Page:Fabella prevalence rate increases over 150 years, and rates of other sesamoid bones remain constant - a systematic review.pdf/1

 J. Anat. (2019)

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

Department of Bioengineering, Imperial College London, London, UK

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. Key words: fabella; Korea; prevalence rate; sesamoid bone.

Correspondence ''Michael A. Berthaume, Department of Bioengineering, Imperial College London, London SW7 2AZ, UK. E: m.berthaume@imperial.ac.uk'' Accepted for publication 7 March 2019

Introduction

The fabella (Latin for 'little bean') is a sesamoid bone located in the knee joint behind the lateral femoral condyle. Embedded in the tendon of the lateral head of the gastrocnemius muscle, it is stabilized by the fabelloﬁbular ligament, connecting the distal insertion of the fabella to the ﬁbular head (Minowa et al. 2004; Piyawinijwong et al. 2012; Driessen et al. 2014; Hauser et al. 2015; Kurtoğlu et al. 2015) and the posterior capsule of the knee. In rare instances, it serves as an additional origin for a muscle bundle of the popliteal muscle (Duc et al. 2004). Fabella prevalence in humans ranges from 3 to 87% (Silva et al. 2010; Zeng et al. 2012), making it a normal variant in human anatomy. The highest rates reported are in Asians and Australians, and the lowest rates in Europeans and South Americans (Minowa et al. 2004; Silva et al. 2010; Zeng et al. 2012; Hauser et al. 2015). Although its exact function is unknown, the fabella is more common in non-human mammals (Pearson & Davin, 1921; Sarin et al. 1999), which has prompted functional and evolutionary debates about the role of the fabella in locomotion (Sarin et al. 1999; Jin et al. 2017).

Most studies reporting on prevalence rates in humans have determined the presence of the fabella through surgeries/dissections (Agathangelidis et al. 2016), X-rays (Pancoast, 1909), computed tomography (CT) scans (Hauser et al. 2015), and magnetic resonance imaging (MRI) scans (Hedderwick et al. 2017). Ultrasound (Sekiya et al. 2002) and PET-CT (Usmani et al. 2017) have been used to examine the fabella, but no studies have employed these methods to calculate fabella prevalence rate. Problems in calculating prevalence rates can occur depending on the method © 2019 Anatomical Society

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