What is fabella syndrome?

Fabella Syndrome

Introduction

Fabella Syndrome is a rare but clinically significant condition characterized by posterolateral knee pain associated with the presence of a small sesamoid bone called the fabella. This bone is located in the tendon of the lateral head of the gastrocnemius muscle, near the posterolateral femoral condyle. Although the fabella is present in approximately 10–30% of the population, in most individuals it is asymptomatic. However, in some cases, it becomes a source of pain, particularly in active individuals or following knee trauma or surgery. Fabella Syndrome is often overlooked, making clinical awareness essential for accurate diagnosis and effective management.

Anatomy and Location of the Fabella

The fabella is a sesamoid bone embedded within the lateral head of the gastrocnemius muscle. It articulates with the lateral femoral condyle and lies posterior to the knee joint. While it is more commonly ossified in some individuals, in others, it remains cartilaginous or absent altogether. The bone is thought to be a remnant of evolutionary anatomy, similar to the patella, serving to reduce friction in tendinous structures. In certain biomechanical contexts, especially in athletes or those who frequently engage in repetitive knee extension and flexion, the fabella can cause mechanical irritation or compression of adjacent structures, such as the lateral femoral condyle or the common peroneal nerve.

Pathophysiology of Fabella Syndrome

Fabella Syndrome arises when the fabella impinges on surrounding soft tissues or the posterior femoral condyle during knee movement. This typically occurs in activities involving repetitive extension of the knee or direct pressure on the posterior aspect of the knee. The condition may be idiopathic or result from post-surgical alterations, such as after total knee arthroplasty (TKA), where the biomechanics of the knee are altered, and the fabella becomes a prominent stress point. Mechanical irritation leads to local inflammation, tissue damage, and pain, which is usually exacerbated by knee extension or prolonged activity.

Clinical Presentation

Patients with Fabella Syndrome typically present with localized pain in the posterolateral aspect of the knee, often described as aching or stabbing. The pain may be aggravated by full knee extension, prolonged standing, or activities that increase pressure on the back of the knee, such as squatting or kneeling. In some cases, a palpable or even visible lump may be noted behind the lateral aspect of the knee, particularly in individuals with a prominent or ossified fabella. Occasionally, the syndrome may also lead to compression of the common peroneal nerve, resulting in radiating pain or neurological symptoms like numbness or tingling in the lateral leg or foot.

Differential Diagnosis

Due to its rarity and nonspecific symptoms, Fabella Syndrome is often mistaken for other posterior knee pathologies. Differential diagnoses include:

• Popliteal cyst (Baker’s cyst)

• Lateral meniscus tear

• Posterolateral corner injuries

• Common peroneal nerve entrapment

• Gastrocnemius tendonitis

• Posterior knee osteoarthritis
  Proper clinical evaluation and imaging are essential to differentiate Fabella Syndrome from these conditions.

Diagnostic Evaluation

Clinical diagnosis involves a thorough patient history and physical examination, with emphasis on palpation of the fabella area. Reproduction of pain upon direct pressure over the posterolateral knee or with resisted knee extension can indicate fabellar involvement.

Imaging studies play a crucial role in confirming the diagnosis:

• X-rays: Can reveal an ossified fabella located posterior to the lateral femoral condyle.

• MRI: Provides detailed visualization of soft tissue structures, shows the relationship between the fabella and surrounding tissues, and helps rule out other causes of posterior knee pain.

• Ultrasound: Useful for dynamic assessment and guided injections.

Management

Treatment of Fabella Syndrome typically begins with conservative methods. These include:

• Activity modification to reduce stress on the posterior knee.

• Physical therapy focused on stretching and strengthening the surrounding musculature.

• Anti-inflammatory medications to alleviate pain and swelling.

• Ultrasound-guided corticosteroid injections to reduce local inflammation.

If conservative treatment fails to relieve symptoms, surgical excision of the fabella (fabellectomy) may be considered. This procedure is usually effective in eliminating symptoms and is associated with low complication rates. Postoperative rehabilitation focuses on restoring range of motion and strength without provoking pain.

Prognosis

The prognosis for Fabella Syndrome is generally favorable, especially when diagnosed early and treated appropriately. Most patients respond well to non-surgical treatment, and those who undergo surgical excision typically experience full resolution of symptoms. Delay in diagnosis, however, can lead to chronic pain and functional limitations, particularly in athletes or physically active individuals.

Relevance in Post-Surgical Patients

Fabella Syndrome has gained clinical attention in the context of total knee replacement surgeries. In some patients, the presence of a fabella becomes symptomatic following TKA due to altered biomechanics or implant positioning. Surgeons must be aware of this potential complication, and preoperative imaging should consider identifying an ossified fabella that might become problematic postoperatively.

Conclusion

Fabella Syndrome is an underdiagnosed cause of posterolateral knee pain that should be considered when evaluating patients with activity-related discomfort in the back of the knee. Though rare, the condition can significantly affect quality of life, particularly in active individuals. With careful clinical examination, appropriate imaging, and a structured treatment plan, most patients can achieve excellent outcomes. Surgical intervention is reserved for persistent cases and is generally curative. Heightened clinical awareness is essential to prevent misdiagnosis and to ensure effective treatment of this often-overlooked condition.

References

• Driessen, M. J., van der Bracht, H., & Martens, M. (2014). Fabella syndrome as a cause of posterolateral knee pain: A report of three cases. Knee Surgery, Sports Traumatology, Arthroscopy.

• Phisitkul, P., & Stannard, J. P. (2006). Posterolateral corner injuries of the knee: Anatomy, diagnosis, and treatment. Sports Medicine and Arthroscopy Review.

• Weiner, D. S., & Macnab, I. (1973). The fabella syndrome: An update and review. Clinical Orthopaedics and Related Research.

• Physio-pedia. Fabella Syndrome. https://www.physio-pedia.com/Fabella_Syndrome