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I've noticed that leg amputations aren't usually done right at the knee joint.

Instead, it seems like leg amputations are often done further up the leg -- up on the thigh bone.

As someone who knows nothing about medical science, I'm curious why the preferred option is to cut midway up a good bone (like the thigh bone), rather than preserve the entire thigh bone, and amputate at the knee instead.

In other words, why don't we see more amputations that have the thigh's knee socket-ball intact? (kind of like what we'd see with a turkey leg at Thanksgiving :)

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    +۱ for making it holiday-themed ;-)
    – adam.baker
    Nov 29, 2021 at 6:53
  • Doesn't this entirely depend on the individual case? In other words, how much of the leg needs to be amputated depends on the degree of injury or whatever else necessitated the amputation in the first place. Maybe it's just that most leg injuries serious enough to require an amputation are likely to damage the knee beyond recoverability? Dec 1, 2021 at 17:46
  • Most amputations in the United States are done due to irreversible ischemic or infectious disease of the lower extremity. The extent of these disease processes determines the level of amputation. MSK traumatic injury does not often result in amputation except in those cases with concomitant vascular injury and irreversible ischemia; it usually has nothing to do with irreversible damage to the knee joint itself. Dec 1, 2021 at 18:06

3 Answers 3

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If you think about prosthetic devices, the answer will soon become clear. You would not wish to have your thigh bone extended by an extra six or eight inches (about the minimum distance that one could practically strap it to) and have the new joint that much lower than the natural knee of the other leg.

Neither would it work well to extend the leg clear to the foot/ankle point without a knee joint at all.

Cutting the bone roughly midway between joints is the most practical way of enabling the amputee to restore some function via a prosthesis.

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    Do you have a source for this theory? To my understanding leg amputations have always been mid thigh (though I lack a source too), even before modern prosthetics with fancy articulated joints were available.
    – Drake P
    Nov 28, 2021 at 17:17
  • Why not joint the prosthesis directly to the upper half of the preexisting knee joint?
    – Vikki
    Nov 28, 2021 at 18:36
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    @Vikki, any time you have something passing through the skin, it's a major infection risk. Skin is a worthless bearing surface for a joint, so you need to either make a fully-implanted artificial limb (we don't know how to do this), or you need to leave room for a fully-external joint.
    – Mark
    Nov 28, 2021 at 20:41
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    @DrakeP My source for this "theory" is practical experience. I have lived in the most bombed country on the planet, where an estimated 30% of the cluster bombs failed to detonate and are now UXOs (unexploded ordnance) which have maimed and killed thousands of people in the years since. I designed a rudimentary prosthesis for one person who lost part of her leg due to infection. But practical knowledge is not valued here--actually, since there are no "credible" sources (e.g. peer-reviewed) to any of the answers for this question, perhaps they should be deleted for not following the rules here.
    – Polyhat
    Nov 29, 2021 at 19:34
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    Thank you Polyhat. Your hints are sufficient and the facts are saddening. I've read a bit about unexploded land and sea ordinances, and I find it all quite disturbing. Years ago, I actually discussed design ideas with others regarding possible devices to remove them. There really is no excuse that any still exist in our world. I'm sorry you've had to witness so much of it firsthand. Nov 30, 2021 at 9:00
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The bone cut must be proximal enough to perform a myodesis (anchoring the muscle through drill holes near the cut end of the bone) or myoplasty (suturing the fascia of antagonistic muscles together) and allow the flaps to cover the end of the femur without tension.

The incision can not simply be moved lower to address the problem of flap coverage because now you are essentially trying to cover an above knee amputation with below knee amputation flaps. There is likely a good reason an above knee amputation was selected in the first place (perhaps inadequate below knee perfusion) and thus it would be unwise to rely on these poorly perfused flaps to adequately heal.

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    I think this answer would be more helpful if you defined "myodesis" so those of us less fluent don't have to look it up; and explained why the surgeon couldn't address the "flaps" problem just by moving the initial surface incision lower to compensate. But I'm still upvoting.
    – TextGeek
    Nov 29, 2021 at 16:01
  • @TextGeek Or just move some skin from elsewhere on the body to cover the severed limb.
    – nick012000
    Nov 29, 2021 at 16:30
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    A stump is a poor place to skin graft for a variety of reasons and it would be unlikely to heal. Good flap coverage is your best bet. Nov 29, 2021 at 16:34
  • I'm a little surprised that with respect to below-the-knee amputations there has been no mention of the difficulty with having two bones, tibia and fibula, to deal with.
    – Polyhat
    Nov 29, 2021 at 19:47
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    From a technical standpoint, it is slightly easier to transect mid femur. But disarticulating the knee would still not be difficult for any competent surgeon - putting it back together is difficult but that wouldn’t apply in this hypothetical situation. The real issue with an amputation through the join as you described is there is very little tissue to cover it. Only a small amount of real muscle belly crosses the joint and it wouldn’t be enough to create a healthy flap. Dec 1, 2021 at 17:59
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Because every piece of biological function that can be retained is one less act for the patient to re-learn and for a prosthetic to replicate.

Removing a patient's knee and below means a more expensive artificial limb with more joints compared to foot removal only.

By retaining the operating knee joint where possible allows the patient as much function as they can have to assist with recuperation, both physical and mental.

Additionally, the knee is a weight bearing joint when assembled. The bare bottom end of the femur is not well-suited to taking loads when not mated with the rest of the knee.

And there is less flesh in this area, so the blood supply is not conducive to growing healthy flesh over the stump either. When the leg is cut above the knee, the bone is cut a little higher and the thigh muscle/flesh is used to cover the stump. There is far less meat in the knee to close the hole.

(source, my Grandad had a foot removed. He'd also had a steel knee installed earlier, which limited blood flow to the stump and it simply starved. So he had an above-knee amputation and could not adjust to a longer leg, ended up in a wheelchair for the rest of his life.)

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    The question is "why cut above the knee?", not below.
    – hobbs
    Nov 29, 2021 at 3:37
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    I most of the answer this misses the question - except for the fifth paragraph. Nov 29, 2021 at 11:23
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    There are many reasons to select an AKA over a BKA. Nov 29, 2021 at 16:35

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