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.

Medial Longitudinal Arch of the Foot: Figure 14-28 shows the locations of the medial longitudinal and transverse arches of the foot. Both arches lend very important elements of stability and resiliency to the loaded foot. The talonavicular joint serves as the keystone to the medial longitudinal arch. For this reason, the structure and function of the medial longitudinal arch is addressed in this section. The transverse arch is described later during the study of the distal intertarsal joints.

The medial longitudinal arch is evident as the characteristic concave “instep” of the medial side of the foot. This arch is the primary load-bearing and shock-absorbing structure of the foot.¹²⁹ The bones that form the medial arch are the calcaneus, talus, navicular, cuneiforms, and associated three medial metatarsals. Without this arched configuration, the large and rapidly produced forces applied against the foot during running, for example, would likely exceed the physiologic weight-bearing capacity of the bones. Additional structures that assist the arch in absorbing loads are the plantar fat pads, sesamoid bones located at the plantar base of the great toe, and superficial plantar fascia (which attaches primarily to the overlying thick dermis, functioning primarily to reduce shear forces.) As will be described, the medial longitudinal arch and associated connective tissues are usually adequate to support the foot during relatively low-stress or near-static conditions—for example, standing at ease. Active muscular forces, however, assist the arch when the stresses and loads on the foot are larger and more dynamic, such as during standing on tiptoes, walking, jumping, or running. The following section describes the passive support mechanism provided by the medial longitudinal arch. The role of muscles in providing active support is described later, in the study of muscles of the ankle and foot.

Early to Mid-Stance Phase of Gait: Kinematics of Pronation at the Subtalar Joint: Immediately after the heel contact phase of gait, the dorsiflexed talocrural joint and slightly supinated subtalar joint rapidly plantar flex and pronate, respectively (compare Figures 14-19 and 14-31, B). Although the data plotted in Figure 14-31, B show only 2 degrees of average maximum eversion (beyond the resting posture),²⁴ other researchers using asymptomatic subjects report higher values, in the range of 5 to 9 degrees.^50,129,142 Differences in defining the 0-degree position of the subtalar joint, dissimilar sample sizes, and the varying measurement techniques account for much of this inconsistency within the literature.¹⁴² For this reason, it is often difficult to define what constitutes “abnormal” eversion (pronation) during walking.

The pronation (eversion) at the subtalar joint during stance occurs primarily by two mechanisms. First, the calcaneus tips into slight eversion in response to the ground reaction force passing upward and just lateral to the midpoint of the posterior calcaneus. The simultaneous impact of heel contact also pushes the head of the talus medially in the horizontal plane and inferiorly in the sagittal plane. Relative to the calcaneus, this motion of the talus abducts and (slightly) dorsiflexes the subtalar joint. These motions are consistent with the formal definition of pronation. A loosely articulated skeletal model aids in the visualization of this motion. Second, during the early stance phase, the tibia and fibula, and to a lesser extent the femur, internally rotate after initial heel contact.^51,105 Because of the embracing configuration of the talocrural joint, the internally rotating lower leg steers the subtalar joint into further pronation. The argument is often raised that with the calcaneus in contact with the ground, pronation at the subtalar joint causes, rather than follows, internal rotation of the leg; either perspective is valid.

The amplitude of pronation at the subtalar joint during early to mid-stance phase of walking is indeed relatively small—about 5 degrees on average—occurring for only one quarter of a second during average-speed walking. The amount and the speed of the pronation nevertheless influence the kinematics of the more proximal joints of the lower extremity. These effects can be readily appreciated by exaggerating and dramatically slowing the pronation action of the rearfoot during the initial loading phase of gait. Consider the demonstration depicted in Figure 14-32. While standing over a loaded and fixed foot, forcefully but slowly internally rotate the lower limb and observe the associated pronation at the rearfoot (subtalar joint) and simultaneous lowering of the medial longitudinal arch.⁹⁵ If sufficiently forceful, this action also tends to internally rotate, slightly flex, and adduct the hip and create a valgus stress on the knee (Table 14-5). These mechanical events are indeed exaggerated and do not all occur to this degree and pattern during walking at normal speed. Nevertheless, because of the linkages throughout the lower limb, excessive or uncontrolled pronation of the rearfoot could exaggerate one or more of these mechanically related joint actions.⁵⁷ Clinically, a person who excessively pronates during early stance may complain of medial knee pain, possibly from excessive valgus stress placed on the knee and subsequent overstretching of the medial collateral ligament. Whether the overpronation causes excessive strain on the medial collateral ligament or vice versa is not always obvious.

Although widely accepted, a predictable kinematic relationship between the magnitude and timing of excessive pronation and excessive internal rotation throughout the lower limb has not been established conclusively.¹⁰⁵ Precise measurements of these kinematic relationships while a subject is walking are technically difficult. The kinematics themselves are highly variable and poorly defined. Some studies report the kinematics as a rotation of a single bone, and others report relative rotations between bones. Additional studies are needed in this area before definite cause-and-effect relationships are known. These relationships are important, as they serve as the basis for many exercises and for the use of orthotics to reduce painful conditions related to excessive or poorly controlled pronation.

Mid-to-Late Stance Phase of Gait: Kinematics of Supination at the Subtalar Joint: At about 15% to 20% into the gait cycle, the entire stance limb reverses its horizontal plane motion from internal to external rotation.^52,105 External rotation of the leg while the foot remains planted coincides roughly with the beginning of the swing phase of the contralateral lower extremity. With the stance foot securely planted, external rotation of the femur, followed by the tibia, gradually reverses the horizontal plane direction of the talus from internal to external rotation. As a result, at about 30% to 35% into the gait cycle, the pronated (everted) subtalar joint starts to move sharply toward supination (inversion) (see Figure 14-31, B). As demonstrated in Figure 14-33, with the rearfoot supinating, the midfoot and forefoot must simultaneously twist into relative pronation in order for the foot to remain in full contact with the ground.^75,96 By late stance, the supinated subtalar joint and the elevated and tensed medial longitudinal arch convert the midfoot (and ultimately the forefoot) into a more rigid lever.^50,68 Muscles such as the gastrocnemius and soleus use this stability to transfer forces from the Achilles tendon, through the midfoot, to the metatarsal heads during the push off phase of walking or running.

A person who, for whatever reason, remains relatively pronated late into stance phase often has difficulty stabilizing the midfoot at a time when it is naturally required. Consequently, excessive activity may be required from extrinsic and intrinsic muscles of the foot to reinforce the medial longitudinal arch. Over time, hyperactivity may lead to generalized muscle fatigue and painful “overuse” syndromes throughout the lower limb and foot.

Basic Structure and Function: As a group, the distal intertarsal joints (1) assist the transverse tarsal joint in pronating and supinating the midfoot, and (2) provide stability across the midfoot by forming the transverse arch of the foot. Motions in these joints are small and typically not formally described.

Anatomic Considerations: The tarsometatarsal joints are frequently called Lisfranc’s joints, after Jacques Lisfranc, a French field surgeon in Napoleon’s army who described an amputation in this region of the foot. As a group, the five tarsometatarsal joints separate the midfoot from the forefoot (review organization of joints in Figure 14-23). The joints consist of the articulation between the bases of the metatarsals and the distal surfaces of the three cuneiforms and cuboid. Specifically, the first (most medial) metatarsal articulates with the medial cuneiform, the second with the intermediate cuneiform, and the third with the lateral cuneiform. The bases of the fourth and fifth metatarsals both articulate with the distal surface of the cuboid.

The articular surfaces of the tarsometatarsal joints are generally flat, although the medial two show slight, irregular curvatures. Dorsal, plantar, and interosseous ligaments add stability to these articulations. Only the first tarsometatarsal joint has a well-developed capsule.¹²⁰

Kinematic Considerations: The tarsometatarsal joints serve as the base joints of the forefoot. Mobility is least at the second and third tarsometatarsal joints, in part because of strong ligaments and the wedged position of the base of the second ray between the medial and lateral cuneiforms (see Figure 14-35, B). Consequently, the second and third rays produce an element of longitudinal stability throughout the foot, similar to the second and third rays in the hand.⁶⁷ This stability is useful in late stance as the forefoot prepares for the dynamics of push off.

Mobility is greatest in the first, fourth, and fifth tarsometatarsal joints, most notably in the first (most medial) joint.³⁴ During walking, the first tarsometatarsal joint normally expresses about 10 degrees of sagittal plane movement; mobility in other planes is usually slight.²³

During the early to mid-stance phase of walking, the first tarsometatarsal joint gradually dorsiflexes about 5 degrees. This motion occurs as body weight depresses the cuneiform region downward as the ground simultaneously pushes the distal end of the first ray upward. This movement is associated with a gradual lowering of the medial longitudinal arch³⁷—a mechanism that helps absorb the stress of body weight acting on the foot. At late stance (push off) phase of gait, however, the first tarsometatarsal joint rapidly plantar flexes about 5 degrees.²³ The plantar flexion of the first ray, controlled in part by pull of the fibularis longus, effectively “shortens” the medial column of the foot slightly, thereby helping to raise the medial longitudinal arch. This mechanism increases the stability of the arch (and medial column of the foot) at a time in the gait cycle when the midfoot and forefoot are under higher loads.

Although the descriptions are dated and still partially unresolved, most literature describes a natural mechanical coupling of the kinematics at the first tarsometatarsal joint: specifically, plantar flexion occurs with slight eversion, and dorsiflexion with slight inversion.37,45,68 Such passive mobility does indeed appear to occur naturally when assessed in a non–weight-bearing condition (Figure 14-36). These movement combinations are atypical, however, because they do not fit the standard definitions of pronation or supination. Nevertheless, the unique mobility at the first tarsometatarsal joint may provide useful functions. Combining plantar flexion and eversion, for example, allows the medial side of the foot to better conform around irregular surfaces on the ground (see Figure 14-35, C). (This motion of the first metatarsal is generally similar to the movement of the thumb metacarpal as the pronated hand attempts to grasp a large spherical object.) Exactly how these atypical movement combinations relate functionally to the overall kinematics of the foot during walking remains uncertain.

Structure and Function: Plantar, dorsal, and interosseous ligaments interconnect the bases of the four lateral metatarsals. These points of contact form three small intermetatarsal synovial joints. Although interconnected by ligaments, a true joint does not typically form between the bases of the first and second metatarsals. This lack of articulation increases the relative movement of the first ray, in a manner similar to the hand. Unlike in the hand, however, the deep transverse metatarsal ligaments interconnect the distal end of all five metatarsals. Slight motion at the intermetatarsal joints augments the flexibility at the tarsometatarsal joints.

Anatomic Considerations: Five metatarsophalangeal joints are formed between the convex head of each metatarsal and the shallow concavity of the proximal end of each proximal phalanx (see Figure 14-23). These joints are located about 2.5 cm proximal to the “web spaces” of the toes. With the joints flexed, the prominent heads of the metatarsals are easily palpable on the dorsum of the distal foot.

Articular cartilage covers the distal end of each metatarsal head (Figure 14-37). A pair of collateral ligaments spans each metatarsophalangeal joint, blending with and reinforcing the capsule. As in the hand, each collateral ligament courses obliquely from a dorsal-proximal to a plantar-distal direction, forming a thick cord portion and a fanlike accessory portion.

The accessory portion attaches to the thick, dense plantar plate, located on the plantar side of the joint. The plate, or ligament, is grooved for the passage of flexor tendons. Fibers from the deep plantar fascia attach to the plantar plates and sheaths of the flexor tendons. Two sesamoid bones located within the tendon of the flexor hallucis brevis rest against the plantar plate of the first metatarsophalangeal joint (Figure 14-38). Although not depicted in Figure 14-38, four deep transverse metatarsal ligaments blend with and join the adjacent plantar plates of all five metatarsophalangeal joints. By interconnecting all five plates, the transverse metatarsal ligaments help maintain the first ray in a similar plane as the lesser rays, thereby adapting the foot for propulsion and weight bearing rather than manipulation. In the hand, the deep transverse metacarpal ligament connects only the fingers, freeing the thumb for opposition.

A fibrous capsule encloses each metatarsophalangeal joint and blends with the collateral ligaments and plantar plates. A poorly defined dorsal digital expansion covers the dorsal side of each metatarsophalangeal joint. This structure (analogous to the extensor mechanism in the digits of the hand) consists of a thin layer of connective tissue that is essentially inseparable from the dorsal capsule and extensor tendons.

Kinematic Considerations: Movement at the metatarsophalangeal joints occurs in two degrees of freedom. Extension (dorsiflexion) and flexion (plantar flexion) occur approximately in the sagittal plane about a medial-lateral axis; abduction and adduction occur in the horizontal plane about a vertical axis. The second digit serves as the reference digit for naming the movements of abduction and adduction of the toes. (The reference digit for naming abduction and adduction in the hand is the third or middle digit.) The axes of rotation for all volitional movements of the metatarsophalangeal joints are through the center of each metatarsal head.

Most people demonstrate limited dexterity in active movements at the metatarsophalangeal joints, especially in abduction and adduction. From a neutral position, the toes can be passively extended about 65 degrees and flexed about 30 to 40 degrees. The great toe typically allows greater extension, to near 85 degrees.¹³¹ This magnitude of extension is readily apparent as one stands up on “tiptoes.”

Deformities or Trauma Involving the Metatarsophalangeal Joint of the Great Toe:

Anterior Compartment Muscles:

Lateral Compartment Muscles:

Posterior Compartment Muscles:

Injury to the Common Fibular Nerve and Its Branches: The common fibular nerve winds around the neck of the fibula, just deep to the fibularis longus. This nerve is injured relatively frequently from lacerations or trauma that involves a fractured proximal fibula. Injury to the deep branch of the fibular nerve can result in paralysis of all the dorsiflexor (pretibial) muscles (see Figure 14-41). With paralysis of the dorsiflexor muscles, the foot rapidly and uncontrollably plantar flexes immediately after the heel contact phase of walking. During the swing phase, the hip and knee must excessively flex to ensure that the toes clear the ground.

Paralysis of the dorsiflexor muscles dramatically increases the likelihood of developing a plantar flexion contracture at the talocrural joint. This deformity is referred to as a drop-foot or pes equinus. In a surprisingly short period, a plantar flexed posture may lead to adaptive shortening and tightening of the Achilles tendon as well as several other collateral ligaments of the ankle. The relentless pull of gravity can also contribute to a plantar flexion contracture, often requiring an ankle-foot orthosis to maintain adequate dorsiflexion during walking.

An injury to the superficial branch of the fibular nerve may result in paralysis of the fibularis longus and fibularis brevis (see Figure 14-41). Over time, paralysis may lead to a supinated or inverted posture of the foot, a condition called pes varus. An injury to the common fibular nerve may involve both deep and superficial nerve branches. The resulting paralysis of all dorsiflexor and evertor muscles strongly predisposes a person to a deformity of combined plantar flexion of the ankle and supination of the foot, a condition referred to as pes equinovarus.

Injury to the Tibial Nerve and Its Branches: Injury to the tibial nerve may cause varying levels of weakness or paralysis in the muscles of the posterior compartment (see Figure 14-42). Isolated paralysis of the gastrocnemius and soleus because of tibial nerve injury is rare. Nevertheless, regardless of underlying pathology, paralysis of these muscles results in profound loss of plantar flexion torque. Over time, a fixed dorsiflexion posture may result at the talocrural joint, a condition known as pes calcaneus. The term calcaneus is used to describe the often prominent heel pad that forms in response to the heel of the chronically dorsiflexed ankle sharply striking the ground at the initiation of the stance phase.

Paralysis involving primarily the supinator muscles of the foot may result in a fixed pronated deformity, primarily the result of the unopposed action of the fibularis longus and brevis muscles. The term pes valgus typically describes both eversion and abduction components of the pronation deformity. Paralysis involving all the muscles of the posterior compartment increases the potential for a fixed deformity called pes calcaneovalgus.

Injury to the tibial nerve within the leg typically involves the more distal medial and lateral plantar nerves (see Figure 14-42). Paralysis of the intrinsic muscles within the foot often results in a “clawing” of the toes: hyperextension of the metatarsophalangeal joints with flexion of the interphalangeal joints. This deformity results primarily from an unopposed pull of the extrinsic toe extensor muscles across the metatarsophalangeal joints. The pathomechanics of clawing caused by weakness of the intrinsic muscles of the foot are similar to those of clawing of the fingers after a combined ulnar and median nerve injury (see Chapter 8).

The common fixed deformities or abnormal postures of the ankle, foot, and toes are summarized according to nerve injury in Table 14-7.

Anatomic and Functional Considerations: Intrinsic muscles are those that originate and insert within the foot. The following discussion highlights the primary attachments and actions of the intrinsic muscles. More detailed attachments of these muscles is presented in Appendix IV, Part D.

The dorsum of the foot has one intrinsic muscle, the extensor digitorum brevis, which is innervated by the deep branch of the fibular nerve. The extensor digitorum brevis originates on the dorsal-lateral surface of the calcaneus, just proximal to the calcaneocuboid articulation. The muscle belly sends four tendons: one to the dorsal surface of the great toe (often designated as the extensor hallucis brevis), and three that join the tendons of the extensor digitorum longus of the second through the fourth toes (see Figure 14-44). The extensor digitorum brevis assists the extensor hallucis longus and extensor digitorum longus in extension of the toes.

The remaining intrinsic muscles originate and insert within the plantar aspect of the foot. These muscles are organized into four layers (Figure 14-53). The plantar fascia is located just superficial to the first layer of muscles.