An accessory navicular bone (ANB) is an additional piece of bone or cartilage located on the inner side of the foot, positioned near the arch. This anatomical variation is a congenital condition. While many individuals with an accessory navicular bone may never experience symptoms, its presence can sometimes lead to discomfort or pain.
How the Bone Develops
An accessory navicular bone stems from a developmental process during fetal growth. Bones initially form as cartilage models, which then gradually harden into bone through a process called ossification.
The main navicular bone typically develops from a single ossification center. However, in some individuals, a secondary ossification center forms near the navicular bone’s tuberosity, a prominent bump on its inner side. Normally, this secondary ossification center would fuse completely with the main navicular bone as a child grows. In cases where an accessory navicular bone develops, this fusion fails to occur or remains incomplete. Instead, the accessory center persists as a distinct, separate bone or piece of cartilage. This developmental anomaly is not caused by injury or lifestyle choices later in life; rather, it is an inherent variation from birth.
Variations in Bone Structure
Accessory navicular bones manifest in three structural forms.
Type I is typically a small, round or oval bone, measuring approximately 2-3 millimeters, and is often embedded within the posterior tibial tendon. This type is considered a sesamoid bone, which is a small, independent bone found within a tendon.
Type II presents as a larger, often triangular or heart-shaped bone, measuring up to 12 millimeters. It is connected to the main navicular bone by a bridge of cartilage or fibrous tissue. This cartilaginous or fibrous connection allows for some movement between the accessory bone and the main navicular.
Type III, also known as a cornuate navicular, represents a complete fusion of the accessory bone with the navicular bone, resulting in an enlarged bony protuberance on the navicular’s inner side. This type is thought to be a fused Type II accessory bone.
When the Bone Causes Pain
While an accessory navicular bone is present from birth, it often becomes symptomatic later in life, particularly during adolescence or early adulthood. Pain arises when the accessory bone or the surrounding soft tissues become irritated.
The posterior tibial tendon, which plays a significant role in supporting the arch of the foot, often attaches to or near the accessory navicular bone. Stress, overuse, or direct trauma can inflame this tendon or the accessory bone itself. Activities involving repetitive motion or increased strain on the foot can lead to irritation where the tendon inserts onto the accessory bone. Friction or pressure from footwear rubbing against the bony prominence of the accessory navicular can cause localized discomfort and inflammation. Abnormal movement at the connection between the accessory bone and the main navicular can also contribute to pain.