Sesamoid Bones: What are they and what do they do?

By Brittany Carrie

A Student’s Perspective

During the first few weeks of my rotation, I was exposed to many new and exciting things that I had not been exposed to in the classroom setting. I observed and helped treat patients who had undergone severe trauma from lacerating tendons to complete amputations, saw different splinting techniques, and learned about some rather unique cases that are often characterized by significant pain with no physical representation on X-ray or other diagnostic imaging. One day, one of our patients who has had significant pain and discomfort in her fingers brought in a remarkable X-ray that, to my eyes, appeared flawless! I could see every bone so vividly; it was unlike any other X-ray image I had seen in a textbook! However, located near the MCP of the patient’s thumb in both hands was a small round bone that I can only describe as looking like a small seed in the hand. I was unsure what this bone was, and my mentor told me it is a sesamoid bone (sesamoid bone thumb)! This led me to further investigation. I wanted to know what sesamoid bones were, what they do and how a sesamoid bone hand therapy is done, BUT through my research, it appears researchers still do not know exactly what they do or why they exist. The following provides some general themes I found through a small literature search.

To begin with, sesamoid bones in hand are small, oval shaped bones that resemble the appearance of sesame seeds (Koo, Song, Sung, Lee, & Jun, 2017; Yammine, 2014; Yammine 2018). They are found within tendons primarily in the hands and feet and are seen radiographically. There are sesamoid bones found in other locations as well including the patella and fabella in the knee and the pisiform on the wrist (Yammine, 2014).

 In most cases, sesamoid bones are found within the MCP joint of the thumb, index, and small fingers; however, the number of sesamoids at each joint varies across gender and population (Koo, Song, Sung, Lee, & Jun, 2017; Yammine, 2014; Yammine 2018). Figure 1 illustrates sesamoid bones located in the MCP and IP joints. In a study done by Yammine (2014), it was found that women often have more sesamoid bones in their hands compared to men. Most people typically have 5 sesamoid bones in each hand—the thumb has 2 in the MCP joint and 1 in the IP joint; the index finger has 1 in the MCP joint; the small finger has 1 in the MCP (Yammine, 2014; Yammine, 2018). However, research has shown that the East Asian and European populations have a higher amount of sesamoid bones in the hand compared to African and Middle Eastern populations, and the Japanese have a higher amount of sesamoid bones in comparison to the Chinese (Koo, Song, Sung, Lee, & Jun, 2017; Yammine, 2014).

Fig 1. Radiographic image of sesamoid bones located at MCP and IP joints. Retrieved from http://www.myhand.com.au/m/index.php?option=com_content&view=article&id=50&Itemid=128&font-size=larger

The presence and function of sesamoid bones still remains unclear; however, there are a few hypotheses that include both functional and phylogenetic ideas. The functional hypothesis presents the idea that sesamoid bones increase tendon leverage to decrease friction and alter the direction of muscle action which increases pinch strength and MCP range of motion (Koo, Song, Sung, Lee, & Jun, 2017). Sesamoids are also believed to assist with joint stability and provide capsular strengthening (Yammine, 2014). The phylogenetic hypothesis describes the presence of sesamoid bones beginning during embryo development (Koo, Song, Sung, Lee, & Jun, 2017; Yammine, 2014). In sum, sesamoid bones are a unique feature to the human body that still remain a mystery as to what they do, why they exist, and why they are more prominent in certain populations.

Koo, B. S., Song, Y., Sung, Y.-K., Lee, S., & Jun, J.-B. (2017). Prevalence and distribution of sesamoid bones in the hand determined using digital tomosynthesis. Clinical Anatomy (New York, N.Y.), 30(5), 608–613. 

Yammine, K. (2014). The prevalence of the sesamoid bones of the hand: A systematic review and meta-analysis. Clinical Anatomy (New York, N.Y.), 27(8), 1291–1303. 

Yammine, K. (2018). The relationship between digit independence and digital sesamoids in humans and a proposal of a new digital sesamoid evolutionary hypothesis. Anatomical Record (Hoboken, N.J.: 2007), 301(6), 1046–1060.