The Foot PDF

Musculoskeletal System The Foot Welcome to this lecture on the foot. We have already briefly covered the bony anatomy of the foot, but we will consider the joints of the foot and consider its organization into arches. This lecture has been divided into 4 se...

Musculoskeletal System The Foot Welcome to this lecture on the foot. We have already briefly covered the bony anatomy of the foot, but we will consider the joints of the foot and consider its organization into arches. This lecture has been divided into 4 sections.  Learning Outcomes After this lecture you should be able to: ▪ Give an outline of the subtalar and midtarsal joints and joints of the forefoot ▪ Recall the meanings of the terms ‘supination’ and ‘pronation’ of the foot ▪ Give an account of the arches of the foot, and state the importance of the tendons and ligaments during standing and walking ▪ Know the bones forming each arch ▪ Know the key structures which help to maintain each arch (particularly the medial longitudinal arch) ▪ Appreciate the principles of the organisation of the intrinsic muscles of the foot ▪ Identify the structures involved in the extensor mechanism of the toes ▪ Give an account of the neurovascular structures in the foot and know the cutaneous distribution of the nerves Here are the learning outcomes for the lecture.  Musculoskeletal System The Foot Part 1: The Joints of the Foot In this part of the lecture, we will focus on the Joints of the Foot.  Learning Outcomes After this lecture you should be able to: ▪ Give an outline of the subtalar and midtarsal joints and joints of the forefoot ▪ Recall the meanings of the terms ‘supination’ and ‘pronation’ of the foot ▪ Give an account of the arches of the foot, and state the importance of the tendons and ligaments during standing and walking ▪ Know the bones forming each arch ▪ Know the key structures which help to maintain each arch (particularly the medial longitudinal arch) ▪ Appreciate the principles of the organisation of the intrinsic muscles of the foot ▪ Identify the structures involved in the extensor mechanism of the toes ▪ Give an account of the neurovascular structures in the foot and know the cutaneous distribution of the nerves The learning outcomes for this part of the lecture are that afterwards you should be able to; Give an outline of the subtalar and mid-tarsal joints and the joints of the forefoot Recall the meanings of the terms supination and pronation of the foot The remaining outcomes will be considered later.  Subtalar and Midtarsal Joints Midtarsal joints Subtalar joints There are two separate subtalar joints, and these are between the talus and calcaneus anteriorly and posteriorly. There are also two midtarsal joints. A medial one between the talus and navicular bone, and a lateral one between the calcaneus and cuboid. If the talus is removed, we can see the subtalar joints clearly. There is one anterior and one posterior. The posterior one is the true ‘anatomical’ subtalar joint. It is a synovial joint with its own capsule. The anterior joint has two articular surfaces on talus and calcaneus (referred to collectively as the clinical subtalar joint), which share a common synovial cavity with the talonavicular joint. The latter is a ball and socket synovial joint. The floor of this joint is strengthened by the spring ligament. The complex joint is referred to as the talocalcaneonavicular (or TCN) joint. The calcaneocuboid joint allows gliding of these two bones on each other, but the key movements of inversion and eversion are formed at the anatomical subtalar joint and TCN complex. Axis of Subtalar and Midtarsal Joints The important point to note is that the axis through which the movements of inversion/eversion take place around, is an axis which runs through the centre of the joint complex. This axis runs upwards and medially. Therefore inversion is a clockwise rotation around this axis (from the subject’s standpoint).  Supination of the Foot Hence, inversion is also accompanied by plantar-flexion and adduction of the forefoot. This is sometimes called ‘supination’ of the foot. When the foot is in supination, the rotation mechanism in the foot tightens the intrinsic ligaments within the foot, and the foot then becomes a rigid lever. This is ideal for lifting the body off the ground during walking or running. It provides a stable structure to provide thrust. It does however put added pressure on the hindfoot, because the force of body weight is shifted to the lateral side of the joint. This causes the subtalar joints to become angled such that the foot is pushed into varus and the calcaneus is no longer directly vertical under the tibia as it should be. Since the weight distribution is more laterally placed, it means that the medial arch of the foot is under less pressure, and it can rise. This though is part of the normal dynamic of the foot. There are certain conditions, such as neurological ones that can change the tone of muscular support, and this results in an exaggerated high arch and varus deformity at the heel. It is a cavovarus foot.  Pronation of the Foot Eversion is accompanied by dorsiflexion and abduction of the forefoot. This is also known as ‘pronation’ of the foot. When the foot is in pronation, the rotation mechanism in the foot relaxes the intrinsic ligaments within the foot, and the foot then becomes loose. This is ideal for absorbing the forces of body weight as the foot hits the ground. The force of body weight is shifted to the medial side of the joint. This causes the subtalar joints to become angled such that the foot is pushed into valgus and again the calcaneus is no longer directly vertical under the tibia as it should be. Since the weight distribution is more medially placed, it means that the medial arch of the foot is under more pressure, and it drops. This though is part of the normal dynamic of the foot. There are clinical scenarios where the medial arch drops too low, perhaps as a result of loss of supporting structures in the foot, or it could be due to an abnormal pull of the Achille’s tendon due to an increase in tone of gastrocnemius. These lead to what is known as a planovalgus deformity of the foot. The precise mechanisms are beyond the scope of this lecture.  Joints of the Midfoot and Forefoot Talonavicular (Ball and Socket) Tarsometatarsal (Gliding) (Lisfranc Joint) Calcaneocuboid (Plane gliding) When walking we transfer our body weight from the heel onto the ball of the foot, which is the head of the 1st metatarsal bone. Hence the joints of the foot in between cannot afford to give way and need to be stable and strong. However, we have already seen that the subtalar and midtarsal joints in fact are capable of quite complex movements. The talonavicular joint is a ball and socket joint, and it together with the subtalar and calcaneocuboid joints permit pronation and supination of the foot. When supinated the foot and its joints become very stable indeed. The tarsometatarsal joints are together referred to as the Lisfranc joint. It was named after one Napolean’s surgeons, who described a quick and efficient way to amputate across this joint to cure gangrene after frostbite of the soldier’s feet. Such was the harshness of the Russian front during his winter invasion. There isn’t much movement there normally, and even less after Lisfranc performs his amputation!  Joints of the Midfoot and Forefoot Talonavicular (Ball and Socket) Tarsometatarsal (Gliding) (Lisfranc Joint) Calcaneocuboid (Plane gliding) Metatarsophalangeal (Condyloid) Interphalangeal (Hinge) The metatarsophalangeal joint is a condyloid joint and is capable of bi- axial movements. Flexion-extension and adduction-abduction. We can spread our toes for fine tuning of balance, but the action is minimal. These joints however can be extended to facilitate walking and running as shown on the illustration on the left. Lastly, the interphalangeal joints are pure hinges. These are either interphalangeal in the case of the hallux or proximal and distal interphalangeal in the case of digits 2-5. Often the names of all of these joints are shortened to their initials. Hence, anatomists and clinicians alike prefer to talk of MTPs, PIPs and DIPs rather than the long-winded equivalent.  Musculoskeletal System The Foot Part 2: Arches of the Foot Welcome to part 2 of the lecture on the Foot. In this section we will focus on the arches of the foot.  Learning Outcomes After this lecture you should be able to: ▪ Give an outline of the subtalar and midtarsal joints and joints of the forefoot ▪ Recall the meanings of the terms ‘supination’ and ‘pronation’ of the foot ▪ Give an account of the arches of the foot, and state the importance of the tendons and ligaments during standing and walking ▪ Know the bones forming each arch ▪ Know the key structures which help to maintain each arch (particularly the medial longitudinal arch) ▪ Appreciate the principles of the organisation of the intrinsic muscles of the foot ▪ Identify the structures involved in the extensor mechanism of the toes ▪ Give an account of the neurovascular structures in the foot and know the cutaneous distribution of the nerves The learning outcomes for this part of the lecture are that afterwards you should be able to: Give an account of the arches of the foot, and state the importance of the tendons and ligaments during standing and walking Know the bones forming each arch Know the key structures which help to maintain each arch (particularly the medial longitudinal arch) The remaining outcomes will be dealt with in subsequent sections.  Arches of the Foot The thing is our feet have an equally impressive design. In fact there are several arched bridges in each foot. There are the medial longitudinal, lateral longitudinal and transverse arches.  A Stable and Flexible Foot Feet ultimately take our body weight and propel it along the ground as we walk and run. Each foot must be both stable and flexible. This is not easy to accomplish. Having many interconnected bones goes a long way to achieving this. Arranging these bones into arches helps with the load bearing that the foot must undertake.  Medial Longitudinal Arch The medial longitudinal is the most important, and is the highest arch. It is also the one that is most capable of deforming and reacting against body weight. The medial longitudinal arch is comprised of the calcaneus posteriorly, the talus, navicular, cuneiforms and the medial three metatarsals. Keystone of the Medial Longitudinal Arch The highest bone is the talus and this behaves like a keystone of a stone bridge. It directs the forces of body weight anteriorly to the heads of the medial three metatarsal bones and posteriorly to the calcaneus. The navicular and cuneiform bones are loaded in the process, and they resist this load by being anchored to each other by ligaments.  Lateral Longitudinal Arch The lateral longitudinal arch is not as pronounced as the medial one. It comprises the calcaneus posteriorly, the cuboid, and the fourth and fifth metatarsals. Keystone of the Lateral Longitudinal Arch The ‘keystone’ of this arch is the cuboid.  Transverse Arch Intermediate cuneiform There is also a transverse arch which is composed of the cuneiforms, the cuboid, and the bases of all of the metatarsal bones. The keystone of this arch is the intermediate cuneiform bone.  Arch Support The state of the arches plays a key feature in the normal dynamics of the foot and is important for normal function. The architecture of the foot is held in a delicate state of balance by the tendons, muscles and ligaments which support these arches. Any disruption to this equilibrium causes a change in the position of the arch. For example, if a supporting muscle becomes insufficient or damaged, then the ligaments will be put under greater strain.  Pes Planus If they are unable to cope with the increased load, then pes planus or flat-foot will follow. The most common cause is posterior tibial tendon dysfunction.  Pes Cavus If the muscles go into spasm or contracture, then pes cavus may result. This is commonly as a result of an increase in the tone of the gastrocnemius muscle, but other muscles can also be involved. It can be genetic (i.e. congenital) or acquired through nerve damage (e.g. a stroke or brain tumour). Regardless of the aetiology of the neuropathy, the muscle imbalance leads to excess stress being placed on the heel and ball of the foot, with consequent problems such as clawing of the toes, and an over-supinated foot.  Arch Support: Plantar Fascia Probably the most important structure in the maintenance of the arches is the plantar aponeurosis (or plantar fascia). Getting back to the analogy of a bridge, this fascia acts as a ‘tie-beam’, holding the ends of the longitudinal arches together. If the fascia is over-stretched or torn, it is almost inevitable that flat-foot (pes planus) will develop. The usefulness of the plantar fascia in lifting the arch is seen when lifting the big toe up; this stretches the fascia and the medial longitudinal arch of the foot rises. This is known as the Windlass mechanism. The plantar fascia is prone to inflammation, and this is known as plantar fasciitis. This commonly occurs in the posterior third of the aponeurosis. A patient with plantar fasciitis will complain of heel pain at the start of the day as the fascia begins to take on the load of body weight. It unfortunately only gets worse as the day goes on, unless the pain is heeded and the foot rested.  Arch Support: Ligaments All of the bones of the foot are anchored together by numerous ligaments. This gives the foot a rigidity which is necessary to support body weight yet keep the foot flexible enough to allow it to adapt to uneven surfaces, and act as shock absorbers. Ligaments are named according to the bones they connect, but also include a reference to their location. For example the dorsal calcaneocuboid ligament or the plantar cuneonavicular ligament. Thankfully though, all you need is to appreciate that they exist. There is little point learning these in detail. There are only a few that need to be identified individually. Arch Support: Longitudinal Ligaments On the plantar surface, the plantar calcaneonavicular ligament or spring ligament has already been mentioned previously. This arises from the calcaneus and runs on the sole of the foot to the navicular bone. On the lateral side the important ligaments are the long and short plantar ligaments. Again, we have met these before.  Plantar Ligaments Short plantar Plantar ligament calcaneonavicular Long plantar (Spring) ligament ligament Indeed, you may recall this image from a previous lecture. This shows these ligaments more clearly on this plantar view of the foot.  Arch Support: Tendons The long tendons from the leg provide a lift and support from above, much like the design of a suspension bridge. On the lateral side (left image), the peroneal tendons hold up the lateral longitudinal set of bones, with the peroneus longus grooved under the cuboid bone (the keystone). As this muscle also crosses under the transverse arch (to reach the medial cuneiform and base of 1st metatarsal), it provides some strength and support to that arch also. On the medial side, the deep flexor tendons provide the required support. The medial longitudinal arch is supported particularly by tibialis posterior and flexor hallucis longus tendons. The former attaches to the tuberosity of the navicular, medial cuneiform and bases of the 2nd and 3rd metatarsals. The fact that it has multiple attachments helps to staple the bones together, and further strengthen the arch.  Musculoskeletal System The Foot Part 3: Muscles of the Foot In part three of this lecture we will consider the intrinsic muscles of the foot.  Learning Outcomes After this lecture you should be able to: ▪ Give an outline of the subtalar and midtarsal joints and joints of the forefoot ▪ Recall the meanings of the terms ‘supination’ and ‘pronation’ of the foot ▪ Give an account of the arches of the foot, and state the importance of the tendons and ligaments during standing and walking ▪ Know the bones forming each arch ▪ Know the key structures which help to maintain each arch (particularly the medial longitudinal arch) ▪ Appreciate the principles of the organisation of the intrinsic muscles of the foot ▪ Identify the structures involved in the extensor mechanism of the toes ▪ Give an account of the neurovascular structures in the foot and know the cutaneous distribution of the nerves The learning outcomes for this section of the lecture are that afterwards you should be able to; Appreciate the principles of the organisation of the intrinsic muscles of the foot Identify the structures involved in the extensor mechanism of the toes The remaining outcome will be dealt with in part 4 Intrinsic Muscles of the Foot - Dorsum Extensor digitorum brevis Extensor hallucis brevis Before we begin, it is worth saying that none of the muscles I am about to tell you about (in this entire lecture) are worth memorising. You should simply get a feel for the way they are organised in layers. Functionally, none are critical and few act independently, so loss of any one is of little consequence. That said, we will look at these muscles, but if you wish to skip this lecture, you have my permission to do so. You won’t miss anything critical. The dorsum of the foot contains two intrinsic muscles, which are functionally not very important. They are extensor hallucis brevis and extensor digitorum brevis. They simply aid the action of the long muscles of similar name. They both arise from the calcaneus. The tendon of extensor hallucis brevis attaches to the proximal phalanx of the hallux. Extensor digitorum brevis normally has three tendons attaching to the middle phalanx of digits 2-4 or the lateral sides of the extensor digitorum longus tendons of those digits. Occasionally, one may find a slip going to the 5th digit in addition. Due to the superficial location of these muscles they are often injured. Footballers for example may bruise the muscles by a direct blow of the ball to the dorsum of the foot. There is little superficial fat here to cushion the blow.  1st Layer of Muscles of Sole of Foot Abductor hallucis Flexor digitorum brevis Abductor digiti minimi The sole of the foot is organised into layers. Before we reach the muscles however, the first notable structure is the plantar fascia which we have described previously. The skin over the sole is very thick but there is no superficial fascia. That’s not surprising, as you wouldn’t want to walk on a layer of fat. The first layer of muscle contains the flexors and abductors; abductor hallucis, flexor digitorum brevis, and abductor digiti minimi. They all arise from the calcaneus and the abductors insert onto the side of the proximal phalanx of digit 1 and 5. The flexor digitorum brevis attaches to the middle phalanges of digits 2- 4 or 2-5. Functionally, they do what they say they will.  2nd Layer of Muscles of Sole of Foot Lumbricals Flexor accessorius (quadratus plantae) The second layer contains muscles associated with the long flexor tendons. Flexor digitorum accessorius attaches from the calcaneus posterior and inserts into the flexor digitorum longus tendons distally. It serves to adjust the line of pull of the long flexors, so that the toes flex along their long axis. The lumbricals arise from these same long tendons and run on the medial side of each of the metatarsals to attach to the distal phalanges dorsally. The flex the MTP joints and extend the interphalangeal joints.  3rd Layer of Muscles of Sole of Foot Adductor hallucis Flexor hallucis (oblique head) brevis Adductor hallucis (transverse head) Flexor digiti minimi brevis The third layer of muscles contains the flexor hallucis brevis, which arises from the cuneiforms and cuboid and has a bifurcate tendon which inserts either side of the proximal phalanx of the hallux. Near its insertion it has a sesamoid bone within each tendinous slip. The adductor hallucis has transverse and oblique heads. The oblique head arises from the bases of the 2rd to 4th metatarsals, whilst the transverse head arises from the heads of the 3rd to 5th metatarsals. The insertion is into the medial sesamoid bone of the hallux. Finally, there is the flexor digiti minimi brevis. This arises from the base of the 5th metatarsal bone and it insets onto the lateral side of the proximal phalanx of the 5th digit.  4th Layer of Muscles of Sole of Foot Interossei muscles Plantar interossei – adduct Dorsal interossei – abduct The final layer contains two sets of muscles, the plantar and dorsal interossei muscles. The plantar ones are adductors and the dorsal ones are abductors. Abduction and adduction take place about an axis drawn through the 2nd digit. Hence movement away from the 2nd digit is abduction, and movement to it is adduction.  Plantar Interossei Muscles There is no need for an adductor of the 2nd digit itself, as it is the axis and any deviation from that axis is abduction. The hallux has its own adductor as we have seen in the 3rd layer of muscles. So there are 3 plantar interossei which attach to the medial side of digits 3-5 and insert in the phalanges of their own digit.  Dorsal Interossei Muscles There are 4 dorsal interossei, since the 2nd digit needs 2 of its own. It can abduct in two directions. The 3rd and 4th digits need an abductor positioned on the lateral side of the digit to pull it away from the 2nd one. The hallux and digiti minimi have their own abductors in the 1st layer.  Interossei and Lumbricals Both lumbricals and interossei attach to an expansion of the extensor digitorum longus tendon. This is called the dorsal expansion. From this diagram you can see how the lumbricals pass on the plantar side of the deep transverse metatarsal ligament, and then runs onto the dorsum of the digit. This means that it flexes the MTP joint, but extends the interphalangeal joint. The interossei on the other hand lie either medial or lateral to the MTP joint and hence cause abduction or adduction as appropriate. Again, plantar ones adduct, dorsal ones abduct. However, they too then run onto the dorsal expansion and hence extend the interphalangeal joints. Complicated isn’t it? As I said at the beginning though it matters little. I’m glad though that you stayed with it, because all of this fairly useless anatomy is replicated almost entirely in the hand, where it is of the utmost importance. So you will have to know it all eventually for that limb!  Musculoskeletal System The Foot Part 4: Neurovasculature We have now reached the final part of this lecture on the foot. In this section we will discuss the nerves and vessels of the foot.  Learning Outcomes After this lecture you should be able to: ▪ Give an outline of the subtalar and midtarsal joints and joints of the forefoot ▪ Recall the meanings of the terms ‘supination’ and ‘pronation’ of the foot ▪ Give an account of the arches of the foot, and state the importance of the tendons and ligaments during standing and walking ▪ Know the bones forming each arch ▪ Know the key structures which help to maintain each arch (particularly the medial longitudinal arch) ▪ Appreciate the principles of the organisation of the intrinsic muscles of the foot ▪ Identify the structures involved in the extensor mechanism of the toes ▪ Give an account of the neurovascular structures in the foot and know the cutaneous distribution of the nerves The single learning outcome is the afterwards you should be able to; Give an account of the neurovascular structures in the foot and know the cutaneous distribution of the nerves  Motor Nerves of the Foot There are two nerves on the dorsum of the foot and 2 nerves on the sole of the foot. On the dorsum are the two fibular nerves, superficial and deep. It is the deep one that supplies the dorsal muscles. The supply to the muscles on the sole of the foot is via the terminal branches of the tibial nerve, the medial and lateral plantar nerves. The medial plantar nerve supplies abductor hallucis, flexor hallucis brevis, flexor digitorum brevis and the first lumbrical muscle. The lateral plantar nerve supplies all other intrinsic muscles. Dorsal Cutaneous Nerves of the Foot The cutaneous supply to the dorsum is largely from the superficial fibular nerve, with the deep fibular supplying the skin between the first two toes (1st web space). Both of these nerves carry the L5 dermatome. The lateral margin of the foot is supplied by the sural nerve from S1. The medial margin of the foot is supplied by the saphenous nerve, a branch of the femoral nerve in the anterior thigh carrying the L4 dermatome. Hence across the dorsum from medial to lateral we have L4, L5 and S1 nerves. Plantar Cutaneous Nerves of the Foot On the sole of the foot, there is a branch from the tibial nerve to the skin over the heel and this is the medial calcaneal nerve. The sural and saphenous nerves again supply the margins. The remainder of the skin of the sole of the foot is divided between the medial plantar nerve and lateral plantar nerve. The latter supplies the skin over the lateral one and a half digits, whilst the medial plantar nerve supplies the rest. Across the sole of the foot the skin is also supplied by the L4, L5 and S1 nerves.  Arteries on the Dorsum of the Foot Dorsal metatarsal Arcuate Anterior tibial Dorsalis pedis Dorsal digital There are three arteries of the foot. On the dorsum is the dorsalis pedis artery, which is a continuation of the anterior tibial artery of the leg. The dorsalis pedis is usually palpable as it crosses over the tarsal bones. It then forms a dorsal arch near the bases of the metatarsal bones, known as the arcuate artery. From the arch arise the dorsal metatarsal arteries, which then divide distally to run alongside adjacent toes.  Plantar Arteries Plantar Plantar arch Posterior Medial plantar digital tibial Lateral plantar Plantar metatarsal The two arteries on the sole of the foot are the medial and lateral plantar arteries. The lateral plantar takes the lion’s share and forms the plantar arch near the bases of the metatarsals. These then also divide into two to give plantar digital arteries to adjacent toes. There are free communicating branches between the dorsal and palmar metatarsal arteries.  Veins of the Foot Dorsal venous Small saphenous vein arch Great saphenous vein Most of the blood from the foot drains superficially into the dorsal venous arch, although some will drain back along the venae comitantes that accompany the arteries. The dorsal venous arch leads to the saphenous veins. The great saphenous vein runs medially in front of the medial malleolus, whilst the small (short) saphenous vein runs posterior to the lateral malleolus. The course of these veins has been dealt with previously.  Musculoskeletal System The Foot And that completes this lecture on the foot. In the final lecture in the series, we will review all of the nerves that we have met along the way, and look at the consequences of their injury.