as osteoarthritis) and be running for at least one hour per week. All the participants also had to be free of any musculoskeletal injuries and have no history of any neurological condition that could affect sensory perception. In addition, the runners had to be free of any acute or chronic pain conditions, and not be taking any medications for pain or that could affect pain perception.

Before testing, all the participants had to complete three questionnaires:

·        The Knee injury and Osteoarthritis Outcome Score for Patellofemoral Pain and Osteoarthritis (KOOS-PF) - a valid and reliable self-report measure for patients with PFP⁽¹⁴⁾.

·        The Brief Pain Inventory short-form (BPI) – a valid and reliable for measuring pain severity and impact on function⁽¹⁵⁾.

·        The University of Wisconsin Running Injury and Recovery Index (UWRI) - a valid and reliable patient-reported outcome measure to assess issues related to injury recovery, frustration, and training progression following a running related injury⁽¹⁶⁾.

The testing itself consisted of:

·        Y-balance test – a functional test designed to assess lower limb strength, flexibility, neuromuscular control, stability, range of motion, balance, and proprioception.

·        Pain-pressure testing – a test where a device applied increasing pressure to various locations, including the kneecap, the outer knee, the shin bone, and the frontal thigh on both the affected limb (in the PFP runners) and unaffected limb (all runners). The force at which the sensation of pressure turned to pain was noted.

·        Heat/testing – in the heat testing, heat pulses were applied the painful (in the PFP runners) and unaffected knees (in all runners). These pulses rose rapidly from 42-51°C at 10°C per second at a rate of one pulse every three seconds. During this test, the participant rated their perception of pain intensity for each of the 10 heat pulses.

·        Cold testing (for central sensitization measurement) - in the cold testing, just enough pressure was applied to the runners’ shins to produce mild pain. While this pressure was maintained, the runners had to plunge their dominant hand into very cold water (6C/43F) and any changes in pain threshold at the shin were recorded. When central pain sensitization is present, pain and discomfort in another region of the body (in this case, the hand) can, in theory, affect pain experienced at the injured location.

What they found

The first fact to emerge was the large inter-group difference between the knee pain group and the healthy group (see table 1). In short, the PFP runners experienced pain more often, more severe pain and more disruption to their training as a result of pain. They also scored worse on the KOOS-PF and UWRI questionnaires, indicating a higher prevalence of knee discomfort, slower recovery from training, slower progression, and more frustration as a result.

Table 1: Inter-group differences between PFP and healthy runners

Key: BPI = Brief Pain Inventory (higher score = worse pain); KOOS-PF = Knee injury and Osteoarthritis Outcome Score for Patellofemoral Pain and Osteoarthritis (higher score = better function); UWRI = University of Wisconsin Running Injury and Recovery score (higher score = less frustration and better training progress); n = number of participants; NPRS, numeric pain rating scale; R = correlation (anything above 0.5 indicates a large effect); p-value = significance of result (any value below 0.05 is considered highly significant).

When it came to the results of the QST testing, the runners with chronic PFP symptoms had a reduced pressure pain threshold in areas around the knee (on the kneecap and on the side of the knee). In short, less pressure produced more pain compared to the healthy runners. The real giveaway however was the finding of hypersensitivity (reduced pressure pain threshold) at remote sites from the involved knee in the PFP runners. This included more pain sensitivity in the shin bone and quadriceps in both legs, even in the leg unaffected by PFP! This finding - of hypersensitivity in regions of the body remote from the injury - is very strongly suggestive of pain due to central sensitization⁽¹⁷⁾. In plain English, these long-term PFP sufferers were likely experiencing pain as a result of an over-excited central nervous system – not as a result of injured tissue in the knee itself.

Implications for runners

These findings provide both bad news and good news for runners with chronic long-term PFP. The bad news is that simply waiting and hoping that the patellar pain will resolve if enough healing takes place and enough time elapses is unlikely to yield results. Likewise, undergoing therapies or ‘treatment’ for patellar pain may well be unsuccessful (because in all probability, there’s no tissue damage to heal!).

The (really) good news however is that runners who suffer from chronic PFP can take heart that there’s a very good chance that they have healthy knee tissue, and that the pain is simply the result of a hypersensitive central nervous system. In short, it’s highly likely that there’s nothing physically wrong enough with the tissues of the knee that would prevent a full return to running.

Maladaptive pain is real pain – pain that can hinder participation in sport. So the obvious question is how to can runners with chronic PFP ‘tone down’ and ‘switch off’ these errant pain signals? Rather than attempting to ‘heal’ a perceived problem or injury to the tissue, a better approach to use with maladaptive pain is one of ‘graded exposure’. Graded exposure involves gradually exposing pain sufferers to the very movements that are actively feared or avoided. Graded exposure requires that patients want to overcome their fear, understand that exercise will help the problem not hinder it, and are willing to put in the effort and time.

In a sense, graded exposure is about gradually exposing the central nervous system to movements that cause unnecessary pain and ‘teaching’ it that these movements are okay, and that pain signals are no longer required because the original injury has healed up. Although it sounds strange, part of this graded exposure process is developing the belief and confidence that the original tissue injury has indeed healed up. This belief and confidence allows the brain to process sensory feedback differently. Remember, the processing of sensory information plays a large part in how the information is interpreted by the brain.

The key for most runners (or indeed any athlete who is suffering from long-term chronic pain long after an original injury) will be to find a physiotherapist who understands the concept of maladaptive pain and how to treat it. He or she will be able to determine if indeed there is any residual tissue damage serious enough to prevent a full return to running (the presence of some scar tissue or weakened tissue due to inactivity do not in themselves prevent a full return to sport). Assuming not, the physiotherapist will then be able to provide a series of graded exercises to progressively challenge and ‘re-educate’ the central nervous system and dampen down any errant pain. He she will work with the athlete to help develop confidence and ensure that the graded exposure program is implemented quickly enough to make progress, but slowly enough so that further pain and hypersensitivity is not provoked.

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