A knee ultrasound (knee sonography) is a non-invasive medical imaging technique that uses high-frequency sound waves to create real-time pictures of the structures within and around the knee joint. This procedure assesses soft tissues, including tendons, ligaments, and muscles, and detects fluid accumulation or irregularities in blood flow. Unlike X-rays, which view bone structure, an ultrasound provides a detailed view of the surrounding soft tissues and their condition. The scan helps physicians accurately diagnose the source of pain, swelling, or limited movement in the knee.
How the Knee Ultrasound Procedure is Performed
The knee ultrasound is a straightforward procedure that requires minimal preparation. Patients may be asked to wear loose clothing that allows easy access to the knee area. During the examination, the patient lies on a table, often with the knee slightly bent, which helps relax the muscles and expose the joint structures for a clearer view.
A clear, water-based gel is applied to the skin over the knee to facilitate the transmission of sound waves and eliminate air pockets. The sonographer uses a small, handheld device called a transducer, which is gently pressed against the skin and moved across the knee. The transducer emits sound waves and captures the returning echoes, which the system converts into a live image on a monitor.
The examination is dynamic; the patient may be asked to slightly flex, extend, or apply gentle pressure to the knee during the scan. This allows the physician to observe structures, such as ligaments and tendons, as they move or are placed under stress. This dynamic assessment can reveal instability or impingement that static imaging might miss. The entire process is painless and generally takes only 15 to 30 minutes to complete.
Diagnosing Tears and Structural Damage to Soft Tissues
Ultrasound provides high-resolution visualization of superficial and mid-depth soft tissues, making it an excellent tool for identifying structural damage. Physicians use the scan to examine major tendons, including the patellar tendon (connecting the kneecap to the shinbone) and the quadriceps tendon. Damage, such as patellar tendinopathy or a partial tear, appears as a localized disruption or altered texture (echogenicity) in the tendon fibers.
The structural integrity of the collateral ligaments, specifically the Medial Collateral Ligament (MCL) and Lateral Collateral Ligament (LCL), is easily assessed due to their superficial location. Injuries like a sprain or partial tear are often seen as swelling of the ligament tissue or an area of inhomogeneous texture. Dynamic assessment during the scan can confirm ligament laxity or instability under stress.
While ultrasound excels at these surface structures, it is limited in visualizing deep, intra-articular structures like the Anterior Cruciate Ligament (ACL) and Posterior Cruciate Ligament (PCL) due to acoustic shadowing from the bone. However, the scan effectively identifies muscle strains and hematomas in the surrounding thigh and calf muscles. It is also useful for finding foreign bodies lodged in the soft tissue near the skin’s surface following an injury.
Detecting Fluid Accumulation and Inflammatory Conditions
Knee ultrasound is highly effective for detecting and characterizing fluid accumulation and signs of inflammation within the joint and surrounding bursae. A joint effusion (excess fluid within the joint space) is readily identified, often appearing as an anechoic (black) or hypoechoic (dark) collection that distends the suprapatellar recess. Visualizing this fluid confirms that the swelling is internal to the joint.
Inflammation of the synovial lining, known as synovitis, is a common finding, especially in patients with arthritis. The inflamed tissue often thickens (synovial hypertrophy) and may exhibit increased blood flow (hyperemia). This hyperemia is detectable using Doppler technology, allowing the physician to gauge the activity level of the inflammation.
Ultrasound is also effective for diagnosing bursitis, which is inflammation of the small, fluid-filled sacs (bursae) that cushion the knee. Common examples include prepatellar bursitis and pes anserine bursitis, which appear as localized, fluid-filled swellings. The scan is also the primary method for identifying a Baker’s cyst (popliteal cyst), a fluid-filled sac that forms behind the knee, often resulting from chronic joint effusion.
When Ultrasound is Used Over Other Imaging Methods
Physicians choose ultrasound over other imaging modalities like X-ray or Magnetic Resonance Imaging (MRI) due to its unique advantages. The ability to provide real-time, dynamic imaging is a benefit, allowing structures to be viewed as the patient moves the joint, which is impossible with static scans. This dynamic assessment is useful for demonstrating joint instability or soft tissue impingement.
Ultrasound is significantly more accessible, portable, and cost-effective than MRI, making it suitable for immediate use in clinics or at the bedside. The procedure is entirely safe, using sound waves with no ionizing radiation. This makes it an ideal choice for repeated studies or for patients who cannot undergo MRI due to metal implants or claustrophobia.
However, the limitations of ultrasound guide the decision to use other tests. Since sound waves cannot penetrate deep into bone, ultrasound cannot provide a complete picture of bone marrow or deep cartilage surfaces. If a physician suspects a complex tear of the menisci, a deep ligament injury like an ACL tear, or internal cartilage damage, MRI remains the preferred method for detailed intra-articular assessment.