Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant: a systematic review

Abstract

The fabella is a sesamoid bone located behind the lateral femoral condyle. It is common in non-human mammals, but the prevalence rates in humans vary from 3 to 87%. Here, we calculate the prevalence of the fabella in a Korean population and investigate possible temporal shifts in prevalence rate. A total of 52.83% of our individuals and 44.34% of our knees had fabellae detectable by computed tomography scanning. Men and women were equally likely to have a fabella, and bilateral cases (67.86%) were more common than unilateral ones (32.14%). Fabella presence was not correlated with height or age, although our sample did not include skeletally immature individuals. Our systematic review yielded 58 studies on fabella prevalence rate from 1875-2018 which met our inclusion criteria, one of which was an outlier. Intriguingly, a Bayesian mixed effects generalized linear model revealed a temporal shift in prevalence rates, with the median prevalence rate in 2000 (31.00%) being ~ 3.5 times higher than that in 1900 (7.64%). In all four countries with studies before and after 1960, higher rates were always found after 1960. Using data from two other systematic reviews, we found no increase in prevalence rates of 10 other sesamoid bones in the human body, indicating that the increase in fabella prevalence rate is unique. Fabella presence/absence is due to a combination of genetic and environmental factors: as the prevalence rates of other sesamoid bones have not changed in the last 100 years, we postulate the increase in fabella prevalence rate is due to an environmental factor. Namely, the global increase in human height and weight (due to improved nutrition) may have increased human tibial length and muscle mass. 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.

Keywords: Korea; fabella; prevalence rate; sesamoid bone.

Figure 1

Large (left), medium (centre), and small (right) ossified fabellas in the right knees of three female subjects.

Figure 2

Lateral (left) and superior (right) views of the fabella (white arrow).

Figure 3

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 (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) represent an outlier for this dataset.

Figure 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.

Figure 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).

Figure 6

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 (2014) (n = 16 studies). Unlike with the fabella, there was no correlation between hand sesamoid bone prevalence and time (Table 5).

Figure 7

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 (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).