Anatomy and Biomechanics





KEY FACTS





  • The foot and ankle comprise a complex “machine” consisting of 26 bones and joints working together. The individual parts do not work in isolation.



  • The ankle and hindfoot are 1 part of this machine, allowing the foot to adapt to uneven terrain, while the rest of the body remains upright.



  • Ankle joint is the principal joint for plantar flexion/dorsiflexion. The hindfoot joints (subtalar, talonavicular, and, to a lesser extent, calcaneocuboid) provide a complex motion that can be simply thought of as inversion and eversion.




    • Ankle and hindfoot joints act as a universal joint, so the foot can be positioned on any irregular surface, while the leg remains vertical for bipedal weight bearing.




  • Loss of motion from either the ankle or hindfoot will lead to overload of the other, as the joints attempt to make up for the lost motion.




    • This is why patients with ankle fusions universally develop hindfoot arthritis on x-rays at late follow-up.




  • The midfoot bridges the universal joint of the hindfoot to the metatarsal heads. At the joints between the navicular, cuneiforms, and medial metatarsals, stability is much more important than flexibility.




    • Arthrodesis of these joints probably does not impair foot function at all.




  • Toes, especially the 1st, provide propulsion during gait. Metatarsophalangeal motion is important in this function. Interphalangeal motion is not essential for walking.



  • In the normal human foot, contraction of the Achilles pulls on the calcaneal tuberosity at a short distance from the ankle. As the ankle begins to rotate in response to the Achilles contraction, the force is transmitted across a rigid foot to the metatarsal heads, at a distance from the ankle (center of rotation). The end result is amplification of the Achilles force for propulsion.



  • Overall, the human foot has changed from a flexible primate appendage used to grasp tree branches into a rigid lever for bipedal gait.







Overall schematic of the human foot is shown. The ankle and hindfoot provide flexibility. The longitudinal arch and the midfoot are important for stability. The toes remain flexible to facilitate propulsion during gait.








In the normal human, the Achilles tendon acts at a short distance from the center of rotation of the ankle . With a rigid arch, the short but strong Achilles contraction passes to the metatarsal heads resulting in a large displacement at the forefoot with strong propulsive forces .








The monkey foot is well adapted for an arboreal lifestyle. The 1st ray (hallux) is mobile so that it can grasp around a tree branch.








The monkey or primate foot does not have a rigid arch to act as a lever so that Achilles forces act more on the midfoot than the forefoot. The displacement of Achilles contractions are not amplified, so there is no great propulsion with bipedal gait .






Evolution of Modern Foot and Comparative Anatomy


Evolution





  • Human evolution diverged from chimpanzees ~ 5 million years ago.



  • Modern apes do not have a rigid arch.




    • The lever arm for the Achilles tendon is much smaller.



    • The ape foot has less propulsive power than the human foot.




  • In fact, the ape foot is better developed for grasping.




    • The 1st metatarsal is quite mobile at its articulation with the medial cuneiform, so the 1st ray (hallux) can be used to grasp tree branches.




  • The foot of the modern chimpanzee or gorilla is a compromise between a weight-bearing organ and a grasping one.




    • The foot retains the mobile hallux.




  • The modern human foot has a tightly packed, immobile 1st ray.




    • The hallux is no longer able to abduct because of increased rigidity at the 1st metatarsocuneiform joint.



    • Adduction of the 1st metatarsal developed along with stability of the longitudinal arch.




  • Fossil footprints of a purely bipedal gait are visible from 3.7 million years ago.




    • At that time, human ancestors ( Australopithecines ) had a brain case very much like that of a chimpanzee.




  • One theory proposes that development of a modern, bipedal gait was the 1st step in human evolution.




    • By freeing up the hands from any weight-bearing or tree-climbing obligations, the hands could specifically evolve for fine motor skills and tool use.



    • Such refinement in use of the hands induced rapid expansion of the cerebral cortex.




  • The foot of early hominids (such as Homo habilis from 1-2 million years ago) probably looks very much like a modern human foot.



Longitudinal Arch





  • Compared with other animals, the human foot is well adapted for prolonged walking but perhaps not as good for climbing or running.



  • A key anatomical feature of the modern human foot is the longitudinal arch.



  • The arch provides some shock absorption while walking and gives room for nerves and vessels to pass to the forefoot without being crushed.



  • More importantly, the arch provides a long lever arm for the Achilles tendon to act on the forefoot.




    • With a stable arch, the joints between the calcaneus and metatarsals are rigid, so that Achilles tendon forces can pass from the calcaneal tuberosity to the metatarsal heads with rotation at the ankle.




  • The rigid lever facilitates propulsion during gait.



  • No other living primate can walk with the sustained bipedal gait of the modern human.





Anatomy of Foot





  • Some articulations are vital for normal function, while others are relatively unimportant for normal walking and running.



Ankle





  • The tibia and fibula together make a tight socket (mortise) for the talar dome.



  • The talar dome is wider anteriorly than posteriorly, so that dorsiflexion tightens up the fit of the talus in the mortise and also causes the fibula to move slightly laterally.



  • The joint surfaces are highly conforming so that weight-bearing forces can be spread out over a broad surface area, minimizing joint pressures.



  • Alteration in these conforming surfaces can dramatically decrease contact area and increase pressure, leading to arthritis.




    • This might occur with syndesmotic widening or intraarticular fracture.




  • Widening of the mortise by 1 mm increases peak contact pressures almost 50%.



  • Several studies have shown that persistent widening of the ankle mortise after injury leads to poorer outcome.



  • The cartilage of the joint is relatively thick.



  • About 1/6 of weight-bearing forces are borne by the fibula. The remainder passes through the tibia.



  • The distal tibial articular surface (plafond) may have as much as 3° of valgus.



  • The mortise is externally rotated 20-30° relative to the knee.



Ankle Ligaments





  • Stability for the ankle during standing is primarily through the conforming shape of the joint surfaces.



  • Collateral ligaments play a role while walking and running.



  • On the medial side, the superficial deltoid ligament has fibers that pass from the medial malleolus to the talus, navicular, and calcaneus.




    • The deep deltoid is most important for stability.




      • It passes from deep inside the medial malleolus to the medial body of the talus.




    • A major source of blood supply to the talus enters the body through these medial ligaments.




  • The lateral collateral ligaments include the anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), and posterior talofibular ligament.




    • The ATFL provides protection against inversion while the ankle is plantar flexed.



    • The CFL is more important when the ankle is dorsiflexed.




Distal Tibiofibular Syndesmosis





  • The distal tibia has a notch posterolaterally for a snug fit with the distal fibula.



  • This syndesmosis is held together by the anterior inferior tibiofibular ligament, posterior inferior tibiofibular ligament, interosseous ligament and membrane, and transverse tibiofibular ligaments.



Talus





  • The talus is the center of the ankle and hindfoot “universal joint.”



  • A large part of the talar surface is covered by articular cartilage.



  • There are no muscular attachments to the bone.



  • The blood supply to the talus is somewhat tenuous.




    • The posterior tibial artery sends an artery to the tarsal canal, which enters the talus through the deltoid medially, and also through the inferior surface in the tarsal canal (between the posterior and middle facets of the subtalar joint).



    • The dorsalis pedis artery provides important blood supply through the dorsal neck.



    • The peroneal artery also sends some contributions to the sinus tarsi.




  • A fracture or dislocation can easily disrupt some or all of this blood supply, leading to avascular necrosis.



  • Surgical trauma can injure the blood supply to the talus as well.




    • Although avascular necrosis is rarely seen after surgery, the lack of vascularity may present as surgical nonunion.



    • With total ankle arthroplasty, surgical damage to the talus may prevent osseous ingrowth into the implant with aseptic loosening of the talar component.




Calcaneus





  • In contrast to the talus, the calcaneus has abundant blood supply and soft tissue attachments.



  • Fractures of the calcaneus often will have complete disruption of the blood supply to multiple fragments, yet tend to heal well.




    • Here is an old surgeons’ joke: That is why it is called the “heal” (heel) bone.




  • Occasionally, calcaneus fractures will show avascular necrosis but less commonly than the talus.



  • The calcaneus contains a large, dense, medial projection: Sustentaculum tali.




    • The superior surface of the sustentaculum contains the middle and anterior facets of the subtalar joint.



    • The spring ligament takes its origin here.




  • The posterior tibial neurovascular structures pass within a few millimeters of the medial calcaneal wall.




    • When performing a calcaneal osteotomy from the lateral side, it is important to avoid penetrating the medial wall with a saw or osteotome.




Os Trigonum





  • The os trigonum is a normal bone found in many patients at the back of the ankle.




    • It may be an ununited posterior process.




  • The os may articulate with the posterior facet on the calcaneus.



  • It lies just deep and lateral to the flexor hallucis longus tendon.



  • It may become irritated, especially in dancers who frequently go into extreme plantar flexion.



  • Resection of an os trigonum can be done easily through a posteromedial approach.




    • The commonly used posterolateral approach can lead to injury of the sural nerve.




Hindfoot Joints





  • The subtalar joint has 3 articular facets: Anterior, middle, and posterior.




    • The anterior and middle are often contiguous, while the posterior is larger and separate.



    • The posterior facet is saddle-shaped.




  • In the past, the subtalar joint was thought to have a single axis of rotation, passing obliquely from posterolateral to anterodorsal.




    • During stance, the axis makes an angle of 41° with the ground.




  • More precise biomechanical studies have shown the joint to behave more like a screw with no 1 axis of rotation.




    • As the calcaneus rotates into eversion, it also passes posteriorly.



    • Inversion is accompanied by forward translation.




  • Talonavicular and subtalar motion are tightly coupled.




    • Fusion of the talonavicular joint eliminates all subtalar motion.



    • Fusion of the subtalar joint leaves ~ 25% of normal talonavicular motion.




  • The calcaneocuboid joint is less critical for hindfoot motion.




    • Isolated fusion of the calcaneocuboid does not limit subtalar motion much at all and leaves 67% of talonavicular motion intact.




  • The calcaneus and navicular (and the rest of the foot) rotate around the talus (peritalar motion).




    • In general, the motion between other tarsal bones is smaller and much less important than that between the talus, calcaneus, and navicular.



    • Although none of these joints have a true single axis of motion, models of hindfoot mechanics often assume they do.




Hindfoot Ligaments





  • There are many interosseous ligaments in and around the subtalar joint.



  • The lateral ligaments provide support against varus stresses.




    • The CFL and inferior extensor retinaculum provide some lateral support.



    • Other lateral subtalar supports include the cervical ligament and the interosseous talocalcaneal ligament.




  • The spring ligament passes from the sustentaculum tali of the calcaneus to the navicular.




    • It is a key ligament in support of the longitudinal arch.




  • The long plantar ligament runs from the calcaneus to the cuboid and is also an arch stabilizer.



Midfoot Joints





  • The joints between the navicular and the cuneiforms have little motion.



  • The metatarsocuneiform joints are very stable as well.



  • There is some motion between the cuboid and 4th and 5th metatarsals.




    • This motion gives some flexibility to the lateral column of the foot, making gait more comfortable.



    • Fusion of these joints should probably be avoided.




Forefoot





  • Motion at the metatarsophalangeal (MTP) joints is essential during gait, and the metatarsal heads are essential for weight bearing.



  • Resection of any metatarsal head should be avoided in general.



  • While standing, ~ 40% of body weight is carried through the 1st metatarsal.




    • The remainder is divided up among the lesser metatarsal heads.




  • The interphalangeal joints are not important for walking.




    • They are important for grasping, although this is not an important task for the human foot.



    • Interphalangeal fusion or resection is well tolerated.




  • The hallucal sesamoids reside in the tendons of the 2 flexor hallucis brevis muscles.




    • Loss of a sesamoid without repair of the flexor hallucis brevis tendon may lead to varus or valgus at the MTP joint.



    • Resection of a single sesamoid is generally well tolerated.




      • Traditional teaching is that resection of both sesamoids should not be performed.





Plantar Fascia





  • The plantar fascia runs from the calcaneal tuberosity to the forefoot.




    • The main (central) band inserts both into the subcutaneous tissue in the ball of the foot and to the septa of the flexor tendons in the toes.




  • The plantar fascia supports the longitudinal arch.




    • Complete division of the fascia leads to mild loss of arch height.




  • With the “windlass” mechanism, extension at the MTP joints leads to tightening of the fascia and support of the arch.



Plantar Fat Pad





  • The plantar subcutaneous layer consists of a specialized collection of adipose tissue within a framework of fibrous lamellae in a complex whorl pattern.




    • This fibrous frame gives the plantar fat structural support, allowing it to cushion the foot from the impact of normal walking.



    • Damage to the fat pad may occur after high-energy trauma, such as a calcaneus fracture or lawn mower injury.




Instability of 1st Ray





  • The 1st ray (medial cuneiform and 1st metatarsal) is tightly packed with the rest of the foot in the normal human.



  • In 1935, Dudley Morton, an anatomist at Columbia University, proposed that instability of the 1st ray was a source of trouble in the foot. He thought this trait was atavistic, implying a reversion to a more primitive state.




    • Possibly because of objections to the concept of evolution in the early 20th century, his theories were not widely accepted. Controversy continues in modern times.




  • Despite continued controversy, it is undeniable that hallux valgus deformity is caused by deformity at the metatarsocuneiform and MTP joints.




    • Because the cuneiform and metatarsal do not change shape with aging, and because hallux valgus is an acquired deformity, there must be instability in the joints to create the deformity.




  • Instability of the 1st ray at the metatarsocuneiform joint leads not only to hallux valgus but can also lead to elevation of the 1st metatarsal.




    • Weight-bearing forces will then be transferred to the 2nd metatarsal head.




      • This is often the cause of transfer metatarsalgia.





Arch Height





  • The medial longitudinal arch passes through the talonavicular, naviculocuneiform, and metatarsocuneiform joints.



  • Instability or sagging at any of these joints can result in a fallen arch or flatfoot.



  • Instability at the 1st metatarsocuneiform joint is seen with hallux valgus deformity.




    • Because this instability is 3D, patients with hallux valgus often have a flatfoot.




  • On a weight-bearing lateral radiograph, one indicator of arch integrity is the talometatarsal angle.




    • This angle is determined by the intersection of the axis of the talus with the axis of the 1st metatarsal.




      • In the normal foot, it is ± 4°.




    • Arch height varies, but whether the arch is low or high, the talometatarsal angle should be within the normal range.



    • A talometatarsal angle outside the normal range suggests a pathologic process.




Tripod Model of Foot





  • One model of foot structure depicts the foot as a tripod.




    • The 3 “legs” of the tripod are the heel, 1st metatarsal head, and 5th metatarsal head.




      • Balance between these 3 is important for foot support.



      • Elevation or depression of the 1st metatarsal will tilt the rest of the tripod.





  • In some flatfeet, subluxation or sag at the 1st metatarsocuneiform joint leads to collapse of 1 leg of the tripod.




    • Without a supporting medial post to balance the foot, the hindfoot can collapse into valgus.




      • The final result is a flatfoot.



      • This has been termed forefoot-driven hindfoot valgus.




    • Collapse at the naviculocuneiform or talonavicular joints can lead to the same end result: Hindfoot valgus.




  • By a similar model, plantar flexion of the 1st metatarsal will drive the hindfoot into varus.




    • This is termed forefoot-driven hindfoot varus.




      • The end result is a cavovarus foot.





Structural Diversity in Human Feet





  • It is clear that there is a wide spectrum of foot shapes in modern humans.




    • Variations in arch height and 1st metatarsal alignment lead to abundant diversity.




  • Interestingly, when fossil specimens from prehistoric hominids are evaluated, there is also structural diversity.




    • Rather than implying that early hominids could not walk upright, it suggests that the human foot is a work in progress.



    • It will be interesting to see whether human feet are more uniform 1 million years from now.




Foot and Ankle in Gait





  • As the heel strikes the ground, the ankle moves from dorsiflexion to plantar flexion (foot flat on the ground).




    • Eccentric contraction of the tibialis anterior controls the descent to foot flat.



    • Tibialis anterior rupture or peroneal nerve palsy leads to a gait pattern with uncontrolled “slapping” of the foot on the ground as the limb moves from heel strike to foot flat.




  • As the leg moves to midstance, the ankle dorsiflexes ~ 10°.



  • Body weight passes over the foot, and strong gastrocnemius and soleus contractions move the foot to heel rise.




    • The ankle once again plantar flexes.




  • The primary functions of the Achilles muscles are to decelerate tibial advance for knee stability and to stabilize the ankle so the limb can rock on to the forefoot.




    • In routine gait, they are not as important for push off.




      • As the stride is lengthened, more work is required of the Achilles.





  • Untreated rupture or overlengthening of the Achilles will prevent effective heel rise.




    • Body weight is kept close to the heel rather than moving to the forefoot.



    • Stride length is shortened, and heel rise is delayed.




  • The toe flexors, especially the flexor hallucis longus, are active during late stance, heel rise, and toe-off stages.



  • Once the foot leaves the ground, the ankle must dorsiflex to clear the ground.




    • The 1st toe clears the ground by < 1 cm during normal gait.



    • Absence of the ankle dorsiflexors (tibialis anterior) leads to a high steppage gait, where the limb is lifted higher off the ground for clearance.




  • Normal cadence for an adult is 101-122 steps per minute.




    • It is slightly higher in women than in men and much higher in small children.



    • Cadence does not change with aging, but stride length does decrease.




Ankle Stability





  • Inversion injuries are the most common ankle athletic injury.




    • This is due in part to the relative strength of the medial (deltoid) ligaments relative to the lateral complex.



    • Furthermore, many athletic maneuvers tend to promote inversion.




  • Stability of the ankle comes in part from static soft tissue ligaments.




    • The lateral ankle ligaments, in particular the ATFL and CFL, provide some mechanical constraint to inversion.




  • The ATFL and CFL are not stout enough to prevent uncontrolled inversion.




    • Rather, their role may be more as proprioceptive sensors than simple static restraints.



    • When the ankle begins to roll into inversion, the ATFL suddenly stretches.



    • In a reflex similar to the patellar deep tendon reflex, stretch of the ATFL leads to reflexive contraction of the peroneal tendons.




      • Peroneal activation prevents uncontrolled ankle inversion.





  • The peroneal tendons also have stretch mechanoreceptors, and these may be more important than the ligaments in providing position sense.




    • Reflex contraction of the peroneals in response to sudden stretch of ankle mechanoreceptors is referred to as “closed-loop” control.



    • Stretch receptors in the ligaments and tendons send a signal to motor cell neurons in the spinal cord with reflexive contraction of the peroneal muscles.



    • Peroneal closed-loop reaction time may be too slow to prevent inversion injuries during athletic activity.




  • There also appears to be “open-loop” controls, consisting of preactivation of stabilizing muscles in anticipation of upcoming stress.




    • In other words, an athlete learns through training and experience to activate appropriate stabilizing muscles at just the right time.




  • Perhaps in the real world, open-loop preactivation fires appropriate stabilizing muscles for the anticipated stress, and then closed-loop stretch reflexes fine-tune the balance as needed.




Oct 29, 2019 | Posted by in ORTHOPEDIC | Comments Off on Anatomy and Biomechanics
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