The human foot is a marvel of anatomical engineering, comprising 26 bones, 33 joints, and more than 100 muscles, tendons, and ligaments working in concert to support the full weight of the body. Yet, for all this complexity, the foot occasionally harbours an additional, uninvited structure: the os tibiale externum, more commonly known as the accessory navicular. Though small and often overlooked, this sesamoid-like bone carries considerable clinical significance and has fascinated anatomists, podiatrists, and orthopaedic surgeons for centuries.
What Is the Os Tibiale Externum?
The os tibiale externum is an accessory ossicle — a supernumerary bone — located on the medial (inner) side of the foot, adjacent to the navicular bone. The navicular itself sits along the inner arch of the foot, articulating with the talus proximally and the three cuneiform bones distally. The os tibiale externum develops as a secondary centre of ossification within the substance of, or immediately adjacent to, the posterior tibial tendon’s insertion site on the navicular. It is encased within this tendon or connected to the navicular by a fibrous or fibrocartilaginous bridge, a junction known as a synchondrosis.
This accessory bone is one of the most common accessory ossicles found in the human foot, with an estimated prevalence of between 4% and 21% of the general population, depending on the imaging modality and study population used. It presents bilaterally — in both feet — in approximately 50 to 90% of affected individuals, suggesting a strong genetic underpinning. It is slightly more common in females than males, and it tends to become symptomatic during adolescence when the foot is undergoing rapid growth and the demands placed on the musculoskeletal system intensify.
Classification and Anatomy
Anatomists and clinicians have long recognised that the os tibiale externum does not present in a uniform fashion. The most widely accepted classification system divides it into three distinct types.
Type I is the smallest variant, a true sesamoid bone fully embedded within the posterior tibial tendon, typically measuring 2 to 3 millimetres. It sits entirely within the tendon substance and is usually an incidental finding on imaging, rarely causing symptoms.
Type II is the most clinically relevant variant and the one most commonly associated with pain. It is a larger ossicle, typically between 8 and 12 millimetres, connected to the navicular by a synchondrosis — a fibrocartilaginous union. This junction is biomechanically vulnerable. Under repetitive loading and torsional stress, the synchondrosis can become inflamed, develop microfractures, or fail entirely, producing the clinical syndrome known as accessory navicular syndrome or os tibiale externum syndrome.
Type III, sometimes called the cornuate navicular, represents a fully fused accessory ossicle that has united with the navicular proper. This produces a characteristically enlarged, horn-shaped navicular visible on plain radiographs. Because fusion has occurred, Type III is generally asymptomatic, though the altered morphology of the navicular may influence the mechanics of the arch.
Clinical Presentation and Symptoms
The os tibiale externum is often asymptomatic throughout an individual’s lifetime, discovered only incidentally on imaging performed for an unrelated reason. However, when symptoms do arise, they typically manifest as medial midfoot pain, tenderness directly over the navicular prominence, and swelling in the region of the inner arch. The overlying skin may become irritated, particularly in individuals who wear tight or poorly fitting footwear.
Symptoms most commonly emerge during adolescence, often coinciding with periods of rapid skeletal growth, increased physical activity, or both. Flat foot deformity — pes planus — is frequently associated with symptomatic accessory navicular, as a lowered medial longitudinal arch places the posterior tibial tendon under greater mechanical strain, transmitting increased stress across the synchondrosis. Activities that involve repeated inversion and eversion of the foot, such as running, dancing, and team sports, are common triggers.
Acute exacerbation can also result from direct trauma to the medial foot. A forced eversion injury — such as a rolled ankle — may stress or rupture the synchondrosis acutely, mimicking or co-existing with a navicular fracture and presenting a diagnostic challenge in the emergency setting. High-resolution imaging, including MRI, is invaluable in these scenarios, revealing bone marrow oedema, synchondral disruption, and tendon pathology that plain radiographs cannot adequately depict.
Diagnosis
Diagnosis begins with a thorough clinical assessment. A clinician will observe the contour of the medial arch, assess for the characteristic medial navicular prominence, and identify point tenderness with direct palpation. Functional assessment includes evaluating heel-rise capacity, assessing for pes planus, and examining the integrity and strength of the posterior tibial tendon.
Plain radiographs, particularly the medial oblique view, reliably demonstrate the ossicle and allow classification by type. MRI provides superior soft tissue resolution and is the gold standard for assessing the degree of synchondral inflammation, bone marrow changes, and posterior tibial tendon integrity. Technetium bone scintigraphy has historically been used to demonstrate increased uptake at the synchondrosis, confirming active stress response, though MRI has largely supplanted it in contemporary practice.
Treatment
Management is stratified according to symptom severity. Conservative treatment is the first-line approach and succeeds in the majority of cases. Rest, activity modification, and the use of non-steroidal anti-inflammatory medications can reduce acute inflammation. Orthotics designed to support the medial arch and offload the navicular prominence are particularly effective, redistributing stress away from the synchondrosis. Immobilisation in a short-leg cast or a removable walking boot is reserved for more severe or refractory presentations, allowing the inflamed synchondrosis to settle over a period of weeks.
When conservative measures fail after three to six months, surgical intervention becomes appropriate. The Kidner procedure — excision of the accessory ossicle with advancement and reattachment of the posterior tibial tendon to the navicular — remains the most commonly performed operation. Modern variations aim to preserve tendon function and restore medial arch support. Outcomes are generally favourable, with the large majority of patients reporting significant pain relief and a return to full activity.
The os tibiale externum is a small but clinically meaningful anatomical variant that sits at the intersection of embryology, biomechanics, and musculoskeletal medicine. Its presence is common, its expression variable, and its capacity to generate significant pain in active individuals well documented. Understanding its classification, the mechanisms by which it becomes symptomatic, and the spectrum of treatment options available allows clinicians to manage affected patients effectively — restoring comfort, function, and quality of life from a bone that, by rights, was never meant to be there at all.