The Relationship Between Digit Independence and Digital Sesamoids in Humans and a Proposal of a New Digital Sesamoid Evolutionary Hypothesis

Abstract

Digital sesamoids are found in the metapodial-phalangeal joints of most mammals and quadrupedal tetrapods, yet their functional significance is still unclear. During primate evolution, a slight decline in their frequency has been associated with brachiation in gibbons, followed by a quasi-complete absence in orangutans then a slight resurgence occurred in gorillas and chimpanzees. Simultaneously, forearm muscles showed a progressive division in hominoid evolution towards a more "individualistic" musculature yielding more mobility and independence to some fingers. In humans, sesamoids are consistently observed in thumbs and big toes and frequently in other hypermobile digits such as the index and little fingers. Using a simple mathematical equation, this paper attempted to quantify a presumed association between hypermobile fingers and sesamoid frequency and distribution in humans. To this, an anatomic definition of digital independence has been formulated which includes three variables; (1) number and (2) frequency of independent flexor/extensor forearm muscles destined to a single finger, (3) and number of free/absent webspace. Results of previous meta-analyses and means of big sample studies were used to evaluate the frequency of such muscles. The expected values obtained via this model were found to be very close to the observed (published) values of the ossified sesamoids in human hands, and that in terms of frequency and distribution. The findings in humans showed a quasi-linear association between the degree of mobility and sesamoid frequency. The more the number of independent muscles destined to a finger, the more its metacarpo-phalangeal joint is likely to bear sesamoids. Based on our results and on a new analysis of primates' forearm/hand muscles and sesamoid evolution, a new hypothesis is proposed to answer two questions; the evolution of digital sesamoid frequency in primates and its sesamoid distribution in human digits. It claims that the number/frequency of independent forearm/hand muscles, and particularly the independent extensors, is likely to be a major factor in sesamoid reversion in hominoids. The argument is based on the link between the metatarsal break induced by the digitigrade locomotion observed in quadrupedal mammals and tetrapods and the amount of extra-extension of the metacarpo-phalangeal joints conferred by the individualization of some forearm extensors. The same rationale yielded similar conclusions when applied to both the hand and foot. The manipulative function of the hand and the plantigrade locomotion of the foot required such extra-extension in specific digits and consequently, a higher frequency of digital sesamoids was associated with these two different functions. The new evolutionary analysis suggests evolutionary pathways for both, the sesamoids of the hands and feet, and speculates that muscle individualization would have induced a very slow re-acquisition of digital sesamoids when compared to their rapid decline after brachiation. It is the first hypothesis offering ...

Sesamoids in tetrapods: the origin of new skeletal morphologies

“…In some cases, evolutionary studies were performed (e.g. Sarin et al, ; Sánchez‐Villagra & Menke, ; Abella et al, ; Samuels et al, ; Amador et al, ; Yammine, ), particularly for the radial sesamoid. During the last decade, molecular and developmental approaches have refreshed and clarified former hypotheses regarding the origin of sesamoids (e.g.…”

“…However, the enormous amount of research published about them over more than a century tells another story. Sesamoids have been intensively studied in the context of clinical research, producing an extensive literature (Gruber, ; Pfitzner, ; Müller, ; Pires de Lima, ; Skillern, ; Bizarro, ; Gunn, ; Kewenter, ; Sindberg, ; Stener, ; Scapinelli, , , ; Ellis et al, ; Helal, ; Weiner & Macnab, ; Wood, ; Hansen & Peterson, ; Goldberg & Nathan, ; Le Minor, , , , ; McBryde Jr & Anderson, ; Gibeault et al, ; Bianchi et al, ; Lennox et al, ; Bareither, Muehleman & Feldman, ; Richardson, ; Duncan & Dahm, ; Mellado et al, ; Feller et al, ; Griffith et al, ; Bakerly & Saleh, ; Seki, Hoshino & Kuroda, ; Chen et al, ; Hauser et al, ; DeCamp & Schaefer, ; Tzatzairis et al, ; Vaz & Trippia, ; Yammine, ; Dalip, Iwanaga & Oskouian, ; Berthaume, Di Federico & Bull, , among many others). This represents an enormous and continued interest over time.…”

Cyamella (a popliteal sesamoid bone) prevalence: A systematic review, meta‐analysis, and proposed classification system

“…They are commonly related to tendons and ligaments, have a genetic basis and, once they are formed, epigenetic stimuli drive their growth and development to the acquisition of their definitive tissue composition, which can be diverse, for example, cartilage, fibrocartilage, or bone. (Abdala et al, 2019, p. 17)Historically, there have been two competing hypotheses for the origin of sesamoid bones., but additional hypotheses exist for specific sesamoid bones, such as the bones in the digits in humans (Yammine, 2018). The stress intensive or mechanical differentiation theory, named the epigenetic theory in Abdala et al (2019), hypothesizes that sesamoid bone formation is controlled by mechanical stimuli (Abdala et al, 2019; Eyal et al, 2015; Parsons, 1908; Samuels et al, 2017; Sarin & Carter, 2000).…”

Sesamoid bones of the hand: A multicenter study

“…Two main hypotheses have been described for the SB development: functional and phylogenetic. [ 2 - 4 ] In 2018, Yammine[ 5 ] added a new hyperextension hypothesis to these two hypotheses and explained the frequency of sesamoids in human digits. There is still no consensus concerning the function of the sesamoids.…”