Page:Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant - a systematic review.pdf/9

Fabella: more common than it once was, M. A. Berthaume et al. 9 Fig. 4 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.

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.

Fig. 5 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).

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.

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. This © 2019 Anatomical Society