| February 2005 Case of the Month |
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Compiled By: Raymond P. Radanovich, D.O.
History: 12 year old boy with microtrabecular infraction or grade III stress injury of a Type II accessory navicular bone.
Impression: 1) Microtrabecular infraction or grade III stress injury of a Type II accessory navicular bone.
2) Diffuse moderate patchy osseous edema of the hindfoot and forefoot structures consistent with an overuse syndrome. The
activity likely has likely secondarily exacerbated the grade III stress injury of the Type II accessory navicular bone,
synchondrosis and the parent navicular bone.
Findings: Extensive osseous edema involves a 10 x 5 mm accessory type II navicular bone. Fibrovascular osseous stress reaction is
present within the parent navicular bone. Patchy edema involves the talus, navicular, cuneiforms and calcaneous without
evidence of marcrofractures.
Discussion: Accessory Navicular
The accessory navicular is considered to be an anatomic ossicle variant and is the largest supernumerary bone of the
foot. There are three types of accessory navicular bones with a reported incidence is approximately 10-21 %.
The type I accessory navicular, also known as the os tibiale externum, occurs when an ossification center forms a sesamoid
bone embedded in, and surrounded entirely by the distal posterior tibial tendon near the navicular insertion. It tends to be oval or
round and may be up to 5 mm in size. However, it is usually about 2 x 3 mm in size located medial and posterior to the navicular.
Some believe that this type of accessory navicular is entirely asymptomatic while others believe that it may result in eventual
failure of the posterior tibial tendon, altered biomechanics and development of pes planus.
In the type II accessory navicular, the ossification center is medial to the navicular within the tubercle of the navicular. In
the child, the type II navicular exists as a cartilaginous structure, continuous with the cartilage of the parent navicular. Ossification occurs, on average, at approximately nine years of age, typically forming a triangular shaped bone up to 12 mm in size. A
residual cartilaginous synchondrosis may persist joining the ossicle approximately 1-2 mm medial and posterior to the parent
navicular. Typically the majority or entire posterior tibialis tendon inserts on this accessory ossicle. The type II navicular is most
often associated with a painful foot in an active patient performing repetitive plantar flexion and inversion and should not be readily
dismissed as an anatomic variant in a symptomatic patient. Chronic traction on the synchondrosis results in osseous stress
reaction and bony hypertrophic changes at the synchondrosis producing pain.
The type III accessory navicular, or cornuate navicular, is a prominent navicular tuberosity, which is essentially a Type II
ossicle fused to the parent navicular. This type of accessory navicular is not associated with traction related symptoms unless
a valgus stress injury fractures the attachment of the ossicle to the navicular leading to abnormal motion. On MRI, this is seen
as bone marrow edema and hypertrophic spurring. Another cause of pain with this type of ossicle is abnormal pressure on the
soft tissues over this osseous protuberance due to friction. (e.g., tight boots).
When a large portion of the posterior tibial tendon inserts onto either a Type II and III accessory navicular ossicle, the
posterior tibialis tendon is displaced resulting in a valgus deviated foot. This anatomic arrangement puts the patient at risk of the development of a pes planus deformity.
Nonsurgical treatment consists of applying doughnut-shaped moleskin to the painful area, use of a special orthotic which
shields the prominence or cast immobilization up to six weeks. Problematic cases may require a Kidner procedure which
consists of surgical excision of the majority of the ossicle (leaving a wafer thin portion of the ossicle for reattachment purposes)
and reinsertion of the posterior tibialis tendon with Harpoon or Mitek anchors to the plantar aspect of the navicular with the foot
in partial inversion. Lengthening of the lateral column may also be necessary to fully restore a fallen arch. |
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References:
Resnick, Bone and Joint Imaging. WB Saunders, 1989. p 1073.
Rolfes RJ, Pomeranz SJ Kim TW. MRI of the Foot and Ankle. Pearls, Pitfalls & Pathology.
MRI EFI Publications. p 19-22. 2002.
Lawson JP, Odgen JA et. al. The Painful Accessory Navicular. Skeletal Radiology 12 (4) : 25 0-62, 1984.
Bennett GL, Weiner DS Surgical Treatment of Symptomatic Accessory Tarsal Navicular. JPO. Vol. 10. 1990. p 445-449.
Veitch JM. Evaluation of the Kidner procedure in the treatment of symptomatic accessory tarsal scaphoid.
CORR. Vol. 131. 1978. p 210-213.
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