Ankle and Foot

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Chapter 14

Ankle and Foot

CHAPTER AT A GLANCE

Walking and running require the foot to be sufficiently pliable to absorb stress and to conform to the countless spatial configurations between it and the ground. In addition, walking and running require the foot to be relatively rigid in order to withstand potentially large propulsive forces. The healthy foot satisfies the seemingly paradoxical requirements of shock absorption, pliability, and strength through a complex functional and structural interaction among its joints, connective tissues, and muscles. Although not emphasized enough in this chapter, the normal sensation of the healthy foot also provides important measures of protection and feedback to the muscles of the lower extremity.

This chapter sets forth a firm basis for an understanding of the evaluation and treatment of several disorders that affect the ankle and foot, many of which are kinesiologically related to the movement of the entire lower extremity. Several of the kinesiologic issues addressed in this chapter are related specifically to the process of walking, or gait, a topic covered in detail in Chapter 15. Figure 15-12 should be consulted as a reference to the terminology used throughout Chapter 14 to describe the different phases of the gait cycle.

OSTEOLOGY

Basic Terms and Concepts

NAMING THE JOINTS AND REGIONS

Figure 14-1 depicts an overview of the terminology that describes the regions of the ankle and foot. The term ankle refers primarily to the talocrural joint: the articulation among the tibia, fibula, and talus. The term foot refers to all the tarsal bones, and the joints distal to the ankle. Within the foot are three regions, each consisting of a set of bones and one or more joints. The rearfoot (hindfoot) consists of the talus, calcaneus, and subtalar joint; the midfoot consists of the remaining tarsal bones, including the transverse tarsal joint and the smaller distal intertarsal joints; and the forefoot consists of the metatarsals and phalanges, including all joints distal to and including the tarsometatarsal joints. Table 14-1 provides a summary of the organization of the bones and joints of the ankle and foot.

The terms anterior and posterior have their conventional meanings with reference to the tibia and fibula (i.e., the leg). When describing the ankle and foot, however, these terms are often used interchangeably with distal and proximal, respectively. The terms dorsal and plantar describe the superior (top) and inferior aspects of the foot, respectively.

OSTEOLOGIC SIMILARITIES BETWEEN THE DISTAL LEG AND THE DISTAL ARM

The ankle and foot have several features that are structurally similar to the wrist and hand. The radius in the forearm and the tibia in the leg each articulates with a set of small bones—the carpus and tarsus, respectively. When the pisiform of the wrist is considered as a sesamoid (in contrast to a separate carpal bone), the carpus and tarsus have seven bones each. The general plan of the metatarsus and metacarpus, as well as the more distal phalanges, is very similar. A notable exception is that the first (great) toe in the foot is not as functionally developed as the thumb in the hand.

As described in Chapter 12, the entire lower extremity progressively internally or medially rotates during embryologic development. As a result, the great toe is positioned on the medial side of the foot, and the top of the foot is actually its dorsal surface. This orientation is similar to that of the hand when the forearm is fully pronated (Figure 14-2). This plantigrade position of the foot is necessary for walking and standing. With the forearm pronated, flexion and extension of the wrist are similar to plantar flexion and dorsiflexion of the ankle, respectively.

Individual Bones

FIBULA

The long and thin fibula is located lateral and parallel to the tibia (Figure 13-3). The fibular head can be palpated just lateral to the lateral condyle of the tibia. The slender shaft of the fibula transfers only about 10% of body weight through the leg; most of the weight is transferred through the thicker tibia. The shaft of the fibula continues distally to form the sharp and easily palpable lateral malleolus (from the Latin root malleus, hammer). The lateral malleolus functions as a pulley for the tendons of the fibularis (peroneus) longus and brevis. On the medial surface of the lateral malleolus is the articular facet for the talus (see ahead Figure 14-11). In the articulated ankle, this facet forms part of the talocrural joint (Figure 14-3).

DISTAL TIBIA

The distal end of the tibia expands to accommodate loads transferred across the ankle. On its medial side is the prominent medial malleolus. On the lateral surface of the medial malleolus is the articular facet for the talus (see ahead Figure 14-11). In the articulated ankle, this facet forms a small part of the talocrural joint. On the lateral side of the distal tibia is the fibular notch, a triangular concavity that accepts the distal end of the fibula at the distal tibiofibular joint (see ahead Figure 14-11).

In the adult the distal end of the tibia is twisted externally around its long axis approximately 20 or 30 degrees relative to its proximal end.141 This natural torsion is evidenced by the slight externally rotated position of the foot during standing. This twist of the lower leg is referred to as lateral tibial torsion, based on the orientation of the bone’s distal end relative to its proximal end.

TARSAL BONES

The seven tarsal bones are shown in four different perspectives in Figures 14-4 through 14-7.

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.”)

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).

RAYS OF THE FOOT

A ray of the forefoot is functionally defined as one metatarsal and its associated set of phalanges.

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.

ARTHROLOGY

The major joints of the ankle and foot are the talocrural, subtalar, and transverse tarsal joints (Figure 14-9). As will be described, the talus is mechanically involved with all three of these joints. The multiple articulations made by the talus help to explain the bone’s complex shape, with nearly 70% of its surface covered with articular cartilage. An understanding of the shape of the talus is crucial to an understanding of the kinesiology of the ankle and foot.

Terminology Used to Describe Movements

The terminology used to describe movements of the ankle and foot incorporates two sets of definitions: a fundamental set and an applied set. The fundamental terminology defines movement of the foot or ankle as occurring at right angles to the three standard axes of rotation (Figure 14-10, A). Dorsiflexion (extension) and plantar flexion describe motion that is parallel to the sagittal plane, around a medial-lateral axis of rotation. Eversion and inversion describe motion that is parallel to the frontal plane, around an anterior-posterior axis of rotation. Abduction and adduction describe motion that is parallel to the horizontal (transverse) plane, around a vertical (superior-inferior) axis of rotation. For at least the three major joints of the ankle and foot, these fundamental definitions are inadequate because most movements at these joints occur about an oblique axis rather than about the three standard, orthogonal axes of rotation depicted in Figure 14-10, A.

A second and more applied terminology has therefore evolved in the attempt to define the movements that occur perpendicular to the prevailing oblique axes of rotation at the ankle and foot (see Figure 14-10, B). Pronation is defined as a motion that has elements of eversion, abduction, and dorsiflexion. Supination, in contrast, is defined as a motion that has elements of inversion, adduction, and plantar flexion. The orientation of the oblique axis of rotation depicted in Figure 14-10, B varies across the major joints but, in general, has a pitch that is similar to that illustrated. The exact pitch of each major joint’s axis of rotation is described in subsequent sections.

Pronation and supination motions have been called “triplanar” motions. Unfortunately, this description is misleading. The term triplanar implies only that the movements “cut through” each of the three cardinal planes, not that the joint exhibiting this movement possesses three degrees of freedom. Pronation and supination occur in one plane. Table 14-2 summarizes the terminology used to describe the movements of the ankle and foot, including the terminology that describes abnormal posture or deformity.

Structure and Function of the Joints Associated with the Ankle

From an anatomic perspective, the ankle includes one articulation: the talocrural joint. An important structural component of this joint is the articulation formed between the tibia and fibula—an articulation reinforced by the proximal and distal tibiofibular joints and the interosseous membrane of the leg (see Figure 13-3). Because of this functional association, the proximal and distal tibiofibular joints are included under the topic of the “ankle.”

PROXIMAL TIBIOFIBULAR JOINT

The proximal tibiofibular joint is a synovial joint located lateral to and immediately inferior to the knee. The joint is formed between the head of the fibula and the posterior-lateral aspect of the lateral condyle of the tibia (see Figure 13-4). The joint surfaces are generally flat or slightly oval, covered by articular cartilage.120

A capsule strengthened by anterior and posterior ligaments encloses the proximal tibiofibular joint (see Figures 13-7 and 13-9). The tendon of the popliteus muscle provides additional stabilization as it crosses the joint posteriorly. Very little gliding motion occurs at this joint; a firm articulation is needed to ensure that forces within the biceps femoris and lateral collateral ligament of the knee are transferred effectively from the fibula to the tibia.

DISTAL TIBIOFIBULAR JOINT

The distal tibiofibular joint is formed by the articulation between the medial surface of the distal fibula and the fibular notch of the tibia (Figure 14-11).6 Anatomists frequently refer to the distal tibiofibular joint as a syndesmosis, which is a type of fibrous synarthrodial joint that is closely bound by an interosseous membrane.120 Relatively little movement is permitted between the distal tibia and distal fibula.

The interosseous ligament provides the strongest bond between the distal end of the tibia and fibula (see Figure 14-3).120 This ligament is an extension of the interosseous membrane between the tibia and fibula. The anterior and posterior (distal) tibiofibular ligaments also stabilize the joint (Figures 14-11 and 14-12). A stable union between the distal tibia and fibula is essential to the stability and function of the talocrural joint.

TALOCRURAL JOINT

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.

SPECIAL FOCUS 14-1   imageAnkle Injury Resulting from the Extremes of Dorsiflexion or Plantar Flexion

The proximal and distal tibiofibular joints and interosseous membrane are functionally and structurally related to the talocrural joint. This relationship is apparent after an injury related to extreme dorsiflexion—for example, landing from a jump. An extreme and violent dorsiflexion of the ankle (leg over the foot) can cause the mortise to “explode” outward, injuring many of the collateral ligaments. The traumatic widening of the mortise can also injure the ligaments that support the distal tibiofibular joint and interosseous membrane—the so-called high ankle or syndesmotic sprain.6 This type of injury occurs less frequently than the common inversion ankle sprain but usually requires a more prolonged recovery time.12

Full plantar flexion—the loose-packed position of the talocrural joint—slackens most collateral ligaments of the ankle and all plantar flexor muscles. In addition, plantar flexion places the narrower width of the talus between the malleoli, thereby releasing tension within the mortise. As a consequence, full plantar flexion causes the distal tibia and fibula to “loosen their grip” on the talus. Bearing body weight over a fully plantar flexed ankle, therefore, places the talocrural joint in a relatively unstable position. Wearing high heels or landing from a jump in a plantar flexed (and usually inverted) position increases the likelihood of destabilizing the mortis and potentially injuring the lateral ligaments of the ankle.35

Structure and Function of the Joints Associated with the Foot

SUBTALAR JOINT

The subtalar joint, as its name indicates, resides under the talus (see Figure 14-9). To appreciate the extent of subtalar joint motion, one need only firmly grasp the unloaded calcaneus and twist it in a side-to-side and rotary fashion. During this motion, the talus remains essentially stationary within the tight-fitting talocrural joint. Pronation and supination during non–weight-bearing activities occur as the calcaneus moves relative to the fixed talus. In weight bearing such as during the stance phase of walking, for example, pronation and supination occur as the calcaneus remains relatively stationary. This situation requires complex kinematics involving the leg and talus (as a common unit) rotating over the stable calcaneus. This mobility at the subtalar joint allows the foot to assume positions that are independent of the orientation of the superimposed ankle and leg. This function is essential to activities such as walking across a steep hill, standing with feet held wide apart, quickly changing directions while walking or running, and keeping one’s balance on a rocking boat.

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.)

TABLE 14-4.

Primary Functions of the Prominent Ligaments of the Subtalar Joint

Ligament Primary Function at the Subtalar joint
Calcaneofibular Limits excessive inversion
Tibiocalcaneal fibers of the deltoid ligament Limits excessive eversion
Interosseous (talocalcaneal)
Cervical
Both ligaments bind the talus with the calcaneus; limit the extremes of all motions, especially inversion

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.

SPECIAL FOCUS 14-2   imageStandard Clinical Measurements of Subtalar Joint Range of Motion

Accurately measuring the extent of pronation and supination at the subtalar joint through standard goniometry is very difficult. Measurement error reflects the inability of a standard, rigid goniometer to follow the oblique arc of pronation and supination, compounded by simultaneous movements in surrounding joints. As a method of improving the usefulness of this measurement, clinicians often report subtalar joint motion as a more simple frontal plane motion of inversion and eversion of the rearfoot (calcaneus).

The rather strict terminology described for subtalar motion is not always adhered to in clinical and research settings. “Shortcuts” in terminology have evolved that, unfortunately, limit the ability to effectively communicate the precise details of foot and ankle kinesiology. Pronation and supination at the subtalar joint are often referred to simply as eversion and inversion of the calcaneus, respectively. Eversion, for example, is only a component of, rather than a synonym of, pronation. Comparisons of range-of-motion data among studies are often difficult, unless the motions are explicitly defined.

Clinically, the expression “subtalar joint neutral” is often used to establish a baseline or reference for evaluating a foot before fabrication of an orthotic device.47 The neutral position of the subtalar joint is attained by placing the subject’s calcaneus in a position that allows both lateral and medial sides of the talus to be equally exposed for palpation within the mortise. In this “neutral” position, the joint is typically one third the distance from full eversion and two thirds the distance from full inversion.

TRANSVERSE TARSAL JOINT (TALONAVICULAR AND CALCANEOCUBOID JOINTS)

The transverse tarsal joint, also known as the midtarsal joint, consists of two anatomically distinct articulations: the talonavicular joint and the calcaneocuboid joint. These joints connect the rearfoot with the midfoot (see organization of joints illustrated in Figure 14-23).

At this particular point in this chapter, it may be instructive to consider the functional characteristics of the transverse tarsal joint within the context of the other major joints of the ankle and foot. As described earlier, the talocrural (ankle) joint permits motion primarily in the sagittal plane: dorsiflexion and plantar flexion. The subtalar joint, however, permits a more oblique path of motion consisting of two primary components: inversion-eversion and abduction-adduction. This section now describes how the transverse tarsal joint, the most versatile joint of the foot, moves through a more oblique path of motion, cutting nearly equally through all three cardinal planes. Among other important functions, the path of pronation and supination at the transverse tarsal joint allows the weight-bearing foot to adapt to a variety of surface contours (Figure 14-24).

The transverse tarsal joint has a strong functional relationship with the subtalar joint. As will be described, these two joints function cooperatively to control most of the pronation and supination posturing of the entire foot.

Articular Structure and Ligamentous Support:

Talonavicular Joint: The talonavicular joint (the medial compartment of the transverse tarsal joint) resembles a ball-and-socket joint, providing substantial mobility to the medial (longitudinal) column of the foot. Much of this mobility is expressed as a twisting (inverting and everting) of the midfoot relative to the rearfoot.74 The talonavicular joint consists of the articulation between the convex head of the talus and the continuous, deep concavity formed by the proximal side of the navicular bone and “spring” ligament (see Figure 14-8). The convex-concave relationship of the talonavicular joint is evident in Figure 14-21. The spring ligament (labeled as SL in Figure 14-21) is a thick and wide band of collagenous connective tissue, spanning the gap between the sustentaculum talus of the calcaneus and the medial-plantar surface of the navicular bone.87 By directly supporting the medial and plantar convexity of the head of the talus, the spring ligament forms the structural “floor and medial wall” of the talonavicular joint. Considerable support is required in this region during standing because body weight tends to depress the head of the talus in plantar and medial directions—toward the earth. The surface of the spring ligament that directly contacts the head of the talus is lined with smooth fibrocartilage.120 (The more formal and precise name of the spring ligament is the plantar calcaneonavicular ligament. The term “spring” is actually a misnomer because it has little, if any, elasticity; its highly collagenous nature offers considerable strength and resistance to elongation. Nevertheless, the term spring remains well established in the clinical and research literature.87)

An irregularly shaped capsule surrounds the talonavicular joint. The ligaments reinforcing this capsule are summarized in the box.

Calcaneocuboid Joint: The calcaneocuboid joint is the lateral component of the transverse tarsal joint, formed by the junction of the anterior (distal) surface of the calcaneus with the proximal surface of the cuboid (see Figure 14-23). Each articular surface has a concave and convex curvature. The joint surfaces form an interlocking wedge that resists sliding. The calcaneocuboid joint allows less motion than the talonavicular joint, especially in the frontal and horizontal planes.76 The relative inflexibility of the calcaneocuboid joint provides stability to the lateral (longitudinal) column of the foot.

The dorsal and lateral parts of the capsule of the calcaneocuboid joint are thickened by the dorsal calcaneocuboid ligament (see Figure 14-15).102 Three additional ligaments further stabilize the joint. The bifurcated ligament is a Y-shaped band of tissue with its stem attached to the calcaneus, just proximal to the dorsal surface of the calcaneocuboid joint. The stem of the ligament flares into lateral and medial fiber bundles. The aforementioned medial (calcaneonavicular) fibers reinforce the lateral side of the talonavicular joint. The lateral (calcaneocuboid) fibers cross the dorsal side of the calcaneocuboid joint, forming the primary bond between the two bones.120

The long and short plantar ligaments reinforce the plantar side of the calcaneocuboid joint (Figure 14-25). The long plantar ligament, the longest ligament in the foot, arises from the plantar surface of the calcaneus, just anterior to the calcaneal tuberosity. The ligament inserts on the plantar surface of the bases of the lateral three or four metatarsal bones. The short plantar ligament, also called the plantar calcaneocuboid ligament, arises just anterior and deep to the long plantar ligament and inserts on the plantar surface of the cuboid bone. By passing perpendicularly to the calcaneocuboid joint, the plantar ligaments provide excellent structural stability to the lateral column of the foot.70

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.

Axes of Rotation and Corresponding Movements: Manter originally described two axes of rotation for movement at the transverse tarsal joint: longitudinal and oblique.80 Movement at this joint therefore occurs naturally in two unique planes, each oriented perpendicular to a specific axis of rotation. The longitudinal axis is nearly coincident with the straight anterior-posterior axis (Figure 14-27, A to C), with the primary component motions of eversion and inversion (see Figure 14-27, D and E). The oblique axis, in contrast, has a strong vertical and medial-lateral pitch (see Figure 14-27, F to H). Motion around this axis, therefore, occurs freely as a combination of abduction and dorsiflexion (Figure 14-27, I), and adduction and plantar flexion (see Figure 14-27, J).

The transverse tarsal joint possesses two separate axes of rotation, with each axis producing a unique kinematic pattern. Although this may be technically correct, the functional kinematics associated with most weight-bearing activities occur as a blending of movements across both axes—a blend that yields the purest form of pronation and supination (i.e., movement that maximally expresses components of all three cardinal planes).76,95 Pronation and supination at the transverse tarsal joint allow the midfoot (and ultimately the forefoot) to adapt to many varied shapes and contours.

Range of motion at the transverse tarsal joint is difficult to measure and isolate from adjacent joints. By visual and manual inspection, however, it is evident that the midfoot allows about twice as much supination as pronation. The amount of pure inversion and eversion of the midfoot occurs in a pattern similar to that observed at the subtalar joint: about 20 to 25 degrees of inversion and 10 to 15 degrees of eversion.

Arthrokinematics: The arthrokinematics at the transverse tarsal joint are best described in context with motion across both the rearfoot and midfoot. Consider the movement of active supination of the unloaded foot in Figure 14-26, D. The tibialis posterior muscle, with its multiple attachments, is the prime supinator of the foot.64 Because of the relatively rigid calcaneocuboid joint, an inverting and adducting calcaneus draws the lateral column of the foot “under” the medial column of the foot. The important pivot point for this motion is the talonavicular joint. The pull of the tibialis posterior contributes to the spin of the navicular, and to the raising of the medial longitudinal arch (instep) of the foot. During this motion, the concave proximal surface of the navicular and the spring ligament both spin around the convex head of the talus.

Pronation of the unloaded foot occurs by similar but reverse kinematics as those described. The pull of the fibularis longus helps lower the medial side and raise the lateral side of the foot.

The previously described arthrokinematics of supination and pronation assume that the foot is unloaded, or off the ground. The challenge is to understand these arthrokinematics when the foot is on the ground, typically during the walking process. This topic is addressed later in this chapter.