employed. For example, fabellae are sometimes so small they are difficult to detect on MRI scans, particularly if the knee is not positioned correctly (Yu et al. 1996; Ehara, 2014). Additionally, as the fabella is occasionally cartilaginous (Jin et al. 2017), its presence may not always be detected by X-rays or CT scans. For example, a recent study on a Chinese population reported that 57.9% of the cartilaginous fabella were not visible on radiographs (Zeng
et al. 2012). This highlights an issue with comparing prevalence rates between studies, as some consider only osseous
fabellae, whereas others also consider cartilaginous ones.
Comparing prevalence rates is further complicated as several studies do not specify whether the prevalence rates included cartilaginous with more recent ones (Hessen, 1946). This is true regardless of whether the more recent studies calculate their prevalence rates using bony or bony and cartilaginous fabellae.
Recent studies often rely on hospital archives of previously gathered X-rays, CT scans or MRIs as a cost-effective way of collecting data. Unfortunately, this has the potential to lead to a skewed sample, as imaging is initially done to investigate knee problems, and the presence of the fabella has been associated with several knee ailments. These include common peroneal neuropathy (Mangieri, 1973; Patel et al. 2013; Cesmebasi et al. 2016), chondromalacia (Goldenberg & Wild, 1952; Grisolia & Bartels, 1959; Robertson et al. 2004), osteoarthritis (Wolf & Bryk, 1959; Hagihara et al. 1994), popliteal artery entrapment syndrome (Ando et al. 2017), nerve palsy (Itoman et al. 1976; Takebe & Hirocardhata, 1981; Kubota et al. 1986; Tabira et al. 2012; De et al. 2017), and rheumatoid arthritis (Uchino et al. 1992). The fabella can also cause pain through dislocation (Frey et al. 1987; Franceschi et al. 2007), fracture (Sagel, 1932; Levowitz & Kletschka, 1955; Ikeuchi & Nagatsuka, 1970; Dashefsky, 1977; Woo, 1988; Marks et al. 1998; Theodorou et al. 2005; Tang et al. 2010; Heideman et al. 2011; Barreto et al. 2012; Cherrad et al. 2015; Kwee et al. 2016; Zhou et al. 2017), and generalized discomfort, a condition known as fabella syndrome (Weiner et al. 1977; Weiner & Macnab, 1982; Erichsen, 1997; Zipple et al. 2003; Segal et al. 2004; Dannawi et al. 2010; Seol et al. 2016; Kim et al. 2018; Rankin et al. 2018). As with any other joint, the interaction between the fabella and the femur can cause degenerative joint diseases, such as fabella-femoral osteoarthritis (Urata et al. 2015).
Finally, the fabella can be problematic in cases of total knee arthroplasty (Larson & Becker, 1993; Wang, 1995; Erichsen, 1997; Segal et al. 2004; Theodorou et al. 2005; Jung et al. 2007; Hou, 2016; Kwee et al. 2016; Okano et al. 2016). The absence of an articulating groove in the back of the lateral femoral condyle, which serves to stabilize the fabella and is present in some anatomical variants (e.g. Chew et al. 2014), leads to a fabella medio-lateral instability, causing it to painfully ‘snap’ over the replacement condyle. The reason for this pain is not known. Hou (2016) recently investigated the effects of the fabella on posterolateral pain and palsy of common peroneal nerve following total knee arthroplasty. During trials, fabellae were excised from some patients but left in others. Post-surgery, posterolateral pain and palsy of common peroneal nerve were only observed in patients who still had fabellae. Accordingly, Hou recommended removing the fabella when knee replacement surgery is performed.
Here, we present the prevalence rate of the fabella in a population of Koreans using a randomized previously gathered dataset. As factors related to sex and length/speed of growth and development are correlated to bone formation (i.e. men are generally taller, and tall people have longer bones that are generally mechanically loaded more heavily), we investigate the effects of sex, age, and height on fabella prevalence rate. In addition, as other studies have reported higher rates for bilateral fabellae than for unilateral ones (Phukubye & Oyedele, 2011; Piyawinijwong et al. 2012; Egerci et al. 2017), we investigate whether bilateral or unilateral fabellae are more common.
To contextualize our prevalence rate results, we performed a systematic review to determine how Koreans compare with other populations, investigated possible changes in prevalence rate through time, and compared this with prevalence rates of other sesamoid bones.
Materials and methods
Prevalence rate
Sample
A randomized sample of previously collected CT scans, totalling 212 knees from 106 individuals (f = 55, m = 51), were investigated for the presence of the fabella (Dai et al. 2012). Scans were gathered as part of a larger project to examine human anatomy, and represent a randomized sample of Koreans. Ages of the individuals ranged from 21 to 60 years (mean/median = 52.45/55 years) and heights from 146 to 178 cm (mean/median = 160.65/160 cm; Table 1). The resolution of the scans ranged from 0.8220 x 0.8220 mm2 to 0.9626 x 0.9626 mm2 with a slice thickness of 1.0000 mm.
CT scans prohibit the distinction between highly dense, cartilaginous and ossified fabellae, and detection of lower density, cartilaginous fabellae. Accordingly, we made no distinction between cartilaginous and bony fabellae. As it is likely that many cartilaginous fabellae are missed by CT scans, this reported prevalence rate represents a minimum rate for this sample.
Data collection
We recorded the presence/absence of the fabella on both right and left knees. Although the fabella is located behind the lateral condyle of the femur, the rest of the knee was inspected for sesamoid bones as (1) fabella presence is often correlated with the presence of other sesamoid bones (Sarin et al. 1999) and (2) some studies have reported fabellae in the medial head of the gastrocnemius (Kawashima et al. 2007; Zeng et al. 2012). Due to the resolution of the CT scans and the miniscule size of some of the fabellae (Fig. 1), fabella dimensions were not measured.
