In this study, fabellae ranged in size from small (just a few pixels) to large (Fig. 1 ). In general, fabellae did not appear to articulate with the lateral femoral condyle. However, the CT scans were acquired postmortem, and soft tissues were severely deformed in most individuals, making it possible that some fabellae would have articulated with the condyle in life but were separated in death. Some large fabellae were still articulated with the posterior surface of the lateral femoral condyle, the most drastic of which was observed in female 005 (Fig. 2 ), which shows a large articulating surface in the femur.

One study investigating prevalence rates in a Japanese population identified a correlation between fabella prevalence rate and age, finding a lower prevalence rate in younger (< 50 years, 31%) than older individuals (> 50 years, 47%) (Kato et al. 2012 ). In our dataset, individuals < 50 years old were no more or less likely to have a fabella than were individuals > 50 years old (younger = 23/94, older = 35/118, χ 2 = 0.7099, P = 4448).

Similarly, individual prevalence rate was not correlated to age (rpbi = 0.0601, t = 0.6143, df = 104, P = 0.5404), or the likelihood of having bilateral (rpbi = −0.0136, t = −0.1384, df = 104, P = 0.8902) or unilateral (rpbi = 0.0973, t = 0.9967, df = 104, P = 0.3212) fabellae. This is not surprising as all individuals in this study were skeletally mature (age 21+ years), and new ossifications do not typically occur during adulthood. Three studies on human foetuses reported the fabella to be common (Jin et al. 2017 ), rare (Minowa et al. 2005 ) or completely absent (Oransky et al. 1989 ) at early stages of development, suggesting fabella initiation time is variable in humans.

Height was not correlated to individual prevalence rate (rpbi = −0.0245, t = −0.2502, df = 104, P = 0.8029), or the likelihood of having bilateral (rpbi = 0.0574, t = 0.5867, df = 104, P = 0.5587) or unilateral (rpbi = −0.106, t = −1.0869, df = 104, P = 0.2796) fabellae (Table 4 ). These results are supported by the substantiated knowledge that the number of ossification centres is not correlated to adult height in humans.

There were no differences between males and females in terms of knee ( f = 52/110, m = 42/102, χ 2 = 0.7970, P = 0.4059) or individual ( f = 32/55, m = 24/51, χ 2 = 1.3138, P = 0.3341) prevalence rates (Table 3 ). Both men and women were equally likely to have bilateral ( f = 20/55, m = 18/51, χ 2 = 0.0132, P = 1) or unilateral ( f = 12/55, m = 6/51, χ 2 = 1.8972, P = 0.2087) fabellae. These results are in agreement with other fabella studies, in which no sex‐based differences in fabella presence/absence were observed (Parsons & Keith, 1897 ; Chew et al. 2014 ; Ortega & Olave, 2018 ). Within unilateral cases, fabellae were equally likely to be present in the right or left knee (right = 8/18, left = 10/18, χ 2 = 0.222, P = 0.8177).

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Fabellae were present in 56/106 individuals (52.83%) and 94/212 knees (44.34%). All fabellae were located in the lateral heads of the gastrocnemius: other than the patella, no other sesamoid bones were observed in the knees. Of the 56 individuals with fabellae, bilateral cases were more prevalent than unilateral ones (bilateral = 38/56, unilateral = 18/56, χ 2 = 7.1429, P = 0.0107). Of the 32 female cases, bilateral cases were as prevalent as unilateral (bilateral = 20/32, unilateral = 12/32, χ 2 = 2, P = 0.2110), but of the 24 male cases, bilateral cases were more prevalent than unilateral ones (bilateral = 18/24, unilateral = 6/24, χ 2 = 6, P = 0.0238). Our prevalence rate of 67.86% falls slightly below the prevalence rate of ~ 80% bilateral cases reported by other studies (Sutro et al. 1935 ; Pritchett, 1984 ), but rates of ~ 50–66% have been reported (Houghton‐Allen, 2001 ; Phukubye & Oyedele, 2011 ; Piyawinijwong et al. 2012 ; Egerci et al. 2017 ). The relatively high prevalence rates of fabellae in this sample were comparable to those reported in other Asian samples (e.g. 28.50–86.69% in Chinese and 15.29–85.85% in Japanese samples; Table 2 ).

Systematic review

Our searches revealed 2631 abstracts on fabella prevalence rates between 1875 and 2018, written in seven languages (English, German, French, Spanish, Italian, Japanese, and Chinese). It should be noted that the authors are not confident they identified all non‐English studies, as it is possible non‐English studies exist without translated titles/abstracts and as such were not detected by our search terms. Also, we are not confident we identified all studies < 75 years old, as we discovered some in bibliographies that did not come up in our scholar.google.co.uk searches.

A total of 185 full‐text articles/conference proceedings were reviewed, 66 of which reported on fabella prevalence rates. Of the 66 studies, five were discarded from further analysis as they did not fit the inclusion criteria. Pancoast (1909) and Hukuda et al. (1983) reported that 67/529 and 11/31 individuals from the USA and Japan, respectively, had fabellae, but these could not be transformed into a knee prevalence rate (Pancoast, 1909; Hukuda et al. 1983). Chew et al. (2014) reported on a prevalence rate of 31.25% (25/80) in ‘Asians’, but we could not determine whether this was an individual or knee rate (Chew et al. 2014). Siina (1931), taken from Table 1 (tabelle I) in Hessen (1946), and Munshi et al. (2003), had a sample sizes of 10 and 8 knees, respectively (Munshi et al. 2003). Finally, three studies claimed to have data on fabella presence/absence, but the data were not present, at least not in the versions of the papers we had access to (Nishimura & Shimizu, 1963; Orzincolo et al. 1987; Osti et al. 2013). Our final analysis included 21 676 knees and represented studies done in 27 countries. It should be noted that Taiwan was part of Japan from 1895 to 1945, at the time of studies of Kitahara (1935) and Hanamuro (1927). According to Hessen (1946), Kitahara's (1935) sample was ‘Formosawilde’, indicating it consisted of the natives of Taiwan. As such, we have classified this sample as being from Taiwan, even though no such political entity existed at the time. According to Hessen (1946), Hanamuro (1927) included individuals from Formosa as well, but classified them as ‘Formosa‐Chinesen’, indicating they were immigrants from mainland China into Taiwan. As such, we classified their sample as being from China. A summary of prevalence rates reported in the literature can be found in Table 2.

We identified one outlier in our dataset (Fig. 3), as the number of fabellae (n = 2) was exceptionally low for that number of knees (n = 62). This is not to say the data are incorrect, only that it is an outlier from the other 56 studies, and thus was excluded from further analyses.

Figure 3 Open in figure viewer PowerPoint y = 0.82350 * x −0.60879; t‐value = 11.149, P = 2.96e‐16), with an intercept that is not statistically different from zero (t‐value = −1.541, P = 0.129). The data for Brazil (Silva et al., 2010 Plot of the natural log of sample size (number of knees) and number of fabellas for the 57 studies considered for this analysis. A Pearson's correlation revealed a statistically significant relationship between the two variables (= 0.82350 * x −0.60879;‐value = 11.149,= 2.96e‐16), with an intercept that is not statistically different from zero (‐value = −1.541,= 0.129). The data for Brazil (Silva et al.,) represent an outlier for this dataset.

There were five studies for which the method remained ‘unknown’, either because the method was not mentioned in the study or we were not able to obtain the original study and identify the method. We assumed Parsons & Keith (1897) used anatomical dissections, as the X‐ray was invented in 1895, making it unlikely they used X‐rays to collect their data. For the four other studies, all imputed datasets yielded consistent results for Sugiyama (1914), Ooi/Oi (1930), and Mikami (1932), classifying the first two as anatomical dissections and the third as X‐ray. According to the imputed data, Pichler (1918) was categorized as X‐ray 15/20 times, MRI 3/20 times, and CT 2/20 times. As MRI and CT scanners were not invented in 1918, we assume Pichler used X‐rays to collect their data.

P slope < 0.01, P intercept < 0.01; Fig. The logistic regression revealed a strong increase in prevalence rates through time (< 0.01,< 0.01; Fig. 4 ). The r code and raw data used to conduct the analysis are available in the Data S1 and Table S1. Assuming median random and fixed effects, the results show that:

Figure 4 Open in figure viewer PowerPoint There is a statistically significant relationship between prevalence rate and time, with people being, on average, nearly 3.5 times more likely to have a fabella in 2018 than in 1918. The confidence intervals are, from widest to narrowest, 99, 95, 75, and 50%. The raw data used to create this figure are available in the Table S2.

Interestingly, recent studies show a higher variance in prevalence rates compared with older studies. This is because there is an increase in maximum prevalence rates, with no real increase in minimum prevalence rates, causing a larger spread of the data. Although different populations were examined before and after 1960, and a genetic component may be involved in population‐related fabella prevalence rates (Sarin et al. 1999), the authors are confident that the observed increase in fabella prevalence rates is not affected by these factors, as described below.

Prevalence rates were reported in four countries both before and after 1960: China, Japan, Korea, and USA. For China and Korea, there was one study before and one study after 1960; in both countries, the more recent study had a higher prevalence rate (Fig. 5). For USA and Japan, there were several studies both before and after 1960, and Pearson's linear regressions revealed positive relationships between prevalence rate and time in both countries. As there were relatively few studies in each country, we chose simpler Pearson's linear regressions in lieu of binomial mixed effect models to provide a visualization of the average change in prevalence rate over time. As random effects were ignored, little faith should be put in the regression equations and their P‐values (Fig. 5). Although it is not possible to hold genetics constant between the older and newer studies, particularly in countries that have large levels of genetic diversity, such as USA, this evidence supports the idea that the increase in prevalence rates is not a by‐product of different populations being used in studies before and after 1960.

Figure 5 Open in figure viewer PowerPoint Four countries (China, Japan, Korea, and USA) had prevalence rates reported both before and after 1960. For China and Korea, there was only one study before and one study after 1960, and the lines connect these studies. For the USA and Japan, there were several, and Pearson's linear regressions were run. There is no statistically significant relationship in the USA (P = 0.0793), but there is a significant relationship in Japan (prevalence rates = 0.5064 * year −947.9; P = 2.25e‐4).

Why would there be an increase in fabella prevalence rate over time? Skeletal phenotypes result from a combination of genetic and environmental factors. Although fabella formation appears to have a genetic component, it is improbable a genetic mutation is responsible for the worldwide increase in prevalence rates; the probability of a mutation occurring in Homo sapiens and spreading throughout the entire species in the past 100 years is an unprecedented and unlikely scenario.

Environmentally, it is possible that the increase in prevalence rates could be due to a hormonal or epigenetic shift. Since the mid‐20th century, there has been a marked increase in plastic usage (Zalasiewicz et al. 2016), and plastics are known to have deleterious effects on growth and development. For example, several chemicals found in plastics are known to disrupt hormonal pathways in vertebrates and other animals. It is therefore possible that plastics could have affected human skeletal growth and development, and be responsible for the increase in fabella prevalence rates. If a hormonal or epigenetic pathway were responsible, it is reasonable to assume the effects would be systematic, influencing all the sesamoid bones in the human body.

To test this idea, we investigated temporal changes in prevalence rates in other sesamoid bones in the human body. We identified two systematic reviews investigating sesamoid bone prevalence rates in the human hand (Yammine, 2014) and foot (Yammine, 2015) with data from 1892 onwards. Using these reviews, we investigated temporal changes in prevalence rate in six sesamoid bones in the hand and four sesamoid bones in the foot.

Due to the low number of studies investigating prevalence rates for these bones (16 across 120 years for the hand and 16 across 121 years for the foot), we ran binomial regressions without random effects using the glm function in R to investigate possible temporal changes. Our analyses revealed there were no temporal changes in sesamoid bone prevalence rates in either the hand or the foot (Tables 5 and 6; Figs 6 and 7). These results imply the increase in fabella prevalence rate does not have a hormonal or epigenetic origin, and the increase in fabella prevalence rate is unique.

Table 5. Results from binomial regressions testing the relationship between time and prevalence rates of six sesamoid bones in the hand P‐value Z‐value Degrees of freedom MCP‐I 0.925 0.094 13 MCP‐II 0.400 −0.842 11 MCP‐III 0.855 −0.183 10 MCP‐IV 0.837 −0.205 10 MCP‐V 0.219 −1.229 11 IP‐I 0.363 −0.91 9

Table 6. Results from binomial regressions testing the relationship between time and prevalence rates of four sesamoid bones in the feet P‐value Z‐value Degrees of freedom MTP‐II 0.939 −0.077 14 MTP‐III 0.101 0.920 14 MTP‐IV 0.937 −0.079 14 MTP‐V 0.986 −0.017 14

Figure 6 Open in figure viewer PowerPoint 2014 n = 16 studies). Unlike with the fabella, there was no correlation between hand sesamoid bone prevalence and time (Table 5). Temporal changes in six sesamoid bone in the hand: the sesamoid bones at the metacarpophalangeal (MCP) joint of the first (MCP‐I), second (MCP‐II), third (MCP‐III), fourth (MCP‐IV), and fifth (MCP‐V) fingers, and at the interphalangeal joint of the first finger (IP‐I). Data from table 2 in Yammine () (= 16 studies). Unlike with the fabella, there was no correlation between hand sesamoid bone prevalence and time (Table 5).

Figure 7 Open in figure viewer PowerPoint 2015 n = 16 studies). Similar to the sesamoid bones in the hand, there was no correlation between foot sesamoid bone prevalence and time (Table 6). Temporal changes in four sesamoid bone in the foot: the sesamoid bones at the metatarsophalangeal (MTP) joint second (MTP‐II), third (MTP‐III), fourth (MTP‐IV), and fifth (MTP‐V) toes. Data from table 6 in Yammine () (= 16 studies). Similar to the sesamoid bones in the hand, there was no correlation between foot sesamoid bone prevalence and time (Table 6).

Sesamoid bones form in areas of high mechanical stimuli, such as pressure, friction or stress (Sarin & Carter, 2000), and act to modify/reduce pressure, friction or stress. It is therefore possible that some change in mechanical loading could have caused an increase in fabella prevalence rate. Differences in loading could be due to differences in kinematics or muscle mass/bone lengths. We do not believe the differences are due to kinematics for the following reasons. First, it is unlikely that all humans, worldwide, have begun to move their lower limbs in a consistently different manner in the last 100 years. Secondly, there appears to be no correlation between magnitude of mechanical loading over one's lifetime and fabella presence in people today, with fabellae being found in both active individuals, such as non‐professional (Dashefsky, 1977; Kuur, 1986) and Olympic level athletes (Zenteno et al. 2010), and inactive individuals, such as foetuses (Minowa et al. 2005; Jin et al. 2017) and the elderly (Laird, 1991; Ando et al. 2017). Finally, unlike in other mammals, the fabella likely offers no significant mechanical advantage in humans, as when excised (common practice to address fabella syndrome), no ill mechanical effects are observed (Weiner & Macnab, 1982; Zenteno et al. 2010; Agathangelidis et al. 2016; Okano et al. 2016). This implies there may be no significant mechanical, evolutionary advantage to having a fabella (Sarin et al. 1999).

It is, however, possible global changes in muscle mass/bone lengths could be responsible. Worldwide, there has been a general increase in dietary quality and nutrition over the last 100 years, which has allowed humans to come much closer to achieving their genetic potential.1 This means people are taller, weigh more, and have bigger muscles today than they did 100 years ago. Increases in tibial length could lead to a larger moment arm acting on the knee and on the tendons crossing it. Coupled with the increased force from a larger gastrocnemius, this could produce the mechanical stimuli necessary to initiate fabella formation and/or ossification. However, these factors do not explain the high prevalence of cartilaginous fabellae in foetuses, or why there was no relationship between presence and height in our sample.

Lastly, it is possible there is no shift in fabella prevalence rate, but the increase in prevalence rates is due to a change in fabella identification, where fabellae that were being previously ignored are now being identified. We believe this is highly unlikely for two reasons. First, there were no other changes in the prevalence of sesamoid bones in the hand or foot, and if there was a change in sesamoid bone identification protocol, it would likely not be isolated to the fabella. Secondly, the inclusion of X‐ray and CT scans to determine prevalence rates in recent studies should lead to a decrease, not an increase, in prevalence rates through time, as cartilaginous fabellae, which may or may not have been included in previous studies, cannot be detected by X‐rays and CT scans.

In this study, we investigated the prevalence rate of the fabella in a Korean population using published CT scans. Our prevalence rate of 52.83 and 44.34% for individuals and knees, respectively, falls within the range of those reported in the literature and shows an increase in fabella prevalence in Koreans over the past 80 years. In addition, we found bilateral fabellae to be more common than unilateral ones, there were no sex differences in prevalence rates, and presence of a fabella was uncorrelated with height and age. We also found a significant increase in fabella prevalence rates through time, but we are unsure why this has occurred and why there has not been an increase in other sesamoid bones in the human body during the same time span.