BACKGROUND PHYSIOLOGY - Knee pain in general

Dr Smallman discusses why the evaluation of pain in the front of the knee is not a simple matter.

First published 2018, and reviewed August 2023 by Dr Sheila Strover (Clinical Editor)

 

Pain is complicated, because there are so many factors in play: those in the knee, and those in the complex messaging system that is the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves running from the spinal cord to the rest of the body), and finally there are psychological aspects -- all of which can modify remarkably one’s response to pain.

A simple listing of the anatomy of an event, belies the complexity. Something happens to the knee, leading to the perception of pain. What is involved starts with activated local noxious (pain-sensitive) nerves in the knee which transmit signals to the spinal cord, where these signals are mixed with all of the other signals entering the cord at that level. There are then multiple levels of further processing in the cord, brain stem and cortex ending with your conscious mind perceiving that pain.

Time, treatment, and the natural history of the event lead to an outcome, which most of the time is successful. What happens in those who are not successful is complex, in part because of how the knee is structured, and because of important aspects of the pain -- what happens to it over time, and to the inherent nature of the pain.

 

Structure of the knee and its nerve supply

The knee is a structure of many tissues with complex anatomy, innervated by every nerve in the region: femoral, tibial, common peroneal, recurrent peroneal, and obturator. The tissues that contain nerve endings that perceive pain, so-called noxious nerves, include the joint capsule, ligaments, synovium, bone, and the outer edges of the menisci. In general, in the knee each nerve supplies a particular part of the knee, with some overlap. However, the fat pad is unique in that it is a deep structure, intra-capsular in location within the anterior compartment, sitting between capsule on the outside, and the very thin synovium on the inside, and is innervated, not by one nerve, but rather by all of the abovementioned nerves (ref. 2).

If you have Anterior Knee Pain (AKP), and it is not resolving, you might see a physician, who would take an appropriate history and examine your knees. Physicians diagnose pain by observing, touching, and moving structures. If pain is present on direct pressure, then one assumes that the problem lies in the underlying structure. If the pain were at the top of your kneecap, the problem might be injury to, or inflammation of the underlying quadriceps tendon (quadriceps tendonitis). If it were over the outside of the knee well above the joint line, it might be overuse related in the iliotibial band which can rub on the underlying femur (iliotibial band syndrome). At the joint line on the inside or outside, it might be meniscal degeneration or tear.

 

We observed as our experience grew with evaluating patients with AKP, that the pain was not always as classically described for AKP, that is, deep and parapatellar. It could be above the knee, on one or the other joint line, or both, or below the knee in the calf, front or back, or in the thigh. Pain in each of these areas brings up a wide variety of diagnoses; hence AKP’s reputation as a mimicker. 

 

 

Aspects of pain, be it referred, chronic, or neuropathic which make AKP difficult to treat include:

Referred pain

I would like to explain the phenomenon of referred pain, wherein the pain may be perceived to be coming from an area other than that causing the pain. The most common example of referred pain is someone who is experiencing a heart attack, but has pain presenting not in the chest, where the heart is physically located, but rather in the shoulder, neck, or even the mouth. The physiology of referred pain overlaps with that of chronic pain, and of neuropathic pain.

The idea is simple: if the patient has AKP, but only perceives pain in the middle of the thigh or and there is some tenderness there as well as some generalized tenderness on the front of the knee and the clinician misses the feature in the history that strongly suggest the AKP symptom complex, then the diagnosis will not be made, and the patient is then stuck with this condition that belies diagnosis. I recently saw a patient who perfectly matches this description. She had the pain for several years. She was “beside herself” because of the pain. She had innumerable visits to the emergency room. She could barely walk because of the pain. Her legs were atrophic. Her pain was mid-thigh and nowhere else. She was generally unhealthy, smoking (+++) and mal-nourished. She had normal radiographs, an MRI that showed only an IPP. Her operation was straightforward – untethering the fat pad. Now several months out she has no pain and in on a new chapter in her life. The problem was not easily identified because of the concept of referred pain.

For AKP, the areas of referred pain, given the multiple innervation of the fat pad, are the hip, thigh, or calf. If the clinician is not aware of this, treatment may be delayed or never given.

Chronic pain

Research now shows that AKP patients share many characteristics with chronic pain patients, including magnifying the extent, duration and intensity of the pain. There is complex neurophysiology in this domain not relevant to this discussion. Our observation is that many patients respond in the only reasonable way to pain that simply will not go away, by altering their lifestyles to avoid the pain. The good news is that in many, despite years of AKP, the pain can be ablated by arthroscopic untethering of the fat pad if the problem can be diagnosed.

Neuropathic pain

Finally, let us consider the nature of the pain itself. Our observation, and that of others, is that AKP for some can become increasingly intrusive, to the point of dominating one’s life. Our model invokes perturbation of activated noxious nerves in the IPP-fat pad complex. Neuropathic pain is described as form of chronic pain that occurs when nerves in the central nervous system become injured or damaged.  Neuropathic pain may be associated with abnormal sensations called dysesthesia or pain from normally non-painful stimuli (allodynia). We have seen this in some patients with AKP.

In AKP there is no actual damage to the contained nerves, but the pain is similar to neuropathic pain in some, in that it is difficult to treat, and its impact is profound, often reducing the ability of those with PFP to perform sporting, physical activity and work-related activities pain-free. Increasing evidence identifies AKP that it is a recalcitrant condition, persisting for many years (ref. 5).

Osteoarthritis

In many adult patients AKP is missed, or is mis-diagnosed as osteoarthritis, or degenerative arthritis. The radiograph may show “osteoarthritis” and so the troubling symptoms which are disturbing your life daily are mis-cast as osteoarthritis. I would like you to understand how osteoarthritis presents so that you will not be mis-diagnosed.

The most common condition affecting the knee as we age is degenerative arthritis, or Osteoarthritis. The key thing about the usual pain of osteoarthritis (OA) is that it comes on insidiously, it is not disabling, people live with it for years, and only after a long time, as the structures of the knee begin to fail mechanically, do they actually present with it saying, "I am having trouble walking". Morning stiffness, and perhaps occasional swelling are the early symptoms. There is one subset of patients who have a form of OA that is more inflammatory in nature, comes on fairly abruptly in the 50’s and does not follow the pattern I am describing for OA. By contrast AKP significantly affects afflicted patients by reducing their capacity to perform daily activities, athletic or work-related activities without pain.

Articular cartilage (or joint cartilage), you see, has no nerves and cannot be a source of pain. Degenerating articular cartilage joint surfaces of early osteoarthritis are not painful, as Dr. Dye demonstrated and as many orthopaedic surgeons understand. Only after years as the articular cartilage is worn away, pressures on the cartilage increasingly are passed to the underlying bone, which does have nerves. The pain of osteoarthritis likely includes pain arising from the subchondral bone. Malalignment, accelerating mechanical wear in the osteoarthric process, contributes to this kind of pain, which clinicians consider 'arthritic' in nature. It does not manifest the characteristics of the Anterior Knee Pain symptom complex that is the focus of this course.

 

References

1. Smallman T, Portner O, Race A, Mann K, Shekitka K. Arthroscopic Untethering of the Fat Pad of the Knee: Release or Resection of the Infrapatellar Plica (Ligamentum Mucosum) and Related Structures for Anterior Knee Pain. Arthroscopy Techniques.2018

2. Gardner E. Anat Rec: The innervation of the knee joint. 1948;101:109-130

3. Grelsamer R, Moss G, Ee G, Donell S. The Knee: The patellofemoral syndrome; the same problem as the Loch Ness Monster? Knee 2009;16:301-302

4. Bennell K, Hodges P, Mellor R, Bexander C, Souvlis T. The nature of anterior knee pain following injection of hypertonic saline into the infrapatellar fat pad. Journal of orthopaedic research. 2004;22(1):116-21

5. Crossley, K, et al, 2016 Patellofemoral pain consensus statement from the 4th International Patellofemoral Pain Research Retreat, British Journal of Sports Medicine, 50:14, 839-43

---

PREVIOUS PART: BACKGROUND PHYSIOLOGY - Kinematics of the knee

NEXT PART: TAKEHOME MESSAGE - The surgical technique of Untethering the Fat Pad