I have been asked this question so many times over the years I have worked in this industry. And like the answer to so many questions the initial answer is it depends.
Orthotics are often prescribed to people who have pain in their feet or who are prone to sprained ankles. They are not that different to running shoes with arch support for people who over pronate.
Now the theory is that by preventing the foot arch from collapsing inwards the risk of ankle and knee injury is reduced. This will be the effect at least in the short term but in many cases it is a case of treating the symptoms of the problem not the cause.
The real problem generally lies in the rotation of the hip. If your hip is externally /laterally rotated as your foot makes contact with the ground whilst walking or running then because your knee wants to move forward your foot flattens to allow movement at the ankle joint. Your knee has very limited sideways movement. Therefore you are very likely to end up swinging your leg around the natural line of movement. This will just increase the stress on your knee. If your foot internally or medially rotates the arch is usually excessive /high in this case a heel wedge is more common to stop the foot rolling outwards. And the knee appears to turns inwards although in a neutral standing position it will be turned outward. Generally the foot should always be pointing forward when moving forward and you should push off the ball of your foot and the toes not the inside or outside of your foot. Then the knee should automatically flex and move in a forward direction without rotation.
The other cause which is usually only one sided is weight transfer (you put all your weight on one leg) when you stand still. This tends to put slightly different stresses on the knee and hip as you move.
I strongly believe that in most cases if you can correct the imbalances further up the body in the hip and lower back so that you move in an anatomically correct manner you should not need orthotics.
There are exceptions where joints have collapsed such as with rheumatoid arthritis or when there has been severe trauma such as a fracture and in some defects at birth.
The pictures below demonstrate my point.
If you compare the two pictures above with the one below you can see the rotation and also the difference in the shape of the arch of the foot. This arching or flattening will be much more pronounced in someone who walks all the time with a large degree of rotation.
You need to address the cause of the problem not just the symptoms.
If you want to know more about how to go about fixing the problem you can email: rehabandrun@gmail.com or
telephone: 07980 695185