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Fabella is a small bone, the so-called the navicular bone, located behind the knee and located in the calf muscle or tendon. Fabella is an extra bone, an anatomical variety that occurs in 39% of people. It tends to be more common in men than in women, and more common in older than younger people. Fabella can lead to posterolateral knee pain due to softening of the cartilage (chondromalacia fabellae) or other degenerative changes in its articular surface.
Fabella – anatomy
Fabella is the so-called accessory bone, i.e. it is not anatomically required for the functioning of the skeleton. It is assumed that its prevalence in the world’s population ranges between 10% and 30%. Interestingly, in 1918, an average of 11,22% of people in the world had a fabelle. Bone is the result of mechanical stress in the calf muscle tendon, which causes ossification within the tendon, causing it to harden. Fabella can be bone or cartilaginous due to the mechanical stress on the tendon.
Fabella consists of a compact bone with a spongy bone core, while a cartilaginous fibrous capsule has the tough fibrous tissue typical of tendons. The fabella is embedded in the lateral part of the gastrocnemius muscle or the tendon that connects the muscle. Fabella can only occur in one or both legs, but this is rarely seen. Another variation in the development of this bone is that it may be behind one of the femoral condyles, which is the rounded portion of the femur on either side of the knee joint.
Some sesamines, also known as heterotopic bones or visceral bones, are natural and are found in tendons that cut across prominent joint surfaces. One of the largest of these bones is the kneecap, which protects the knee joint during strong or repeated knee bending actions. The heterotopic bones under the big toe play a similarly important role in protecting the toe joint during repetitive situations such as running or jumping. Overuse of the sesamoid (which is very common in dancers) can cause inflammation of the sesamoid in the muscle tissue surrounding the heterotopic bones around the toes.
Fabella – causes
The presence of fabella may have genetic and / or environmental causes. It is assumed with high probability that genetics are to a great extent responsible for the presence of fabella. This is due to the frequency of the appearance of this bone both within and between genetically distinct populations.
The average person is better fed today, which means that people today are taller and heavier. This likely lengthened: the length of the tibia, creating more torque or rotational force around the knee, and the size of the gastrocnemius muscle, increasing the strength of the muscles. All of this can cause stronger mechanical stimuli in the tendon of the lateral head of the gastrocnemius muscle.
It is worth adding that the sesamoid (Latin. sesamoid bone) tend to form in response to mechanical stimuli, these increased stimuli can signal fibril ossification. Not every person has a fabelle, which may be due to genetics, as there may be a genetic component that controls the ability to form a fabelle. To prove this claim, look at the prevalence of this bone in the worldwide population.
Anatomical studies suggest that the presence of fabella is higher in the Asian population. This may be due to different kneeling and squatting habits and therefore increased stretching forces. Another study, published in 2003 in the Journal of Orthopedic & Sports Physical Therapy, suggests a relationship between the occurrence of fabella and body type. Ectomorphic body types are more prone to developing neuropathy than endomorphic types. This is potentially due to less isolation by subcutaneous adipose tissue in people with ectomorphic somatotypes.
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Fabella – functions
The function of fabella in humans is not fully known. In mammals, such as monkeys, it appears to act like the kneecap, increasing the mechanical advantage of the muscles. For people who walk on two straight legs, this does not have to happen as the fabella may stop connecting to the femoral condyle when the knee bends. The presence of fabella in humans is interesting because it is rare in great apes compared to other primates.
Can fabella cause pain?
The presence of fabella is believed to be associated with diseases of the knee joint (e.g. osteoarthritis, nerve paralysis, popliteal entrapment syndrome). Moreover, fabella can cause pain itself (displacement, fracture) and can cause generalized discomfort known as fabella syndrome.
Another problem with fabella occurs during total knee arthroplasty because the knee does not have a dimple (depression) that is sometimes located on the back of the lateral femoral condyle that stabilizes the fabella. While problems can be handled conservatively, fabella removal procedures are common, which appear to have no long-term side effects.
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Fabella and the fabella team
Fabella syndrome is a rare but significant cause of pain due to mechanical irritation of the posterolateral tissues of the knee joint. Symptoms of fabella syndrome include posterolateral knee pain and a ‘clicking’ feeling as the knee bends. The aforementioned study from the Journal of Orthopedic & Sports Physical Therapy suggests a link between the presence of fabella and an increased risk of osteoarthritis of the knee.
The symptoms of fabella syndrome include pain in the posterolateral area, which hurts even more when the knee is fully extended, and local tenderness may occur due to pressure on the femoral condyle. Most of these symptoms are the result of the repeated rubbing of the fabella against the posterolateral femoral condyle.
Fabella diagnosis is based on palpation, magnetic resonance imaging (MRI) and ultrasound. Due to the irregular structure of the fabella, it can sometimes be mistaken for a foreign body. On MRI, fabella can appear as a posterior femoral condyle abnormality, which can sometimes be interpreted as an osteochondral defect or a loose body, but which can be easily distinguished from the ossicles because the fabella moves away from the lateral femoral condyle when the knee is bent.
Treatment for fabella syndrome is conservative at first, but if symptoms persist, surgery may be needed. Nonsurgical treatments include steroid injections, immobilization with splints and dressings, temporary restriction of activity, manual therapy, and painkillers. Research suggests relieving symptoms with physical therapy. Manual therapy may be a temporary solution.
Increasingly, extracorporeal shock wave therapy (ESWT) is also used, which consists in applying three thousand shock waves with a frequency of 12 Hz to the painful area. This procedure may be performed at two-week intervals for a total of one to four times. According to a study published in 2018 in the Cureus Journal of Medical Science, patients noticed a sudden drop in pain intensity after just one series of ESWT. The decreases in pain intensity scale were maintained during the two-month follow-up clinical visit.
Surgical treatment is used when inoperative treatment does not bring the expected results. Fabella excision can be successfully performed both as an open procedure and as an arthroscopic procedure. The operation to remove fabella is called a fabellectomy.
According to a 2017 study published in Arthroscopy techniques, there are no restrictions on movement after a fabelle excision, and flexion / extension exercises are started immediately after surgery to avoid loss of movement. The patient will walk with orthopedic crutches until he can move without limping. Usually crutches are needed in the first 2 weeks after surgery. However, the use of crutches is at the discretion of the patient. Exercises for the ankle, simple leg raises and quadriceps exercises are initiated immediately after surgery when tolerated, with the frequency gradually increasing to 3 to 5 times a day. Returning to normal everyday life is possible after about 3 to 4 months, when the capsule and soft tissues have healed sufficiently.
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