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The anatomy of the navicular and periarticular structures
Foot Ankle Clin N Am 9 (2004) 1 – 23
The anatomy of the navicular and periarticular structures Pau Golano, MD*, Oscar Farin˜as, MD, Ivan Sa´enz, MD Laboratory of Arthroscopy and Surgical Anatomy, Department of Human Anatomy and Embryology, Faculty of Medicine, University of Barcelona, C/ Feixa Llarga s/n, 08907 L’Hospitalet de Llobregat, Barcelona, Spain
Bone morphology In general, the navicular bone has a roughly pyriform shape whose major oblique axis is oriented in the dorsoplantar and lateromedial directions, adapting itself to the angle of rotation of the head of the talus ( 45°) [1]. Its round base is situated dorsolaterally, whereas its apex is oriented plantarmedially. From its morphology, it is possible to distinguish four faces and two ends (Fig. 1). Posterior aspect The posterior aspect articulates with the head of the talus, although the articular surface does not completely cover it [2]. It has a biconcave surface that is completely covered with articular cartilage (Fig. 2). The degree of concavity is variable; in some cases, the articular surface is nearly flat [3]. Anterior aspect The anterior aspect has a nephroid appearance with plantar concavity. Two slight crests divide the anterior aspect into three articular surfaces. These crests extend dorsoplantarly and converge at the plantar margin. Although the articular surfaces are oriented in different directions, overall, the anterior surface is convex (Fig. 3). The largest articular surface, the medial articular surface, is convex and articulates with the medial cuneiform bone. It has a roughly triangular shape,
* Corresponding author. E-mail address: [email protected] (P. Golano). 1083-7515/04/$ – see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/S1083-7515(03)00155-4
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Fig. 1. Posterior view of the navicular bone showing its different parts. 1, dorsal aspect; 2, lateral end; 3, inferior aspect; 4, Medial end. The medial end corresponds to the navicular tuberosity (black star). The navicular beak (white star), when present, transforms the posterior articular surface of navicular bone to a quadrangular shape.
Fig. 2. (A) Sagittal T1-weighted MRI at level of talocalcaneal and talonavicular joints and (B) transverse CT image at level of talonavicular and cuneonavicular joints. Note the biconcavity of the posterior aspect of the navicular bone. 1, talus; 2, os calcis; 3, navicular; 4, cuboid; 5, medial cuneiform; 6, middle cuneiform.
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Fig. 3. Transverse CT image at the level of navicular bone. Note the convexity of the anterior aspect of the navicular bone that is formed by the convergence of the three articular surfaces of the cuneiform bones (white arrows). 1, navicular bone; 2, Os calcis.
with a dorsally convex base. The middle or intermediate articular surface, which is flat or slightly convex, also is triangular with a dorsal base. It articulates with the intermediate or middle cuneiform bone. The lateral articular surface, which is flat or slightly convex, is the smallest of the three and has roughly quadrangular morphology. The three articular surfaces converge plantarly and make up the transverse tarsal arch of the foot. As mentioned by Sarrafian [4], the navicular bone plays a role in the change in direction of the medial bone column. The neck and head of the talus initiate a medial deviation, whereas the navicular bone orients this column laterally and plantarly. The ‘‘zigzag’’ arrangement maintains the axial alignment of the foot, despite the initial divergence. Dorsal aspect Markedly convex, the dorsal aspect is wider in its medial portion. The highest point of the convexity coincides with the intermediate or middle articular surface. This aspect provides insertion for many capsulo-ligamentous structures.
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Fig. 4. Fluoroscan image of two different navicular bones: (A) with navicular beak (black arrow) and (B) without it.
Plantar aspect The plantar aspect is irregular and is continuous medially with the navicular tuberosity. It often presents an osseous prominence called the ‘‘navicular beak’’ (Fig. 4). Some investigators consider this prominence to be a fused os cuboides secundarium [5,6]. When this prominence is present, the posterior articular surface of the navicular bone extends downwards, thus adopting a quadrangular morphology. Like the dorsal aspect, the plantar aspect provides insertion for many capsulo-ligamentous structures. Medial end The medial end is made up of an osseous prominence, the navicular tuberosity. The plantar and medial avicular ligaments, as well as the tendon of the posterior tibialis muscle, insert into the navicular tuberosity. The size of this osseous prominence is variable. When it is separated from the rest of the bone, it is known as the ‘‘naviculare secundarium’’ or ‘‘accessory navicular bone.’’ Lateral end The lateral end is convex and has two discernible segments—the inferior or plantar segment and the superior or dorsal segment. The superior segment provides insertion for the medial component of the bifurcate ligament or lateral calcaneonavicular ligament. A small, inconstant articular surface [3] for the cuboid occupies nearly all of the inferior segment and is continuous with the articular surface for the lateral cuneiform bone.
Joints and ligaments From a functional and anatomical point of view, the subtalar articular complex is formed by the posterior talocalcaneal joint and the acetabulum pedis [7], which contains the head of the talus. The talocalcaneonavicular joint, or acetabulum pedis, thus named for its morphologic similarity to the hip joint [8], is made up of a series of skeletal and ligamentous structures. The skeletal elements
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include the posterior articular surface of the navicular and the anterior and middle calcaneal articular surfaces, which articulate with the head and the anteromedial surface of the anterior aspect of the talus. These osseous structures are joined or stabilized by the inferior or plantar and superomedial calcaneonavicular ligaments. The lateral calcaneonavicular ligament, a component of the bifurcate ligament, provides the lateral limit (Fig. 5). There is a lot of confusion and disagreement in the literature [9– 13] regarding the description of the plantar calcaneonavicular ligament, also known as the spring ligament. According to Testut and Jacob [8], the plantar calcaneonavicular ligament is made up of a fibrocartilaginous and a fibrous component in which it is possible to differentiate two types of fascicles, an anterior fascicle that is inserted into the plantar aspect of the navicular and a transverse fascicle that is
Fig. 5. Dorsal view of the acetabulum pedis after removing the talus. The components of the acetabulum pedis are classified as osseous (posterior articular surface of navicular bone, anterior and middle calcaneal articular surface) and ligamentous (superomedial, inferior, and lateral calcaneonavicular ligaments). 1, posterior articular surface of navicular bone; 2, anterior calcaneal articular surface; 3, middle calcaneal articular surface, 4; posterior calcaneal articular surface; 5, superomedial calcaneonavicular ligament; 6, inferior calcaneonavicular ligament; 7, lateral calcaneonavicular ligament (component of bifurcate ligament).
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fused with the deltoid ligament. Weitbreck [14] differentiated two fascicles of the plantar calcaneonavicular ligament. The main one, plantar and rounded, inserts into the plantar aspect of the navicular; the second, medial and flat, inserts into the medial end of the navicular bone. Moreover, in 1993, Sarrafian [4] described two differentiated ligaments: the superomedial and inferior calcaneonavicular ligaments. It is possible that the confusion about the description of these structures lies in their indivisibility; the medial margin of the inferior calcaneonavicular ligament is continuous with the superomedial calcaneonavicular ligament [15]. Also it is difficult to dissecting these ligaments because of their fibrocartilaginous characteristics, although they can be delimited artificially. Some investigators speak of a spring ligament complex that includes the superomedial and inferior calcaneonavicular ligaments, as well as the talonavicular fascicle of the superficial component of the deltoid ligament [2,16].
Fig. 6. (A) Medial view of an osseous rearfoot and midfoot. Note the origin and insertion sites of superomedial calcaneonavicular ligament (dark green lines). This ligament joins laterally with fibers of the tibionavicular component of the deltoid ligament (pink lines), and plantarly with the inferior calcaneonavicular ligament (light green lines). (B) Medial view of navicular and os calcis. The superomedial calcaneonavicular ligament attaches to the superior, medial, and inferior articular margins of the medial third of the navicular bone and sustentaculum tali. 1, navicular tuberosity; 2, sustentaculum tali.
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Superomedial calcaneonavicular ligament The superomedial calcaneonavicular ligament, also known as the ‘‘ligamentum neglectum,’’ was initially described as a component of the tibiocalcaneonavicular ligament [17], but was later classified by Lane [18] as a separate ligament. It has a quadrangular shape and its fibers intermingle with those of the inferior calcaneonavicular ligament. Its origin is at the anterior and medial margins of the sustentaculum tali, as well as at the anterior margin of the anterior articular surface of the os calcis. This ligament shares its origin with the tibiocalcaneal part of the superficial deltoid ligament. From its origin, the superomedial calcaneonavicular ligament follows the perimeter of the margin of the anterior calcaneal articular surface, fans out anteriorly, dorsally, and laterally, with a lateral concavity, and ends in a broad insertion at the margin of the posterior articular surface of the navicular. Its insertional surface covers the superior, medial, and inferior articular margins of the medial third of the navicular (Figs. 6 and 7) [16]. There are discrepancies in the anatomical studies concerning the insertion of this ligament into the navicular tuberosity [2,4,16]. This ligament is fused along its course with the different components of the deltoid ligament, as well as with the superior talonavicular ligament. At its dorsal aspect, it is in intimate relation with a thick fibrous or fibrocartilaginous layer that forms the floor of the fibrous tunnel of the tendon of the posterior tibialis muscle (see Fig. 7). Its articular surface presents a triangular fibrocartilaginous surface that corresponds in size and shape to the triangular plantar-medial articular surface of the head of the talus (Figs. 8 and 9). The fibrocartilaginous surface occasionally is ossified, which may be one of the reasons for confusion about the existence of a possible accessory
Fig. 7. Sequential transverse T1-weighted MRI at level of talocalcaneonavicular joint showing the location of superomedial calcaneonavicular ligament (white arrows) (A – C) cross sections in dorsoplantar direction. Note its close relation with the posterior tibialis tendon. 1, navicular; 2, head of the talus; 3, os calcis; 4, posterior tibialis tendon.
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Fig. 8. Fluoroscan dorsoplantar view of an osteoarticular fresh-frozen cadaveric specimen. The talus has been removed to show the osseous components of the acetabulum pedis. Note the presence of a fibrocartilaginous tissue (black star) that corresponds with the superomedial calcaneonavicular ligament.
Fig. 9. Medial (A), dorsal (B), and anterior (C) view of the talus. Note the triangular plantarmedial articular surface of the head of the talus (arrows) for the superomedial calcaneonavicular ligament.
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Fig. 10. Dorsal view of spring ligament. The spring ligament is composed of the superomedial and inferior calcaneonavicular ligament. Note the two fascicles of the inferior calcaneonavicular ligament. 1a, Medial fascicle; 1b, lateral fascicle; 2, superomedial calcaneonavicular ligament; 3, posterior articular surface of navicular bone; 4, anterior calcaneal articular surface; 5, middle calcaneal articular surface.
Fig. 11. Medial view of the os calcis. The inferior calcaneonavicular ligament arises from coronoid fossa (arrow). This fossa corresponds to a small depression that is located between the anterior and middle calcaneal articular surface, just anterior to the sustentaculum tali.
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navicular bone. The concave structure of this ligament provides a gentle transitional surface for the head of the talus. Inferior calcaneonavicular ligament The inferior calcaneonavicular ligament is trapezoidal in shape. It is in relation with the inferior segment of the head of the talus, which is not supported by articular surfaces (Fig. 10). It arises from the most superior part of the coronoid fossa, located in the space between the anterior and middle calcaneal articular surfaces, at the level of the anterior aspect of the sustentaculum tali (Fig. 11). This ligament inserts into the plantar aspect of the navicular bone, laterally to the navicular beak, and just lateral to the insertion of the superomedial calcaneonavicular ligament (Fig. 12). This ligament has a fascicular morphology; the lateral fascicle, which inserts into the navicular beak, is the strongest. Between the fiber bundles there is a series
Fig. 12. (A) Plantar view of an osseous rearfoot and midfoot. The inferior calcaneonavicular ligament (light green lines) arises from the coronoid fossa and attaches to the plantar aspect of the navicular bone. There is a confluence of fibers with the superomedial calcaneonavicular ligament (dark green lines) medially. (B) Plantar view of fresh-frozen cadaveric specimen showing the inferior calcaneonavicular ligament after the posterior tibialis tendon has been removed. 1, coronoid fossa; 2, navicular bone; 3, inferior calcaneonavicular ligament; 4, plantar cubonavicular ligament.
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Fig. 12 (continued).
of longitudinal intervals, with an adipose layer spread through them that covers the plantar aspect of this ligament (Fig. 13). This fat pad is located intra-articularly but extrasynovially. The medial margin of this ligament is continuous with the superomedial calcaneonavicular ligament; in most cases, a small triangular space that is occupied by fatty tissue, separates the two ligaments at their navicular attachments [16]. Its dorsal surface is fibrocartilaginous for the support of the head of the talus and appears a triangular morphology. Medially, its plantar surface is in contact with the tendon of the posterior tibialis muscle, and, laterally, with the tendons of the flexor hallucis longus and flexor digitorum longus muscles [12]. Together with the anterior superficial fibers of the deltoid ligament, the long plantar ligament, and the plantar fascia, the inferior calcaneonavicular ligament is one of the main static stabilizers of the longitudinal arch of the foot [16]. Bifurcate ligament The bifurcate ligament (or Chopart’s ligament) is made up of the lateral calcaneonavicular ligament and the medial calcaneocuboid ligament, which are arranged in a ‘‘Y’’ or ‘‘V’’ with different origins at the os calcis.
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Fig. 13. Dorsal view of acetabulum pedis. The talus has been removed. (A) A fat pad covers the inferior calcaneonavicular ligament. (B) After the fat tissue has been removed the different fascicles of the inferior calcaneonavicular ligament can be seen. Note that the anterior and middle calcaneal articular surfaces can be fused. 1, posterior articular surface of navicular bone; 2, anterior and middle calcaneal articular surface; 3, superomedial calcaneonavicular ligament; 4a and 4b, inferior calcaneonavicular ligament; 5, lateral calcaneonavicular ligament (component of bifurcate ligament).
The lateral calcaneonavicular ligament arises from the anteromedial angle of the sinus tarsi, just lateral to the anterior talar articular surface, and reaches the lateral aspect of the intermediary tubercle [19]. It extends anteriorly, dorsally, and medially to its insertion in the superior segment of the lateral end of the navicular (see Figs. 5 and 13). Barclay-Smith [20] described this ligament as being formed by two fiber bundles. The inferior fibers are short and are separated from the most lateral portion of the inferior calcaneonavicular ligament by a fatty interval. The superior fibers, which are the most superficial, are long and resistant and constitute the main part of this ligament. The medial calcaneocuboid ligament arises from the anterior aspect of the intermediary tubercle, lateral to the origin of the lateral calcaneonavicular ligament. It runs anteriorly and slightly inferiorly and attaches into the dorsal aspect of the cuboid (approximately 1.5 cm anterior to the posterior margin of the
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Fig. 13 (continued).
Fig. 14. Medial view of the ankle showing the location of the PTT toward the foot sole. Note its relationship with the deltoid ligament and superomedial calcaneonavicular ligament. 1, tendon of posterior tibialis muscle; 2, deltoid ligament; 3, navicular tuberosity; 4, superomedial calcaneonavicular ligament.
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cuboid). The two components of the bifurcate ligament form an angle of approximately 30° in the transverse plane and 20° in the sagittal plane [21].
Tibialis posterior tendon The navicular bone provides tendinous insertion only to the posterior tibialis muscle. The posterior tibialis muscle is located in the deep compartment of the leg,
Fig. 15. Plantar view of the foot. Drawing (A) and fresh-frozen cadaveric specimen dissection (B). The anterior component of the PTT, continuity of the main tendon, attaches to the navicular tuberosity (1), the inferior aspect of the medial cuneiform, and the inferior capsule of the medial cuneonavicular joint (2).
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Fig. 15 (continued).
between the flexor hallucis longus and flexor digitorum longus muscles. From its broad surface of origin, it runs distally, forming a long tendon. At the level of the inferior third of the leg, the posterior tibialis tendon (PTT), which initially runs laterally to the flexor digitorum longus muscle, crosses this muscle and runs medially, forming the sural decussation. With respect to the ankle joint, the PTT is located just behind the medial malleolus, within an osteofibrous groove, and on top of the deltoid ligament (Fig. 14). With respect to the foot, the PTT is in contact with the inferior aspect of the inferior calcaneonavicular ligament. Here, the tendon becomes flattened and acquires fibrocartilaginous characteristics, or, may even have a sesamoid bone within it; this sesamoid bone represents one of the possible types of accessory navicular bones. Right at the navicular tuberosity, the tendon of the posterior tibialis muscle is divided into three components: anterior, middle, and posterior. The anterior component is the largest of the three and is the continuation of the main tendon.
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It inserts into the navicular tuberosity and the inferior capsule of the medial cuneonavicular joint, as well as the inferior aspect of the medial cuneiform bone (Fig. 15). The middle component is attached into the middle and lateral cuneiform bones, the cuboid, and the bases of the second to fifth metatarsal bones (metatarsal extension) (Fig. 16). In all specimens, a bursa was found in relation to the metatarsal extension of the PTT. This bursa is independent of the synovial sheath of the tendon of the posterior tibialis muscle and a small fibrous septum separates them [22]. Its proximal limit of the bursa is located near the insertion of the tendon into the navicular tuberosity. Finally, the posterior component, with its recurrent path, inserts into the sustentaculum tali [23].
Fig. 16. Plantar view of the foot. Drawings showing the middle component attachments at the level of the middle and lateral cuneiform and cuboid (A) and to the bases of the second to fifth metatarsal bones (B). (C) Fresh-frozen cadaveric specimen dissection of the attachments of the middle component of the PTT. 1, attachment to middle cuneiform bone; 2, attachment to lateral cuneiform bone; 3, attachment to cuboid; 4 – 6, metatarsal extension (attachments to the bases of second to fifth metatarsal bones).
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Fig. 16 (continued).
The posterior tibialis muscle-tendon unit is the main dynamic stabilizer of the rearfoot; it maintains the structure formed by the longitudinal arch of the foot [24]. Many factors are implicated as causes of failure of the PTT, including: (1) impingement in the osteofibrous groove, (2) compression by the flexor retinaculum, (3) presence of an accessory navicular bone, (4) weakening in the area of insertion, (5) hypovascularization of the tendon at the level of the medial malleolus [25], (6) inflammatory arthropathy, (7) acute traumatism [26], (8) corticosteroid injection, and (9) chronic mechanical overload [27,28]. A dysfunction of the tendon of the posterior tibialis muscle causes an inversion of the rearfoot which blocks the transverse tarsal joint (Chopart’s joint) during the middle and late phases of gait. As a result, the contractive force of the triceps surae must act through the talonavicular joint instead of the metatarsal heads. In this situation, the head of the talus acts repeatedly on the spring ligament. A lengthening or rupture of this ligament enables the talus to carry out plantar flexion with a valgus alignment of the os calcis which results in an adult acquired flatfoot deformity [8,29 –34].
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Accessory navicular bone At the posterior margin of the posteromedial navicular tuberosity, a supernumerary ossiculum, that serves as insertion for the PTT, along with the navicular bone, is found occasionally [35]. Initially described by Bahuin in 1605, the accessory navicular bone is considered to be a radiographic and anatomical variant present in 4% to 21% of the population [36], although the highest rates are found in patients who have flat foot (19%) [37]. It is more frequent in women and is visible radiographically from the age of 9 years [35]. Depending on the accessory navicular bone’s morphology and position, as well as on whether there is synchondrosis with the navicular bone, Weitch in 1978, divided it into three types [38]: Type I. Os tibiale externum, naviculare secundarium or accessory navicular. This type represents 30% of the total. Located within the tendon of the posterior tibialis muscle, this sesamoid bone presents an ovoid morphology of about 2 mm to 3 mm (Fig. 17).
Fig. 17. Posteroanterior (PA) radiograph of bilateral type I accessory navicular (arrows).
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Fig. 18. PA radiograph of type III accessory navicular (arrows).
Type II. Prehallux or bifurcate hallux. This type is found in 50% to 60% of cases and shows a triangular morphology of about 12 mm at its major axis. Formed from a secondary ossification center of the navicular bone, it is found joined to this bone by fibrocartilage or hyaline cartilage [39,40], which creates a synchondrosis. Type III. The accessory navicular bone is a prominent navicular tuberosity known as a cornuated navicular [41] and is considered to be the last stage in the fusion of type II (Fig. 18) [36]. This bone is considered to be an asymptomatic variant, although when it suffers trauma, it can cause painful symptoms.
Fig. 19. Lateral radiograph of type II accessory navicular (arrow).
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In 1986, Sella et al [40] divided type II into two subtypes (IIa and IIb) depending on its localization with respect to the navicular and on the angle formed between the synchondrosis and the lateral process of the talus (the SOT angle). In subtype IIa, the SOT angle is an average of 56.3° and the ossiculum is located dorsally. In subtype IIb, the SOT angle is much more acute and the ossiculum is situated more plantarly (Figs. 19 and 20). When the fibrocartilage of the synchondrosis of types IIa and IIb is subjected to tension or compressive forces, it can present histologic findings that are similar to bone fractures [40,41]. Radiographically, the changes are imperceptible and require 99mTc (99mtechnetium methylene diphosphonate) in follow-up testing to detect the increase in activity. Occasionally, surgery is necessary in symptomatic cases of this supernumerary bone when orthopedic treatments have failed [36,42]. The presence of an accessory navicular has been associated with several pathologies, such as flatfoot [43,44], or, more recently, hallux limitus [45].
Fig. 20. PA radiograph of type II accessory navicular (arrows).
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Vascularization of the navicular bone The navicular bone has vascular branches at its dorsal and plantar aspects, as well as its tuberosity. The dorsal vascularization arises from a vascular branch of the dorsalis pedis artery which, upon crossing the dorsum of the navicular, divides into three to five branches [46]. Branches that arise directly from the dorsalis pedis artery also are found occasionally. The plantar vascularization depends on vessels that arise from the medial plantar artery, whereas the navicular tuberosity receives vessels from an anastomotic network that is formed by the dorsalis pedis and medial plantar arteries. This arterial network provides a rich vascularization to the medial and lateral portions of the navicular, although the central portion of the bone shows a lower level of vascularization [47]. As described by Zchakaja [48], with increasing age, there is a decrease in the number of supplementary arteries that supply the navicular bone. This may be a cause of osteonecrosis or stress fractures at the middle third of this bone, as described initially by Torg et al [47] and, later, by other investigators [49 –53].
Acknowledgments We would like to thank to Joan Angel Clavero, MD from Diagnosis Medica (Barcelona, Spain) for his support and the use of the MR and CT images that appear in this article.
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[13] Chen JP, Allen AM. MR diagnosis of traumatic tear of the spring ligament in a pole vaulter. Skeletal Radiol 1997;26:310 – 2. [14] Weitbreck J. Syndesmology. Philadelphia: WB Saunder; 1969. [15] De Palma L, Santucci A, Ventura A, et al. Anatomy and embryology of the talocalcaneal joint. Foot Ankle Surg 2003;9:7 – 18. [16] Davis WH, Sobel M, DiCarlo EF, et al. Gross, histological, and microvascular anatomy and biomechanical testing of the spring ligament complex. Foot Ankle 1996;17:95 – 102. [17] Henle J. Handbuch der systematischen anatomie des menschen (Handbook of the human systematic anatomy), vol 3. Braunschweig (Germany): Von Friedrich Vieweg und John; 1868. [18] Lane AS. The causation, pathology and physiology of several of the deformities wich develop during young life. Guys Hosp Rep 1887;44:254. [19] Ko¨ktu¨rk C. Remarques sur le ligament de Chopart (lig. Bifurcatum) [Remarks on Chopart’s ligament (lig. bifurcatum)]. Comptes-Rendus Assoc Anat 1957;44:380 [in French]. [20] Barclay-Smith E. The astragalo-calcaneo-navicular joint. J Anat Physiol 1896;30:390. [21] Hovelacque A, Sourdin A. Note au sujet de quelques ligaments de l’articulation me´dio-tarsienne [Note on the subject of some ligements of mediotrsal articulation]. Ann Anat Pathol Anat Normal 1933;10:469. [22] Jones FW. Structure and function as seen in the foot. London: Baillie`re, Tindall & Cox; 1944. [23] Martin BF. Observations on the muscles and tendons of the medial aspect of the sole of the foot. J Anat 1964;98:437 – 53. [24] Kitaoka HB, Luo ZP, An K-A. Effect of the posterior tibial tendon on the arch of the foot during simulated weightbearing: biomechanical analysis. Foot Ankle 1997;18:43 – 6. [25] Frey C, Shereff M, Greenidge N. Vascularity of the posterior tibial tendon. J Bone Joint Surg Am 1990;72A:884 – 8. [26] Marks RM, Schon JC. Posttraumatic posterior tibialis tendon insertional elongation with incompetency: a case report. Foot Ankle 1998;19:180 – 3. [27] Rosenberg ZS. Chronic rupture of the posterior tibial tendon. Magn Reson Imaging Clin North Am 1994;2:279 – 86. [28] Pomeroy GC, Pike RH, Beals TC, et al. Acquired flatfoot in adults due to dysfunction of the posterior tibial tendon. J Bone Joint Surg Am 1999;81-A:1173 – 82. [29] Jahss MH. Tendons disorders of the foot and ankle. In: Jahss MH, editor. Disorders of the foot & ankle: medical & surgical management. 2nd edition. Philadelphia: W.B. Saunders; 1991. p. 1321 – 71. [30] Mann RA. Flatfoot in adults. In: Coughlin MJ, Mann RA, editors. Surgery of the foot & ankle. 6th edition. St. Louis (MO): Mosby-Year Book; 1999. p. 733 – 67. [31] Funk DA, Cass JR, Johnson KA. Acquired adult flatfoot secondary to posterior tibial tendon pathology. J Bone Joint Surg Am 1986;68A:95 – 102. [32] Goldner JL, Geats PK, Bassett FH, et al. Progressive talipes equinovalgus due to trauma or degeneration of the posterior tibial tendon and medial plantar ligaments. Orthop Clin North America 1974;5:39 – 49. [33] Balen PF, Helms CA. Association of posterior tibial tendon injury with spring ligament injury, sinus tarsi abnormality, and plantar fasciitis on MR imaging. AJR 2001;176:1137 – 43. [34] Gazdag AR, Cracchiolo A. Rupture of the posterior tibial tendon. Evaluation of injury of the spring ligament and clinical assessment of tendon transfer and ligament repair. J Bone Joint Surg Am 1997;79A:675 – 81. [35] Sever JW. Clinical importance of the os tibiale externum, or accessory tarsal scaphoid. JAMA 1927;89:359 – 61. [36] Miller TT, Staron RB, Feldman F, et al. The symptomatic accessory tarsal navicular bone: assessment with MR imaging. Radiology 1995;195:849 – 53. [37] Wood WA, Spencer AM. Incidence of os tibiale externum in clinical pes planus. JAPA 1970;60:276 – 7. [38] Weitch JM. Evaluation of the Kidner procedure in treatment of syntomatic accessory tarsal scaphoid. Clin Orthop 1978;131:210 – 3.
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[39] Zadek I, Gold AM. The accessory tarsal scaphoid. J Bone Joint Surg Am 1948;30:957. [40] Lawson JP, Ogden JA, Sella EJ, et al. The painful accessory navicular. Skeletal Radiol 1984;12: 250 – 62. [41] Sella EJ, Lawson JP, Ogden JA. The accessory navicular synchondrosis. Clin Orthop 1986; 209:280 – 5. [42] Ray S, Goldberg VM. Surgical treatment of the accessory navicular. Clin Orthop Rel Res 1983; 177:61 – 6. [43] Kidner FC. The prehallux (accesorie scaphoid), in its relation with flat foot. J Bone Joint Surg Am 1929;11A:831 – 7. [44] Hatoff A. Bipartite navicular bone as a cause of flat foot. Am J Dis Child 1950;80:991. [45] Lee Evans RD, Averett R. The association of hallux limitus with the accessory navicular. J Am Podiatr Med Assoc 2002;92:359 – 65. [46] Velluda C. Sur la vascularisation du scaphoid du tarse [[On the vascularization of the tarsal scaphoid]]. Ann Anat Pathol 1928;5:1016 [in French]. [47] Torg J, Pavlov H, Cooley L, et al. Stress fractures of the tarsal navicular. J Bone Joint Surg Am 1982;64A:700 – 12. [48] Zchakaja MJ. Blutversorgung der knochen des fusses (Vascularisation of the feb bones) (ossa pedis). Fortschr Gebiete Ro¨ntgenol 1932;45:160. [49] Fitch K, Blackwell J, Gilmour W. Operation for non-union of stress fracture of the tarsal navicular. J Bone Joint Surg Br 1989;71B:105 – 10. [50] Haller J, Sartoris DJ, Resnick D, et al. Spontaneous osteonecrosis of the tarsal navicular in adults. AJR 1988;151:355 – 8. [51] Kiss ZS, Khan KM, Fuller PJ. Stress fractures of the tarsal navicular bone: CT findings in 55 cases. AJR 1993;160:111 – 5. [52] Gu¨nal I, Yo¨ru¨koglu K. Osteonecrosis of the accessory navicular bone. Arch Orthop Trauma Surg 2001;121:546 – 7. [53] Coughlin MJ. Tarsal navicular stress fractures. Tech Foot Ankle Surg 2002;1:112 – 22.
The anatomy of the navicular and periarticular structures Pau Golano, MD*, Oscar Farin˜as, MD, Ivan Sa´enz, MD Laboratory of Arthroscopy and Surgical Anatomy, Department of Human Anatomy and Embryology, Faculty of Medicine, University of Barcelona, C/ Feixa Llarga s/n, 08907 L’Hospitalet de Llobregat, Barcelona, Spain
Bone morphology In general, the navicular bone has a roughly pyriform shape whose major oblique axis is oriented in the dorsoplantar and lateromedial directions, adapting itself to the angle of rotation of the head of the talus ( 45°) [1]. Its round base is situated dorsolaterally, whereas its apex is oriented plantarmedially. From its morphology, it is possible to distinguish four faces and two ends (Fig. 1). Posterior aspect The posterior aspect articulates with the head of the talus, although the articular surface does not completely cover it [2]. It has a biconcave surface that is completely covered with articular cartilage (Fig. 2). The degree of concavity is variable; in some cases, the articular surface is nearly flat [3]. Anterior aspect The anterior aspect has a nephroid appearance with plantar concavity. Two slight crests divide the anterior aspect into three articular surfaces. These crests extend dorsoplantarly and converge at the plantar margin. Although the articular surfaces are oriented in different directions, overall, the anterior surface is convex (Fig. 3). The largest articular surface, the medial articular surface, is convex and articulates with the medial cuneiform bone. It has a roughly triangular shape,
* Corresponding author. E-mail address: [email protected] (P. Golano). 1083-7515/04/$ – see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/S1083-7515(03)00155-4
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Fig. 1. Posterior view of the navicular bone showing its different parts. 1, dorsal aspect; 2, lateral end; 3, inferior aspect; 4, Medial end. The medial end corresponds to the navicular tuberosity (black star). The navicular beak (white star), when present, transforms the posterior articular surface of navicular bone to a quadrangular shape.
Fig. 2. (A) Sagittal T1-weighted MRI at level of talocalcaneal and talonavicular joints and (B) transverse CT image at level of talonavicular and cuneonavicular joints. Note the biconcavity of the posterior aspect of the navicular bone. 1, talus; 2, os calcis; 3, navicular; 4, cuboid; 5, medial cuneiform; 6, middle cuneiform.
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Fig. 3. Transverse CT image at the level of navicular bone. Note the convexity of the anterior aspect of the navicular bone that is formed by the convergence of the three articular surfaces of the cuneiform bones (white arrows). 1, navicular bone; 2, Os calcis.
with a dorsally convex base. The middle or intermediate articular surface, which is flat or slightly convex, also is triangular with a dorsal base. It articulates with the intermediate or middle cuneiform bone. The lateral articular surface, which is flat or slightly convex, is the smallest of the three and has roughly quadrangular morphology. The three articular surfaces converge plantarly and make up the transverse tarsal arch of the foot. As mentioned by Sarrafian [4], the navicular bone plays a role in the change in direction of the medial bone column. The neck and head of the talus initiate a medial deviation, whereas the navicular bone orients this column laterally and plantarly. The ‘‘zigzag’’ arrangement maintains the axial alignment of the foot, despite the initial divergence. Dorsal aspect Markedly convex, the dorsal aspect is wider in its medial portion. The highest point of the convexity coincides with the intermediate or middle articular surface. This aspect provides insertion for many capsulo-ligamentous structures.
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Fig. 4. Fluoroscan image of two different navicular bones: (A) with navicular beak (black arrow) and (B) without it.
Plantar aspect The plantar aspect is irregular and is continuous medially with the navicular tuberosity. It often presents an osseous prominence called the ‘‘navicular beak’’ (Fig. 4). Some investigators consider this prominence to be a fused os cuboides secundarium [5,6]. When this prominence is present, the posterior articular surface of the navicular bone extends downwards, thus adopting a quadrangular morphology. Like the dorsal aspect, the plantar aspect provides insertion for many capsulo-ligamentous structures. Medial end The medial end is made up of an osseous prominence, the navicular tuberosity. The plantar and medial avicular ligaments, as well as the tendon of the posterior tibialis muscle, insert into the navicular tuberosity. The size of this osseous prominence is variable. When it is separated from the rest of the bone, it is known as the ‘‘naviculare secundarium’’ or ‘‘accessory navicular bone.’’ Lateral end The lateral end is convex and has two discernible segments—the inferior or plantar segment and the superior or dorsal segment. The superior segment provides insertion for the medial component of the bifurcate ligament or lateral calcaneonavicular ligament. A small, inconstant articular surface [3] for the cuboid occupies nearly all of the inferior segment and is continuous with the articular surface for the lateral cuneiform bone.
Joints and ligaments From a functional and anatomical point of view, the subtalar articular complex is formed by the posterior talocalcaneal joint and the acetabulum pedis [7], which contains the head of the talus. The talocalcaneonavicular joint, or acetabulum pedis, thus named for its morphologic similarity to the hip joint [8], is made up of a series of skeletal and ligamentous structures. The skeletal elements
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include the posterior articular surface of the navicular and the anterior and middle calcaneal articular surfaces, which articulate with the head and the anteromedial surface of the anterior aspect of the talus. These osseous structures are joined or stabilized by the inferior or plantar and superomedial calcaneonavicular ligaments. The lateral calcaneonavicular ligament, a component of the bifurcate ligament, provides the lateral limit (Fig. 5). There is a lot of confusion and disagreement in the literature [9– 13] regarding the description of the plantar calcaneonavicular ligament, also known as the spring ligament. According to Testut and Jacob [8], the plantar calcaneonavicular ligament is made up of a fibrocartilaginous and a fibrous component in which it is possible to differentiate two types of fascicles, an anterior fascicle that is inserted into the plantar aspect of the navicular and a transverse fascicle that is
Fig. 5. Dorsal view of the acetabulum pedis after removing the talus. The components of the acetabulum pedis are classified as osseous (posterior articular surface of navicular bone, anterior and middle calcaneal articular surface) and ligamentous (superomedial, inferior, and lateral calcaneonavicular ligaments). 1, posterior articular surface of navicular bone; 2, anterior calcaneal articular surface; 3, middle calcaneal articular surface, 4; posterior calcaneal articular surface; 5, superomedial calcaneonavicular ligament; 6, inferior calcaneonavicular ligament; 7, lateral calcaneonavicular ligament (component of bifurcate ligament).
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fused with the deltoid ligament. Weitbreck [14] differentiated two fascicles of the plantar calcaneonavicular ligament. The main one, plantar and rounded, inserts into the plantar aspect of the navicular; the second, medial and flat, inserts into the medial end of the navicular bone. Moreover, in 1993, Sarrafian [4] described two differentiated ligaments: the superomedial and inferior calcaneonavicular ligaments. It is possible that the confusion about the description of these structures lies in their indivisibility; the medial margin of the inferior calcaneonavicular ligament is continuous with the superomedial calcaneonavicular ligament [15]. Also it is difficult to dissecting these ligaments because of their fibrocartilaginous characteristics, although they can be delimited artificially. Some investigators speak of a spring ligament complex that includes the superomedial and inferior calcaneonavicular ligaments, as well as the talonavicular fascicle of the superficial component of the deltoid ligament [2,16].
Fig. 6. (A) Medial view of an osseous rearfoot and midfoot. Note the origin and insertion sites of superomedial calcaneonavicular ligament (dark green lines). This ligament joins laterally with fibers of the tibionavicular component of the deltoid ligament (pink lines), and plantarly with the inferior calcaneonavicular ligament (light green lines). (B) Medial view of navicular and os calcis. The superomedial calcaneonavicular ligament attaches to the superior, medial, and inferior articular margins of the medial third of the navicular bone and sustentaculum tali. 1, navicular tuberosity; 2, sustentaculum tali.
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Superomedial calcaneonavicular ligament The superomedial calcaneonavicular ligament, also known as the ‘‘ligamentum neglectum,’’ was initially described as a component of the tibiocalcaneonavicular ligament [17], but was later classified by Lane [18] as a separate ligament. It has a quadrangular shape and its fibers intermingle with those of the inferior calcaneonavicular ligament. Its origin is at the anterior and medial margins of the sustentaculum tali, as well as at the anterior margin of the anterior articular surface of the os calcis. This ligament shares its origin with the tibiocalcaneal part of the superficial deltoid ligament. From its origin, the superomedial calcaneonavicular ligament follows the perimeter of the margin of the anterior calcaneal articular surface, fans out anteriorly, dorsally, and laterally, with a lateral concavity, and ends in a broad insertion at the margin of the posterior articular surface of the navicular. Its insertional surface covers the superior, medial, and inferior articular margins of the medial third of the navicular (Figs. 6 and 7) [16]. There are discrepancies in the anatomical studies concerning the insertion of this ligament into the navicular tuberosity [2,4,16]. This ligament is fused along its course with the different components of the deltoid ligament, as well as with the superior talonavicular ligament. At its dorsal aspect, it is in intimate relation with a thick fibrous or fibrocartilaginous layer that forms the floor of the fibrous tunnel of the tendon of the posterior tibialis muscle (see Fig. 7). Its articular surface presents a triangular fibrocartilaginous surface that corresponds in size and shape to the triangular plantar-medial articular surface of the head of the talus (Figs. 8 and 9). The fibrocartilaginous surface occasionally is ossified, which may be one of the reasons for confusion about the existence of a possible accessory
Fig. 7. Sequential transverse T1-weighted MRI at level of talocalcaneonavicular joint showing the location of superomedial calcaneonavicular ligament (white arrows) (A – C) cross sections in dorsoplantar direction. Note its close relation with the posterior tibialis tendon. 1, navicular; 2, head of the talus; 3, os calcis; 4, posterior tibialis tendon.
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Fig. 8. Fluoroscan dorsoplantar view of an osteoarticular fresh-frozen cadaveric specimen. The talus has been removed to show the osseous components of the acetabulum pedis. Note the presence of a fibrocartilaginous tissue (black star) that corresponds with the superomedial calcaneonavicular ligament.
Fig. 9. Medial (A), dorsal (B), and anterior (C) view of the talus. Note the triangular plantarmedial articular surface of the head of the talus (arrows) for the superomedial calcaneonavicular ligament.
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Fig. 10. Dorsal view of spring ligament. The spring ligament is composed of the superomedial and inferior calcaneonavicular ligament. Note the two fascicles of the inferior calcaneonavicular ligament. 1a, Medial fascicle; 1b, lateral fascicle; 2, superomedial calcaneonavicular ligament; 3, posterior articular surface of navicular bone; 4, anterior calcaneal articular surface; 5, middle calcaneal articular surface.
Fig. 11. Medial view of the os calcis. The inferior calcaneonavicular ligament arises from coronoid fossa (arrow). This fossa corresponds to a small depression that is located between the anterior and middle calcaneal articular surface, just anterior to the sustentaculum tali.
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navicular bone. The concave structure of this ligament provides a gentle transitional surface for the head of the talus. Inferior calcaneonavicular ligament The inferior calcaneonavicular ligament is trapezoidal in shape. It is in relation with the inferior segment of the head of the talus, which is not supported by articular surfaces (Fig. 10). It arises from the most superior part of the coronoid fossa, located in the space between the anterior and middle calcaneal articular surfaces, at the level of the anterior aspect of the sustentaculum tali (Fig. 11). This ligament inserts into the plantar aspect of the navicular bone, laterally to the navicular beak, and just lateral to the insertion of the superomedial calcaneonavicular ligament (Fig. 12). This ligament has a fascicular morphology; the lateral fascicle, which inserts into the navicular beak, is the strongest. Between the fiber bundles there is a series
Fig. 12. (A) Plantar view of an osseous rearfoot and midfoot. The inferior calcaneonavicular ligament (light green lines) arises from the coronoid fossa and attaches to the plantar aspect of the navicular bone. There is a confluence of fibers with the superomedial calcaneonavicular ligament (dark green lines) medially. (B) Plantar view of fresh-frozen cadaveric specimen showing the inferior calcaneonavicular ligament after the posterior tibialis tendon has been removed. 1, coronoid fossa; 2, navicular bone; 3, inferior calcaneonavicular ligament; 4, plantar cubonavicular ligament.
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Fig. 12 (continued).
of longitudinal intervals, with an adipose layer spread through them that covers the plantar aspect of this ligament (Fig. 13). This fat pad is located intra-articularly but extrasynovially. The medial margin of this ligament is continuous with the superomedial calcaneonavicular ligament; in most cases, a small triangular space that is occupied by fatty tissue, separates the two ligaments at their navicular attachments [16]. Its dorsal surface is fibrocartilaginous for the support of the head of the talus and appears a triangular morphology. Medially, its plantar surface is in contact with the tendon of the posterior tibialis muscle, and, laterally, with the tendons of the flexor hallucis longus and flexor digitorum longus muscles [12]. Together with the anterior superficial fibers of the deltoid ligament, the long plantar ligament, and the plantar fascia, the inferior calcaneonavicular ligament is one of the main static stabilizers of the longitudinal arch of the foot [16]. Bifurcate ligament The bifurcate ligament (or Chopart’s ligament) is made up of the lateral calcaneonavicular ligament and the medial calcaneocuboid ligament, which are arranged in a ‘‘Y’’ or ‘‘V’’ with different origins at the os calcis.
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Fig. 13. Dorsal view of acetabulum pedis. The talus has been removed. (A) A fat pad covers the inferior calcaneonavicular ligament. (B) After the fat tissue has been removed the different fascicles of the inferior calcaneonavicular ligament can be seen. Note that the anterior and middle calcaneal articular surfaces can be fused. 1, posterior articular surface of navicular bone; 2, anterior and middle calcaneal articular surface; 3, superomedial calcaneonavicular ligament; 4a and 4b, inferior calcaneonavicular ligament; 5, lateral calcaneonavicular ligament (component of bifurcate ligament).
The lateral calcaneonavicular ligament arises from the anteromedial angle of the sinus tarsi, just lateral to the anterior talar articular surface, and reaches the lateral aspect of the intermediary tubercle [19]. It extends anteriorly, dorsally, and medially to its insertion in the superior segment of the lateral end of the navicular (see Figs. 5 and 13). Barclay-Smith [20] described this ligament as being formed by two fiber bundles. The inferior fibers are short and are separated from the most lateral portion of the inferior calcaneonavicular ligament by a fatty interval. The superior fibers, which are the most superficial, are long and resistant and constitute the main part of this ligament. The medial calcaneocuboid ligament arises from the anterior aspect of the intermediary tubercle, lateral to the origin of the lateral calcaneonavicular ligament. It runs anteriorly and slightly inferiorly and attaches into the dorsal aspect of the cuboid (approximately 1.5 cm anterior to the posterior margin of the
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Fig. 13 (continued).
Fig. 14. Medial view of the ankle showing the location of the PTT toward the foot sole. Note its relationship with the deltoid ligament and superomedial calcaneonavicular ligament. 1, tendon of posterior tibialis muscle; 2, deltoid ligament; 3, navicular tuberosity; 4, superomedial calcaneonavicular ligament.
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cuboid). The two components of the bifurcate ligament form an angle of approximately 30° in the transverse plane and 20° in the sagittal plane [21].
Tibialis posterior tendon The navicular bone provides tendinous insertion only to the posterior tibialis muscle. The posterior tibialis muscle is located in the deep compartment of the leg,
Fig. 15. Plantar view of the foot. Drawing (A) and fresh-frozen cadaveric specimen dissection (B). The anterior component of the PTT, continuity of the main tendon, attaches to the navicular tuberosity (1), the inferior aspect of the medial cuneiform, and the inferior capsule of the medial cuneonavicular joint (2).
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Fig. 15 (continued).
between the flexor hallucis longus and flexor digitorum longus muscles. From its broad surface of origin, it runs distally, forming a long tendon. At the level of the inferior third of the leg, the posterior tibialis tendon (PTT), which initially runs laterally to the flexor digitorum longus muscle, crosses this muscle and runs medially, forming the sural decussation. With respect to the ankle joint, the PTT is located just behind the medial malleolus, within an osteofibrous groove, and on top of the deltoid ligament (Fig. 14). With respect to the foot, the PTT is in contact with the inferior aspect of the inferior calcaneonavicular ligament. Here, the tendon becomes flattened and acquires fibrocartilaginous characteristics, or, may even have a sesamoid bone within it; this sesamoid bone represents one of the possible types of accessory navicular bones. Right at the navicular tuberosity, the tendon of the posterior tibialis muscle is divided into three components: anterior, middle, and posterior. The anterior component is the largest of the three and is the continuation of the main tendon.
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It inserts into the navicular tuberosity and the inferior capsule of the medial cuneonavicular joint, as well as the inferior aspect of the medial cuneiform bone (Fig. 15). The middle component is attached into the middle and lateral cuneiform bones, the cuboid, and the bases of the second to fifth metatarsal bones (metatarsal extension) (Fig. 16). In all specimens, a bursa was found in relation to the metatarsal extension of the PTT. This bursa is independent of the synovial sheath of the tendon of the posterior tibialis muscle and a small fibrous septum separates them [22]. Its proximal limit of the bursa is located near the insertion of the tendon into the navicular tuberosity. Finally, the posterior component, with its recurrent path, inserts into the sustentaculum tali [23].
Fig. 16. Plantar view of the foot. Drawings showing the middle component attachments at the level of the middle and lateral cuneiform and cuboid (A) and to the bases of the second to fifth metatarsal bones (B). (C) Fresh-frozen cadaveric specimen dissection of the attachments of the middle component of the PTT. 1, attachment to middle cuneiform bone; 2, attachment to lateral cuneiform bone; 3, attachment to cuboid; 4 – 6, metatarsal extension (attachments to the bases of second to fifth metatarsal bones).
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Fig. 16 (continued).
The posterior tibialis muscle-tendon unit is the main dynamic stabilizer of the rearfoot; it maintains the structure formed by the longitudinal arch of the foot [24]. Many factors are implicated as causes of failure of the PTT, including: (1) impingement in the osteofibrous groove, (2) compression by the flexor retinaculum, (3) presence of an accessory navicular bone, (4) weakening in the area of insertion, (5) hypovascularization of the tendon at the level of the medial malleolus [25], (6) inflammatory arthropathy, (7) acute traumatism [26], (8) corticosteroid injection, and (9) chronic mechanical overload [27,28]. A dysfunction of the tendon of the posterior tibialis muscle causes an inversion of the rearfoot which blocks the transverse tarsal joint (Chopart’s joint) during the middle and late phases of gait. As a result, the contractive force of the triceps surae must act through the talonavicular joint instead of the metatarsal heads. In this situation, the head of the talus acts repeatedly on the spring ligament. A lengthening or rupture of this ligament enables the talus to carry out plantar flexion with a valgus alignment of the os calcis which results in an adult acquired flatfoot deformity [8,29 –34].
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Accessory navicular bone At the posterior margin of the posteromedial navicular tuberosity, a supernumerary ossiculum, that serves as insertion for the PTT, along with the navicular bone, is found occasionally [35]. Initially described by Bahuin in 1605, the accessory navicular bone is considered to be a radiographic and anatomical variant present in 4% to 21% of the population [36], although the highest rates are found in patients who have flat foot (19%) [37]. It is more frequent in women and is visible radiographically from the age of 9 years [35]. Depending on the accessory navicular bone’s morphology and position, as well as on whether there is synchondrosis with the navicular bone, Weitch in 1978, divided it into three types [38]: Type I. Os tibiale externum, naviculare secundarium or accessory navicular. This type represents 30% of the total. Located within the tendon of the posterior tibialis muscle, this sesamoid bone presents an ovoid morphology of about 2 mm to 3 mm (Fig. 17).
Fig. 17. Posteroanterior (PA) radiograph of bilateral type I accessory navicular (arrows).
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Fig. 18. PA radiograph of type III accessory navicular (arrows).
Type II. Prehallux or bifurcate hallux. This type is found in 50% to 60% of cases and shows a triangular morphology of about 12 mm at its major axis. Formed from a secondary ossification center of the navicular bone, it is found joined to this bone by fibrocartilage or hyaline cartilage [39,40], which creates a synchondrosis. Type III. The accessory navicular bone is a prominent navicular tuberosity known as a cornuated navicular [41] and is considered to be the last stage in the fusion of type II (Fig. 18) [36]. This bone is considered to be an asymptomatic variant, although when it suffers trauma, it can cause painful symptoms.
Fig. 19. Lateral radiograph of type II accessory navicular (arrow).
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In 1986, Sella et al [40] divided type II into two subtypes (IIa and IIb) depending on its localization with respect to the navicular and on the angle formed between the synchondrosis and the lateral process of the talus (the SOT angle). In subtype IIa, the SOT angle is an average of 56.3° and the ossiculum is located dorsally. In subtype IIb, the SOT angle is much more acute and the ossiculum is situated more plantarly (Figs. 19 and 20). When the fibrocartilage of the synchondrosis of types IIa and IIb is subjected to tension or compressive forces, it can present histologic findings that are similar to bone fractures [40,41]. Radiographically, the changes are imperceptible and require 99mTc (99mtechnetium methylene diphosphonate) in follow-up testing to detect the increase in activity. Occasionally, surgery is necessary in symptomatic cases of this supernumerary bone when orthopedic treatments have failed [36,42]. The presence of an accessory navicular has been associated with several pathologies, such as flatfoot [43,44], or, more recently, hallux limitus [45].
Fig. 20. PA radiograph of type II accessory navicular (arrows).
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Vascularization of the navicular bone The navicular bone has vascular branches at its dorsal and plantar aspects, as well as its tuberosity. The dorsal vascularization arises from a vascular branch of the dorsalis pedis artery which, upon crossing the dorsum of the navicular, divides into three to five branches [46]. Branches that arise directly from the dorsalis pedis artery also are found occasionally. The plantar vascularization depends on vessels that arise from the medial plantar artery, whereas the navicular tuberosity receives vessels from an anastomotic network that is formed by the dorsalis pedis and medial plantar arteries. This arterial network provides a rich vascularization to the medial and lateral portions of the navicular, although the central portion of the bone shows a lower level of vascularization [47]. As described by Zchakaja [48], with increasing age, there is a decrease in the number of supplementary arteries that supply the navicular bone. This may be a cause of osteonecrosis or stress fractures at the middle third of this bone, as described initially by Torg et al [47] and, later, by other investigators [49 –53].
Acknowledgments We would like to thank to Joan Angel Clavero, MD from Diagnosis Medica (Barcelona, Spain) for his support and the use of the MR and CT images that appear in this article.
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