Closer Look at Metatarsalgia
Local Foot Anatomy
The foot can be neatly divided into three anatomical segments that have slightly different roles in walking:
- The rearfoot - also known as the heel
- The midfoot - also called the instep. It makes up the highest point of the arches of the feet
- The forefoot - also called the balls of the feet, also usual considered to include the toes
The rearfoot consists of two foot bones that make the heel shape, one of which articulates with the two leg bones that join together to make the ankle. The rearfoot allows the foot to contact the ground safely avoiding too much impact shock, adapt to the ground surface, and to act as an interface with the leg to allow the leg to pass over the heel while the heel stays on the ground. The rearfoot is normally the first part of the foot to touch the ground and first to come off the ground when walking. It also makes up the start of the foot arches.
The midfoot consists of five small bones that should never contact the ground. They sit just in front of the ankle and are easiest to feel over the top of the foot. These work as part of a system that allows rotation movements and gliding up and down movements that allow the forefoot to adapt to an uneven ground. This is especially useful for any irregularities between the surfaces that the heel is on compared to the ball of the foot. The midfoot is a very important part of making the arch of the foot work. The midfoot bones depress down as our body weight passes over the supporting foot. This drop in arch height stretches all the ligaments, other fibrous tissue like the plantar fascia and tendons under the foot. Just like stretching an elastic band, this stores energy in the foot, which is released as the heel lifts off the ground.
The forefoot consists of five metatarsal bones, and all the bones that make up the toes, called the phalanges. The phalanges can consist of 17 or 18 bones with all, but the big toes having three bones inside, although some people do just have two in the little toes as well. The big toe (known as the hallux) has different anatomy to the other toes, because it’s evolutionary story in different to the other toes, as for a long time our ancestors had an opposable big toes like thumbs. As a result we have two little bones under the big toe joint called sesamoids.
What the forefoot does in gait
Long bones are used as levers in terrestrial animals and are the most important bones used to create large motion. Our legs and are arms are made of long bones. They give us levers which muscles can pull on to allow us to move from A to B, or in the case of the arms, move objects from A to B.
The long bones found crossing the palm of our hands to our fingers are called the metacarpals, while those crossing the arches of our feet into the balls and joining to the toes are called metatarsals. Metatarsals are much longer relatively than to our size than metacarpals and as a consequence produce much greater movement via their longer leverage than if they were shorter. The fossil record clearly shows shortening of our rearfoot, midfoot and toe bones of our feet while our metatarsals have elongated, which is a much better shape to produce leverage for bipedialism.
The bones in are fingers and toes are all called phalanges and these are long bones. In the hand they are still used to create a lot of motion so our hand phalanges are long, to allow great flexibility. Our toes are changed their role since we became bipedal and we now have enough specimens of hominid feet going back 4.4 million years to see the toes bones getting gradually smaller and the big toes straightening up and loosing its mobility, to be pretty sure what has happened. The changes are very simple to explain and make engineering sense when you are becoming bipedal. We don’t want too much flexibility in the toes when we walk. We need the toes to act as stabilisers for the joints on the ball of the foot, so we can effectively roll the foot over the toes as we move our weight from one foot towards the other. The longer the toes are, the harder it is to roll over them. However without toes we have nothing to pull against to stabilise the metatarsal heads to control how much the arch lowers. Toes also hold the metatarsal heads from sliding forward as we roll off the ball on our foot. The toes act like the hole the pole goes in during a pole vault.
In essence then, our forefoot helps give us a stable base in conjunction with the rearfoot, while our midfoot drops down, to allow our body weight to pass over the foot, stretching the soft tissues in the bottom of the foot and back of the leg. This allows us to store energy in the foot like a spring. The action of the toe muscles pushing back against the metatarsal head controls how much the arch flattens, making sure the spring isn’t over stretched. As the rearfoot comes off the ground the energy to propel us forward is released and with some muscle contract from the calf and under the foot allows us to use the long levers of the metatarsals to roll forward towards our other foot. The toe muscles keep a force applied to the metatarsal heads to create much the same effect as a the hole a pole vault athlete needs to get the pole into to allow him to use the pole as a lever to propel himself over the bar! Sounds complicated, but it is really very simple.
Why are we prone to Metatarsalgia?
Walking seems like such a simple task, yet to do it successfully on two legs takes great co-ordination. Just look at a toddler. It takes a very sophisticated balance system to allow us to walk up right on just two legs, and it takes roughly 6 years for a child to develop an adult like gait. Because of the size of our large brains and therefore heads, humans are born at a very immature state compared to other mammals including chimps and other great apes. Children continue to develop parts of their spinal cord to the age of around 8.
Many major changes have taken place to allow us to stand upright including developing a curve of the spine in the lower back and neck, making the pelvis wider and shorter, rearranging the pelvic muscles making the knees less bowed outwards, making the feet stick out a bit, creating mobile arches in the feet and totally changing toe function from grasping structures to forefoot stabilisers. To achieve this we have largely altered how long we turn certain genes on rather than creating new genes. You may have heard we have only 1.2% difference in our genes from chimps, but despite this we look and behave extremely differently to chimps. This is because although we use the same genes we express them very differently by changing the amount of time they are used as we develop as foetuses and children, then other apes do.
Evolution is often about compromises. Human bipedal gait has involved a lot of compromises. Most of the changes that have taken place could have been avoided if apes hadn’t already evolved without tails. Without a nice tail to balance the trunk of our bodies we have no choice but to take up a position where our centre of mass sits right above our pelvis, and that has necessitated the spinal and pelvic changes already mentioned. But most importantly for metatarsalgia, it has meant us adapting a structure used originally for grasping in an arboreal environment.
The toes, like the fingers have spent most of there development developing the ability to bend downwards towards the plantar surface of the foot, much like hands bending fingers towards our palms of the hand. As we walk we now use our metatarsophalangeal joints as our fulcrums to roll off our feet, which means they bend upwards far more than downwards. To achieve this, the soft tissue anatomy around the metatarsophalangeal has changed a little to allow the toes to bend up more. Try bending your fingers back as far as you do with your toes. It hurts doesn’t it? The structures under the foot need the muscles that pull the toes down (the flexors or plantarflexors) to stay quite strong to prevent the structures that limit the upward movement of the toes from being injured by this increased extension or dorsiflexion. Walking on rough uneven ground tends to help because every step is different, some steps the toes will get bent up, sometimes the toes need to be bent down, to grip the ground, but generally walking barefoot on natural surfaces is unlikely to require a great deal of upward toe extension.
Another consideration for the metatarsals and the toe joints is the way in which we use an arch of the foot as an energy storage mechanism. The metatarsal heads load much earlier and for longer than they would if they didn’t slope downwards with a declination angle from the midfoot, which is held off the ground. As a result the metatarsals have rotated in a different way to other great apes, and are now shaped to resisting bending forces (moments). They therefore have a concave bottom surface along their length. When walking, as we lift the heel off the ground the metatarsals are subjected to a force that bends them the opposite way to the concave surface. The concavity therefore makes them much stronger. The strongest metatarsal is the first one, which joins to the big toe. The next strongest is the 2nd and 3rd metatarsals and finally the 4th and 5th. These last two although normally weight-bearing earlier than the others, are the first to come off the ground, and generally they are loaded with little force after the other three are loaded. These three are usually taking the greatest weight from the body as it passes forward over the foot. It is variable whether the 1st metatarsal head or the 2nd and 3rd metatarsal heads receive the most pressure, but generally the 2nd is loaded for the longest. It can be no surprise that this area is most frequently associated with metatarsalgia.
The last set of issues in the forefoot is the placement of some of the structures, such as the nerves and blood vessels were positioned before the foot became a bipedal structure as its primary role. There are a few delicate nerves around the toes that can get irritated and injured. As we get older we also tend to loose some of the natural padding on the bottoms of our feet, although this is probably more a result of loss of muscle strength than loss of the fat. The tissues of the forefoot actually tend to thicken, but they also become much stiffer. This is especially true under the 2nd metatarsal head. The skin under our forefoot is subjected to a lot of shearing stresses. Thicker, stiffer soft tissue is less able to cope with shearing stresses, which can lead to burning pain, callus and in diabetic patients, even ulcers.
Things have changed
The foot today finds itself encountering two very novel environments. The first is shoes. Evidence of humans wearing shoes from burial sites in Northern Eurasia starts around 45,000 years ago. In these areas the foot bones of these populations start to change with the metatarsals becoming less robust and the little toe getting smaller. In the last 500 years more of us have been wearing shoes for fashion not just for keeping us warm and protecting the foot from trauma. More humans have also been wearing shoes from childhood, and that could have profound effects of foot development, especially as younger children are tending to wear more restrictive and higher heeled shoes from an earlier age.
The other novel environment is a rapidly spreading one - hard flat man-made surfaces. As the world becomes more urbanised, more of us are spending more of our time on a very unforgiving surface that tends to demand that the toes do more extension than they would be expected to do on an uneven surface. They never get a chance to flex down on concrete, especially when inside shoes.
Walking on hard flat surfaces is tough enough for the forefoot, but modern trends since the early part of the 20th century has been to wear fashion dress shoes even at work. The higher the heel, the more upwards bend the toes have to make. This stretches the structures under the ball of the foot that try to stop the bones extending upwards, and over time they will become stretched out and weakened. The muscles that flex the toes will also weaken allowing the extensor muscles over the top of the foot to tighten up. The tightening of the extensor muscles causes more stretch on the ligaments that hold the toe down so that a gradual cycle of continued weakening and stretching of the structures under the forefoot occurs. Given time the toes will become retracted, clawed, hammer toed, or dislocated depending on which muscles are affected and which soft tissues rupture. To make matters worse, the extensor muscles pulling the toe upwards are 2-3x stronger than the muscles under the foot pulling the toes down when you have a hammertoe. Normally are plantarflexors pulling down are stronger than our dorsiflexors that pull up.
The less space there is in the toe box of the shoe the more the muscles and other soft tissues can get crushed. This certainly can cause corns and callus, but can also weaken the foot muscles which effectively can’t work when held too tight. So shoes with heels that have restricted toe and forefoot space are particularly problematic in causing metatarsalgia, and treatment without shoe change is very difficult.