The extra bones in the foot result from the formation of extra ossification nuclei during the developmental period. They can only occur in one or both feet. In most cases, the accessory ankles are accidentally recognized on X-rays and their presence is asymptomatic. In some people, however, additional ankles in the foot can cause pain due to their fracture and / or irritation of the surrounding soft tissues. The most common accessory ankles of the foot are: the triangular bone (os trigonum), the additional navicular bone, the additional fibula, the vesalius bone (os vesalianum), and the additional sub-sagittal bone. Some ankles run along the tendons of the muscles and function as a sesamoid. In post-traumatic conditions, it is very important to distinguish between the naturally occurring sesamoid and the tendon rupture that occurs with the detachment of a bone fragment (avulsion fracture). The remaining additional ankles lying outside the tendons may also make it difficult to properly diagnose a fracture due to their proximity to the bone, a fragment of which is usually displaced. A good radiologist can differentiate both conditions and make a diagnosis that affects the choice of the right treatment.
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Triangular ankle (os trigonum)
The triangular bone is present in about 7-25% of people. Its presence can be confirmed on radiographs in the lateral view - it is visible as an oval or triangular ankle often connected by a cartilage with a posterolateral part of the talus. The triangular bone may cause pain symptoms depending on its size, location and mobility. Pain occurs during the maximum plantar flexion of the foot and is a symptom of the so-called posterior ankle conflict. The additional triangular ankle should be differentiated from the fracture of the lateral process of the talus, the so-called Shepherd's fracture. The triangular ankle itself is very rarely broken.
Additional navicular bone
The additional navicular bone lies on the anteromedial side of the foot. There are three types of it:
I - round or oval ankle located on the course of the tendon of the posterior tibia muscle, it can be separated from the actual navicular bone with a 5 mm long tendon,
II - an ankle in the shape of a pyramid or a heart, located 1-2 mm from the navicular bone and connected with it by a cartilage,
III - excessively pronounced tuberosity of the navicular bone - treated as type II fusion of bones.
Symptoms of the accessory navicular bone are often associated with the squamous valgus foot. The medial slope of the hindfoot emphasizes the medial side of the tarsus, leading to the conflict of the extra ankle with the footwear. Pain most often results from damage to the synchondrosis in type II of the additional navicular bone. Relief in ailments can be brought by wearing insoles correcting excessive valgus.
Additional fibula
Dodatkowa kość strzałkowa jest usytuowana na bocznym brzegu stopy w okolicy kości sześciennej na przebiegu ścięgna mięśnia strzałkowego długiego – pełni rolę trzeszczki w miejscu, gdzie ścięgno przechodzi na podeszwową stronę stopy. U większości ludzi trzeszczka ta posiada budowę chrzęstną, u niektórych jednak występuje jako dodatkowa kość strzałkowa. Jest ona najlepiej widoczna na skośnych radiogramach stopy jako owalna lub okrągła kostka. U 30% ludzi może przybierać postać trzeszczki dwudzielnej czyli podzielonej na dwie części. W 60% przypadków występuje w formie kostnej w obu stopach. Dodatkowa kość strzałkowa może wywoływać dolegliwości bólowe z powodu jej złamania lub usidlenia w okolicy guzka strzałkowego kości piętowej. Przemieszczenie dodatkowej kości strzałkowej widoczne na rtg może wskazywać na przerwanie ciągłości ścięgna mięśnia strzałkowego długiego. Przewlekłe przeciążenia można próbować leczyć rehabilitacją, natomiast ostre urazy przebiegające z utratą funkcji mięśnia strzałkowego należy zaopatrywać chirurgicznie.
Vesalius bone
The Vesalius bone is located within the tendon of the short fibula, in the vicinity of the attachment of this muscle to the tuberosity of the fifth metatarsal bone. Pain symptoms resulting from the presence of an additional ankle may mimic a fracture at the base of the fifth metatarsal bone.
Sub-sagittal bone
The sub-sagittal bone is located around the anterior talofibular ligament (ATFL) on the anterolateral side of the ankle joint. Torsion injuries and lateral instability of the ankle joint can cause pain due to the displacement of the additional sub-sagittal bone. The presence of the ankle significantly exacerbates the symptoms of anterolateral conflict, leading to painful restriction of mobility, especially dorsiflexion and foot eversion.
Intermetatarsus
]The extra bone is located between the first and second metatarsal bones and is best seen on radiographs in antero-posterior projection. The metatarsal may be misinterpreted as a fracture of the base of the second metatarsal in injuries to the Lisfranc joint.
Calcaneus "secondary" (os calcaneus secundarium)
An additional ankle is located between the calcaneus and the cubic bone or talus bone. It is very rarely the cause of pain, but its presence may be mistaken for a fracture of the anterior-superior process of the calcaneus. The evaluation of X-rays is often insufficient in this case and it is necessary to perform computed tomography or magnetic resonance imaging (MRI). Magnetic resonance imaging allows you to visualize the bone marrow edema accompanying a calcaneus fracture.
Treatment of a painful extra bone in the foot
To confirm local irritation from the accessory ankle, a lidocaine test is often performed by administering an anesthetic to the soft tissues surrounding the accessory ankle. In selected cases, it is possible to attempt conservative treatment in the form of rehabilitation and wearing corrective orthopedic insoles. Surgical excision of the accessory ankle allows for permanent pain relief.
Source:
Nwawka, O. Kenechi et al. “Sesamoids and Accessory Ossicles of the Foot: Anatomical Variability and Related Pathology.” Insights into Imaging 4.5 (2013): 581–593.
Frequently asked questions about the extra bones in the foot:
The accessory triangular bone is found in 7-25% of people and is located at the back of the talus. In most cases, its presence does not cause any discomfort. In athletes who perform repeated plantar flexion movements (such as ballerinas who climb toes), the triangular ankle may be irritated as manifested by overload pain at the back of the ankle.
An additional navicular bone lies on the medial side of the tarsus. The pain is exacerbated by the conflict between the protruding ankle and the upper of the shoes. In the case of a flat valgus foot, the discomforts can be alleviated by using an insole correcting the incorrect position of the foot. Failure to achieve the desired effect may be an indication for the surgical removal of the additional navicular bone.
Surgical excision of the accessory ankle is performed in the event of an acute injury that resulted in a fracture of the accessory ankle. Surgical treatment also includes conditions where medical treatment has not been successful.
