TNSSThere are plenty of sesamoid bones in the bod

2O it? What is that again? There are plenty of sesamoid bones in the body, 42 of them to be precise. Sesamoids are imbedded within tendons and help improve the mechanical motion of tendons, often by decreasing friction. Most sesamoids are ossified and covered by c


artilage, which makes them visible on radiographs. However, some sesamoids are not ossified/calcified and therefore not easily seen on imaging studies, including MRI. It’s important to know they exist and their anatomical location. The posterior tibialis tendon (PTT) often has a fibrocartilaginous nodule distal to the medial malleolus, which causes focal thickening of the tendon prior to its division. In MRI, we often see a very thickened distal PTT and may be confused for tendinopathy/inflammation. Furthermore, some variability in signal intensity may be seen within the tendon which is related to the composition of the fibrocartilaginous nodule. If you encounter a markedly thickened PTT distal to the medial malleolus with some intermediate signal intensity, it’s probably just a fibrocartilaginous nodule. However, if there is associated peritendinous edema (high T2 signal intensity), then inflammation is likely (as we see in this case). This case shows a PTT intratendinous nodule with superimposed inflammation and associated sprain of the Spring ligament.