What is biotribology?
Biotribology is the study of lubrication in the human body. There are a lot of studies on the mechanism of lubrication of joints like the hip joint. How does the hip joint stay lubricated?
Another question to ask would be, how does the hip joint stay lubricated under a wide range of operating conditions. For example, how is it kept lubricated when supporting longs of weight, say while doing weight squats, versus bearing the weight of the body while standing on two legs, versus standing on one? How does it deal with the high forces of impact experienced while running. How do our hip joints keep lubricated in all these different situations?
Marine engine lubrication
I've talked about this elsewhere, but in the study of marine engine lubrication, there are three main methods of lubrication.
Boundary layer lubrication
One type of lubrication is boundary layer lubrication. This is where mating surfaces rub against each other. It generally results in parts needing to be replaced after a time.
So its not an ideal lubrication system. However, with our joints, since they can be replenished, it's perhaps not such a bad thing
Another type of lubrication is hydrodynamic lubrication. In this type of lubrication method, the movement of the shaft relative to the bearings creates a build up of lubricant between mating surfaces. This means less wear and tear. However, it requires higher operating speeds. And as a side-note, this effect is driven by the same principles that cause hydroplaning.
This type of lubrication can happen in our hip joints with relatively fast movement, say walking briskly or running.
The third type of lubrication is hydrostatic lubrication. Here an pump is used to pressurize lubricant. This again prevents mating surfaces from rubbing against each other. With marine engines, the disadvantage is that you have to pay extra for the pump. However, if you have the money, the advantage is that you don't have to worry so much about operating speed. You prevent mating surfaces from rubbing away at each other both at low speed and high.
The third method can occur as a result of increasing joint capsule tension. I'll talk about how this can happen, next.
Varying hip joint capsule tension to keep the hip joints lubricated
With synovial joints like the hips, the idea of pressurizing lubricating fluid so that it isn't squeezed out from between mating surfaces can be achieved by varying joint capsule tension.
This might not seem possible if ligaments (which tend to attach to joint capsules) are viewed as passive structures.
Ligaments are active structures
The idea of active ligaments was proposed by Jaap van der Wal.
It basically means that ligaments are affected by muscle tension, just like tendons are.
(There are ligaments that aren't affected by muscle activation, but that is the exception rather than the rule, and he explains why in his paper and in his talk linked to above).
If we view both ligaments and tendons as active structures, then both can affect joint capsule tension and then joint capsule tension can be used to pressurize synovial fluid so that it keeps the hip joint hydrostatically lubricated.
Pressurizing synovial fluid when joints are under compression
If muscles that work on the hip joint are active when the hip joint is bearing weight then the hip joint capsule envelope can be tensioned to resist joint surfaces contacting each other.
Note that while the joint capsule will pull joint surfaces closer together, at the same time it adds pressure to the synovial fluid contained within the capsule. It acts to squeeze synovial fluid between the mating surfaces of the joint and that's what keeps those mating surfaces from rubbing.
If a joint bears weight while the muscles that act on the joint are relaxed, then the joint capsule may not have sufficient tension. Synovial fluid will be pressed from between mating joint surfaces, allowing the to run together.
If the joint isn't moving, say when you are standing still, this isn't such a bad thing. However, if you are moving, and say you are not using muscles of the hip to cause the movement, then that can cause joint surfaces to rub.
Is boundary layer lubrication in the hip joint a bad thing?
If hip muscles aren't active and the hip joint is moving while bearing weight, boundary layer lubrication is probably sufficient, for short periods of time, particularly in young bodies. As we get older, boundary layer lubrication may not be so reliable, particularly if we don't exercise, or we over use some muscles while negleting others.
What happens if boundary layer lubrication (and all other lubrication methods) fail?
If boundary layer lubrication fails, for whatever reason, and hydrodynamic lubrication doesn't come into play (because we aren't active enough) and hydrostatic lubrication doesn't come online either, because the muscles that are supposed to produce aren't doing their job, then the hip joint won't be lubricated.
If lubrication fails during movement while the hip is loaded, then tension within the joint capsule can't be distributed throughout the joint capsule. That's what joint lubrication does. It allows the bones to move relative to each other and that in turn allows tension to be distributed throughout the joint capsule.
When lubrication fails, it is probably only a matter of time before the joint capsule fails also. And then you have no lubrication what's so ever because the very structure that's supposed to contain the lubricant has failed.
How the hip resists being pulled apart
Where muscle tension can be used to reduce compressive forces, it can also be used to help resist tensile or pulling forces.
If the leg is lifted, but hanging down from the pelvis, then there's a pulling action on the hip joint. This is resisted by suction of the fluid in the hip joint. It resists a vacuum being created and so prevents the hip joint from being pulled apart.
If the force of the pull is increased then muscle activation could be used to resist this pulling and again help to keep the joint capsule intact.
Henry Gray wrote about the shoulder joint, a far less stable joint than the hip, seemingly being held together by suction.
So how do you keep the hip joint lubricated
Knowing (or understanding) that muscle activation is what can keep the hip joint lubricated, it seems fairly obvious that regular exercise can help keep your hip joints safe.
To be sure that you are helping your hip joints versus working againt them, it helps to be able to feel your hips.
How do you feel your hips?
Muscle activation is what allows us to feel our hips. If we activate our hip muscles, they allow us to feel our hips.
Muscles are responsible for proprioception
Muscles are actually what allow us to proprioceive our bodies.
Two types of sensation that we can learn to feel are muscle activation sensation and connective tissue tension.
So while exercise is the first thing you can do to help keep your hip joints safe, something else you can do is learn to feel the muscles that act on your hip joint.
What type of person needs to be careful with their hip joints
Learning to feel the muscles of the hip joint can be particularly important if you are someone who, for instance can can do a forward bend or the splits without feeling a thing.
If you don't feel anything while "stretching" it's a good sign that you aren't activating muscles, and that in turn means that your joints aren't being lubricated, or rather that you are relying on boundary layer lubrication. So, top tip, exercise, but also learn to feel your muscles.