A below-knee amputation, formally a transtibial amputation, removes the leg below the knee joint while preserving the knee itself. Keeping the knee matters a lot for the patient: walking with a prosthesis is far easier and less energy-demanding than after an above-knee amputation.
The most common indications are severe trauma, peripheral vascular disease (often with diabetes), infection that can't be controlled, and congenital limb deformities.
How it's done (details vary between surgeons, but the general sequence is similar):
Under anaesthesia, a curved or straight incision is made around the leg below the knee, and the surgeon dissects through skin, subcutaneous tissue and muscle to expose the tibia. The muscles are divided and retracted to get clear access to the bone.
The tibia is then cut with a saw – that's the step in this clip, with a metal shield protecting the soft tissues from the blade. The goal is a smooth, even bone end. Depending on the technique, the fibula is usually divided as well (typically a little shorter than the tibia), though some surgeons leave it in place to add stability to the stump.
The major vessels are ligated to control bleeding. The nerves are pulled down gently, cut and allowed to retract into the soft tissue, so the cut ends don't sit at the tip of the stump – an important step for reducing neuroma formation and residual limb pain.
Finally, muscle and skin are closed over the bone end to create a well-padded stump. The skin flaps (classically a long posterior flap, since the calf has a better blood supply) are planned before the first incision to make sure the bone is properly covered and to reduce the risk of wound complications.
Rehabilitation and prosthetic fitting follow, and that's where the patient gets their mobility back.