Medical Sciences Stack Exchange is a question and answer site for professionals in medical and allied health fields, students of those professions, related academics, and others with a sound understanding of medicine and healthcare-related sciences. It only takes a minute to sign up.
Sign up to join this community
I've noticed that leg amputations aren't usually done right at the knee joint. Instead, the amputation is done further up the leg -- up on the thigh bone.
Why cut midway up a good bone like the thigh bone instead of amputating at the knee?
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 :)
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.
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.
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.)
