Edge loading from sports or exercises

[Bionic]

We know that edge loading on the implants can lead to early wear and high ion levels.  Usually we think of edge loading as a consequence of walking.  A normal gait applies maximum wear to the acetabular implant at roughly the 1 o'clock position (relative to the sagittal plane).  If the cup is implanted at too steep an angle (generally believed to be greater than 55-degrees), then the point of maximum wear can actually overlap the edge of the cup, causing problems.

But what about normally installed cups?  Is there some activity that can still promote edge loading?  For example, bicycling moves the hip through a very different range of motion than walking.  The point of maximum stress is bound to be very different.  For long distance bicyclists, do you think it's possible for edge loading to occur because of the normal bicycling stroke?

To answer this question, it would be really helpful to see a model of the hip so we can visualize where the forces are applied.  Does anyone know if there's a virtual model of the hip that can be manipulated online to explore this?

[obxpelican]

Remember the pilot and the plane and letting him fly the plane?    :D

You crack me up Bionic.

;D


Chuck

We know that edge loading on the implants can lead to early wear and high ion levels.  Usually we think of edge loading as a consequence of walking.  A normal gait applies maximum wear to the acetabular implant at roughly the 1 o'clock position (relative to the sagittal plane).  If the cup is implanted at too steep an angle (generally believed to be greater than 55-degrees), then the point of maximum wear can actually overlap the edge of the cup, causing problems.

But what about normally installed cups?  Is there some activity that can still promote edge loading?  For example, bicycling moves the hip through a very different range of motion than walking.  The point of maximum stress is bound to be very different.  For long distance bicyclists, do you think it's possible for edge loading to occur because of the normal bicycling stroke?

To answer this question, it would be really helpful to see a model of the hip so we can visualize where the forces are applied.  Does anyone know if there's a virtual model of the hip that can be manipulated online to explore this?

[sroberts]

There is much more impact force on he hip when walking vs. cycling. It's why cyclist are at such a high risk of osteoporosis. Floyd Landis won the Tour de France with advanced AVN. He couldn't walk but he could ride over 2000 miles on a damaged hip. Think about it.

[Bionic]

Remember the pilot and the plane and letting him fly the plane?

But the flight's over Chuck!  Now I'm trying to find my way around this new destination. :)

There is much more impact force on he hip when walking vs. cycling. It's why cyclist are at such a high risk of osteoporosis. Floyd Landis won the Tour de France with advanced AVN. He couldn't walk but he could ride over 2000 miles on a damaged hip. Think about it.

Biking has less impact, for sure, than walking, but not always less sustained force.  Think about hill climbing and sprints.  Biking also requires greater ROM than walking, and holds the joint under compression through much of that ROM.  Although walking involves impact, the impact lasts only an instant and the hip is fairly static during that time.

It seems to me that people who bike an hour a day, for example, would show a different wear pattern in their implants, what I would call a "secondary wear pattern," which is in addition to the primary one they show as a result of walking and other daily activities.  My question/concern is whether that secondary wear pattern could involve edge loading.

During my walk through the mall yesterday I came upon a model human skeleton (at a skin care store, for some reason).  He was called "Mr Bones."  After getting permission from the sales clerk, I grabbed hold of Mr. Bones and flexed his hip to see what would happen to the head of the femur.  Sure enough, the edge of the femoral head crossed under the edge of the acetabulum when flexing the hip 90-degrees.  90-degrees is a typical maximum flex angle for bicycling (I think), and the edges of the femoral head and acetabulum approximately line up with the edges of the cap and cup in a normal resurfacing.  This means that edges probably do cross during bicycling.

But is that a problem?  Edge loading isn't just about edges crossing; it's about stress being applied to one of the implants on its edge.  It seems to me that maximum stress in the acetabulum during the normal biking stroke would be more posterior than normal, since the foot and knee are brought forward.  It seems to me that this would not present an edge loading problem, provided that the cup is placed at a slightly forward-pointing angle (anteversion?).  Still, that's just a wild-assed guess.

[sroberts]

nope, compressive forces are on the muscles when cycling. Forces are generated by muscle not the joints.

[Bionic]

But don't muscles work by applying tension across joints?  Every time I flex my quads or my glutes, I pull the femur more forcibly into the hip socket.

Also, if I apply 100 pounds of force against the pedal, isn't every ounce of that force transmitted through my leg and into my hip joint?  The muscles across the hip joint cannot guard the joint from receiving the force, since they have no ability to expand, only to contract.  And their contraction only adds to the force on joint; it does not subtract.

[sroberts]

You're wrong on several counts but I'll address the main points:It only takes 14 pounds of pressure to turn a bike crank. The muscles are exerting force on the bones via the tendons. The joint is just going through a range of motion when sitting on a bike. If you are talking about running, then yes, your hip and knee are absorbing about 4 times your body weight.
You may want to read, "Exercise Physiology", by Brooks and Fahey.

Muscles exert force on tendons which move bones. Ligaments (non contactile tissue, hold the joint stable.

[Bionic]

So, you're saying that if I push down on a bicycle pedal with 100 pounds of force, my hip joint will experience only 14 pounds of force?  What happens to the other 86 pounds? 

When I stand on one foot, isn't the weight of my torso and other leg borne by the hip socket of the foot I'm standing on?  Why would it be different if I were standing over a bicycle pedal?

And don't the muscles that cross over the hip joint from my femur or tibia to my pelvis compress the hip joint when they contract?

[sroberts]

If you push on the pedal w/ 100bls of force you're going to jam you're leg down so hard you'll probably fall over and since you're standing up with no hands on the handlebars (your example), you'll definitely fall over.

When you are riding a bike and standing up, neither you hip or your knee is locked out. The load is taken up by the muscles while the ligaments are stabilizing the joints.

Do you think for one minute if your argument made any sense, that any orthopod would prescribe cycling as a primary mode of exercise. It's even the most prescribed exercise for THR's. Geez, at least educate yourself on the basics of exercise physiology and biomechanics.

[Bionic]

I'm not trying to tick you off, Spencer, but I'm really not buying it.  I think I can probably push the pedals with 100 pounds of force.  But assume for argument it's only half that.  50 pounds. 

I usually ride a recumbent outdoor bike, so I can't stand up--I only push back onto the seat.  When I push the pedal with 50 pounds of force, the seat pushes against by back with 50 pounds of force.  It has to in order to maintain static equilibrium.  Otherwise I would fly off the bike.

That means the entire side of my body, from the foot on the pedal to my pelvis pushing into the seat, is under 50 pounds of compression.  At my hip, the only rigid structure that can bear that compression is my hip joint.  It's not the tendons or the muscles or the ligaments.  It's the bone-on-bone, or in my case, metal-on-metal, joint that bears the force.  It has to see the entire 50 pounds.

But I believe it sees much more than that, because in addition to the force from the pedal, there's also all the muscular contraction going on across the hip joint.  Muscles can only apply tension; they can relax but never actively expand. So all those muscles that cross the hip joint, e.g., the gluteus maximus, rectus femoris, vastus externalis, sartorius, ilio-tibial band, and all the rest, can add more compression to the joint anytime they flex.

Understand that I'm not talking about easy rides on a stationary bike.  I'm talking about dragging my 200 pound carcass over hills at 30 mph outdoors.

I'm happy to read any references you find that explain the biomechanics, but it would obviously be helpful if they were available online so I could find them quickly and we could discuss them.

[sroberts]

Well you are dong a good job of it. Your description of how you think the muscles work is laughable. Go to the library or order Brooks and Fahey's book from Amazon.com. 

You are not going to go 30mph up any hill at 200lbs on a recumbant, downhill yes, uphill no.
The compression forces on you hip are going to be minimal wherever you ride.

Read up on muscle physiology and biomechanics.

[obxpelican]

Guys.... I'm going to lock this thread, it's not going the way many threads go on this forum.

I'll leave it up to Pat as whether or not to delete the whole thread.  I would suggest you guys take it to email and settle it there.  I would hate for it to get any more hostile.

You both contribute well on here, maybe agree to disagree?



Chuck

[Pat Walter]

Hi Guys

Thanks for the insight into muscles and biking.  I am not sure too many people will want to find out much more about it all.  Most just enjoy peddling and feeling the wind in their hair.

I don't want anyone to get upset or leave the group from too deep of a discussion about any topic.

If you want to delve deeper into muscles and biking, why don't you go to private email.  I know you are both great guys and appreciate both of your responses.

I think Chuck is right - just agree to disagree and get on with the ride.  You both have great new hips and can enjoy any activity now - regardless of how the muscles work, your hips work great.

Thanks for the info.  Good Luck.

Pat