Introduction

Accessory ossicles and the sesamoid bones are the skeletal variations of the ankle and foot. Accessory ossicles are a developmental variation and it appears as a secondary center, which originates from the ossification center of the main bone. It may exist adjacent to the main bone or separated. They are often confused with avulsion fractures. As a result of painful fractures, these bones may be infected or dislocated and also can cause connective tissue diseases [4, 12, 13, 20, 23].

Sesamoid bones are the 5–10 mm round or oval-shaped bones that developed from their own ossification center. They are partially or totally embedded in the substance of a corresponding tendon. The sesamoid bones are the part of a gliding mechanism that reduces friction and protects the tendon [5, 19, 20, 22]. Anatomically, the sesamoid bones of the first metatarsophalangeal joint are considered as a normal part of the skeleton, additionally sesamoids of the lesser toes are seen rarely [4, 11].

Accessory ossicles and the sesamoid bones cause various diseases at foot and mimic fractures of foot bones. These bones and its clinic importance must be well known to reduce unnecessary orthopedic consultations and misdiagnosis [2, 14]. Several case reports of accessory ossicles are available in the literature [17, 25, 26]. However, there are insufficient number of studies with large sample sizes reported in the orthopedic and anatomic literature. In the present study, we aimed to document a detailed investigation on the accessory ossicles and sesamoid bones of Turkish subjects in both the feet according to the sex, frequencies and divisions of the bones, coexistence and bilaterality by radiography.

Materials and methods

A double-centered study was performed retrospectively to determine the incidence of the accessory ossicles and sesamoid bones in the ankle and foot. In this study, a total number of 984 cases was identified. In the first center, radiographs of 534 cases (290 men, 244 women, age range 14–72 years) which are evaluated between 2005 and 2007 in Radiology Department of Akdeniz University Faculty of Medicine (Antalya) were examined. Radiographs of 450 cases (238 men, 212 women, age range 14–60 years) were obtained from the second center (Hacettepe University Faculty of Medicine, Ankara which were evaluated between 1974 and 1991).

Anteroposterior, oblique and lateral foot radiographs of 984 cases were examined with regard to presence, incidence, coexistence and distribution of accessory ossicles and sesamoid bones in both feet.

Statistical analysis

The results were analyzed using the Statistical Package for Social Science (SPSS) version 13.0. There was no statistical significance between data of the accessory ossicles and sesamoid bones according to Chi-Square test.

Results

Accessory ossicles (Fig. 1) were detected in 209 of the 984 cases. In our study, the most common accessory ossicle of the ankle and the foot region was accessory navicular bone. It was found in 116 cases. We also detected os peroneum in 46 cases, os trigonum (Fig. 2) in 23 cases, os supranaviculare in 16 cases, os vesalianum (Fig. 3) in 4 cases, os supratalare in 2 cases, os intermetatarseum (Fig. 4) in 2 cases. Accessory ossicles were seen in 10.9% of all female participants and 10.2% of all male participants. The accessory ossicles were seen in 7.6% of cases bilaterally, in 7.3% of cases right unilaterally and in 6.3% of cases left unilaterally (Table 1).

Fig. 1
figure 1

Schematic illustration showing the accessory ossicles of the ankle and the foot region and their incidence

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Fig. 2
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Lateral ankle radiograph shows os trigonum (arrow) on the right foot (20–year-old man)

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Fig. 3
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Lateral ankle radiograph show Os vesalianum (arrow) and bipartite medial hallucal sesamoid (arrowhead) (20–year-old man—unilateral-right)

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Fig. 4
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Dorsoplantar radiograph shows a small os intermetatarseum (arrow) (35-year-old woman—left)

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Table 1 Accessory ossicles of the ankle and foot region
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In all cases hallucal sesamoid bones were present (100%) normally. In 95 cases extra sesamoid bones were identified. We observed bipartite hallucal sesamoid in 25 cases (Figs. 3, 5). In 21 cases the medial hallucal sesamoid and in four cases the lateral hallucal sesamoid were found to be bipartite. Interphalangeal sesamoid bone of the hallux was seen in 20 cases (Fig. 6). None of the radiographs showed sesamoid bones in the proximal and distal interphalangeal joints of second, third, fourth, and fifth toes. Sesamoid bones of the foot region are detailed in Table 2.

Fig. 5
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Dorsoplantar radiograph shows coexistency in a 29-year-old woman. Bipartite medial hallucal sesamoid (arrowhead) and fifth metatarsophalangeal sesamoid (arrow)

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Fig. 6
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Dorsoplantar radiograph shows hallux interphalangeal sesamoid bone (arrow) in a 34-year-old man

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Table 2 Sesamoid bones of the foot region
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We also identified the coexistencies of two different accessory ossicles as 6% (Fig. 7), accessory ossicles and sesamoid bones as 7% (Fig. 8), and bipartite sesamoid bones and sesamoid bones as 1.9% (Fig. 5). Distribution of the most common accessory ossicles in male and female cases was similar.

Fig. 7
figure 7

The coexistence of accessory navicular bone and os peroneum in an asymptomatic case 38-year-old woman. a Oblique radiograph shows accessory navicular bones (arrowhead), b oblique radiograph shows os peroneum (arrow)

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Fig. 8
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The coexistence of accessory navicular bone, os peroneum and metatarsophalangeal sesamoid bones in a 27-year-old woman. a Dorsoplantar radiograph show accessory navicular bones (white arrowhead) and the fifth sesamoid bone and a fourth sesamoid bone (black arrow). b Lateral radiograph shows os peroneum (white arrow)

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Differences according to the side, sex, presence or absence of accessory ossicles and sesamoid bones were statistically analyzed. There were no significant differences in the prevalence of the accessory ossicles and sesamoid bones according to the side (right, left) and sex. There was no correlation between the side, unilaterality, bilaterality or the sex with the presence or absence of accessory ossicles and sesamoid bones in the foot.

Discussion

Many skeletal variations in the ankle and foot may be found, including different accessory ossicles and sesamoid bones, bipartitions and coalitions [20, 23]. Most accessory ossicles and sesamoid bones do not cause any complaint and remain asymptomatic. Generally, they are detected by routine radiologic examinations after trauma or overuse leading to degenerative changes or pain. They may also suffer or stimulate fractures and restrict the range of motion [1, 13, 18, 20, 24, 27]. In literature reported, incidence of the accessory ossicles in the foot and ankle is 18–36.3% in general populations [4, 20]. In the present study, the incidence of the accessory ossicles was 21.2%. The accessory navicular bone, also known as os tibiale, os tibiale externum and naviculare secundarium is adjacent to the posteromedial tuberosity of the navicular bone, and the incidence of the accessory navicular bone is 4–21%. Some authors had declared the marked association of accessory navicular bone with flat-foot deformity [18, 20, 23]. In our study, as accessory navicular was the most common accessory bone with an incidence of 11.7%, but the flat-foot deformity was not identified in the cases. The os peroneum is a round or oval-shaped sesamoid bone imbedded within the peroneus longus tendon. It is located near the calcaneocuboid joint, and its prevalence is only 9%. This latter entity may easily be misinterpreted as an avulsion fracture [20, 26]. The os trigonum is one of the largest and most common accessory ossicles in the ankle and foot region, with an estimated prevalence of 1–25% [4, 20]. The accessory navicular bone, os peroneum and os trigonum are the most common bones and in different studies the incidence shows variability. Tsuruta et al. [27] reported the incidence of os accessory navicular, os trigonum and os perenoum, in a 3460 radiographs study in Japan. According to Kruse and Chen [12] the most common accessory ossicles of foot and ankle were os perenoum, os accessory navicular and os trigonum. The incidence of accessory ossicles in Turkish subjects was reported as 18.3% by Cilli and Akcaoglu [3] and os peroneum, os accessory navicular and the os trigonum were the most common accessory bones. This study was performed only in males and coexistence and bilaterality were absent. In our study, incidence of os accessory navicular was 11.7%, os peroneum was 4.7% and os trigonum was 2.3% as the most common accessory bones.

In parallel with the current literature, the less common accessory ossicles were os supranaviculare (1.6%), os vesalianum (0.4%), os supratalare (0.2%) and os intermetatarseum (0.2%) in our study. The supranaviculare, also known as the os talonaviculare dorsale, talonavicular ossicle or Pirie’s bone, is found on the dorsal aspect of the talonavicular joint, close to the midpoint. It has an estimated prevalence of 1% and is a rare incidental skeletal variant. Os vesalianum is a small accessory ossicle adjacent to the tip of a well-developed tuberosity of the fifth metatarsal. It is a very rare accessory bone, with an estimated prevalence of 0.1–1%. The accessory ossicle located along the superior surface of the talar head is known as the os supratalare. It is typically located over the ridge along the talar head/neck, but may be seen distally over the head. It can easily simulate an old, ununited avulsion fracture and is only identifed in the lateral view. It has an estimated prevalence of 2% and is a rare incidental skeletal variant. The os intermetatarseum is found between the medial cuneiform and the base of the first and second metatarsals. This ossicle may be round, oval, kidney-shaped, linear, or even resemble a rudimentary metatarsal, and its estimated prevalence is 1.2–10%. Also, os intermetatarseum may be seen together with hallux valgus deformity. Its size varies and it is frequently asymptomatic [3, 4, 20]. This ossicle is found in our study only in two cases and, therefore, association with hallux valgus deformity was not assessed.

Additionaly it was also aimed to show the incidence of the sesamoid bones. The number of sesamoid bones in the adult human skeleton can vary greatly among individuals. The reason for pathogenesis of these ossicles in the human skeleton is still unknown. The precise number of these ossicles is also unknown [5, 11, 22]. Although sesamoid bones of the first metatarsophalangeal joint (hallucal sesamoid) are considered as a normal part of the skeleton, sesamoids of the lesser toes are seen rarely. Congenital absence of the hallux sesamoids are an uncommon variation [11, 16]. In our cases, absence of the hallux sesamoids was not detected. Divisions of the hallucal sesamoid are difficult for surgeons to interpret on radiographs because they can mimic sesamoid fracture. Partitationed hallucal sesamoids may suffer or simulate fractures [11, 20]. The frequency of partitioning of the hallucal sesamoids is reported between 7.8 and 33% and bilateral involvement occurs in 13.5–90% of the observed populations [7]. The incidence of partite sesamoids as well as their etiology has been the subject of significant discussion in the literature. Medial division was reported as 7.2–30.6% and the lateral division was reported as 0.6–2.5% [4, 19]. Bilateral involvement occurs in 13.5–90% of observed populations. According to Inge and Ferguson [9] division of the sesamoids of the hallux is often unilateral. Kiter et al. [11] reported a lower rate of division of the hallucal sesamoids (4%) and the divisions were mostly bilateral. In the present study, the bipartite hallucal sesamoid was seen in 2.7% of cases. The bipartite medial hallucal sesamoid was 2.1% and the bipartite lateral hallucal sesamoid was 0.4%.

In the foot, lesser toe sesamoids are much more common on the medial side than on the lateral side, and lesser toe sesamoids are always more predominant in the second and fifth toes than in other lesser toes [5, 14, 15]. Possible reasons for these differences in frequency are the asymmetry of the metatarsal head at the plantar surface of the foot and the transverse plantar arch, both of which lead to unequal stress on the metatarsal heads. Kiter et al. [11] reported that metatarsophalangeal (MTP) sesamoid bones were seen in the second, third, fourth and fifth digits in 2.8, 0.5, 1 and 15.1%, respectively, in 371 turkish subjects. The occurrence of sesamoids in the fifth digit was reported higher (2.7 times) in males. In the present study, MTP sesamoid bones were seen in the second, third, fourth and fifth digits in 0.4, 0.2, 0.1 and 4.3% of all cases. In this study, we did not find a significant difference between males and females.

The hallucal interphalangeal (IP) sesamoid is considered by some to represent an anatomical rarity that possesses little clinical significance. However, the location of this seemingly innocuous sesamoid bone directly inferior to the IP joint of the hallux is associated with the development of several important anatomical, biomechanical, and clinical pathologies. The hallucal IP sesamoid has a detrimental effect on the biomechanical functions of the first MTP and hallucal IP joints. IP sesamoids may cause painful callosities plantar to the joint or may become incarcerated in a dislocated joint [6, 8, 21]. Its occurrence has been reported to be 3.9% by Dharap et al. [7], 5% by Bizzaro [2], and 13% by Jahss [10]. The incidence of the hallucal IP sesamoid was 2% in our study.

In conclusion, the present study represents the first detailed report with particular emphasis on the incidence of accessory ossicles and sesamoid bones of the feet in Turkish subjects. The results of the study seem to differ considerably from similar reports in other populations. It also provides anatomical data that could help clinicians in the diagnosis and management of disorders of accessory ossicles and sesamoid bones, which are often overlooked in patients who present with pain and discomfort in the feet.