Accessory Navicular
1. Definition—Malformation, “Too large” (see Table 1-1, Chapter 1)
a. Medial/plantar–medial enlargement of the tarsal navicular bone with a secondary ossification center that eventually coalesces with the main body of the navicular in most affected individuals (Figure 6-18).
b. Prevalence is 10% to15% of the population. Most do not hurt.
c. Accessory navicular may be coincident with a flexible flatfoot. Both conditions have high individual prevalence rates. A cause and effect relationship has not been established between the two conditions.
2. Elucidation of the segmental deformities
a. Firm/bony prominence on the medial/plantar–medial aspect of the navicular/midfoot
b. Forefoot—neutral or supinated (if associated with a valgus/everted hindfoot)
c. Midfoot—neutral
d. Hindfoot—neutral or valgus/everted
e. Ankle—neutral or plantar flexed (equinus)
3. Imaging
a. Standing AP, lateral, and both obliques of foot
i. The lateral (nonstandard) oblique is the best view for revealing an accessory navicular (Figure 6-19).
ii. Three types of accessory naviculars
• Type I—small, separate ossicle in the posterior tibialis tendon adjacent to the main body of the navicular
• Type II—bullet-shaped ossification center on the proximal medial/plantar–medial aspect of the navicular with a synchondrosis to the main body of the navicular
• Type III—cornuate-shaped navicular—either a primary malformation or the result of metaplasia of a type II synchondrosis to a synostosis with the main body of the navicular
Figure 6-18. An accessory navicular (black arrows) creates a bony prominence on the medial/plantar–medial aspect of the midfoot. It moves with the navicular/acetabulum pedis during inversion and eversion of the subtalar joint. That is in contrast to the bony prominence on the medial/plantar–medial aspect of the midfoot in a flatfoot. The bony prominence in a flatfoot is the head of the talus. It does not move with inversion and eversion of the subtalar joint. In fact, the prominence of the head of the talus becomes obscured by the navicular when the subtalar joint in a flatfoot is inverted.
4. Natural history
a. All three types create a bony prominence along the medial/plantar–medial midfoot that presses the overlying skin against the shoe or the ground, with the possible development of painful callus formation.
b. The incidence of pain is not known, but is low.
c. Pain can also be experienced in a type II accessory navicular if a crack develops in the synchondrosis. Such cracks typically result from repetitive stress rather than from an acute injury. Cartilage has poor vascularity. If a crack develops, it might not heal. The cyclic tension stress on the synchondrosis during weight-bearing leads to painful inflammation at the site. In these cases, maximum tenderness is elicited by plantar-to-dorsal (upward) pressure under the accessory navicular, rather than by direct medial-to-lateral pressure on the ossicle (Figure 6-20).
5. Nonoperative treatment
a. Accommodative shoe wear
b. Over-the-counter arch supports to move the navicular to a different position in relation to the shoe, and thereby decrease the pressure on the overlying skin, as well as to decrease tension stress on the posterior tibialis–accessory navicular complex
c. If particularly inflamed and painful, temporary immobilization in a cast or CAM boot with or without nonsteroidal anti-inflammatory drugs
Figure 6-19. An accessory navicular is best seen on the lateral (nonstandard) oblique x-ray, as in A and B. A. Type I. B. Type II. Dashed black line is the site of resection of the accessory navicular and the enlarged medial body of the navicular. C. Type III.
Figure 6-20. Adolescent male with a painful type II accessory navicular. A. Maximum tenderness to palpation is elicited by plantar-to-dorsal (upward) pressure under the accessory navicular. B. There is less tenderness to direct medial-to-lateral pressure over the ossicle.
6. Operative indications
a. Pain at the site of the accessory navicular that is not relieved by prolonged attempts at nonoperative treatment
7. Operative treatment with reference to the surgical techniques section of the book for each individual procedure
a. Accessory navicular resection (see Chapter 8)—perform this for a painful accessory navicular in a well-aligned foot with normal ankle dorsiflexion.
b. Accessory navicular resection (see Chapter 8) and gastrocnemius recession (see Chapter 7)—perform this combination of procedures for a painful accessory navicular in a well-aligned foot with a gastrocnemius contracture
c. Accessory navicular resection (see Chapter 8) and calcaneal lengthening osteotomy (see Chapter 8) and gastrocnemius recession (see Chapter 7)—perform this combination of procedures for a painful accessory navicular in a severe flatfoot with a gastrocnemius contracture