Fabella Injection

Indications and Clinical Considerations

Accessory bones of the knee are relatively common, with a reported incidence of the fabella of approximately 25%. The fabella, which is Latin for “little bean,” is asymptomatic in the vast majority of patients. However, in some patients, the fabella becomes painful because 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 ( Figs. 167.1 and 167.2 ). It may be either unilateral or bilateral and may be partite or tripartite, further adding to the clinician’s confusion. The fabella may exist as an isolated asymptomatic or symptomatic finding. There are reports of fracture and dislocation of the fabella as well as hypertrophy of this accessory bone causing compression of the peroneal nerve ( Fig. 167.3 ). Because the fabella is covered in hyaline cartilage to facilitate its articulation with the femoral condyle, it is subject to chondromalacia as well as the development of osteoarthritis.

Knee pain secondary to the fabella is characterized by tenderness and pain over the posterolateral knee. The patient often feels that he or she has gravel in the knee and may report a severe grating sensation with range of motion of the knee. The pain from the fabella worsens with activities that require repeated flexion and extension of the knee. The 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.

Plain radiographs are indicated for all patients with a fabella to rule out fractures and to identify other accessory ossicles that may have become inflamed ( Fig. 167.4 ). Plain radiographs will also often identify loose bodies or joint mice that may also be present. On the basis of 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 are indicated if bursitis, tendinitis, Baker cyst, joint instability, occult mass, or tumor is suspected and to further clarify the diagnosis ( Fig. 167.5 ). Radionuclide bone scanning may be useful in identifying stress fractures or tumors of the knee that may be missed on plain radiographs ( Fig. 167.6 ). Arthrocentesis of the knee joint may be indicated if septic arthritis or crystal arthropathy is suspected.

Fabella pain syndrome is a clinical diagnosis supported by a combination of clinical history, physical examination, radiography, ultrasound, radionuclide scanning, and MRI ( Fig. 167.7 ). Pain syndromes that may mimic fabella pain syndrome include primary disease of the knee, including gout and occult fractures, as well as bursitis and tendinitis of the knee, both of which may coexist with the fabella. Baker cyst rupture may also mimic the pain associated with the fabella. Primary and metastatic tumors of the knee may also manifest in a manner analogous to knee pain secondary to fabella pain syndrome.

Clinically Relevant Anatomy

The popliteal fossa is posterior to the knee joint. The fossa contains the popliteal artery and vein, the common peroneal and tibial nerves, and the semimembranosus bursa ( Fig. 167.8 ). The fabella may compress the common peroneal nerve within the popliteal fossa. The knee joint capsule is lined with a synovial membrane that attaches to the articular cartilage and gives rise to a number of bursae, including the suprapatellar, prepatellar, infrapatellar, and semimembranosus bursae. When these bursae become inflamed, they may overproduce synovial fluid, which can become trapped in saclike cysts because of a 1-way valve phenomenon. This occurs commonly in the popliteal fossa.

The gastrocnemius muscle has two heads, with the lateral head finding its origin on the lateral condyle of the femur and the medial head finding its origin from the medial condyle of the femur. The fabella is located in the lateral portion of the gastrocnemius muscle. The gastrocnemius muscle coalesces with the soleus muscle to form a common tendon known as the Achilles tendon, which attaches to the posterior calcaneus.