What Are the Sesamoids in the Ball of the Foot?
The sesamoids in the ball of the foot are two small bones (medial and lateral) embedded within the tendons beneath the first metatarsophalangeal (MTP) joint of the great toe (hallux), where they are nearly always present and serve critical functions in weightbearing and biomechanics. 1, 2
Anatomical Location and Structure
The hallucal sesamoids are positioned:
- Beneath the head of the first metatarsal bone at the MTP joint of the big toe
- Embedded within the flexor hallucis brevis tendon
- Consist of two distinct bones: the medial (tibial) sesamoid and lateral (fibular) sesamoid
- Form part of the hallucal-sesamoid complex, which is critical for normal foot function 2
These bones are present in nearly 100% of the population at this location, making them a consistent anatomical feature rather than an accessory bone 3, 4.
Function and Clinical Significance
The sesamoids serve several biomechanical roles:
- Weightbearing: They bear significant load during walking and running
- Mechanical advantage: They increase the leverage of the flexor hallucis brevis tendon
- Protection: They protect the flexor tendon from excessive pressure
Development and Ossification
Ossification timing varies by population but generally:
- Begins at age 8-9 years in most populations
- Completes by age 10-14 years 4
- Earlier ossification in females compared to males
Normal Variants
Bipartite sesamoids (divided into two or more parts) occur as a normal variant:
- Medial sesamoid partition: approximately 10.7% of the population
- Lateral sesamoid partition: approximately 1.3% of the population
- Bipartism (two parts) is most common at 92%, followed by tripartism (7.5%) 3
Important Clinical Pitfall
Distinguishing a bipartite sesamoid from a fracture can be challenging on radiographs alone. Key differentiating features include:
- Bipartite sesamoids have smooth, well-corticated margins
- Fractures show irregular, sharp edges
- When uncertain on radiographs, negative bone scintigraphy suggests bipartition rather than fracture 1
- CT imaging provides more precision than radiography for this distinction 1
Common Pathologies
The sesamoids are susceptible to multiple conditions that can cause pain in the ball of the foot:
- Sesamoiditis (inflammation)
- Stress fractures and acute fractures
- Avascular necrosis
- Osteoarthritis
- Dislocation (associated with turf toe injuries)
- Inflammatory or deposition arthropathies 1, 2
Imaging Approach
When evaluating sesamoid pathology:
- Radiographs are the initial study, including sesamoid axial views 1
- MRI is most useful for diagnosing fractures, stress changes, and avascular necrosis 1
- CT helps confirm stress fractures and distinguish them from bipartite sesamoids with greater precision than radiography 1
- Bone scintigraphy can detect pathology not evident on radiographs, though it has low anatomic resolution 1
Lesser Metatarsal Sesamoids
While the hallucal sesamoids are nearly universal, sesamoids can also occur at the lesser MTP joints (2nd through 5th toes), though much less commonly:
- 5th MTP joint: 13% prevalence (most common of the lesser toes)
- 2nd MTP joint: 1.9% prevalence
- 3rd and 4th MTP joints: 0.32-0.9% prevalence 3
These are considered accessory bones rather than constant anatomical features and show significant variation by ancestry and population 3, 5, 4.