Causes / typical injury mechanism: Quadriceps tendinopathy is caused by repetitive, high-impact contractions of the knee's extensor muscles against resistance, resulting in microtears and inflammation of the tendon proximal to the patella.

Tendinopathy includes tendonitis (acute inflammation of the tendon) and tendinosis (chronic degeneration with abnormal healing and less inflammation; healthy tendon is replaced by fibrous scar tissue). These conditions are closely related and grouped together as tendinopathy.

Classic history and presentation: This overuse injury is most commonly seen in individuals who have repeatedly loaded the extensor mechanism of the knee. Individuals who play jumping sports such as volleyball, basketball, and skiing, or who participate in activities that involve squatting and climbing, are at the highest risk.

Patients present with pain localized to the quadriceps tendon, near the superior border of the patella, usually after a recent increase in exercise frequency or intensity. Pain is worsened by activity (active, resisted knee extension, as seen in jumping or stair climbing) and improves with rest (knee extended, extensor mechanism unloaded). Patients often have concomitant symptoms at the patellofemoral joint and patellar tendon, at its origin or insertion.

Prevalence:

• Age – This can usually be seen in young or middle-aged individuals.
• Sex / gender – A higher prevalence is seen in nonelite male athletes (10.2%) than in their female counterparts (6.4%).

Risk factors: Activities with high-impact loading of the knee (eg, jumping) create the highest risk. Other factors are greater weight, taller height, younger age, and increased weight training despite weak core muscles and tight hamstrings.

Pathophysiology: This is caused by structural changes that occur in response to repetitive, high-impact loading of the tendon. The mechanism is not well understood. Increased vascularization of the stressed tendon is an early change, which is later accompanied by thickening. The degree of structural change does not correlate with the severity of clinical symptoms.

Grade / classification system: No generally accepted classification system is used for quadriceps tendinopathy. Most practicing physicians simply classify this as mild, moderate, or severe, which is somewhat subjective. However, there are a few classifications based on symptoms.

The Blazina classification:

• Grade I – Pain after activity only, without functional impairment.
• Grade II – Pain during and after activity with satisfactory performance levels.
• Grade III – Prolonged pain during and after activity with decreasingly satisfactory performance.

Roels et al made modifications to the Blazina classification scheme, including consideration of tendon rupture:

• Grade I – Pain at the infrapatellar or suprapatellar region after practice or event.
• Grade II – Pain at the beginning of activity that disappears after warming up and reappears after completion of activity.
• Grade III – Pain remains during and after activity, and the patient is unable to participate in sports.
• Grade IV – Complete rupture of the tendon.

Ferretti et al modified the Blazina classification to revolve around intensity of pain:

• Stage 0 – No pain.
• Stage 1 – Pain only after intense sports activity with no functional impairment.
• Stage 2 – Moderate pain during sports activity with no restriction on sports performance.
• Stage 3 – Pain with slight restriction on performance.
• Stage 4 – Pain with severe restriction on performance.
• Stage 5 – Pain during daily activity with absolute inability to participate in sports.