fabella

{{short description|Accessory bone behind the lateral condyle of the femur}}

{{use mdy dates|date=April 2019}}

{{Infobox anatomy

|Name = Fabella

|Latin = os fabella

|Image = Fabella with arrow.jpg

|Caption =

}}

The fabella is a small sesamoid bone found in some mammals embedded in the tendon of the lateral head of the gastrocnemius muscle behind the lateral condyle of the femur. It is an accessory bone, an anatomical variation present in 39% of humans.{{Cite journal |title=Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review |journal=Journal of Anatomy |volume=235 |issue=1 |pages=67–79 |last1=Berthaume |first1=Michael A. |last2=Di Federico |first2=Erica |last3=Bull |first3=Anthony M. J. |doi=10.1111/joa.12994 |pmid=30994938 |pmc=6579948 |date=April 17, 2019 |publisher=Wiley}}{{cite news|url=https://www.bbc.co.uk/news/health-47950258|work=BBC News|title=Sore knee? Maybe you have a fabella|date=19 April 2019}} Rarely, there are two or three of these bones (fabella bi- or tripartita). It can be mistaken for a loose body or osteophyte. The word fabella is a Latin diminutive of faba 'bean'.{{cite journal | last1 = Egerci | first1 = OF | last2 = Kose | first2 = O | last3 = Turan | first3 = A | last4 = Kilicaslan | first4 = OF | last5 = Sekerci | first5 = R | last6 = Keles-Celik | first6 = N | year = 2017 | title = Prevalence and distribution of the fabella: a radiographic study in Turkish subjects | journal = Folia Morphol (Warsz). | volume = 76 | issue = 3| pages = 478–483 | doi = 10.5603/FM.a2016.0080 | doi-access = free }}

In humans, it is more common in men than women, older individuals compared to younger, and there is high regional variation, with fabellae being most common in people living in Asia and Oceania and least common in people living in North America and Africa. Bilateral cases (one per knee) are more common than unilateral ones (one per individual), and within individual cases, fabellae are equally likely to be present in right or left knees. Taken together, these data suggest the ability to form a fabella may be genetically controlled, but fabella ossification may be environmentally controlled.{{Cite journal |title=Human biological variation in sesamoid bone prevalence: the curious case of the fabella |journal=Journal of Anatomy |last1=Berthaume |first1=Michael A. |last2=Bull |first2=Anthony M. J. |doi=10.1111/joa.13091 | pmc=6956444 |date=October 17, 2019 |publisher=Wiley|doi-access=free }}

Although the fabella seems to have disappeared with the evolution of Hominidae, it reappeared in humans sometime after they diverged from chimpanzees. It is unknown whether it reappeared soon after this divergence, 5–7 million years ago, or more recently in human evolution.{{Cite journal|title=Coincident development of sesamoid bones and clues to their evolution |journal=The Anatomical Record |volume=257 |issue=5 |pages=174–180 |last1=Sarin |first1=Vineet K. |last2=Erickson |first2=Gregory M. |last3=Giori |first3=Nicholas J.|last4=Bergman |first4=A. Gabrielle|last5=Carter|first5=Dennis R.|doi=10.1002/(SICI)1097-0185(19991015)257:5<174::AID-AR6>3.0.CO;2-O| date=1999|publisher = Wiley|doi-access=free }} The re-emergence of the lateral fabella may be correlated with "straight-legged, bipedal locomotion" in humans.{{Cite journal |last=Fragoso Vargas |first=Nelly A. |last2=Berthaume |first2=Michael A. |date=2024-09-11 |title=Easy to gain but hard to lose: the evolution of the knee sesamoid bones in Primates—a systematic review and phylogenetic meta-analysis |url=https://royalsocietypublishing.org/doi/10.1098/rspb.2024.0774 |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=291 |issue=2030 |doi=10.1098/rspb.2024.0774 |issn=0962-8452|url-access=subscription }}

"The fabella can lead to posterolateral knee pain either due to cartilage softening (chondromalacia fabellae) or other osteoarthritic changes on its articular surface."{{cite journal |last1=Dannawi |first1=Z. |last2=Khanduja |first2=V. |last3=Vemulapalli |first3=K. |last4=Zammit |first4=J. |last5=El-Zebdeh |first5=M. |title=Arthroscopic Excision of the Fabella – |journal=Journal of Knee Surgery |date=20 January 2010 |volume=20 |issue=4 |pages=299–301 |doi=10.1055/s-0030-1248063 |pmid=17993073}}