Michael A. Berthaume, Erica Di Federico, Anthony M. J. Bull

“ 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.
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) . ”

“ 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%) . ”

“ 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. ”

“ For those with fabellae, the percentage of bilateral and unilateral cases was calculated. Pearson's chi-square tests were performed using the chisq.test function (simulate P-value = TRUE, B = 10 000) to investigate the correlation between sex and prevalence rates. The simulate-P-value simulates datasets using Monte Carlo simulations to estimate P-values for chi-square tests. A Pearson's chi-square test was performed with the unilateral data to investigate bilateral asymmetry. Point biserial correlations with ”

“ Sonographic imaging of the posterolateral structures of the knee: findings in human cadavers. Arthroscopy 18, 872–881. Seol P-H, Ha KW, Kim YH, et al. (2016) Effect of radial extracorporeal shock wave therapy in patients with fabella syndrome. Ann Rehabil Med 40, 1124–1128. Silva JG, Chagas CAA, Torres DFM, et al. (2010) Morphological analyisis of the fabella in Brazilians. Int J Morphol 28, 105–110. Stroup DF, Berlin JA, Morton SC, et al. (2000) Meta-analysis of observational studies in epidemiology: a proposal for reporting. ”

“ In the case of the fabella a comparison view of the other knee is unlikely to be helpful. Australas Radiol 45, 318–319. Hukuda S, Mochizuki T, Ogata M, et al. (1983) The pattern of spinal and extraspinal hyperostosis in patients with ossification of the posterior longitudinal ligament and the ligamentum flavum causing myelopathy. Skeletal Radiol 10, 79–85. Ikeuchi H, Nagatsuka Y (1970) Fracture of the fabella, Seikeigeka. Orthop Surg 21, 559–563. Itoman M, Yamamoto M, Okuda T, et al. (1976) Peroneal nerve palsy caused by compression of the fabella a case report. ”

“ 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. ”