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Most anatomy textbooks will tell you that the human body has 206 bones. 

This may be true generally, but the reality of the situation is that the number of bones people may have varies from individual to individual.  Some people have a few Extra Bones beyond the 206 most authors quote--others have fewer.  And a great deal of this variation in the number of bones occurs in the feet, which alone count for approximately one quarter of the bones in your body.  

Most additional bones in the body are called Accessory Bones or Accessory Ossicles.

While there are quite a few different possible locations for these bones to develop in the foot, we'll discuss the three most common locations.

Accessory Navicular 
(Os Tibiale Externum / Os Naviculare)

Finding an Accessory Navicular bone is not uncommon.  It is seen in approximately 5% of the population.   It's found on the medial (inside) part of the foot, near the Navicular bone.  It represents one of the Navicular bone's two growth centres (or ossification centres), the part of each bone that hardens from cartilage in childhood.

This is the Navicular bone, where the Accessory Navicular is found.   (It is absent in this picture.)

Accessory Navicular bone

While in most patients the bone's two growth centres fuse in development, occasionally the bones remain separate, as in the example to the left.

There are several different appearances and sizes that this ossicle may take on, described by the Dwight Classification System.  

Causes:  The most common reason the two different growth centres fail to unite is an excessively flattened foot-type, (a flexible flat foot).  This is usually (but not always) because the major tendon that prevents the foot from flattening, the Posterior Tibial tendon, attaches to this part of the Navicular bone.  If the foot flattens more than it should, the posterior tibial tendon pulls on the Navicular bone excessively, sometimes preventing fusion of the two growth centres. 

Because having an Accessory Navicular bone is associated with an overworked Posterior Tibial tendon, this condition is associated with Posterior Tibial dysfunction.

Diagnosis:  The condition can usually be diagnosed with X-ray or bone scan, and care must be taken to differentiate an Accessory Navicular from other conditions (like fracture, certain types of arthritis, and Posterior Tibial tendon rupture). 

Treatment:  For pain that results from shoe pressure on the bone, we can simply redistribute weight from the bony bump. 

For pain that results from an excessive pull of the Posterior Tibial tendon, application of a cast to immobilize the tendon will frequently resolve symptoms in the short term. 

In the long term, we will usually need to control any excessive pronation that may be associated with the foot--most frequently with an orthosis.  We have had excellent results with a special type of orthosis known as an inverted orthosis. 

Surgical involvement is reserved for non-responsive patients, and usually involves removal of the Accessory Navicular bone.  If the Posterior Tibial tendon has been compromised, it may be necessary to reroute or reattach it.  If the foot is crooked or not functioning normally from being excessively flattened  for too long, it may be necessary to fuse certain joints in the foot, or to straighten other bones in the foot to help realign the foot.   

Os Trigonum

The Os Trigonum is quite similar to the Accessory Navicular, only occurring in a different bone on a different part of the foot. 

The Os Trigonum is found just behind the ankle bone (Talus).

Causes:  Just as the Accessory Navicular represents a portion of bone that never united with the Navicular bone, the Os Trigonum is  a portion of bone (the lateral tubercle) that never fused with the main portion of the ankle bone (the Talus).

This condition is quite common--some authors report its presence in as many half of all people--but symptoms usually only develop when the bony fragment interferes with ankle joint motion. 

Diagnosis is usually made on X-ray, but care must be taken to differentiate the condition from a fractured tubercle (known as a Shepard's fracture) and tendonitis of tendons that pass in the area.

Treatment usually consists of anti-inflammatory medications, steroid injections, casting the foot until it calms down, changes in activities (some movements and activities cause symptoms more than others) or surgically removing the piece of bone.   

Os Peroneum

The Os Peroneum is a small bone that is sometimes seen on the edge of the cuboid bone.  Intimately associated with the Peroneus Longus Tendon as it courses under the Cuboid bone, across the bottom of the foot to the base of the first metatarsal. 

Causes:  This bone is a normal variant.  It is intimately associated with the Peroneus Longus tendon as it courses around and under the Cuboid bone, across the bottom of the foot to the base of the first metatarsal.  The bone is thought to help redirect the pull of the Peroneus Longus tendon.   Diagnosis is usually made on X-ray.  It must be differentiated from a fractured cuboid bone.

Treatment is rarely necessary, as these bones are rarely problematic.  They may be surgically removed if troublesome.
  

Os Vesalinum

The Os Vesalinum is quite similar to the Accessory Navicular and the Os Trigonum, only it occurs on a different bone and in a different part of the foot. In this case, the condition occurs on the lateral (outside) portion of the foot, in an area known as the base of the fifth metatarsal.

The portion of the foot  in which the Os Vesalinum  develops.

Causes:  Again, this condition represents a small portion of bone that never united with the main bone, in this case the fifth metatarsal bone.  As with the case of the Accessory Navicular the site of this condition is associated with the attachment of tendons, in this case the Peroneus Brevis and the Peroneus Tertius tendons. 

Diagnosis is usually made on X-ray, but care must be taken to differentiate the condition from a fractured fifth metatarsal base and a tendon rupture of one of the tendons attaching to the bone in this area. 

Treatment usually consists of anti-inflammatory medications, steroid injections, casting the foot until it calms down, changes in activities (some movements and activities cause symptoms more than others) or surgically removing the bone.   

Other Locations

There are several other locations accessory ossicles may form in the foot.   Some examples:  

Os Supranaviculare is present on the dorsum (top surface) of the navicular bone.  

Os Intermetatarseum is classically used to describe an ossicle present on the dorsum (top surface) of the foot at the junction of the medial cuneiform, 1st Metatarsal base, and 2nd metatarsal base. The phrase "os intermetatarseum", however, is sometimes used to describe the much more common condition of accessory bones at the other end of the metatarsal--near where it attaches to the toe bones.  (See example to the right.)

Os Calcaneus Secondarius is present on the calcaneus (heel bone), near the junction with the cuboid, talus, and navicular bones.  

Os Subfibulare is seen below the tip of the fibular malleolus (the bump on the outside of the ankle).  It must be differentiated from a chip fracture off the fibula bone.  

Os Subtibiale is seen below the tip of the tibial malleolus (the bump on the inside of the ankle).  It must be differentiated from a chip fracture off the tibia bone.