K N E E S

K N E E S

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  1. I’m an undergrad mechanical engineer who just spent a lot of time studying knee implants for a class. The issue is that we ask an incredible amount out of our knees. My girlfriend (isn’t particularly active) has a stepcounter and usually walks over 10,000 steps a day. 10,000 a day x 365 days a year= 3.65 million steps a year, crude estimate. Basically the only way that any device can survive is self repair and/or infinite life operation. Infinite life operation is basically when a part doesn’t wear out while being used. At all. usually something steel considered infinite once past 10^7 cycles, but bones aren’t steel, and we Easily get at least 3×10^7 cycles in one year.

    *edit, someone corrected me we actually get 3×10^6, but my point stands that we get there quickly in our lives. We still get 10^7 in 3 years*

    But the infinite life of equipment is a hell of a lot lower than the maximum a part can hold, and so we tend to cause our knees to fail through fatigue. So if you do something more extreme than walking or running like say running on pavement, falling, jumping, things which put sudden loads on your knee, you can do more damage than infinite life + self repair healing factor can deal with. And you’ll gradually work your way closer to failure (by which I mean injury)

    Compounding this is our knees are a compromise of a lot of features, which means it can do lots of things alright but nothing perfectly. To allow knee rotation for that extra mobility we pay a high price in structural integrity. The cushioning part of our knee handles impacts better but comes at a cost of wear resistance that 2 harder materials would have. There’s the material science limits of just what materials our body works in (bone, cartilage, gels, etc). People think compromise means multipurpose, which is good, but for the knee it really means it’s ok at lots of stuff, but not great at any.

    The longest life total joint replacement knees right now are double ceramic surfaces, and they last 30 years ish. That’s with crazy material engineering and comes at the cost of impact resistance. Compounding the problem is biocompatibility, ideally we’d just stick a steel teflon coated hinge in the persons knee and call it a day, but you can’t do that, the body will reject it. Also pretty sure teflons poisonous. A normal knee replacement lasts 10 years, and has better impact resistance. So in reality the knee isn’t all that bad, it’s still twice as good as our best alternative.

    **edit 2: teflons not poisonous, but it’s not good for the wear surfaces of the implant because it flakes and the immune system attacks those flakes, requiring surgery to fix.**

    TLDR; there’s several reasons knee is a weak point, but it’s still better than anything we’ve figured out so far.

    If you’re curious about fatigue stuff
    https://www.efatigue.com/constantamplitude/background/stresslife.html

  2. There’s a level between “overactivity” and “occasional activity” – that’s where you want to be.

    Run / bike / etc regularly but don’t go for a marathon every month, do some resistance but don’t be aiming to squat 500lbs. Mix up your activity so you don’t get a repetitive stress type injury. And in doing so you’ll also keep off the weight which is a big factor for joint health.

  3. I’m a weightlifter and have dislocated the same knee twice, years before I started lifting when I was a kid and just this year.

    It just plain sucks. Both times the doc has told me I just have weak joints and they’ll only get weaker unless I get surgery 🙁

  4. Speaking for weightlifting, knee problems usually arise from doing exercises wrong. Squats and deadlifts require a fuck ton of practice with good form and low weight before you can properly transfer weight of the knees more to the glutes and hips

  5. Humans only needed to evolve to live for a few decades before we inevitably got mauled by a wild animal or died of a disease, so our bodies start falling apart pretty early. That’s why our knees suck.

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