Articular joints, filled with synovial fluid, are typically modeled as allowing the bones they connect to touch.
Articular cartillage, which covers the ends of bones within a joint, acts via various mechanisms to reduce friction and wear through boundary lubrication1.
But what if synovial articular joints are designed to use hydrostatic lubrication which prevents articular surfaces from contacting by maintaining a film of lubricating fluid between them?
As pressure increases (arrow), muscle activation increases to increase ligament and tendon tension (deeper blue color) which in turn increases synovial fluid pressure (yellow) to resist bones contacting.
Boundary lubrication basically means that mating surfaces rub against each other.
In marine engines 2, boundary lubrication is described as coming into play at start up and stopping, low speeds, high contact pressures etc.
A problem with boundary lubrication in machinery is that it results in a lot of wear over time because the surfaces of moving parts are in direct contact.
Hydrodynamic lubrication occurs when a shaft in a bearing is supported by a wedge or layer of oil so that the two surfaces don't contact. The term "dynamic" is used to indicate that the pressure that drives lubricant between the two surfaces is generated by the movement of one surface relative to another.
Aquaplaning is an example of this. At just the right speed, aquaplaning, or hydroplaning as it is also known, causes vehicle tires to ride up on a film of water. When it happens at all four wheels of a car, control of the car relative to the road is lost. It doesn't respond to steering inputs and sudden braking that results in wheel lock won't help.
Its not a good thing. It's even something that aircraft have had to contend with on landing5.
For aquaplaning to occur, the car has to be travelling at a certain speed. More precisely, wheels have to be turning at a particular speed. Likewise for hydrodynamic lubrication to occur between a spinning shaft and it's bearing, the shaft has to be spinning at or above a certain speed. At the right speed, an oil wedge is created that separates shaft and bearing so that the shaft rides on a layer of oil instead of rubbing and wearing against the bearing surface.
Hydrodynamic lubrication is driven by relative movement. And it keeps mating surfaces from touching6.
Hyrdostatic lubrication also creates a layer of lubricant between mating surfaces. However, instead of fluid pressure being created by relative movement, it is created by an external pump. And this could be a mechanism employable in biological articular joints, particularly synovial joints, where lubrication is required to keep mating surfaces from frictioning and wearing out over time.
At articular joints, bones don't spin relative to each othesr (at least not in the same way that a prop shaft spinning relative to a bearing does). So it is a bit difficult to create a hyroplaning affect via spinning. However, articular joints are surrounded by an envelope of connective tissue called the joint capsule.
This capsule contains synovial fluid.
With sufficient tension added to the joint capsule, synovial fluid could be pressurized so that it maintains space between mating bone surfaces, even despite changes in pressure on the joint. This could help to prevent cartilage wear from friction, potentially lengthening the operational lifetime of that joint.
Wear is actually one of the main problem with hip implants3.
With artificial hip joints, the wear will depend on the age of the person receiving the hip joint replacement and their activity intensity and duration. This helps to decide the type of hip joint chosen based on the materials it is built with.
An article on articular joint lubrication1 talks about articular cartillage as providing boundary layer lubrication for the lifetime of the joint, which is ideally the same as the lifetime of the person the joint belongs to.
But what if our articular joints rely on boundary lubrication only as a failsafe mode. Or as a transitionary mode that is roughly equivalent to engine start up or shut off, were shaft spinning speed isn't sufficient to create hydrodynamic lubrication. What if instead of boundary lubrication, were wear is a problem, our articular joints are actually designed to take advantage of hydrostatic lubrication so that joints last longer?
How might out natural hip joints and other "provided at birth" articular joints adapt to the various stresses of use? More specifically, how would they deal with the changes in pressure that result from activity and changes in position?
Joint capsules can include ligament and tendon attachments.
Ligaments are generally considered to be passive structures. They only come into play, for instance, near the end range of a joints motion.
Jan Van der waals argues in one of his papers 7 that ligaments are, for the most part, active structures. What that means is that ligaments, just like tendons, are directly affected by muscle activity. So if a muscle activates, the tension it creates not only affects the relationship between bones, via tendons, it also affects the joint between bones, via ligaments.
His view is that tendons and ligaments are actually part of the same structure.
So how does this relate to hydrostatic lubrication? If changes in muscle tension can directly affect ligaments, it can mean that changes in muscle tension also directly affects joint capsules. And this offers a potential mechanism for pressurizing synovial fluid to achieve hydrostatic lubrication.
Muscles are designed create force when they activate. They activate to either withstand forces and resist change or, or to create forces that produce change.
Muscles can act against each other across a joint to keep a joint stable or to move it in one direction or another.
Muscles can also act against the weight of the body or some external force in one direction or another.
In any case, the force generated by muscles not only affects the relationship of the bones is connects, it also directly affects the tension in the joint capsule that connects those bones (assuming the muscle in question is one that does have connections to that joint capsule). And if hydrostatic lubrication is the intended lubrication mechanism for synovial joints, then muscle tension can be used to affect joint capsule tension in such a way that articular joints are continually lubricated via hydrostatic lubrication as opposed to via boundary lubrication.
So instead of relying on speed as a hydroplaning car or rapidly spinning engine shaft does to create and maintain hydrodynamic lubrication, articular synovial joints could rely on variable tension joint capsules to maintain a layer of lubricating fluid between mating surfaces so that articular cartillage is subject to less wear.
It should be pointed out here that not all muscles may be directly involved in affecting joint capsule tension. However, the way that muscles are networked means that muscle tension in one muscle affects other muscles. So even if one muscle doesn't directly act on a joint capsule, it's activation can trigger a muscle that does so that hydrostatic lubrication is maintained.
With this understanding, its feasible that a failure of a hip joint is the result of muscle failure. If the muscles that work on the hip aren't doing the job of maintaining joint capsule tension, hydrostatic lubrication isn't achieved and boundary lubrication is relied on until articular cartillage wears out.
Once that mode of lubrication fails within an articular joint, bones will stick instead of slide relative to each other, and that means that portions of the joint capsule will be under excessive stress, possibly tearing over time. Then there is neither the option for boundary or hydrostatic lubrication because the piece of equipment designed to contain the lubricating fluid has ruptured.
To activate effectively, muscles have to work against some opposing force. When there is no external force to act against opposing muscles can act against each other. As an example, when flexing your biceps, your biceps isn't working in isolation. It's working against the triceps also. And together these two sets of muscles help to stabilize the elbow joint. The force these muscles generate not only stabilizes the elbow joint, making it stiff, it also potentially adds tension to the joint capsule in such a way that the fluid inside is pressurized sufficiently to keep the bones of the elbow joint from contacting.
Using muscles in opposition can stiffen or stabilize a joint. At the same time, this muscle activation can add tension to the joint capsule of that joint and thus prevent the bones of that joint from touching.
During movements where muscle works against mass, hydrostatic can still come into play, however, instead of being driven by muscle working against muscle it is driven by muscle working against the forces generated by moving limbs.
With articular synovial joints, an important ability is that of being able to adapt to changes in pressure. The knee and hip joints are going to have to deal with more pressure if you stand on one leg as opposed to doing a handstand.
They also have to deal with situations where the foot is suspended, or where a person is hanging from their feet then they have to be strong enough resist being pulled apart by the weight of the rest of the body. Lubrication isn't so much of a problem here, however the fluid is still important since it creates suction that helps to keep the joint together.
To maintain the same space in all likely situations, joint capsule tension has to be variable. And it needs to be actively variable.
In the left picture tension in the abdominal wall squeezes the abdominal container pressing the ribcage up (the diaphragm is active so that instead of it being pushed up into the ribcage, it pulls the ribcage up with it, away from the pelvis). In the right picture, tension in the abdominal wall is reduced to the point that the weight of the upper body, again via an active respiratory diaphragm, pushes down against the abdominal contents, resulting in the belly bulging forwards as the ribcage moves towards the pelvis.
As an example of how tension in an articular joint capsule can be actively variable we could look at the abdomen and the surrounding musculature of the Transverse abdominus and Respiratory Diaphragm.
The abdomen could be considered a fluid filled bag that connects the ribcage to the pelvis. The transverse abdominus wraps around the abdomen between the ribcage and the pelvis. The respiratory diaphragm sits like a cap on top of the abdominal cavity creating a muscular division between it and the cavity of the ribcage.
To pressurize the abdomen, the transverse abdominus muscles and diaphragm could be activated against each other. The feeling is similiar to the pressurization that occurs when you trying to void your bowels.
Increase the inwards squeeze of the transverse abdominus muscle while maintaining diaphragm tension and you can cause your ribcage to move away from your pelvis. This is because the abdominal organs act like an incompressible fluid. Squeezing the transverse abdominus inwards squeezes the abdominal organs upwards. Since the diaphragm is engaged and is anchored by the bottom of the ribcage, the ribcage then moves upwards.
Now if tension is reduced in the transverse abdominus sufficiently, the weight of the body, via the still-engaged diaphragm, presses downwards on the abdominal organs with enough pressure to cause the belly to bulge outwards allowing the ribcage to sinks downwards towards the pelvis.
Doing a back squat with a barbell sitting across the shoulders, the goal might be to prevent the ribcage from sinking towards the pelvis. One possible mechanism for achieving this is to pressurize the abdomen by using the transverse abdominus and diaphragm against each other.
This would be similiar to the mechanism that prevents synovial fluid being squeezed from between bones in a articular synovial joint. With muscle activation adding tension to the joint capsule via ligaments and/or tendons, a joint capsule could have enough tension to resist fluid being squeezed out from between the bones.
One of the reasons that the abdomen is a good example of how hyrdostatic lubrication can work within articular joints is that it's easy to change the shape of the torso. You could contract your transverse abdominus inwards whether your spine is straight or bent backwards, forwards, leftwards or rightwards.
In the same way, no matter what shape changes a joint is undergoing, muscle tension could be varied to maintain hydrostatic lubrication of that joint.
One problem with using the abdomen as a model for an articular joint is that the abdomen is quite tall. In contrast, the volume of fluid in an articular joint is quite small. Despite that, the volume of fluid in a synovial joint should be enough to maintain space between the bones of a joint provided there is enough tension in the joint capsule envelope.
If hydrostatic lubrication is the intended method for lubrication, then this could be one reason for joints with excessive fluid. If muscle tension can't be used to tension a joint capsule enough to keep bones apart, then another option is to fill it with more fluid.
While this can prevent wear on the bones it means the joint is less functional.
The beauty of using muscles to control joint capsule tension and joint lubrication is that muscle activity is generally in response to some intent, to withstand a change or create a change. The joints are going to have to deal with that same change, and with muscles connected to joint capsules, joint capsules get information first hand on the changes that they'll have to contend with. And that same information also provides the power, (in the form of tension) to deal with that change.
Not all muscles directly connect to or directly influence a joint capsule. However, there are bursae, fluid filled sacks that in some cases connect to joint cavities or in others are situated between ligaments in tendons. This is particularly notable with the knee joint4.
As an example, the lateral gastrocnemius subtendinous bursa is located between the lateral head of the gastrocnemius and the joint capsule. There is also a medial gastrocnemius bursa. Gastrocnemius tension can affect knee joint tension via these bursae.
The fibular bursa is located between the lateral fibular collateral ligament and the tendon of the biceps femoris.
The fibulopopliteal bursa is located between the fibular collateral ligament and the tendon of the popliteus.
The anerine bursa is located between the pes anserinus and the medial collateral ligament.
For these latter bursae, tension in the muscle overlaying it can press on the bursae which then presses on the ligament beneath it. These are possible avenues of varying tension within a joint capsule to maintain hydrostatic lubrication.
One very important reason for understanding articular joints in this way is that it can form a framework of understanding for fault finding routines. If a muscle won't activate is or is constantly active, the reason for that may be to protect a joint that it works on in order to maintain hydrostatic lubrication or to prevent circumstances where hydrostatic lubrication isn't possible.