Biomechanics of the Foot and Ankle

The human foot is composed of 28 bones and 33 joint articulations. Together, the feet account for more than 25% of the total number of bones in the human body! These numbers only hint at the intricate anatomic and physiologic relationships that make bipedal locomotion possible. The foot and ankle surgeon needs a firm understanding of these relationships in order to diagnose, counsel, and treat patients.

This chapter, focused on foot and ankle biomechanics, is intended to provide the reader a foundation upon which to build an understanding of subsequent topics. First, a review of the gait cycle will be presented. Next, the biomechanics of the foot and ankle as it relates to gait will be outlined. And from these, surgical implications regarding the foot and ankle will be drawn. For greater details, the final sections in the chapter touch upon the kinematics and kinetics of human locomotion.

This chapter is based on the assumption that the reader has accurate knowledge of the anatomy of the foot and ankle. For review, the reader is encouraged to refer to anatomy textbooks that depict in detail the precise anatomic structures constituting the foot and ankle.

Gait Cycle

At the most fundamental level, gait is defined as the manner in which a person walks. Incredible advancements in computing, sensor, and imaging technologies have made it possible for nuances in human motion to be captured and mimicked in two- and three-dimensional (3D) spaces. One need to only watch some of the latest Hollywood movies to see how seamlessly computer-generated movements integrate with natural human locomotion. At the root of these advancements are the basic principles of gait that were initially studied and outlined by surgeon scientists in the last century. Outlined below are the fundamental aspects of the gait cycle that a surgeon should understand. More detailed descriptions of gait analysis can be found in the literature.

The walking cycle, being one of continuous motion, is difficult to appreciate in its entirety because so many events occur simultaneously. For simplicity, the cycle can first be considered from the standpoint of one limb. The gait cycle begins when the heel of that limb contacts the ground and ends when it again contacts the ground on the subsequent step. Within this cycle, are two phases: stance phase occurs while the foot is in contact with the ground and swing phase occurs while the foot is not in contact with the ground. For a given limb, more time is spent in stance phase (about 62%) than in swing phase (about 38%) ( Fig. 1-1 ).

Fig. 1-1, Phases of the walking cycle. Stance phase constitutes approximately 62% and swing phase 38% of cycle. During stance phase of walking, there are two periods of double limb support and one period of single limb support. Stance phase is further divided into three intervals: from heel strike to foot flat at approximately 7% of the gait cycle, foot flat to heel rise at approximately 34% of the gait cycle, and heel rise to toe-off at approximately 62% of the gait cycle.

Each phase can further be divided into intervals. During stance phase, the first interval begins when the heel strikes the ground (initial contact) and continues to when the foot lies flat (loading response). The second interval occurs as the foot lies flat and the body weight passes over and is briefly aligned with the foot (mid stance). The third interval happens as the body “falls” over the foot, the heel rises from the floor (terminal stance), and the toes begin to leave the floor (pre-swing).

Continuing to follow the same limb, swing phase then occurs. It begins with initial swing, or toe-off, when the foot leaves the ground to when the knee is at maximum flexion during gait. Then mid swing happens between when the knee is at maximum flexion to when the tibia is vertical. This continues to terminal swing, from when the tibia is vertical to just before the heel hits the ground again to begin another cycle.

Of course, as one limb is experiencing one phase of the gait cycle, the other limb is in some part of the opposite phase. It is helpful now to think of what is occurring with the contralateral limb during the phases of gait. Let’s consider the left and right limbs. During the first interval of stance phase, as the left foot experiences a loading response, the right foot is experiencing initial swing, or toe-off, both feet are momentarily touching the ground, providing double limb support. Then, as the body weight passes over the left foot, and the left foot experiences mid stance during the second interval, the right leg is in mid swing. During this time, there is single-limb support on the left leg. In the third interval, as the left foot experiences heel rise and pre-swing, the right foot is in terminal swing. Again, at this point, there is double limb support, where both feet are touching the ground.

Foot and Ankle Biomechanics

Now, with a basic understanding of the gait cycle, let’s consider the mechanical principles that govern gait. The foot is often viewed as a rigid base that supports the rest of the body, when in fact, it is a dynamic structure that changes to accommodate different needs during movement.

Consider the gait cycle described in the previous section. During stance phase, at heel strike, the foot needs to be supple in order to absorb the impact energy that comes with the weight of the body contacting the floor. And within moments, at the end of stance phase, that same foot transforms into a rigid structure over which the weight of the body can pass and fall into heel strike of the other foot. How does this transition happen?

Just as the gait cycle itself is the result of multiple concurrent events, the structural changes that occur during locomotion are the result of a confluence of anatomic relationships. There are passive relationships based on bone and joint anatomy, and there are also active relationships between muscle groups and their actions over each joint. The following sections explore these relationships separately before putting the information together so that it is more easily appreciated how the foot transitions from being supple to being rigid over the course of a gait cycle.

Joint Mechanics

To start, it is simplest to focus attention on one joint at a time, then to put together the knowledge about each joint to see how one affects the others during the foot’s transition from suppleness to rigidity.

The Ankle Joint

The ankle joint is oriented obliquely in the transverse (or axial) plane as well as in the coronal plane. In the transverse, or axial plane, the ankle joint is externally rotated in relation to the sagittal plane. In the clinical literature, this rotation is described as tibial torsion and affects the degree to which the foot is internally or externally rotated, or the degree to which there is in-toeing or out-toeing.

In the coronal plane, the ankle axis is best approximated by a line connecting the tips of the medial and lateral malleoli ( Fig. 1-2 ). The actual axis passes just distal to the tip of each malleolus. In the coronal plane, Inman found that the axis of the ankle may deviate 88 to 100 degrees from the vertical axis of the leg, with an average of 93 degrees. It slants from proximal medial to distal lateral ( Fig. 1-3 ).

Fig. 1-2, Estimation of obliquity of empirical ankle axis by palpating tips of malleoli.

Fig. 1-3, A , Variations in angle between midline of tibia and plafond of mortise. B , Variations in angle between midline of tibia and empiric axis of ankle. SD , Standard deviation; x̄ , arithmetic mean.

The obliquity of the ankle joint contributes to the relative rotation of the leg and the foot depending on the position of the joint. When the leg is held still and the foot is allowed to move, dorsiflexion of the ankle causes the foot to deviate outward. Plantarflexion causes the foot to deviate inward ( Fig. 1-4 ). The amount of rotation varies with the obliquity of the joint and the degree of dorsiflexion and plantar flexion. Conversely, when the feet are fixed to the floor, ankle dorsiflexion causes the tibia to rotate internally, and plantar flexion causes the tibia to rotate externally ( Fig. 1-5 ).

Fig. 1-4, Effect of obliquely placed ankle axis on rotation of foot in horizontal plane during plantar flexion and dorsiflexion, with foot free. Displacement is reflected in shadows of the foot.

Fig. 1-5, Foot fixed to floor. Plantar flexion and dorsiflexion of ankle produce horizontal rotation of leg because of obliquity of the ankle axis.

It is important to note that an oblique ankle axis does not fully account for all of the leg rotation and foot positions during movement. For example, when the magnitudes of the various displacements are studied, it becomes clear that the rotation of the leg attributable to ankle axis obliquity is much smaller than the degree of horizontal rotation of the leg that actually occurs. This additional rotation occurs due to the interplay of joints both proximal and distal to the ankle joint as discussed below.

The Subtalar Joint

The subtalar joint works in cooperation with the more proximal joints of the lower limb to account for the additional leg rotation not explained by the obliquity of the ankle joint axis. The subtalar joint is a sliding single-axis joint that acts like a mitered hinge connecting the talus and the calcaneus. In the coronal plane, the axis passes from medial to lateral at an angle of approximately 16 degrees. In the sagittal plane, the axis is angled about 42 degrees from horizontal. In the transverse plane, the joint deviates approximately 23 degrees medially from the long axis of the foot ( Fig. 1-6 ).

Fig. 1-6, Variations in subtalar joint axes. A , In transverse plane, subtalar axis deviates approximately 23 degrees medial to long axis of foot, with range of 4 to 47 degrees. B , In horizontal plane, axis approximates 41 degrees, with range of 21 to 69 degrees. x̄ , Arithmetic mean.

Mechanically, this hinge joint can be modeled by two boards joined by a hinge as in Fig. 1-7A . The vertical board represents the tibia and the horizontal board the foot. If the axis of the hinge is 45 degrees, then a simple torque converter has been created. Rotation of the vertical member causes equal rotation of the horizontal member. Changing the angle of the hinge alters this one-to-one relationship such that a more horizontally placed hinge causes a greater rotation of the horizontal member for each degree of rotation of the vertical member. And the reverse is true such that a more vertically oriented hinge causes a smaller rotation of the horizontal member for each degree of rotation of the vertical member. External rotation of the leg is converted to hindfoot inversion through the oblique axis of the subtalar joint.

Fig. 1-7, Simple mechanism demonstrating functional relationships. A , Action of mitered hinge. B , Addition of pivot between two segments of mechanism.

The importance of this mechanical relationship is apparent when comparing the angle of the subtalar joint in the sagittal plane in individuals with congenital pes planus and those with cavovarus deformity. In individuals with congenital pes planus, the subtalar joint tends to be more horizontally oriented. In these individuals, a more horizontally angled subtalar joint results in greater suppleness of the hindfoot.

Conversely, in individuals with cavovarus feet, the subtalar joint tends to be oriented more vertically. This means that for each degree of rotation of the leg, there is relatively less inversion or eversion of the hindfoot. Individuals with cavovarus feet therefore tend to have stiffer, more rigid joints in the hindfoot.

The Transverse Tarsal Joint Complex

Traveling distally from the subtalar joint, the calcaneocuboid and talonavicular articulations together make up the transverse tarsal joint complex. While each possess some independent motion, from a functional standpoint, they perform together.

The simple model described above for the subtalar joint can be refined further to include the transverse tarsal joint complex (see Fig. 1-7B ). The horizontal “foot” segment is divided into a short proximal and a long distal segment, with a pivot between the two segments. This pivot represents the transverse tarsal joint complex. Keeping the longer distal segment fixed to the floor in this model, the rotation at the pivot allows for inward and outward tilt at the shorter proximal segment. Analogously in the foot, the transverse tarsal joint is like a pivot that allows for hindfoot inversion and eversion while the forefoot remains in contact with the ground.

A closer look at the two joints of the transverse tarsal joint complex, the calcaneocuboid and talonavicular joints, reveals that beyond being a pivot between the hind and forefoot, they are responsible for the transition from suppleness to rigidity during gait. Elftman demonstrated that the axes of these two joints are parallel when the calcaneus is in an everted position and are nonparallel when the calcaneus is in an inverted position. The importance of this is that, when the axes are parallel, there is flexibility within the transverse tarsal joint, whereas when the axes are nonparallel, there is rigidity at the transverse tarsal joint. Imagine a door where the hinges all line up and will open and close easily, whereas if the hinges of a door diverge, then the door will be stuck in one position. In other words, when the heel everts, the transverse tarsal joint is “unlocked” (supple), and when the heel inverts, the joint is “locked” (rigid) ( Figs. 1-8 and 1-9 ).

Fig. 1-8, Function of transverse tarsal joint (as described by Elftman H: The transverse tarsal joint and its control, Clin Orthop Relat Res 16:41–46, 1960) demonstrates that when the calcaneus is in eversion, the resultant axes of talonavicular (TN) and calcaneocuboid (CC) joints are parallel. When the subtalar joint is in an inverted position, the axes are nonparallel, giving increased stability to the midfoot.

Fig. 1-9, Anatomic specimen with the foot removed at the transverse tarsal joint complex, demonstrating the relationship between talus and calcaneus during hindfoot motion. The talar head (T) and calcaneal side of the calcaneocuboid joint (C) are shown. The vertical line highlights motion of the calcaneus relative to the talus. K-wires mark the axes of the respective joints. When the calcaneus is in the everted position, the talonavicular and calcaneocuboid joint axes are parallel, and the transverse tarsal joint complex is mobile. When the calcaneus is in an inverted position, following the direction of the arrow , the talonavicular and calcaneocuboid joint axes diverge, and the transverse tarsal joint complex is locked.

Another clinical correlation can be drawn here. The dependency of transverse tarsal joint suppleness on hindfoot position further contributes to the suppleness of the foot in an individual with congenital pes planovalgus compared to someone with cavovarus deformity. Those with a valgus hindfoot will more likely have an “unlocked” and supple midfoot. (A caveat here is that this is not true in patients with advanced adult acquired flatfoot deformity, which is discussed in Chapter 29 .) Those with a varus hindfoot will likely have a locked and rigid midfoot.

The talonavicular joint morphology adds additional stability to the longitudinal arch when force is applied across it during the last half of the stance phase. The joint surface has different curvature of radius in the anteroposterior and lateral projections ( Fig. 1-10 ). When force is applied across a joint of this shape, stability is enhanced. This occurs at toe-off, when the plantar aponeurosis (described below) has stabilized the longitudinal arch and most of the body weight is being borne by the forefoot and medial longitudinal arch.

Fig. 1-10, Talonavicular joint. Left , Anterior view. Right , Lateral view. Relationship of head of talus to navicular bone shows differing diameters of head of talus.

Tarsometatarsal Joints and Columns of the Midfoot

Traveling distally from the transverse tarsal joint complex, the midfoot can be divided further into two columns. The medial column consists of the first through third rays, including the cuneiform-metatarsal joints and their respective metatarsals. The lateral column consists of the fourth and fifth rays, including the cuboid-metatarsal joints, and the fourth and fifth metatarsals.

Anatomically, the tarsal-metatarsal joints in the medial column are relatively stiff compared to those at the lateral column. The medial column is therefore considered rigid, while the lateral column is supple. If the model from the previous section is further elaborated to incorporate the foot columns, then the long distal “foot” segment is divided into a medial and a slightly shorter lateral column ( Fig. 1-11 ). External rotation of the vertical component of the model leads to inversion of the hindfoot portion, which then leads to elevation of the rigid medial forefoot and depression of the lateral forefoot. Internal rotation of the leg produces the opposite effect on the foot such that the hindfoot is everted, the medial forefoot is depressed, and the relatively flexible lateral forefoot remains in contact with the floor.

Fig. 1-11, Distal portion of horizontal member replaced by two structures. A and B , Mechanical analog of principal components of foot. C and D , Mechanical components inserted into foot and leg.

Metatarsophalangeal Joints

Finally, at the distal portion the foot are the metatarsaophalangeal joints. The distinguishing feature of the metatarsophalangeal joints is the axis formed by the unequal forward extension of the metatarsals ( Fig. 1-12 ). This is referred to as the metatarsophalangeal break . The head of the second metatarsal is the most distal head; that of the fifth is the most proximal. Although the first metatarsal usually is shorter than the second (because the first metatarsal head is slightly elevated and is supported by the two sesamoid bones), it often functionally approximates the length of the second.

Fig. 1-12, Variations in metatarsal break in relation to longitudinal axis of foot.

In the model above, the metatarsophalangeal break is modeled by a gentle taper or cascade in the length of the two columns going from medial to lateral. As the heel inverts and the medial column elevates, the axis of the metatarsophalangeal break allows all of the metatarsal heads to be in contact with the ground, thus evenly distributing body weight across the forefoot. If the metatarsals were all the same length, then as the heel inverts, only the metatarsal head of the fifth ray would be in contact with the ground. This concept is modeled more simply in Fig. 1-13 . The angle between the metatarsal break and the longitudinal axis of the foot may vary from 50 to 70 degrees. The more oblique the metatarsal break, the more the foot must supinate and deviate laterally after heel rise.

Fig. 1-13, Supination and lateral deviation of foot during raising of heel caused by oblique metatarsophalangeal break. A , Wooden mechanism without articulation. If no articulation is present, the leg deviates laterally. B , Model including the subtalar joint articulation. In addition to its other complex functions, the subtalar joint functions to permit the leg to remain vertical.

Progression from a Supple to Rigid Platform

At this point, the reader can take a moment to synthesize the relationships among the joints described in the models above to understand the cascade of events that allow the foot to transition from a supple to a rigid platform during gait.

The first interval in the gait cycle begins with heel strike and continues to when the foot lies flat. Just prior to heel strike, the hindfoot is inverted due to the pull of the tibialis anterior tendon. At heel strike, the anatomic position of the calcaneus being lateral to the tibial mechanical axis leads to eversion at the hindfoot, which leads to internal rotation of the leg proximally. Distally this rapid eversion of the hindfoot unlocks the transverse tarsal joint, leaving a supple midfoot that allows both columns to contact the ground, absorbing the impact energy.

Then during the second interval of the gait cycle, the limb transitions through stance phase and the leg externally rotates, causing the hindfoot to invert, which then locks the transverse tarsal joint, leaving a rigid midfoot wherein the relatively stiff medial column is elevated and the lateral column is depressed.

In the third interval, the body weight passes over the rigid midfoot while the heel remains inverted and transverse tarsal joints locked. And since there is an oblique axis to the metatarsophalangeal break, the forefoot remains in contact with the ground, with body weight evenly distributed across the metatarsal heads. In this way, as the heel lifts, the body weight can pass over a rigid and supportive foot.

The next section will introduce the modulators of the joint mechanics described above. Figs. 1–14 to 1–16 provide a visual representation of the multiple events that occur during the intervals of stance phase.

Fig. 1-14, Composite of events of first interval of walking, or period that extends from heel strike to foot flat, and occurs during the first 15% of the walking cycle. The heel’s impact and body’s center of gravity shift results in vertical floor reaction that transitions from zero to exceeding body weight by 15% to 25% ( A ). The ankle begins in dorsiflexion and progresses through plantarflexion ( B ). The anterior tibial muscles are active as seen in electromyograph (EMG) tracings ( C ), as they control the progressive plantar flexion at the ankle. The heel is mostly pronated to allow the foot to absorb the energy as the foot hits the ground ( D ). Accordingly, there is increased external rotation at the tibia as the hindfoot pronates ( E ).

Fig. 1-15, Composite of events of second interval of walking, or period of foot flat, which extends from 15% to 40% of the walking cycle. As the body passes over the foot in stance phase, force plate recordings show that the load on the foot may be as low as 70% to 80% of actual body weight ( A ). The ankle progresses from plantar to dorsiflexion ( B ), and the posterior tibial muscles contract eccentrically, along with the intrinsic foot muscles, to control the forward movement of the tibia ( C ), allowing the contralateral leg to take a longer step. The hindfoot supinates, and the tibia begins to come out of internal rotation ( D and E ).

Fig. 1-16, Composite of all events of third interval of walking, or period extending from foot flat to toe-off, and extends from 40% to 62% of the walking cycle. At the end of stance phase, as the other foot begins heel strike, the stance phase foot accordingly does not bear force toward the end of heel rise ( A ). The ankle plantar flexes at toe-off ( B ), driven by concentric contraction of the posterior calf musculature ( C ). The anterior compartment muscles become active in the last 5% of this interval. The subtalar joint remains supinated, and the tibia goes into external rotation ( D and E ).

Passive and Active Modulators of Joint Biomechanics

The mechanical relationships at the joints described above are modulated by both passive and active means. The anatomic characteristics at each joint described above are key passive contributors to the typical movements of the foot. The plantar aponeurosis, described below, is another important passive contributor to achieving a rigid platform in the foot during gait. The multiple tendons that cross the ankle into the foot are part of active systems that control foot biomechanics.

The Plantar Aponeurosis

An important passive contributor to foot biomechanics is the plantar aponeurosis, a band of fibrous tissue arising from the tubercle of the calcaneus and passing distally to insert into the base of the proximal phalanx ( Fig. 1-17 ). As the plantar aponeurosis passes the plantar aspect of the metatarsophalangeal joints, it combines with the joint capsule to form the plantar plate.

Fig. 1-17, Plantar aponeurosis. A , Division of plantar aponeurosis around flexor tendons. B , Components of plantar pad and its insertion into base of proximal phalanx. C , Extension of toes draws plantar pad over metatarsal head, pushing it into plantar flexion.

The plantar aponeurosis is the most significant stabilizer of the longitudinal arch between heel rise and toe off. As the body moves over the fixed foot and the heel begins to rise, the proximal phalanges dorsiflex, pulling the plantar aponeurosis over the metatarsal heads. This tightens the plantar fascia, resulting in a depression of the metatarsal heads and an elevation of the longitudinal arch ( Fig. 1-18 ). This mechanism is passive in that no muscle function per se brings about this stabilization.

Fig. 1-18, Dynamic function of plantar aponeurosis. A , Foot at rest. B , Dorsiflexion of metatarsophalangeal joints, which activates windlass mechanisms, brings about elevation of longitudinal arch, plantar flexion of metatarsal heads, and inversion of heel. C , Superimposed tracing of lateral radiographs of the foot at rest (outline) and with first ray dorsiflexion (gray figure) . Notice that dorsiflexion of the first toe tightens the plantar aponeurosis, which results in depression of the metatarsal heads, elevation and shortening of the longitudinal arch, inversion of the calcaneus, and elevation of the calcaneal pitch.

The plantar aponeurosis is most functional on the medial side of the foot and becomes less functional as one moves laterally toward the fifth metatarsophalangeal articulation. Based on its medial attachment to the calcaneus, plantar fascia tightening also contributes to hindfoot inversion, tibial external rotation, and transverse tarsal joint stabilization. These changes stabilize the midfoot and further allow the foot to act as a rigid lever during the toe-off phase of gait.

The function of the plantar aponeurosis is referred to as the windlass mechanism of the foot, likening its structure and function to an anchor windlass on a boat. The mechanics of the windlass mechanism can be demonstrated clinically by having an individual stand and forcing the great toe into dorsiflexion. As this occurs, one observes elevation of the longitudinal arch by the depression of the first metatarsal by the proximal phalanx, and, at the same time, inversion of the calcaneus. Careful observation of the tibia demonstrates that it externally rotates in response to this calcaneal inversion.

The Posterior Calf Muscles

The function of the posterior calf group during stance phase is to control the forward movement of the tibia on the fixed foot. Control of the forward movement of the stance leg tibia is critical to normal gait because it permits the contralateral leg to take a longer step, increasing stride length and improving walking efficiency. In pathologic states in which the calf muscle is weak, the stride length shortens, and dorsiflexion occurs at the ankle joint after heel strike because it is a position of stability. Paradoxically, the ankle is held more rigidly by secondary stabilizers to make up for the inability to control ankle dorsiflexion.

The posterior calf muscles basically function as a group, although the tibialis posterior and peroneus longus muscles usually begin functioning by about 10% of the stance phase, whereas the other posterior calf muscles tend to become functional at about 20% of the stance phase. As the ankle joint undergoes progressive dorsiflexion from foot flat until heel rise at 40% of the cycle, these muscles contract eccentrically. After heel rise, as ankle plantar flexion begins, they continue to contract, but now via a concentric contraction. It is interesting to note, however, that by 50% of the cycle, the electrical activity in these muscles ceases, and the remainder of the plantar flexion of the ankle joint is a passive event. High-speed motion pictures have demonstrated that during steady-state walking, at the time of toe-off, the foot is lifted from the ground, and the toes do not actively push off.

Muscle activity in the deep posterior compartment contributes to hindfoot inversion (see Fig. 1-15 ). As the posterior tibial muscle–tendon complex contracts, the hindfoot is pulled into inversion. Activity of the intrinsic muscles of the foot also contributes to midfoot stability and correlates fairly closely with the degree of subtalar joint rotation. In the normal foot, the intrinsic muscles become active at about 30% of the walking cycle, whereas in flatfoot, they become active during the first 15% of the walking cycle ( Fig. 1-19 ).

Fig. 1-19, Subtalar joint motion in normal foot and flatfoot. Shaded areas indicate period of activity of intrinsic muscles in normal foot and flatfoot.

The Anterior Calf Muscles

The anterior calf muscles contract eccentrically and function to slow the rapid ankle plantarflexion as the foot goes from heel strike to flatfoot during the first interval of the gait cycle. Anterior compartment musculature weakness results in a footdrop gait, characterized by accentuated hip flexion or circumduction of the hip during swing phase to avoid the toes of the dropped foot hitting the floor during swing-through.

During swing phase, dorsiflexion occurs at the ankle joint. Beginning at about 55% of the cycle and throughout swing phase, the anterior compartment muscles contract concentrically to dorsiflex the ankle. The medial insertion of the tibialis anterior tendon pulls the hindfoot into slight inversion during swing phase such that the calcaneus is slightly inverted at initial heel strike, before everting as described above (see Figs. 1-14–1-16 ). This is why most people will wear down the outer edge of the heel in their shoes asymmetrically.

Ligaments of the Ankle

The ankle joint is stabilized by ligaments whose configuration and alignment permit free movement of the ankle and subtalar joints to occur simultaneously. Because the configuration of the trochlear surface of the talus is curved to produce a cone-shaped articulation whose apex is directed medially, the single fan-shaped deltoid ligament is adequate to provide stability to the medial side of the ankle joint ( Fig. 1-20 ). However, on the lateral aspect of the ankle joint, there is a larger area to be covered by a ligamentous structure. The lateral ligaments are divided into three bands: the anterior and posterior talofibular ligaments, and the calcaneofibular ligament.

Fig. 1-20, Curvature of trochlear surface of talus creates cone whose apex is based medially. From this configuration, one can observe that the deltoid ligament is well suited to function along the medial side of ankle joint, whereas laterally, where more rotation occurs, three separate ligaments are necessary.

Fig. 1-21 demonstrates the anterior talofibular and calcaneofibular ligaments in relation to the subtalar joint axis. The calcaneofibular ligament is parallel to the subtalar joint axis in the sagittal plane. As the ankle joint is dorsiflexed and plantar flexed, this relationship between the calcaneofibular ligament and the subtalar joint axis does not change. It is important to appreciate that, when the ankle joint is in neutral position, the calcaneofibular ligament is angulated posteriorly, but as the ankle joint is brought into more dorsiflexion, the calcaneofibular ligament is brought into line with the fibula, thereby becoming a true collateral ligament. Conversely, as the ankle joint is brought into plantar flexion, the calcaneofibular ligament becomes horizontal to the ground. In this position, it provides little or no stability for resisting inversion stress.

Fig. 1-21, Calcaneal fibular ligament and anterior talofibular ligament. A , In neutral position of ankle joint, both anterior talofibular and calcaneofibular ligaments provide support to joint. B , In plantar flexion, anterior talofibular ligament is in line with fibula and provides most of support to lateral aspect of ankle joint. C , In dorsiflexion, calcaneofibular ligament is in line with the fibula and provides support to the lateral aspect of ankle joint.

The anterior talofibular ligament, on the other hand, is brought into line with the fibula when the ankle joint is plantar flexed, thereby acting as a collateral ligament. When the ankle joint is brought up into dorsiflexion, the anterior talofibular ligament becomes sufficiently horizontal so that it does not function as a collateral ligament. It can thus be appreciated that, depending on the position of the ankle joint, either the calcaneofibular or the anterior talofibular ligament will be a true collateral ligament with regard to providing stability to the lateral side of the ankle joint.

The relationship between these two ligaments has been quantified and is presented in Fig. 1-22 . This demonstrates the relationship of the angle produced by the calcaneofibular and the anterior talofibular ligaments to one another. The average angle in the sagittal plane is approximately 105 degrees, although there is considerable variation, from 70 to 140 degrees. This is important because, from a clinical standpoint, it partially explains why some persons have lax collateral ligaments. If we assume that when the ankle is in full dorsiflexion the calcaneofibular ligament provides most of the stability and that in full plantar flexion the anterior talofibular ligament provides stability, then as we pass from dorsiflexion to plantar flexion and back there will be a certain period in which neither ligament is functioning as a true collateral ligament. If we assume there is an average angle of approximately 105 degrees between these ligaments, then generally speaking, an area in which an insufficient lateral collateral ligament is present is unusual; however, if we have angulation of 130 to 140 degrees between these two ligaments, there is a significant interval while the ankle is passing from dorsiflexion to plantar flexion and back in which neither ligament is functioning as a collateral ligament. This may explain why some persons are susceptible to chronic ankle sprains. Some patients who are thought to have ligamentous laxity may, in reality, possess this anatomic configuration of lateral collateral ligaments.

Fig. 1-22, Average angle between calcaneofibular and talofibular ligaments in sagittal plane. Although the average angle is 105 degrees, there is considerable variation, from 70 to 140 degrees.

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