Fabella is an accessory sesamoid bone located within the substance lateral head of gastrocnemius muscle in the posterior knee in ˜25% of patients (Fig. 153.1). It is covered with hyaline cartilage, which makes it susceptible to the development of chondromalacia or osteoarthritis. Fabella, which is Latin for little bean, is asymptomatic in the vast majority of patients, but it can occasionally become symptomatic due to repeated rubbing of the fabella on the posterolateral femoral condyle. Fabella is often mistaken for a joint mouse or osteophyte, or it is simply identified as a serendipitous finding on imaging of the knee, especially when it is displaced posteriorly by swelling of the joint. Fabella may be either unilateral or bilateral and may be bipartite or tripartite, which can further confuse the diagnosis. The fabella is subject to fracture and dislocation and has the propensity to compress the peroneal nerve.

Patients suffering from symptomatic fabella will complain of pain and tenderness over the posterolateral knee. The patient will often complain of a sensation of having loose gravel in their knee and may note a grating sensation with range of motion of the knee. The pain of fabella worsens with activities that required repeated flexion and extension of the knee. Fabella may coexist with tendonitis 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 in all patients who present with posterior knee pain both to aid in the diagnosis and to rule out occult bony pathology (Fig. 153.2). Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging or ultrasound imaging of the affected area may also confirm the diagnosis and help delineate the presence of other knee bursitis, Baker cyst, internal derangement, calcific tendonitis, synovial disease, and tendinopathy. Radionuclide bone scanning may be useful in identifying stress fractures of the fabella that may have been missed on other imaging modalities.

The popliteal fossa is posterior to the knee joint. The boundaries of the popliteal fossa are the skin, the superficial fascia, the popliteal fascia and the popliteal surface of the femur, the capsule of the knee joint, the oblique popliteal ligament, and the fascia of the popliteus muscle. The fossa contains the popliteal artery and vein, the common peroneal and tibial nerves, and the semimembranosus bursa (Fig. 153.3). 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, which lie between the medial head of the gastrocnemius muscle, the medial femoral epicondyle, and the semimembranosus tendon. The gastrocnemius muscle is located within the posterior compartment of the leg. The lateral head gastrocnemius muscle finds its origin from the lateral condyle of the femur, while the medial head of the gastrocnemius muscle finds its origin from the medial condyle of the femur (Fig. 153.4). Along with the soleus muscle, the gastrocnemius muscle forms a common tendon, which is known as the calcaneal tendon or Achilles tendon that inserts onto the posterior calcaneus. The soleus muscle lies deep to the gastrocnemius muscle.