From the Guidelines
A fabella behind the knee on X-ray is generally considered a normal anatomical variant and requires no treatment when incidentally discovered on imaging, unless it is associated with fabella syndrome, which presents as posterolateral knee pain that worsens with knee extension. This finding is present in approximately 10-30% of the population, with higher prevalence in certain ethnic groups, particularly those of Asian descent 1. The fabella is typically asymptomatic, but in some cases, it can cause pain, and treatment options include conservative measures such as physical therapy, anti-inflammatory medications, or in persistent cases, surgical excision of the fabella.
When evaluating knee pain, radiography is the initial imaging study used to evaluate chronic pain in a native knee, and MRI is usually considered the next imaging study if pain persists but the initial radiographs are normal or demonstrate a joint effusion 1. However, the use of MRI may be premature and unnecessary in some patients, and it is estimated that approximately 20% of patients with chronic knee pain have had an MRI performed without recent radiographs 1.
Some key points to consider when evaluating a fabella on X-ray include:
- The fabella is a small, round sesamoid bone that can be found behind the knee, specifically embedded within the lateral head of the gastrocnemius muscle.
- The fabella appears as a small, dense, oval or round structure posterior to the lateral femoral condyle on X-ray.
- The bone's function is not entirely clear, though it may contribute to knee joint stability or alter the biomechanics of the gastrocnemius muscle.
- Fabella syndrome is a condition that presents as posterolateral knee pain that worsens with knee extension, and it can be associated with the fabella.
In terms of treatment, conservative measures such as physical therapy and anti-inflammatory medications are usually the first line of treatment for fabella syndrome, and surgical excision of the fabella may be considered in persistent cases 1. It is also important to note that radiographs should be obtained and the clinical decision rule should not be applied for patients with gross deformity, a palpable mass, a penetrating injury, prosthetic hardware, an unreliable clinical history or physical examination, altered mental status, neuropathy, or a history suggesting increased risk of fracture 1.
Overall, a fabella behind the knee on X-ray is a normal anatomical variant that requires no treatment unless it is associated with fabella syndrome, and treatment options include conservative measures and surgical excision in persistent cases.
From the Research
Flabula Behind the Knee on X-ray
- The fabella is a sesamoid bone located in the posterolateral knee, which can be visible on X-ray 2.
- It is a common normal ossicle lying behind the knee, but its presence can be associated with knee pain or functional impairment 3.
- The incidence of the fabella is not 100% bilateral, with a study finding it to be bilateral in only 63% of people 2.
- This means that a comparison view of the other knee may not be helpful in confirming the presence of a fabella 2.
Clinical Significance of Fabella
- Fabella syndrome is a condition that causes posterolateral knee pain and swelling, as well as issues with flexion and/or extension of the knee 4.
- The fabella can impinge on the adjacent common peroneal nerve, causing pain and numbness in the leg 5.
- Fabella excision can be an effective treatment for fabella syndrome, with significant improvements in clinical outcome scores and high patient satisfaction 6.
Diagnosis and Treatment
- Diagnostic imaging such as X-ray and MRI can be used to confirm the presence of a fabella and evaluate the degree of injury 3.
- Treatment options for fabella syndrome include activity modification, rest, physical therapy, and cortisone injections, with surgical excision considered when these fail 4.
- Sonography can be used to diagnose and treat common peroneal neuropathy secondary to fabellae 5.