The navicular bone is a small, boat-shaped bone in the midfoot that forms a fundamental structure of the foot’s arch. It connects the ankle to the forefoot and serves as the attachment point for the posterior tibial tendon, which supports the arch. An accessory navicular bone is an extra piece of bone or cartilage that develops alongside the main navicular bone, representing a congenital anatomical variation. This extra bone is often referred to as os naviculare or os tibiale externum.
Anatomical Nature and Prevalence
The accessory navicular is situated on the medial, or inner, side of the foot, near the arch. It develops from an unossified portion of the navicular bone that fails to fuse with the main bone during development. Although present from birth, this congenital anomaly often only becomes noticeable in adolescence when the foot bones begin to calcify.
The extra bone is embedded within the distal fibers of the posterior tibial tendon, just before the tendon inserts into the main navicular bone. It is a common finding, occurring in an estimated 4 to 21% of the general population. The condition is frequently bilateral, found in both feet in about half of affected individuals.
Classification of Accessory Navicular Bones
Accessory navicular bones are categorized into three main types based on their size and connection to the main navicular bone.
Type I
Type I is the smallest, presenting as a small, round or oval fragment, typically measuring only 2 to 3 millimeters. This type is a sesamoid bone suspended entirely within the posterior tibial tendon and lacks a direct connection to the navicular bone.
Type II
Type II is the most frequently symptomatic category. It is a larger, triangular or heart-shaped ossicle, sometimes measuring up to 12 millimeters. It connects to the main navicular bone by a layer of fibrocartilage or hyaline cartilage, known as a synchondrosis. The posterior tibial tendon often inserts directly onto this accessory ossicle, which contributes to irritation and pain.
Type III
The third category, Type III, is also called a cornuate navicular. This type occurs when the accessory bone has fully fused to the navicular bone by a bony bridge. This results in a prominent, hook-shaped enlargement of the navicular tuberosity, essentially representing a fused Type II accessory bone.
Symptoms and When Treatment is Needed
Most people who have an accessory navicular bone remain without symptoms throughout their lives. When the condition becomes problematic, it is referred to as accessory navicular syndrome, which often appears in older children and adolescents. Symptoms include pain and tenderness on the inner side of the midfoot, localized over the arch, where the extra bone creates a noticeable prominence.
The pain is aggravated by physical activities, especially repetitive motion or high impact, or by pressure from tight-fitting footwear. Discomfort arises from irritation and inflammation of the cartilage connection or from strain on the posterior tibial tendon stretched over the bony mass. Medical attention is sought when the pain is persistent, interferes with daily activities or sports, or is accompanied by localized swelling and redness.
Management and Treatment Options
Initial management for symptomatic accessory navicular syndrome focuses on reducing inflammation and strain on the tendon.
Conservative Care
Rest and activity modification are the first steps, combined with applying ice and taking non-steroidal anti-inflammatory drugs (NSAIDs). For significant pain, immobilization using a short leg cast or walking boot may be necessary for several weeks to allow irritated structures to calm down. Custom orthotics or arch supports are common treatments, as they help reduce pressure on the accessory bone and minimize tension on the posterior tibial tendon. Physical therapy is also beneficial, focusing on calf stretching and exercises to stabilize the ankle and foot.
Surgical Intervention
When conservative treatments fail to provide lasting relief, surgical intervention may be recommended. The most common procedure is the Kidner procedure, which involves surgically removing the accessory navicular bone. Since the extra bone is not required for normal foot function, its removal often alleviates the pain. The Kidner procedure also involves reattaching the posterior tibial tendon to the remaining main navicular bone. This step ensures the tendon is properly aligned and restores its function in supporting the arch. The procedure has a high success rate, offering long-term relief for patients, particularly active individuals and adolescents who experience recurrent pain.