Autosomal Dominant Transmission of Accessory Navicular

Abstract

The accessory navicular bone is one of the most symptomatic bones of the foot. Although it has been reported to be present in various members of the same family, there is a lack of knowledge about its inheritance pattern. We report two large pedigrees in which accessory navicular is inherited in an autosomal dominant fashion with incomplete penetrance.

INTRODUCTION

Accessory navicular is a separate ossification center for the tuberosity of the navicular that is present in approximately 5 to 14% of the general population.^2,8 The accessory navicular produces a firm prominence on the plantar-medial aspect of the midfoot. There may be a coexistent flexible flatfoot⁷, but there is no conclusive evidence of a cause-and-effect relationship between the two conditions. Individuals with an accessory navicular may present for evaluation because of the prominence, but more commonly they present because of pain at the site. The accessory navicular has been classified into three types.³ Type I is a rarely symptomatic, pea-sized sesamoid bone located within the distal portion of the posterior tibial tendon; type II, the most frequently symptomatic type, is an accessory bone united to the navicular by a syndesmosis or synchondrosis; and type III is a large accessory bone that results from fusion of a type II with the body of the navicular.

The inheritance of accessory navicular was first reported to occur in an autosomal dominant fashion by McKusick⁶, but there was no published data at that time to support this claim. More recently, Kiter et al.⁴ described the inheritance of accessory navicular in three families to occur in an autosomal dominant pattern with incomplete penetrance. The ethnicity of these families was not reported. We describe vertical transmission of accessory navicular in two families and confirm that autosomal dominant inheritance should be considered in the differential diagnosis of accessory navicular.

MATERIALS AND METHODS

A retrospective review was conducted for the years 1980 to 2003 to identify patients treated surgically for a painful accessory navicular at Saint Louis Shriners Hospital for Children and Saint Louis Children's Hospital. Institutional Review Board approval was obtained for the retrospective chart review. A total of 123 patients with 164 painful accessory navicular bones met the inclusion criteria. Charts were reviewed for documentation of the presence an accessory navicular. A total of 60 patients were contacted by phone to obtain information regarding whether any first-degree relatives were also affected with a painful accessory navicular.

RESULTS

Of the 60 patients we were able to contact by phone, 20 reported that at least one first-degree relative had the diagnosis of an accessory navicular. An additional 20 patients reported other family members that were treated for foot pain, but they were uncertain of a diagnosis. We were able to examine clinically and radiographically 19 and 12 members of two separate families and found accessory navicular in 10 and 4 members respectively. All had type II accessory navicular. Their ages ranged from 10-74 years (mean, 32.4 years).

The pedigrees of the families were consistent with an autosomal dominant inheritance with incomplete penetrance (Figure 1).

Figure 1.

Pedigree of the first family demonstrating autosomal dominant transmission of accessory navicular with incomplete penetrance.

DISCUSSION

Accessory navicular is one of the most symptomatic accessory bones of the human skeleton. Though it has been observed in multiple family members, data are lacking about the mode of its inheritance. The first report in the literature demonstrated an autosomal recessive pattern, based on only one family pedigree.¹ Macnicol et al.⁵ reported accessory navicular in three members of the same family, but they did not speculate on an inheritance pattern. McKusick found the inheritance to be autosomal dominant, without referring to any family studies.⁶ Finally, Kiter et al.⁴ reported on the autosomal dominant transmission of accessory navicular with incomplete penetrance based on three pedigrees, one with three generation involvement.

Based on our report of vertical transmission of accessory navicular in two large pedigrees of Scandinavian descent (Figure 1) and the findings of Kiter et al.⁴, we support the theory that accessory navicular can be transmitted as an autosomal dominant trait with incomplete penetrance. In all patients with accessory navicular, a family history should be obtained to look for affected family members. Appropriate counseling should be provided to families in regards to the inheritance pattern.

Since this disorder follows a Mendelian inheritance pattern, it is likely that a single gene is responsible; this makes a search for this gene an attractive proposition. We are currently performing a genome-wide scan to localize the gene for this disorder. Once chromosomal localization is made, candidate genes will be identified in that area based on their known or probable role in limb development. Mutational analysis studies will then be performed on these chosen genes in the hopes of identifying the one responsible for this disorder.