It is well known that if more tissue is resected in a lower extremity amputation, the metabolic cost of walking is greater. For example, the energy requirements for walking with a transfemoral prosthesis are significantly higher than walking with a transtibial prosthesis. Nonetheless, a surgeon addressing an irreparable distal tibia fracture (an injury close to the ankle) might nonetheless perform a below-the-knee amputation (at the proximal tibia). Why is it reasonable to remove more bone?
When deciding the level of amputation in adults, the surgeon’s goal is to optimize the patient’s rehabilitation potential.
Factors that are important to the function of the residual limb are the soft tissue envelope, how the residual limb will bear load with the prosthesis and what type of prosthesis may be used.
In general, the metabolic cost of walking is inversely proportional to the length of the remaining limb. That is to say, a longer residual limb carries a lower metabolic cost. Given that, a surgeon would–all things equal– prefer to perform an amputation at the most distal level possible.
On the other hand, a far distal tibial amputation, just above the malleoli at the ankle, is less likely to heal well. It is also less likely to accommodate a prosthesis.
Thus, if a patient sustains a very severe injury of the distal tibia requiring amputation, it may be better to perform a traditional below the knee amputation. The additional metabolic demands of the shorter residual limb would be offset by better healing and better potential for rehabilitation.
The optimal length of a below the knee amputation leaves approximately 12-15 centimeters of residual tibia bone (as shown in Figure 1).
If the residual limb is too short, there is a loss of leverage and the knee will lack power; also, a significant flexion contracture might develop.
Alternatively, if much more than 15 centimeters is retained, there is limited soft tissue to cover the bone. (At this level of the leg, the bone is surrounded by tendons, not muscles, which limits the healing of the soft tissue envelope.)
Also, if the residual limb is too long, the prosthesis might not clear the ground.
Ensuring there is an adequate amount of soft tissue padding at the end of the residual limb will reduce the risk of skin breakdown with prosthetic use.
Additional Points to Consider
In the US, approximately 80% of amputations are for vascular disease.
For vascular disease, the level of amputation is determined by where
blood flow is present or not, and in turn an area’s potential for wound
healing. Thus the level of amputation for vascular disease may not
follow the outline above.
Note that a trans-metatarsal (partial foot) amputation might impose greater energy costs than a trans-tibial amputation, even though in the former far more bone is retained. That is because even though much more of the limb is preserved, the remaining foot is too short to provide any power in push-off and there is not enough space to allow for a functional, energy-transmitting prosthesis (Figure 2). The net effect is that more energy is needed to walk.
Energy Expenditure of Walking with Prostheses: Comparison of Three Amputation Levels https://doi.org/10.3109/03093640903433928