What is the best initial imaging modality for a patient with a suspected knee injury and inability to extend the knee, with swelling and a palpable defect at the suprapatellar region?

From the Guidelines

The first imaging modality that should be ordered for this patient is a plain radiograph (X-ray) of the knee. This patient's presentation strongly suggests a patellar tendon rupture, given the history of inability to extend the knee, swelling, and a palpable defect in the suprapatellar region. X-rays are the appropriate initial imaging study as they are readily available, cost-effective, and can help rule out fractures or joint effusions, as recommended by the American College of Radiology 1. They may also show indirect signs of patellar tendon rupture such as patella alta (high-riding patella). While MRI would provide definitive diagnosis of the tendon rupture, it is not typically the first-line imaging study. After obtaining X-rays, an MRI would be the appropriate next step to confirm the diagnosis and assess the extent of the injury, as it has many distinct advantages for the evaluation of the injured knee in the setting of negative radiographs 1. The use of MRI can diagnose the patterns and severity of bone marrow contusions, predict associated soft-tissue injuries, and facilitate diagnosis of anterolateral ligament injuries, which can be associated with ACL injuries and anterolateral knee instability 1. Overall, the initial use of X-rays followed by MRI as needed is the most appropriate approach for evaluating this patient's suspected knee injury. Key benefits of this approach include:

• Rapid evaluation and ruling out of fractures or joint effusions
• Cost-effectiveness and availability of X-rays
• Ability of MRI to provide definitive diagnosis and assess extent of injury
• Potential to improve patient outcomes and reduce morbidity by guiding appropriate treatment decisions 1.

From the Research

Initial Imaging Modality for Suspected Knee Injury

The best initial imaging modality for a patient with a suspected knee injury and inability to extend the knee, with swelling and a palpable defect at the suprapatellar region is:

• Ultrasonography, as it can reliably visualize the rupture and the resulting hematoma 2
• Magnetic resonance imaging (MRI) is also a suitable option, with high sensitivity and specificity for detecting quadriceps tendon ruptures, although its limited availability should not delay therapy 2

Characteristics of Quadriceps Tendon Rupture

Key characteristics of quadriceps tendon rupture include:

• Suprapatellar hematoma and the inability to actively straighten the knee 2
• A typical examination finding is the "suprapatellar gap sign" 2
• Clinical findings typically include the triad of acute pain, impaired knee extension, and a suprapatellar gap 3

Comparison of Imaging Modalities

Comparing different imaging modalities:

• MRI is the modality of choice for the imaging evaluation of the knee, with advances in magnetic field strength, gradient strength, and coil design facilitating new pulse sequences 4
• Ultrasonography is the diagnostic imaging modality of first choice for quadriceps tendon rupture, while MRI has a high sensitivity and specificity for detecting quadriceps tendon ruptures 2