Accessory Navicular

  • Most common accessory bone of the foot
  • Located at Medial plantar border of the navicular
  • Associated with Tibialis Posterior tendon
  • Accessory ossicles are derived from unfused ossification centers
  • Synonyms: Os tibiale; Os tibiale externum; Naviculare secundum

Incidence

  • 21% incidence; 89% of cases are bilateral
  • One of the most common accessory ossicles in the foot It is seen over the medial pole of the navicular bone, usually in adolescent patients
  • It is most commonly symptomatic in the 2nd decade of life and causes medial foot pain
  • <1% of patients become symptomatic.
  • Usually affects teens and young adults
  • More frequent in females

Etiology

  • variant of normal anatomy.
  • It may become symptomatic from the bony prominence impinging against shoe wear.
  • The patient may have diffuse medial and plantar arch pain.
  • It may cause problems by destabilizing the insertion and diminishing the pull of the PTT.
  • In patients with associated severe flatfoot deformity, lateral pain may occur secondary to impingement of the calcaneus against the fibula.
  • A traumatic event can cause injury to the fibrocartilaginous synchondrosis that attaches the ossicle to the main navicular.

Associated Conditions

  • Flatfoot deformity
  • Secondary Achilles tendon contracture

Symptoms

  • Considered an incidental finding on radiographs, but may become symptomatic
  • bursa, redness, irritation, local tenderness
  • Often presents in adolescent patients or young adults, with flatfoot deformity and arch pain

Classification

3 major types of accessory navicular adjacent to the posteromedial navicular tuberosity

Type DescriptionIncidence
ISmall, 2 – 3 mm sesamoid bone in the PTT (os tibiale externum)~30%
IILarger ossicle than type I
Secondary ossification center of the navicular bone		~50%
IIIEnlarged navicular tuberosity (cornate navicular)
considered a fused variant of a type II, often with pointed shape		~20%
Classification of Accessory Navicular

Xray

  • AP, Lat & 45 degree eversion oblique
  • Navicular is the last tarsal bone to ossify from multiple ossification centers
  • Females – 1-3.5 yr
  • Male – 3.0- 5.5 yr
  • Smooth margins with well-formed cortex differentiate this condition from acute fracture
  • There is no evidence that the longitudinal arch is any different, essentially an incidental finding

Bone scan

  • May show increased activity over an accessory navicular
  • May be needed if a navicular stress fracture is suspected in the differential diagnosis

MRI

  • Useful when plain films are unremarkable
  • Often, a type-II accessory navicular is attached to the tuberosity by a fibrocartilage or hyaline cartilage layer, and MRI may show soft-tissue edema consistent with a synchondrosis sprain or tear.
  • Altered signal intensity and bone marrow edema, suggestive of chronic stress and/or osteonecrosis
  • Also helpful in showing PTT degeneration

Differential Diagnosis

  • Navicular fracture may mimic an acute avulsion fracture of the tuberosity of the navicular.
  • Posterior tibial tendinitis
  • Stress fracture of navicular

Treatment

  • Most patients assymptomatic or are successfully managed conservatively
    • rest and avoid athletics or aggravating activities.
    • Anti-inflammatory medication
    • Shoe-wear modification
      • use of a softer, wider shoe
      • If flatfoot is present, a medial arch support may be useful, but often the patient may not tolerate it because of direct pressure on the ossicle.
    • Below-the-knee walking cast or removable fracture boot may be used for 3-6 weeks for persistent symptoms.
    • Physiotherapy – strengthening exercises
  • Surgical
    • Rarely simple surgical excision
      • Options
        1. Kidner procedure, the accessory navicular is excised, and the PTT is rerouted into a more plantar position
        2. excision of the ossicle and reattachment of the PTT insertion to the navicular, with suture anchors or sutures passed through drill holes
    • Severe flatfoot deformity with lateral impingement symptoms
      • may require concomitant osteotomy of the calcaneus and/or medial column of the foot to improve alignment and decrease mechanical stress of the PTT insertion.