Can a patient still extend the knee with a patellar tendon rupture?

Knee Extension in Patellar Tendon Rupture

Patients with a complete patellar tendon rupture cannot actively extend the knee against gravity due to disruption of the extensor mechanism. 1, 2

Clinical Presentation and Diagnosis

When evaluating a suspected patellar tendon rupture, the following key findings are typically present:

• Inability to perform a straight leg raise: This is the most reliable clinical sign of a complete rupture 1
• Palpable defect in the substance of the patellar tendon 2
• Patella alta (high-riding patella) visible on lateral knee radiographs 2
• Pain, swelling, and tenderness at the site of rupture 1

Physical Examination Findings

The physical examination should focus on:

1. Extensor mechanism testing:

   • Ask the patient to actively extend the knee from a flexed position
   • Test for straight leg raise ability
   • Observe for extension lag (inability to maintain full extension)

2. Palpation:

   • Feel for a gap or defect in the patellar tendon
   • Assess for tenderness and swelling

3. Special tests:

   • Thompson/Simonds squeeze test: While this is typically used for Achilles tendon rupture, a similar principle can be applied to assess patellar tendon integrity 3
   • Matles test: Evaluates for increased passive ankle dorsiflexion in Achilles rupture but can be modified to assess patellar position 3

Pathophysiology of Extensor Mechanism Failure

The patellar tendon is a critical component of the knee extensor mechanism, connecting the patella to the tibial tuberosity. When ruptured:

• The quadriceps muscle can no longer transmit force to the tibia
• The patella loses its distal anchor and migrates proximally
• The patient cannot generate the force needed to extend the knee against gravity 2

Imaging Confirmation

While clinical examination is often diagnostic, imaging can confirm the diagnosis:

• Radiographs: Lateral views show patella alta (high-riding patella) 2
• Ultrasound: Can identify tendon discontinuity 3
• MRI: Gold standard for confirming the diagnosis and assessing the extent of rupture 3

Treatment Implications

The inability to extend the knee has significant implications for treatment:

• Acute ruptures (less than 2 weeks): Primary repair is recommended 1
• Chronic/neglected ruptures (more than 2 weeks): Often require reconstruction techniques using autografts such as hamstring tendons 4
• Surgical repair should be performed promptly for optimal outcomes 2

Rehabilitation Considerations

Following surgical repair:

• Early protected range of motion exercises are crucial 2
• Progressive quadriceps strengthening is essential to restore knee extension function
• Protected weight bearing during the initial healing phase
• Return to full activity only after complete restoration of quadriceps strength and knee range of motion 2

Common Pitfalls

• Misdiagnosis: Patellar tendon ruptures can be misdiagnosed as patellar fractures or other knee injuries
• Delayed treatment: Results in poorer outcomes due to muscle retraction and tendon scarring 2
• Inadequate rehabilitation: Can lead to persistent weakness and functional limitations

In summary, the inability to actively extend the knee is a hallmark sign of patellar tendon rupture and should prompt immediate orthopedic evaluation for surgical repair to restore extensor mechanism function and optimize long-term outcomes.