The Science of Patellar Tendinopathy (Jumper's Knee)

Patellar tendinopathy represents a complex degenerative condition affecting the patellar tendon, predominantly at its attachment to the inferior pole of the patella. The condition involves progressive collagen fiber disorganization and failed healing response rather than true inflammation, which fundamentally changes our approach to treatment. The pathology begins with repetitive microtrauma from jumping and landing activities that overwhelm the tendon's capacity to repair. This creates microscopic failures within the tendon structure, leading to alterations at the cellular level that undermine its mechanical properties. The normal parallel arrangement of type I collagen fibers becomes disrupted, replaced by areas of mucoid degeneration and increased ground substance that weakens the tendon's tensile strength. At the cellular level, tenocytes undergo significant changes in response to repetitive loading. These cells alter their protein and enzyme production, increasing prostaglandin E2 and leukotriene B4, which contribute to the degenerative process. Matrix metalloproteinase activity increases, breaking down the extracellular matrix faster than it can be rebuilt. Simultaneously, vascular endothelial growth factor production leads to neovascularization, bringing new blood vessels and nerve fibers into areas that are normally avascular, contributing to pain sensation. The tendon's appearance changes dramatically at the microscopic level. Instead of tightly packed, parallel collagen bundles, affected tendons show areas of fibrinoid necrosis, pseudocyst formation, and random collagen orientation. There's hypercellularity with atypical fibroblast proliferation and areas of cell death through apoptosis. This creates the characteristic thickened, painful tendon seen clinically, often described as having a "mucoid" appearance on imaging. Importantly, this is primarily a degenerative rather than inflammatory condition. While acute inflammation may occur with initial injury, chronic patellar tendinopathy shows minimal inflammatory cells. This understanding has shifted treatment away from anti-inflammatory approaches toward loading programs that stimulate proper tendon remodeling and collagen synthesis.