“The front of my knee below my knee cap hurts!” How often do we hear that complaint in our clinics after injury, surgical procedure, or insidiously?
One possible cause of this type of pain that may go undiagnosed is anterior interval impingement of the knee, a condition where compression of the infra-patellar fat pad (IPFP) results in pain.
In the typical healthy knee, the infra-patellar fat pad will displace posteriorly during knee flexion owing to pressure from the patellar tendon as the angle formed by the patellar tendon and anterior tibia decreases. Conversely, extension moves the fat pad anteriorly away from the tibia. Essentially in lay terms, the IPFP is sucked under the patellar tendon during flexion and is required to squish laterally and anteriorly around the patellar tendon to achieve full knee extension.
Teichtahl, in 2015, reported that a larger Cross Sectional Area of IPFP predicts reduced lateral tibial cartilage volume loss and development of knee pain and might act as a shock absorber for the knee. The IPFP is highly innervated by nocioceptive nerve fibers, including C fibers and the neuropeptide substance P, which may make this particular tissue especially prone to a sensitized state, resulting in a pain experience when the tissue is compressed during flexion or hyperextension. Their conclusions encouraged the consideration of not removing the IPFP during the TKA procedure as is common during this procedure.
Clockaerts, in 2010, found that IPFP is also a tissue capable of modulating inflammatory and destructive responses in knee OA. So several positive benefits of the IPFP have been found beyond its obvious cushioning of forces to the anterior aspect of the knee.
Fat pad impingement can be easily confused with patellar tendonitis. However, patellar tendonitis tends to cause pain only at the patellar tendon, especially at the inferior pole of the patella. Fat pad impingement will cause pain on either side of the patellar tendon, where the fatty tissue sits. The pain may be worse with jumping, prolonged standing or any other position that causes the knees to hyperextend. Also, the area around the patellar tendon may be slightly swollen. Fat pad impingement is not associated with clicking, locking or instability.
Meneghini, in the J Arthroplasty 2007, found that patients whose fat pad had been removed during TKA were nearly twice as likely to experience postoperative pain. The IPFP has also been show to contain vascular endothelial growth factor. The release of this factor after breeching the synovial lining of the fat pad during trauma or surgery may lead to vascular ingrowth or eventual scarring (Inflamed synovium produces inflammatory cytokines which promote the proliferation of fibroblasts and the production of collagen in synovial tissue, which can lead to tissue fibrosis).
Dr. Steadman of the Steadman Clinic in Vail, CO has reported that scarring of the anterior interval changes the mechanics of the anterior structures of the knee and may lead to refractory anterior knee pain. Arthroscopic Anterior Interval Release successfully provides relief in this patient population. The anterior interval describes the anterior interval of the knee as the space between the infrapatellar fat pad and patellar tendon anteriorly, and the anterior border of the tibia and the transverse meniscal ligament posteriorly. It extends from the lateral border of the medial meniscus to the medial border of the lateral meniscus. Normally, bridging scar tissue is not present in this area. The Structures of the anterior interval are typically mobile. If fibrosis courses from the fat pad to the transverse ligament it may lead to closure and dysfunction of the anterior interval. Closure of the interval causes decreased excursion of the patellar tendon in relation to the tibia. The less elastic nature the IPFP, the more likely tethering of the patellar tendon may occur. This tethering may cause stretching around the surrounding synovium during knee extension. Tensioning/stretching of the synovium may lead to anterior knee pain and potential loss of full extension.
Scarring of the anterior interval can be seen on MR finding seen on both T1 and T2 images differentiated from hemorrhage or edema. Scarring has been shown to be prominent after previous surgical procedure (arthroscopy and open). Steadman has observed that little or no mobility occurred during interval closure in patients with IPFP fibrosis or scarring strongly suggesting decreased mobility of the fat pad when extending the knee.
To finish this part I introduction to the location, possible mechanisms, and clinical importance of the anterior interval, this is a highly innervated, vascular, and sensitive tissue! Clinicians should give strong considerations to its’ mobility and irritability during rehabilitation of the knee.
Join me for Part II, where we will discuss clinical significance and potential treatment options!