Summary
ANB is an accessory bone which is located medially to the main navicular bone of the foot. It has three reported types based on morphology. Type I (30 %), type II (50 %) and type III (20 %).
The ANB is the largest supernumerary bone of the foot and clinically the most important. ANB is present in 5%–20% of the population and it is more common in female patients. It is bilateral in 50%–90%.
Type II is found to be the symptomatic variant of ANBs due to traction between the ossicle and the main navicular bone and also as a result of stresses applied on the synchondrosis of ANB type II.
Proper diagnosis of symptomatic ANB depends on clinical and radiological assessment. Plain radiographs, ultrasonography, bone scintigraphy and MRI are helpful tools to investigate ANB.
Non-operative management should be tried for at least three months before deciding surgery and it includes analgesics, soft pads insoles, physiotherapy, foot orthoses, immobilization in a short-leg cast and steroid injection.
Surgical management aims to improve pain and enhance patient activities either by simple excision of the ANB without tendon advancement, removal of the accessory ossicle with reattachment of the P.T.T. to the plantar surface of the navicular bone (Kidner procedure), fusion of the accessory navicular to the main bone using screws (bone union) or percutaneous drilling of synchondrosis under radiological guidance to induce osteosynthesis of the symptomatic ANB. Recently, fusion can be achieved endoscopically.
Patients with flexible flatfoot and symptomatic ANB can be successfully managed by subtalar arthroereisis with modified Kidner procedure with the patients found to have good results in improvement of pain and function and correction of the deformity.