Which magnetic resonance imaging protocol should be ordered for a suspected patellar tendon injury?

MRI Knee Without Contrast for Suspected Patellar Tendon Injury

Order a standard MRI of the knee without contrast when you suspect a patellar tendon injury, as this is the definitive imaging modality for evaluating soft tissue injuries of the extensor mechanism including tendon tears, partial tears, and associated intra-articular injuries. 1

Initial Imaging Algorithm

Start with Plain Radiographs First

• Obtain anteroposterior (AP) and lateral knee radiographs as your initial imaging study, even when patellar tendon injury is suspected 1
• Add a patellofemoral (sunrise/Merchant) view specifically when evaluating the extensor mechanism to assess patellar position and rule out fracture 1, 2
• The lateral view should be obtained at 25-30 degrees of flexion to visualize the patella in profile and assess for joint effusion 1, 2

When to Proceed Directly to MRI

While radiographs are technically the first-line study per ACR guidelines, in clinical practice when patellar tendon rupture is strongly suspected based on physical examination (palpable defect, inability to extend knee against gravity, high-riding patella), you should order MRI immediately rather than waiting for radiographs to be negative, as MRI is necessary for surgical planning. 3, 4

MRI Protocol Specifications

Standard Non-Contrast MRI Knee

• Order a standard MRI knee without IV contrast 1
• MR arthrography and MRA are not indicated for patellar tendon evaluation 1
• No special protocol modifications are needed beyond standard knee MRI sequences 5, 6

Critical MRI Assessment Points

• Axial sequences are essential for measuring tendon thickness and determining tear percentage, which directly impacts treatment decisions 5
• Tendon thickness >8.8 mm on axial MRI strongly correlates with presence of a partial tear 5
• Tendon thickness >11.5 mm and/or tear involving >50% of tendon thickness on axial views predicts failure of conservative management and need for surgery 5
• 91% of partial-thickness tears involve the posterior and posteromedial regions of the proximal tendon 5

Why MRI is Essential for Patellar Tendon Injuries

High Rate of Associated Injuries

• 30% of patellar tendon ruptures have associated intra-articular injuries that require identification for proper treatment planning 3
• The most common associated injuries are ACL tears (18%) and medial meniscus tears (18%) 3
• 75% of high-energy direct impact patellar tendon ruptures have associated injuries, making MRI nearly mandatory in this mechanism 3
• Distal patellar tendon avulsions are associated with multi-ligamentous knee injuries and occult knee dislocations requiring comprehensive MRI evaluation 4

Surgical Planning Requirements

• MRI is necessary for preoperative detection of meniscal injury even when CT shows bony avulsion 1
• Tear location (proximal vs. mid-substance vs. distal), size, and percentage of tendon involvement directly determine surgical approach 5, 4
• Quadriceps tendon ruptures have a 10% rate of associated injuries, lower than patellar tendon but still clinically significant 3

Common Pitfalls to Avoid

• Do not rely on ultrasound as your primary imaging modality for suspected patellar tendon rupture, despite its utility in experienced hands, because MRI provides comprehensive evaluation of associated injuries 1, 6
• Do not order CT as the next study after negative radiographs when soft tissue injury is suspected—CT is for occult fractures, not tendon evaluation 1
• Do not skip MRI in high-energy mechanisms (motor vehicle collision, fall from height, dashboard injury) even if radiographs appear normal, as these have 75% rate of associated injuries 3, 4
• Always assess for compartment syndrome and neurovascular injury in distal patellar tendon avulsions, as these indicate high-energy multi-ligamentous knee trauma 4

Clinical Context for Ordering

Order MRI When:

• Physical examination suggests complete or partial patellar tendon rupture (palpable defect, inability to straight leg raise, high-riding patella) 3, 5, 4
• High-energy trauma mechanism with extensor mechanism injury 3, 4
• Chronic patellar tendinopathy failing conservative management, to assess for partial tear and guide treatment escalation 5
• Any patellar tendon injury with concern for multi-ligamentous knee injury 3, 4

Timing Considerations

• MRI can be obtained acutely or subacutely depending on clinical urgency and surgical planning needs 3, 5
• For chronic tendinopathy, MRI helps differentiate tendinosis from partial tears and quantifies tear burden to predict treatment response 5