A 56-year-old male presented with a painful flat foot. Radiographs in AP and lateral views showed a bean-shaped, well-corticated bony mass adjacent to the medial side of the navicular bone. This is an accessory navicular, an extra bone in the foot that originates from an ossification center that fails to fuse with the main navicular bone. These are usually asymptomatic.
Symptoms
The os tibiale externum is an extra bone on the inside of the foot. It can cause pain if it rubs against shoes, especially when the person walks or runs for long periods of time. The condition is most commonly seen in teenagers and young adults. It is most often associated with flat feet and can lead to problems with the posterior tibial tendon. The symptoms can usually be treated with analgesics and shoe inserts for support of the medial arch.
A 28-year-old man presented to the clinic complaining of pain on the medial side of his left foot for 1 day, which was aggravated by bearing weight and relieved by rest. On examination, there was swelling over the medial aspect of the foot and dorsum of the ankle, but no tenderness or crepitus. Radiographs of the ankle in AP and oblique views were normal.
The os tibiale externeum, also known as accessory navicular, os naviculare secundarium, and os (tarsal) peroneum is a small sesamoid bone found within the tibialis posterior tendon near its insertion on the navicular bone. It is present in 2-14% of the general population. Most cases are asymptomatic and discovered incidentally on radiographs. In a small percentage of cases, the accessory navicular can irritate the posterior tibial tendon, leading to pain on the medial side of the foot.
Diagnosis
The accessory navicular (Os tibiale externum) is an extra bone on the inside of the foot. This bone is within the posterior tibial tendon and attaches to the navicular bone, one of the tarsal bones in the middle of the arch. About 14 percent of people have this extra bone in one or both feet. Usually it causes no pain. When it causes pain, it is called accessory navicular syndrome.
Three types of Os tibiale externum have been described:
Type 1 is a 2-3 mm oval ossicle that acts as a sesamoid bone in the distal part of the posterior tibialis posterior tendon. This is the most common type and is asymptomatic.
Type 2 is a triangular or heart-shaped ossicle that measures up to 12 mm and represents a secondary ossification center connected to the navicular tuberosity by a 1-2 mm layer of fibrocartilage or hyaline cartilage. Portions of the posterior tibialis tendon often insert into this accessory ossicle, which can cause dysfunction and symptoms.
Type 3 is a synostosed Type 2 accessory bone that appears as an enlarged navicular tuberosity and is occasionally symptomatic due to bunion formation. It may also cause heel pain because it is positioned directly over the metatarsophalangeal joint.
Treatment
The accessory navicular (also called os naviculare secundarium or os tibiale externum) is an extra bone on the inside of the foot near the arch. It is incorporated within the posterior tibial tendon that attaches in this area, and is a common cause of pain symptoms. It is congenital, but most people have no symptoms and do not know that they have it. The presence of an accessory navicular is also associated with flat feet, and the combination can cause pain from increased stress on the tibialis posterior tendon.
An os tibiale externum can be diagnosed through physical examination and multimodal imaging. Physiotherapy and shoe inserts are effective conservative treatment options to relieve pain. When these methods do not provide symptom relief, shock wave therapy can offer significant and sustained pain reduction.
The os peroneum is another accessory ossicle that develops in the lateral midfoot area, proximal to the cuboid bone. It is a small sesamoidean bone and occurs in 5-26% of the population. It is a bean-shaped bone with one or two concave articular surfaces. It is usually asymptomatic, but when painful, it can lead to a fracture of the ossicle, or a tear in the peroneus longus tendon. Treatment is typically surgical and includes excision of the ossicle with repair of the peroneus longus tendon, or a Kidner procedure.