What is the mechanism to tear the patella tendon?

Mechanism of Patellar Tendon Tear

The primary mechanism of patellar tendon tear is forceful contraction of the quadriceps muscle on a partially flexed knee, which places excessive tensile load on the tendon beyond its structural capacity. 1

Common Mechanisms of Injury

• Sudden eccentric contraction of the quadriceps muscle is the most common mechanism leading to patellar tendon rupture 2
• Sports-related activities are frequently implicated in traumatic patellar tendon tears 1
• The tendon typically fails when subjected to forces that exceed its tensile strength, often during activities requiring explosive knee extension or deceleration 2

Risk Factors Contributing to Patellar Tendon Tears

• Local factors that increase risk include:

  • Pre-existing patellar tendinopathy (degeneration of the tendon) 3
  • Prior corticosteroid injections directly into the tendon 4
  • Previous knee surgeries or trauma 3

• Systemic factors that increase susceptibility include:

  • Obesity (increases mechanical load on the tendon) 3
  • Diabetes mellitus (affects collagen structure) 3
  • Chronic renal failure 3
  • Hyperparathyroidism 3
  • Use of fluoroquinolone antibiotics or statins 3

Anatomical Considerations

• The patellar tendon connects the patella to the tibial tuberosity and is a critical component of the knee extensor mechanism 2
• The tendon experiences significant tensile forces during activities requiring knee extension 2
• Anatomical deformities such as forefoot and heel varus, excessive pes planus, or foot pronation may contribute to abnormal loading patterns on the patellar tendon 4

Diagnostic Features

• Complete tears typically present with:

  • Inability to perform a straight leg raise 2
  • Palpable gap at the site of tendon rupture 2
  • Patella alta (high-riding patella) visible on radiographs compared to the uninjured knee 2

• Radiographic signs of patellar tendon tears include:

  • Patella alta (high-riding patella) 5
  • Localized soft-tissue swelling 5
  • Bony avulsions in cases where the tendon pulls off a fragment of bone 5
  • Dystrophic calcifications within the tendon in chronic cases 5

Clinical Pearls and Pitfalls

• The incidence of patellar tendon tears after total knee arthroplasty is relatively low at 0.17% to 2.5% 5
• Bilateral patellar tendon ruptures are extremely rare but more commonly associated with systemic risk factors 3, 6
• Corticosteroid injections directly into the tendon substance should be avoided as they may inhibit healing, reduce tensile strength, and predispose to spontaneous rupture 4
• Early diagnosis and prompt surgical intervention are crucial for optimal outcomes, particularly in complete tears 2, 7

Imaging Evaluation

• MRI is the gold standard for evaluating patellar tendon tears with high sensitivity (95%) and specificity (95%) 4
• Ultrasound is a useful alternative with high specificity (94%) but lower sensitivity (58%), making it better for confirming rather than ruling out tendinopathy 4
• Ultrasound can be used for evaluation of patellar tendinopathy in patients with total knee arthroplasty 5