Ankle and Foot Biomechanics 9 - Plantar arches

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Biomechanics of the Ankle and Foot Complex : 9 Plantar Arches :

Biomechanics of the Ankle and Foot Complex : 9 P lantar Arches Dr. Dibyendunarayan Bid [PT] The Sarvajanik College of Physiotherapy, Rampura, Surat

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The foot typically is characterized as having three arches: medial and lateral longitudinal arches and a transverse arch, of which the medial longitudinal arch is the largest.

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Although we may think of and refer to the two longitudinal and the transverse arches as if the arches were separate, the arches are fully integrated with one another (being more analogous to a segmented continuous vault) and enhance the dynamic function of the foot.

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The arches are not present at birth but evolve with the progression of weight-bearing. Gould and associates described flattened longitudinal arches in all children examined between 11 and 14 months of age. By 5 years of age, as children approached gait parameters similar to those of adults, the majority of children had developed an adult-like arch.

Structure of the Arches:

Structure of the Arches The longitudinal arches are anchored posteriorly at the calcaneus and anteriorly at the metatarsal heads. The longitudinal arch is continuous both medially and laterally through the foot, but because the arch is higher medially, the medial side usually is the side of reference. The lateral arch (Fig. 12-33A) is lower than the medial arch (see Fig. 12-33B).

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The talus rests at the top of the vault of the foot and is considered to be the “key-stone” of the arch. All weight transferred from the body to the heel or the forefoot must pass through the talus.

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The transverse arch, like the longitudinal arch, is a continuous structure. It is easiest to visualize in the mid-foot at the level of the TMT joints. At the anterior tarsals (Fig. 12-34A), the middle cuneiform bone forms the keystone of the arch. The transverse arch still can be visualized at the distal metatarsals but with less curvature (see Fig. 12-34B).

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The second metatarsal, recessed into its mortise, is at the apex of this part of the arch. The transverse arch is completely reduced at the level of the metatarsal heads, with all metatarsal heads parallel to the weight-bearing surface.

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The shape and arrangement of the bones are partially responsible for stability of the plantar arches. As illustrated in Figure 12-34A, the wedge-shaped mid-tarsal bones provide an inherent stability to the transverse arch. The inclination of the calcaneus and first metatarsal contribute to stability of the medial longitudinal arch, particularly in standing (see Fig. 12-33B).

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Although the structure of the tarsal bones provides a certain inherent stability to the arches, the arches would collapse without additional support from ligaments and muscles.

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Because the three arches can be thought of as a segmented vault or one continuous set of interdependent linkages, support at one point in the system contributes to support throughout the system. The plantar calcaneonavicular (spring) ligament, the interosseous talocalcaneal ligament, and the plantar aponeurosis have been credited with providing key passive support to the plate (Fig. 12-35).

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The “ articular ” ( superomedial portion) portion of the spring ligament provides particularly important support as it directly supports the head of the talus and the keystone of the longitudinal arch. Likewise, the cervical ligament is credited with contributing particularly important support of the posterior aspect of the longitudinal arch.

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According to one study conducted on cadavers, support from the more laterally located long and short plantar ligaments (see Fig. 12-35 ) appeared to be important but less influential than support from the spring and cervical ligaments.

Function of the Arches:

Function of the Arches Although the archlike structures of the foot are similar to the palmar arches of the hand, the purpose served by each of these systems is quite different. The arches of the hand are structured predominantly to facilitate grasping and manipulation but must also assist the hand in occasional weight-bearing functions. In contrast, the foot in most individuals is rarely called on to perform any grasping activities.

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The plantar arches are adapted uniquely to serve two contrasting mobility and stability weight-bearing functions. First, the foot must accept weight during early stance phase and adapt to various surface shapes.

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To accomplish this weight-bearing mobility function, the plantar arches must be flexible enough to allow the foot to: (1) dampen the impact of weight-bearing forces, (2) dampen superimposed rotational motions, and (3) adapt to changes in the supporting surface.

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To accomplish weight-bearing stability functions, the arches must allow: (1) distribution of weight through the foot for proper weight-bearing and (2) conversion of the flexible foot to a rigid lever.

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The mobility-stability functions of the arches of the weight-bearing foot may be examined: by looking at the role of the plantar aponeurosis and by looking at the distribution of weight through the foot in different activities.

Plantar Aponeurosis:

Plantar Aponeurosis Although other passive structures contribute to arch support, the role of the plantar aponeurosis (the plantar fascia ) is particularly important. The plantar aponeurosis is a dense fascia that runs nearly the entire length of the foot.

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It begins posteriorly on the medial tubercle of the calcaneus and continues anteriorly to attach by digitations to the plantar plates and then, via the plates, to the proximal phalanx of each toe (see Fig. 12-35).

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From the beginning to the end of the stance phase of gait, tension on the plantar aponeurosis increases, with in vivo experiments using radiographic fluoroscopy to show that the plantar fascia deforms, or stretches, 9% to 12% during this time. For this reason, the function of the aponeurosis in supporting the arches has been compared to the function of a tie-rod on a truss .

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The truss and the tie-rod form a triangle (Fig. 12-36); the two struts of the truss form the sides of the triangle and the tie-rod is the bottom. The talus and calcaneus form the posterior strut, and the remaining tarsal and metatarsals form the anterior strut. The plantar aponeurosis, as the tie-rod, holds together the anterior and posterior struts when the body weight is loaded on the triangle.

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This structural design is efficient for the weight-bearing foot because the struts (bones) are subjected to compression forces, whereas the tie-rod (aponeurosis) is subjected to tension forces.

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Bending moments to the bone that can cause injury are minimized. The fibrocartilaginous plantar plates of the MTP joints are organized not only to resist compressive forces from weight-bearing on the metatarsal heads but also to resist tensile stresses presumably applied through the tensed plantar aponeurosis. Therefore, each biological structure is positioned to maximize its optimal loading pattern and minimize the opportunity for injury.

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The plantar aponeurosis and its role in arch support are linked to the relationship between the plantar aponeurosis and the MTP joint. When the toes are extended at the MTP joints (regardless of whether the motion is active or passive, weight-bearing or non-weight-bearing), the plantar aponeurosis is pulled increasingly tight as the proximal phalanges glide dorsally in relation to the metatarsals or as the metatarsal heads glide in a relatively plantar direction on the fixed toes).

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The metatarsal heads act as pulleys around which the plantar aponeurosis is pulled and tightened (Fig. 12-37).

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As the plantar aponeurosis is tensed with MTP extension, the heel and MTP joint are drawn toward each other as the tie-rod is shortened, raising the arch and contributing to supination of the foot. This phenomenon allows the plantar aponeurosis to increase its role in supporting the arches as the heel rises and the foot rotates around the MTP joints in weight-bearing (during the metatarsal break).

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The tension in the plantar aponeurosis (the tie-rod) in the loaded foot is evident if active or passive MTP extension is attempted while the triangle is flattened (that is, when the subtalar and transverse tarsal joint are pronated). The range of MTP extension will be limited. Alternatively, raising the height of the triangle by acting on the struts can unload the tie-rod.

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For example, when the tibia is subjected to a lateral rotatory force, the hindfoot will supinate , the posterior strut will become more oblique, the height of the medial longitudinal arch will increase, and the plantar aponeurosis (the tie-rod) will be relatively unloaded. The reduction in tension in the plantar aponeurosis will allow an increase in the range of MTP extension.

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Through the pulley effect of the MTP joints on the plantar aponeurosis, the plantar aponeurosis acts interdependently with the joints of the hindfoot to contribute to increasing the longitudinal arch ( supination of the foot) as the heel rises during the metatarsal break, thus contributing to converting the foot to a rigid lever for effective push-off.

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The tightened plantar aponeurosis also increases the passive flexor force at the MTP joints, preventing excessive toe extension that might stress the MTP joint or allow the LoG to move anterior to the toes.

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Finally, the passive flexor force of the tensed plantar aponeurosis also assists the active toe flexor musculature in pressing the toes into the ground to support the body weight on its limited base of support.

Weight Distribution:

Weight Distribution Because the foot is a flexible rather than fixed arch, the distribution of body weight through the foot depends on many factors, including the shape of the arch and the location of the LoG at any given moment. Distribution of superimposed body weight begins with the talus, because the body of the talus receives all the weight that passes down through the leg.

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In bilateral stance, each talus receives 50% of the body weight. In unilateral stance, the weight-bearing talus receives 100% of the superimposed body weight.

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In standing, at least 50% of the weight received by the talus passes through the large posterior subtalar articulation to the calcaneus, and 50% or less passes anteriorly through the talonavicular and calcaneocuboid joints to the fore-foot. The pattern of weight distribution through the foot can be seen by looking at the trabeculae in the bones of the foot (Fig. 12-38).

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Because of the more medial location of the talar head, about twice as much weight passes through the talonavicular joint as through the calcaneocuboid joint.

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The somewhat lesser roles of the more laterally located long and short plantar ligaments in supporting the longitudinal arch may be attributable to the reduced weight-bearing compression through the calcaneocuboid joint in comparison with the more medially located talonavicular joint.

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In static standing, the distribution of weight-bearing on the plantar foot is highly variable and depends on a number of postural and structural factors. In one heterogeneous sample of feet (n= 107), peak pressures under the heel (139 kPa ) were, on average, 2.6 times greater than peak pressures under the forefoot (53 kPa ).

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Furthermore, load distribution analysis during quiet standing showed that: the heel carried 60%, the midfoot 8%, and the forefoot 28% of the weight-bearing load. The toes were minimally involved in bearing weight.

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Plantar pressures are much greater during walking than during standing, with the highest pressures typically under the metatarsal heads and occurring during the push-off phase of walking (~80% of stance), when only the forefoot is in contact with the ground and is pushing to accelerate the body forward. Excessive plantar pressures can contribute to pain and injury in otherwise healthy people or contribute to skin break-down in patients with diabetes and peripheral neuropathy.

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Structural and functional factors such as: hammer toe deformity, soft tissue thickness, hallux valgus, foot type, and walking speed have been shown to be important predictors of forefoot plantar pressures during walking in people without impairments and in people with diabetes .

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In general, the increased extension of the MTP joint seen in hammer toe deformity reduces pressures on the toes and increases pressure under the metatarsal heads. Pressure under the first metatarsal head also increases as arch height increases (as indicted by the inclination of the calcaneus or first metatarsal). As one might expect, the soft tissue under the forefoot and the heel acts as a cushion, and as this soft tissue thickness decreases, pressures increase.

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The greatest stresses to the heel during walking occur at heelstrike and typically are 85% to 130% of body weight. Running with a heel contact pattern increases this force to 220% of body weight. These large forces on the calcaneus are partially dissipated by the heel pad that lies on the plantar surface of the calcaneus.

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The heel pad is composed of fat cells that are located in chambers formed by fibrous septa attached to the calcaneus above and the skin below. The effectiveness of the cushioning action of the heel pad decreases with age and with concomitant loss of collagen, elastic tissue, and water. The change is evident in most people older than 40 years.

Muscular Contribution to the Arches:

Muscular Contribution to the Arches Muscle activity appears to contribute little to arch support in the normal static foot. The small intrinsic muscles of the foot (i.e., those that arise and insert within the foot) contract periodically during quiet stance, presumably to provide brief periods of unloading for the many ligaments supporting the foot.

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In gait, however, both the longitudinally and transversely oriented muscles become active and contribute support to the arches of the foot.

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Key muscular support is provided to the medial longitudinal arch during gait by the extrinsic muscles that pass posterior to the medial malleolus and inserting on the plantar foot: namely, the tibialis posterior, the flexor digitorum longus , and the flexor hallucis longus muscles.

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The peroneus longus muscle provides important lateral stability as its tendon passes behind the lateral malleolus, glides along the lateral cuboid just behind the base of the fifth metatarsal, and then courses the entire length of the transverse arch to insert into the base of the first metatarsal.

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These medial and lateral muscles provide a dynamic sling to support the arches of the foot during the entire stance phase of walking and enhance adaptation to uneven surfaces.

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End of Part - 9

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