Anterior Knee Pain

Anterior Knee Pain

This section is detailed in nature, as it attempts to explain in layman’s terms the perplexing phenomenon of Anterior Knee Pain (AKP), which is defined as a symptom complex commonly presenting as a deep-seated ache usually perceived under and about the patella.  The source of the pain is noxious nerves in the infrapatellar fat pad (IFP) which have been sensitized by as a result of an injury, overuse, knee surgery, or any pathologic process that produces pain.  In most instances the problem is self-limited and simply heals with non-operative treatment (rest followed by controlled exercises, physical therapy, analgesics and anti-inflammatories, bracing/taping). But in a disturbing number of patients, it does not recede, and the individual has to adjust life to avoid the pain, and most do. In so doing, however, their lives are restricted, as this characteristic pain tends to be associated with particular activities that involve motion and activity, such as the following:

• any activities that involve force activation — squatting, weight-bearing activities, running;
• prolonged sitting – producing stiffness, pain, or both, relieved by straightening (extending) the knee;
• on getting up from sitting, the pain may be severe, then diminishes and even disappears as the individual keeps walking;
• the “theatre sign” in some – where the individual seeks to sit at the end of any row of chairs, to allow the relief afforded by knee extension and even standing up;
• problems with walking – where the person may exhibit weakness, giving way of the knee, and ‘catching’;
• stair climbing producing pain when weight is applied on the leg, when going up or downstairs;
• snapping, popping, or crunching in the knee occurring with activity as simple as walking, that may be accompanied by pain, which may be so severe that the individual feels that the knee locks.

The symptoms arise from complex physiology and anatomy, which is described in our research (see Contributions), but the explanation can be simplified. The physiology of the contents of the space below the patella is a common factor that underlies these symptoms in most. When the knee moves, forces are applied to the tissues that totally fill this space, called the anterior compartment, which has firm borders that are either bone, or dense fibrous tissue. These tissues include the IFP and its attached, constraining ligament, the infrapatellar plica (IPP). Together they form a complex that is, in essence, a hydraulic shock absorber, that acts as a unit to hold the fat pad in place and fill the space. With knee motion the borders of the space change in contour, and the fat pad, which is deformable, semi-liquid and filled with noxious nerves, simply alters shape to fill the space, much like children’s slime slips out of the way to the sides when squeezed between the compressed palms of their hands. This system works flawlessly in day to day activities when the knee is pain-free.

Nature, however, is not perfect and there are 2 problems in the design of the system. First, the ligament that restrains the fat pad is buried deep and is so attached that the ligament must stretch remarkably as the knee extends, and the highly-innervated fat pad, tethered centrally, has to deform. Secondly, the fat pad is a sensory organ, and contains noxious nerves (those that send pain signals to the brain) which are highly sensitive to mechanical pressure (Scott Dye, URL – https://doi.org/10.1177/03635465980260060601). When any clinical condition induces pain, these nociceptive nerves in the IFP are activated, and one perceives pain.  Then, because of knee motion, these sensitized nerves, transmitting pain signals and contained within the fat pad, must move slightly by the inherent distortion and stretch described above. These nerves, already “sensitized” and passing pain signals, undergo further stretch and deformation. An analogous situation can be described: if you came to see me with a “boil” on your forearm, it would be swollen, red, hot and painful. If, by mistake, I pressed on the area about the boil, you would experience severe added pain. The slightest motion or pressure, at a microscopic level, on activated noxious nerves constitutes a second mechanism further activating the nerves. The added pain in a boil is immediate and severe. In the knee the pain is occurring from nerves in the IFP, buried deep, its outer border accessible and sensitive to the physician’s examination with direct pressure below the knee cap on either side. This local tenderness, along with the the history which includes the typical characteristics listed above indicates that AKP is likely present as part of the clinical problem. Every individual responds uniquely to pain.  In some this added neuralgic pain, if prolonged over months, may lead to avoidance behavior, interfering significantly with one’s quality of life.

AKP is very common, with incidence rates far exceeding those of osteoarthritis: it is found in 1 in 14 adolescents and 1 in 10 military members. The most common physical finding that a physician observes is tenderness about the knee cap, and fully 80% of patients have pain with squatting. AKP can coexist with, and indeed mimic, other pathology in the knee and is likely multifactorial. No specific investigation is diagnostic. Recent literature has increasingly suggested that there are few structural abnormalities and that the cause of pain is likely neural damage as the “provoking factor.”

Our clinical approach to APK has undergone peer review published May 2018 in an open-text article in Arthroscopy Techniques (URL https://www.arthroscopytechniques.org/article/S2212-6287(18)30016-1/fulltext). It describes the basic science underlying, as well as the specific technique for performing this operation, that addresses AKP associated with derangement of the infrapatellar plica (IPP) and fat pad (FP). If your situation warrants, an operation can be considered in which your knee would undergo arthroscopic evaluation with the goal of identifying and treating, wherever possible, all pathology. Rarely, usually if deformity is present, further surgery may be necessary.  The decision to proceed is yours alone.  The discussion to obtain consent must be thorough, as no operation is without the potential of complication.  My task is to outline the risks, benefits, and alternatives to surgery and to answer all of your questions. The operation untethers the fat pad by release/resection of the IPP, often performed for other reasons, primarily to allow visualization, during routine arthroscopy. The kinematic studies show that the stretch and deformation of the IFP, the presumed link to AKP, is eliminated by the operation. The procedure has a high probability of eliminating the pain, with a complication rate as low as that of diagnostic arthroscopy.