Fabella Syndrome – The Clinical Syndrome
Accessory bones of the knee are relatively common, with a reported incidence of the fabella of approximately 25%. Fabella, which is Latin for “little bean,” is asymptomatic in the vast majority of patients. However, in some patients, the fabella becomes painful as a result of repeated rubbing of the fabella on the posterolateral femoral condyle.
Located in the lateral head of the gastrocnemius, the fabella is often mistaken for a joint mouse or osteophyte, or it is simply identified as a serendipitous finding on imaging of the knee. It may be either unilateral or bilateral and may be bipartite or tripartite, further adding to the clinician’s confusion. Fabella may exist as an isolated asymptomatic or symptomatic finding. Fracture and dislocation of the fabella have been reported, as well as hypertrophy of this accessory bone, causing compression of the peroneal nerve. The fabella is covered in hyaline cartilage to facilitate its articulation with the femoral condyle; thus it is subject to chondromalacia and the development of osteoarthritis.
What are the Symptoms of Fabella Syndrome
Knee pain secondary to fabella is characterized by tenderness and pain over the posterolateral knee. Patients often think they have gravel in their knee and may report a grating sensation with range of motion of the knee. The pain of fabella worsens with activities that require repeated flexion and extension of the knee. Fabella may coexist with tendinitis and bursitis of the knee. On physical examination, pain can be reproduced by pressure on the fabella. A creaking or grating sensation may be appreciated by the examiner, and locking or catching on range of motion of the knee may occasionally be present. Rarely, an enlarged fabella can compress adjacent vascular structures.
How is Fabella Syndrome diagnosed?
Plain radiographs are indicated in all patients with fabella to rule out fractures and identify other accessory ossicles that may have become inflamed or fractured. Plain radiographs and computed tomography also will often identify loose bodies or joint mice. Based on the patient’s clinical presentation, additional testing, including complete blood cell count, sedimentation rate, and antinuclear antibody testing, may be indicated. Magnetic resonance imaging (MRI) and ultrasound imaging of the knee joint is indicated if bursitis, tendinitis, Baker cyst, joint instability, occult mass, or tumor is suspected and to further clarify the diagnosis. Radionucleotide bone scanning may be useful in identifying traumatic and stress fractures or tumors of the knee that may be missed on plain radiographs. Arthrocentesis of the knee joint may be indicated if septic arthritis or crystal arthropathy is suspected. If compression of adjacent vascular structures is suspected, angiography is indicated.
Fabella pain syndrome is a clinical diagnosis supported by a combination of clinical history, physical examination, radiography, ultrasound, radionucleotide scanning, and MRI. Pain syndromes that may mimic fabella pain syndrome include primary pathological conditions of the knee, including gout and occult fractures, as may bursitis and tendinitis of the knee, both of which may coexist with fabella. Baker cyst rupture may mimic the pain associated with fabella. Primary and metastatic tumors of the knee may present in a manner analogous to knee pain secondary to fabella.
Initial treatment of the pain and functional disability associated with fabella should include a combination of nonsteroidal antiinflammatory drugs (NSAIDs) or cyclooxygenase-2 (COX-2) inhibitors and physical therapy. Local application of heat and cold also may be beneficial. For patients who do not respond to these treatment modalities, injection of the fabella with a local anesthetic and steroid may be a reasonable next step.
The goals of this injection technique are explained to the patient. The patient is placed in the prone position with the anterior ankle resting on a folded towel to slightly flex the knee. The popliteal fossa is identified and, at a point two fingers lateral and two fingers below the popliteal crease, the skin is prepped with antiseptic solution. With a sterile gloved finger, the lateral head of the gastrocnemius is palpated for the point of maximal tenderness. A syringe containing 2.0 mL of 0.25% preservative-free bupivacaine and 40 mg of methylprednisolone is attached to a 2-inch, 22-gauge needle.
The needle is carefully advanced through the previously identified point at a 45-degree angle from the medial border of the popliteal fossa directly toward the painful area containing the fabella. With continuous aspiration, the needle is advanced very slowly to avoid trauma to the peroneal nerve or popliteal artery or vein. On impinging on the fabella with the needle tip, if no paresthesia is experienced in the distribution of the common peroneal or tibial nerve, the contents of the syringe are then gently injected. Minimal resistance to injection should be felt. A pressure dressing is then placed over the cyst to prevent fluid reaccumulation. Ultrasound guidance may improve the accuracy of needle placement and decrease the incidence of needle-related complications. Occasionally, surgical excision of the fabella will be required to provide long-lasting pain relief.
Complications and Pitfalls
The proximity to the common peroneal and tibial nerves, as well as the popliteal artery and vein, makes it imperative that this procedure be carried out only by those well versed in the regional anatomy and experienced in performing injection techniques. Many patients also report a transient increase in pain after the injection. Although rare, infection may occur if careful attention to sterile technique is not followed.
Pain emanating from the knee is a common problem encountered in clinical practice. Fabella must be distinguished from other more common causes of knee pain, including Baker cyst, bursitis, tendonitis, and synovitis. Careful differential diagnosis will help the clinician distinguish symptomatic fabella from other causes of knee pain.