Fabella syndrome

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Original Editors

Top Contributors - Vidya Acharya, Admin, Claire Knott, Chelsea Mclene, Rachael Lowe, Florence Brachotte, Charlotte Moortgat, WikiSysop and Kim Jackson  

Search Strategy

My first step towards writing this text was consulting the VUB library website. I used Web of Knowledge and Pubmed to type in following key-words : fabella, syndrome, posterior knee pain, fabellofibular, therapy. The most succesfull combination of key-words was fabella syndrome.

Definition/Description

Syndrome is a term given to a group of symptoms that collectively indicate or characterize a disease, psychological disorder, or other abnormal condition. In the case of a fabella syndrome it indicates an abnormal condition.
The fabella syndrome is caused by a little bone posterolateral in the knee joint. The syndrome should be considered when tenderness is found over the posterior aspect of the lateral femoral condyle.

Clinically Relevant Anatomy

The fabella literally means 'litte bean' and is a sesamoid bone that can be found in the anterior gliding surface of the lateral head of the gastrocnemius muscle. In some cases it may be big, irregular of shape and have an abnormal density. Only people with a fabella also have a fabellafibular ligament that originates on the fabella and inserts onto the proximal end of the fibula. This ligament may take part in the stability of the knee joint. The bone may also be present close to the common fibular nerve.

Epidemiology /Etiology

The fabella can be found in 10 to 30 percent of the population and if it is present there is a 50 percent chance that it is bilaterally. This syndrome occurs at all ages but mostly in the early adolescence. Some of the symptoms are pain in the posterolateral region which hurts even more by full knee extension and because of compression against the femoral condyle there can be local tenderness. Most of these symptoms are the result of repetitive friction of the fabella over the posterolateral femoral condyle.

Microscopic examination of histological samples from excised fabellae showed two visible trends.
The first was a bony fabellae which showed characteristics similar to that of a typical long bone. Compact bone surrounded a core of spongy bone, which had a bone marrow or medullary cavity. Thorough examination at high magnification showed that this trend has adipocytes and connective tissue. There was also found fibrocartilage at the periphery and collagen fibers. The second trend existed out of eosinophilic hyaline cartilage with flattened chondrocytes within lacunae. This was identified superficial to the subchondral bone.

Characteristics/Clinical Presentation

In most cases the fabella does not hurt. If it does hurt, it is called fabella syndrome. It is recognized by a sharp pain, local tenderness, and intensification of pain in the area of the fabella by full extension of the knee. It can also cause pain during knee flexion, cross-legged sitting and athletic activities. When the fabella is too close to the common fibular nerve, it can be the reason of tinglings, foot drop and steppage gait (= while lifting a leg during walking the foot will hang with the toes pointing down. The person will have to lift the leg higher so that the toes don’t scrape the floor.)

Differential Diagnosis

Diagnosis of postero-lateral knee pain and dysfunction can be difficult but is important for a good intervention. Baker’s cyst, lateral ligamentous instability, meniscal tears, and proximal tibiofibular joint hypomobility should also be considered. There are other multiple anatomical structures that could be the source including postero-lateral corner structures, the ilio-tibial band and the biceps femoris tendon.

Irregular appearance of fabella may be confused with foreign body. On MRI it can appear like a posterior abnormality of the femoral condyle which sometimes can be interpreted as osteochondral defect or loose body. However this loose body is easy to differentiate from the fabella because the fabella moves away from the lateral femoral condyle during knee flexion.

Diagnostic Procedures

The fabella bone can be detected by palpation, Magnetic resonance imaging (MRI) and ultrasound.

Outcome Measures

add links to outcome measures here (also see Outcome Measures Database)

Examination

The examination can be divided in 3 parts. The inspection, an active investigation and a passive investigation. Possible findings of the inspection can be swelling and redness. This is not always the case depending on whether there is inflammation or not.

Exercises of the active investigation which are most likely to be sensitive are deep squatting, full extension of the knee, stair ascent(the hurting knee ascents first) and stair descent(the good knee descents first).

Passive investigation includes postero-lateral palpation of the knee, Mc Murray test to confirm that there are no meniscal tears and other tests to exclude other differential diagnosis.

Medical Management

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Physical Therapy Management

Manual therapy may be a temporary solution. The use of mobilization of the fabella and the soft tissue of the lateral gastrocnemius followed by medial, lateral and inferior glides of the fabella causes immediate reduction in posterolateral knee pain. It increases tolerance for activities involving knee flexion, extension and rotation. Also the limited active knee flexion can be improved to a full range of motion of the knee joint. But the symptoms will come back.

Key Research

add links and reviews of high quality evidence here (case studies should be added on new pages using the case study template)

Resources

databases searched : Pubmed, Web of Knowledge, PEDro

Clinical Bottom Line

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Recent Related Research (from Pubmed)

see tutorial on Adding PubMed Feed

References

Zipple, JT; Hammer RL; Loubert PV. Treatment of fabella syndrome with manual therapy : A case report. Journal of orthopaedic & sports physical therapy, jan 2003, 33 : 33-39.
(Level of evidence : 3B)

Phukubye, P ; Oyedele, O. The incidence and structure of the fabella in south african cadaver sample. Clinical anatomy, jan 2011, 24(1) : 84-90
(Level of evidence : 2C)

Garcia-German, D ; Sanchez-Guttierez, S ; Bueno, A ; Carballo, F. Intra-articular osteoid osteoma simulating a painful fabella syndrome. Knee, Aug 2010, 17 (4) : 310-312.
(Level of evidence : 3A)

Roberston, A; Jones, SCE; Paes, R; Chakrabarty, G. The fabella : a forgotten source of knee pain? Knee, jun 2004 , 11 (3) : 243-245.
(Level of evidence : 3A)