Talus: The talus is the most superiorly located bone of the foot. Its dorsal or trochlear surface is a rounded dome: convex anterior-posteriorly and slightly concave medial-laterally (see Figures 14-4 and 14-6). Cartilage covers the trochlear surface and its adjacent sides, providing smooth articular surfaces for the talocrural joint.¹²⁰ The prominent head of the talus projects forward and slightly medially toward the navicular. In the adult the long axis of the neck of the talus positions the head of this bone about 30 degrees medial to the sagittal plane. In small children the head is projected medially about 40 to 50 degrees, partially accounting for the often inverted appearance of their feet.

Figure 14-8 shows three articular facets on the plantar (inferior) surface of the talus. The anterior and middle facets are slightly curved and often continuous with each other. The articular cartilage that covers these facets also covers part of the adjacent head of the talus. The oval, concave posterior facet is the largest facet. As a functional set, the three facets articulate with the three facets on the dorsal (superior) surface of the calcaneus, forming the subtalar joint. The talar sulcus is an obliquely running groove between the anterior-middle and posterior facets.

Lateral and medial tubercles are located on the posterior-medial surface of the talus (see Figure 14-4). A groove formed between these tubercles serves as a pulley for the tendon of the flexor hallucis longus (see ahead Figure 14-12).

Calcaneus: The calcaneus, the largest of the tarsal bones, is well suited to accept the impact of the heel striking the ground during walking. The large and rough calcaneal tuberosity receives the attachment of the Achilles tendon. The plantar surface of the tuberosity has lateral and medial processes that serve as attachments for many of the intrinsic muscles and the deep plantar fascia of the foot (see Figure 14-5).

The calcaneus articulates with other tarsal bones on its anterior and dorsal surfaces. The relatively small, curved anterior surface of the calcaneus joins the cuboid at the calcaneocuboid joint (see Figure 14-7). The more extensive dorsal surface contains three facets that join the matching facets on the talus (see Figure 14-8). The anterior and middle facets are relatively small and nearly flat. The posterior facet is large and convex, conforming to the concave shape of the equally large posterior facet on the talus. Between the posterior and medial facets is a wide oblique groove called the calcaneal sulcus. Located within this sulcus are the attachments of several strong ligaments that bind the subtalar joint. With the subtalar joint articulated, the sulci of the calcaneus and talus form a canal within the subtalar joint, known as the tarsal sinus (see Figure 14-7).

The sustentaculum talus projects medially as a horizontal shelf from the dorsal surface of the calcaneus (see Figure 14-6). The sustentaculum talus lies under and supports the middle facet of the talus. (Sustentaculum talus literally means a “shelf for the talus.”)

Navicular: The navicular is named for its resemblance to a ship (i.e., referring to “navy”). Its proximal (concave) surface accepts the head of the talus at the talonavicular joint (see Figure 14-4). The distal surface of the navicular bone contains three relatively flat facets that articulate with the three cuneiform bones.

The medial surface of the navicular has a prominent tuberosity, palpable in the adult at about 2.5 cm inferior and distal (anterior) to the tip of the medial malleolus (see Figure 14-6). This tuberosity serves as one of several distal attachments of the tibialis posterior muscle.

Medial, Intermediate, and Lateral Cuneiforms: The cuneiform bones (from the Latin root meaning “wedge”) act as a spacer between the navicular and bases of the three medial metatarsal bones (see Figure 14-4). The cuneiforms contribute to the transverse arch of the foot, accounting, in part, for the transverse convexity of the dorsal aspect of the midfoot.

Cuboid: As its name indicates, the cuboid has six surfaces, three of which articulate with adjacent tarsal bones (see Figures 14-4, 14-5, and 14-7). The distal surface articulates with the bases of both the fourth and fifth metatarsals. The cuboid is therefore homologous to the hamate bone in the wrist.

The entire, curved proximal surface of the cuboid articulates with the calcaneus (see Figure 14-4). The medial surface has an oval facet for articulation with the lateral cuneiform and a small facet for articulation with the navicular. A distinct groove runs across the plantar surface of the cuboid, which in life is occupied by the tendon of the fibularis longus muscle (see Figure 14-5).

Metatarsals: The five metatarsal bones link the distal row of tarsal bones with the proximal phalanges (see Figure 14-4). Metatarsals are numbered 1 through 5, starting on the medial side. The first metatarsal is the shortest and thickest, and the second is usually the longest. The second and usually the third metatarsals are the most rigidly attached to the distal row of tarsal bones. These morphologic characteristics generally reflect the larger forces that pass through this region of the forefoot during the push off phase of gait. Each metatarsal has a base at its proximal end, a shaft, and a convex head at its distal end (see Figure 14-4, first metatarsal). The bases of the metatarsals have small articular facets that mark the site of articulation with the bases of the adjacent metatarsals.

Longitudinally, the shafts of the metatarsals are slightly concave on their plantar side (see Figure 14-6). This arched shape enhances the load-supporting ability of the metatarsals, and provides space for muscles and tendons. The plantar surface of the first metatarsal head has two small facets for articulation with two sesamoid bones that are imbedded within the tendon of the flexor hallucis brevis (see Figure 14-5). The fifth metatarsal has a prominent styloid process just lateral to its base, marking the attachment of the fibularis brevis muscle (see Figure 14-7).

Phalanges: As in the hand, the foot has 14 phalanges. Each of the four lateral toes contains a proximal, middle, and distal phalanx (see Figure 14-4). The first toe—more commonly called the great toe or hallux—has two phalanges, designated as proximal and distal. In general, each phalanx has a concave base at its proximal end, a shaft, and a convex head at its distal end.

Articular Structure: The talocrural joint is the articulation of the trochlea (dome) and sides of the talus with the rectangular cavity formed by the distal end of the tibia and both malleoli (see Figures 14-3 and 14-9). The talocrural joint is often referred to as the “mortise,” owing to its resemblance to the wood joint used by carpenters (Figure 14-13). The concave shape of the proximal side of the mortise is maintained by connective tissues that bind the tibia with the fibula. The confining shape of the talocrural joint provides a major source of natural stability to the ankle.¹²⁸

The structure of the mortise must be sufficiently stable to accept the forces that pass between the leg and foot. Although variable, approximately 90% to 95% of the compressive forces pass through the talus and tibia; the remaining 5% to 10% pass through the lateral region of the talus and the fibula.¹⁸ The talocrural joint is lined with about 3 mm of articular cartilage, which can be compressed by 30% to 40% in response to peak physiologic loads.¹³⁵ This load-absorption mechanism protects the subchondral bone from damaging stress.

Ligaments: A thin capsule surrounds the talocrural joint. Externally, the capsule is reinforced by collateral ligaments that help maintain the stability between the talus and the rectangular “socket” of the mortise.

The medial collateral ligament of the talocrural joint is called the deltoid ligament, based on its triangular shape. This ligament is broad and expansive (Figure 14-14). Its apex is attached to the medial malleolus, with its base fanning into three sets of superficial fibers (see box). Deeper tibiotalar fibers blend with and strengthen the medial capsule of the talocrural joint.

The primary function of the deltoid ligament is to limit eversion across the talocrural, subtalar, and talonavicular joints. Sprains of the deltoid ligament are relatively uncommon, in part because of the ligament’s strength and because the lateral malleolus serves as a bony block against excessive eversion.

The lateral collateral ligaments of the ankle include the anterior and posterior talofibular and the calcaneofibular ligaments. Because of the relative inability of the medial malleolus to block the medial side of the mortise, the overwhelming majority of ankle sprains involve excessive inversion, often involving injury to the lateral collateral ligaments.⁸

The anterior talofibular ligament attaches to the anterior aspect of the lateral malleolus, then courses anteriorly and medially to the neck of the talus (Figure 14-15). This ligament is the most frequently injured of the lateral ligaments. Injury is often caused by excessive inversion or (horizontal plane) adduction of the ankle, especially when combined with plantar flexion—for example, when inadvertently stepping into a hole or onto someone’s foot while landing from a jump.¹¹⁵ The calcaneofibular ligament courses inferiorly and posteriorly from the apex of the lateral malleolus to the lateral surface of the calcaneus (see Figure 14-15). This ligament resists inversion across the talocrural joint (especially when fully dorsiflexed) and the subtalar joint. As a pair, the calcaneofibular and anterior talofibular ligaments limit inversion throughout most of the range of ankle dorsiflexion and plantar flexion.²¹ About two thirds of all lateral ankle ligament injuries involve both of these ligaments.^36,54

The posterior talofibular ligament originates on the posterior-medial side of the lateral malleolus and attaches to the lateral tubercle of the talus (see Figures 14-12 and 14-15). Its fibers run horizontally across the posterior side of the talocrural joint, in an oblique anterior-lateral to posterior-medial direction (Figure 14-16). The primary function of the posterior talofibular ligament is to stabilize the talus within the mortise. In particular, it limits excessive abduction of the talus, especially when the ankle is fully dorsiflexed.²¹

The inferior transverse ligament is a small thick strand of fibers considered part of the posterior talofibular ligament (see Figure 14-12). The fibers continue medially to the posterior aspect of the medial malleolus, forming part of the posterior wall of the talocrural joint.

In summary, the medial and lateral collateral ligaments of the ankle limit excessive eversion and inversion, respectively, at every joint that the fibers cross. Because most of the ligaments course, to varying degrees, from anterior to posterior, they also limit anterior-to-posterior translation of the talus within the mortise. As described in the section on arthrokinematics, the movements of plantar flexion and dorsiflexion are kinematically linked to anterior and posterior translation of the talus, respectively. For these reasons, several of the collateral ligaments are stretched at the extremes of dorsiflexion or plantar flexion of the talocrural joint.

Several of the major ligaments that cross the talocrural joint also cross other joints of the foot, such as the subtalar and talonavicular joints. These ligaments therefore provide stability across multiple joints. Table 14-3 provides a summary of the movements that stretch the major ligaments of the ankle. This information helps explain the mechanisms that frequently injure these ligaments, as well as the rationale behind the manual stress tests performed to evaluate the structural integrity of the ligaments after injury.

Osteokinematics: The talocrural joint possesses one degree of freedom. Motion occurs around an axis of rotation that passes through the body of the talus and through the tips of both malleoli. Because the lateral malleolus is inferior and posterior to the medial malleolus (which should be verified by palpation), the axis of rotation departs slightly from a pure medial-lateral axis. As depicted in Figure 14-17, A and B, the axis of rotation (in red) is inclined slightly superiorly and anteriorly as it passes laterally to medially through the talus and both malleoli.⁷⁸ The axis deviates from a pure medial-lateral axis about 10 degrees in the frontal plane (see Figure 14-17, A) and 6 degrees in the horizontal plane (see Figure 14-17, B). Because of the pitch of the axis of rotation, dorsiflexion is associated with slight abduction and eversion, and plantar flexion with slight adduction and inversion.¹¹⁸ By definition, therefore, the talocrural joint produces a movement of pronation and supination. Because the axis of rotation deviates only minimally from the pure medial-lateral axis, the main components of pronation and supination at the talocrural joint are overwhelmingly dorsiflexion and plantar flexion (see Figure 14-17, D and E).^76,119 The horizontal and frontal plane components of pronation and supination are indeed small,⁸⁸ and usually ignored in most clinical situations.

The 0-degree (neutral) position at the talocrural joint is defined by the foot held at 90 degrees to the leg. From this position, the talocrural joint permits about 15 to 25 degrees of dorsiflexion and 40 to 55 degrees of plantar flexion, although reported values differ considerably based on type and method of measurement.10,40,118 Accessory movements at the nearby subtalar joint may contribute to about 20% of the total reported range of motion.⁴⁰ Dorsiflexion and plantar flexion at the talocrural joint need to be visualized when the foot is off the ground and free to rotate, and when the foot is fixed to the ground as the leg rotates forward, such as during the stance phase of gait.

Arthrokinematics: The following discussion assumes that the foot is unloaded and free to rotate. During dorsiflexion, the talus rolls forward relative to the leg as it simultaneously slides posteriorly (Figure 14-18, A). The simultaneous posterior slide allows the talus to rotate forward with only limited anterior translation.^22,134 Figure 14-18, A shows the calcaneofibular ligament becoming taut in response to the posterior sliding tendency of the talus-calcaneal segment. Generally, any collateral ligament that becomes increasingly taut on posterior translation of the talus also becomes increasingly taut during dorsiflexion. Maximal dorsiflexion elongates the posterior capsule and all tissues capable of transmitting plantar flexion torque, such as the Achilles tendon.

Full dorsiflexion of the ankle is often limited after a sprain of the lateral ankle. One therapeutic approach aimed at increasing dorsiflexion involves passive joint mobilization of the talocrural joint. Specifically, the clinician applies a posterior-directed translation of the talus and foot relative to the leg.^38,134 An appropriately applied posterior slide is designed to mimic the natural arthrokinematics of dorsiflexion at the talocrural joint.

During plantar flexion, the talus rolls posteriorly as the bone simultaneously slides anteriorly (see Figure 14-18, B). Generally, any collateral ligament that becomes increasingly taut on anterior translation of the talus also becomes increasingly taut during plantar flexion. As depicted in Figure 14-18, B, the anterior talofibular ligament is stretched in full plantar flexion. (Although not depicted, the tibionavicular fibers of the deltoid ligament would also become taut at full plantar flexion [review Table 14-3]). Plantar flexion also stretches the dorsiflexor muscles and the anterior capsule of the joint.

Progressive Stabilization of the Talocrural Joint throughout the Stance Phase of Gait: At initial heel contact during walking, the ankle rapidly plantar flexes in order to lower the foot to the ground (Figure 14-19; from 0% to 5% of the gait cycle). As soon as the foot flat phase of gait is reached, the leg starts to rotate forward (dorsiflex) over the grounded foot.⁶⁸ Dorsiflexion continues until after just after heel off phase. At this point in the gait cycle, the ankle becomes increasing stable owing to the increased tension in many stretched collateral ligaments and plantar flexor muscles (Figure 14-20, A). The dorsiflexed ankle is further stabilized as the wider anterior part of the talus wedges into the tibiofibular component of the mortise (see Figure 14-20, B).¹⁸ The wedging effect causes the distal tibia and fibula to spread apart slightly. This action is resisted by tension in the distal tibiofibular ligaments and interosseous membrane.⁶ At the initiation of the push off phase of walking (just after about 40% of the gait cycle; see Figure 14-19), the fully dorsiflexed talocrural joint is well stabilized to accept compression forces that may reach over four times body weight.¹²¹ This inherent stability may partially account for the relatively low frequency of idiopathic osteoarthritis at the talocrural joint.^18,81 Posttraumatic arthritis at the talocrural joint is, however, relatively common. Residual incongruity within the mortise after trauma can increase intra-articular stress to damaging levels.

The slight natural spreading of the mortise at maximal dorsiflexion causes slight translation of the fibula.¹³ The line of force of the stretched anterior and posterior (distal) tibiofibular ligaments and interosseous membrane produces a slight superior translation of the fibula that is transferred proximally to the proximal tibiofibular joint. For this reason, the proximal tibiofibular joint is related more functionally to the ankle (talocrural joint) than to the knee.

Articular Structure: The large, complex subtalar joint consists of three articulations formed between the posterior, middle, and anterior facets of the calcaneus and the talus. These articulations are depicted in yellow in Figure 14-21.

The prominent posterior articulation of the subtalar joint occupies about 70% of the total articular surface area. (Some anatomy texts limit the description of the subtalar joint to the prominent posterior facets only, referring to it as the talocalcaneal joint.¹²⁰) The concave posterior facet of the talus rests on the convex posterior facet of the calcaneus. The articulation is held tightly opposed by its interlocking shape, ligaments, body weight, and activated muscle. The closely aligned anterior and middle articulations consist of smaller, nearly flat joint surfaces. Although all three articulations contribute to movement at the subtalar joint, clinicians typically focus on the more prominent posterior articulation when performing mobilization techniques to increase the flexibility of the rearfoot.

Ligaments: The posterior and anterior-middle articulations within the subtalar joint are each enclosed by a separate capsule. The larger, posterior capsule is reinforced by three slender thickenings: medial, posterior, and lateral talocalcaneal ligaments (see Figures 14-12, 14-14, and 14-15). These ligaments are often indistinguishable from the capsule and serve as secondary stabilizers of the joint. Other more prominent ligaments provide the primary source of stabilization to the joint as a whole (Table 14-4). The calcaneofibular ligament limits excessive inversion, and the deltoid ligament (tibiocalcaneal fibers) limit excessive eversion. (The anatomy of these ligaments was described previously with the talocrural joint.)

The interosseous (talocalcaneal) and cervical ligaments attach directly between the talus and calcaneus¹²⁰ and therefore provide the greatest nonmuscular stability to the subtalar joint. These broad and flat ligaments cross obliquely within the tarsal sinus and therefore are difficult to view unless the joint is disarticulated, as depicted previously in Figure 14-8. The interosseous (talocalcaneal) ligament has two distinct, flattened, anterior and posterior bands. These bands arise from the calcaneal sulcus and course superiorly to attach within the talar sulcus and adjacent regions. The larger cervical ligament has an oblique fiber arrangement similar to the interosseous ligament but attaches more laterally within the calcaneal sulcus. From this attachment the cervical ligament courses superiorly and medially to attach primarily to the inferior-lateral surface of the neck of the talus (hence the name “cervical”) (see Figure 14-15). The interosseous and cervical ligaments limit the extremes of all motions—most notably inversion.^61,120,127

Although the ligaments within the tarsal sinus are recognized as primary stabilizers at the subtalar joint, a precise anatomic description and a full understanding of their function are unclear.^53,127 This lack of knowledge has limited the development of standard clinical “stress tests” to aid in the diagnosis of ligamentous injury. Cadaveric study suggest that a lateral-to-medial translational force applied to the calcaneus specifically stresses the interosseous ligament.¹²⁷ This finding is consistent with the ligament’s proposed function of resisting inversion at the subtalar joint.

Kinematics: The arthrokinematics at the subtalar joint involve a sliding motion among the three sets of facets, yielding a curvilinear arc of movement between the calcaneus and the talus. Although considerable variation exists from one person to another,⁷¹ the axis of rotation is typically described as a line that pierces the lateral-posterior heel and courses through the subtalar joint in anterior, medial, and superior directions (Figure 14-22, A to C, red).^51,80,103 The axis of rotation is positioned 42 degrees from the horizontal plane (see Figure 14-22, A) and 16 degrees from the sagittal plane (see Figure 14-22, B).⁸⁰

Pronation and supination of the subtalar joint occur as the calcaneus moves relative to the talus (or vice versa when the foot is planted) in an arc that is perpendicular to the axis of rotation (see the red circular arrows in Figure 14-22, A to C). Given the general pitch to the axis, only two of the three main components of pronation and supination are strongly evident: inversion and eversion, and abduction and adduction (see Figure 14-22, A and B). Pronation, therefore, has main components of eversion and abduction (see Figure 14-22, D); supination has main components of inversion and adduction (see Figure 14-22, E). The calcaneus does dorsiflex and plantar flex slightly relative to the talus; however, this motion is small and usually ignored clinically. Overall, the kinematic pattern expressed at the subtalar joint is much greater than at the talocrural joint.⁷⁶

For simplicity, the osteokinematics of the subtalar joint have been pictorially demonstrated by the calcaneus moving relative to a fixed and essentially immobile talus. During walking, however, when the calcaneus is relatively immobile because of the load of body weight, a significant portion of pronation and supination occur by horizontal plane rotation of the talus and leg. Because of the inherent stability and fit provided by the mortise, the majority of the horizontal plane rotation of the talus is mechanically coupled to the rotation of the leg. Small horizontal plane accessory motions within the talocrural joint absorb a small component of this rotation.⁹⁶

Range of Motion: Grimston and colleagues reported active range of inversion and eversion motions at the subtalar joint across 120 healthy subjects (aged 9 through 79 years).⁴⁰ Results showed that inversion exceeds eversion by nearly double: inversion, 22.6 degrees; eversion, 12.5 degrees. Although these data include accessory rotations at the talocrural joint, the much greater ratio of inversion to eversion is typical of that reported for the subtalar joint alone.^5,125 Studies that measure passive range of motion usually report greater magnitudes of motion, with inversion-to-eversion ratios approaching 3 : 1.¹⁴² Regardless of active or passive motion, the distally projecting lateral malleolus and the relatively thick deltoid ligament naturally limit eversion.

Articular Structure and Ligamentous Support:

Kinematics: The transverse tarsal joint rarely moves without associated movements at nearby joints, especially the subtalar joint. To appreciate the mobility that occurs primarily at the transverse tarsal joint, hold the calcaneus firmly while maximally pronating and supinating the midfoot (Figure 14-26, A and C, respectively). During these motions the navicular spins within the talonavicular joint.⁷⁴ Combining motions across both the subtalar and transverse tarsal joints accounts for most of the pronation and supination throughout the foot (see Figure 14-26, B and D, respectively). As evident throughout Figure 14-26, mobility of the forefoot contributes to the pronation and supination of the entire foot.

Three noteworthy points should be made before the detailed kinematics of the transverse tarsal joint are addressed. First, two separate axes of rotation have been identified. Second, the amplitude and direction of movement is typically different during weight-bearing as compared with non–weight-bearing activities. Third, the ability of the transverse tarsal joint to stabilize the midfoot depends strongly on the position of the subtalar joint. The upcoming sections discuss each of these factors.