It is well known that the more tissue resected in an amputation, the higher the metabolic cost of walking. For example, the energy requirements for walking with a transfemoral prosthesis are significantly higher than walking with a transtibial prosthesis. Why, then, would a surgeon perform a below-the-knee amputation for a mangled tibia fracture at the level of the distal tibia? Also, why might it be the case that a trans-metatarsal (partial foot) amputation has greater energy costs than a trans-tibial amputation?
When deciding the level of amputation, the surgeon’s goal is to optimize the patient’s rehabilitation potential by producing a residual limb that will heal well and allow for the best prosthetic options to assist in restoring mobility and ambulation.
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. Therefore, a surgeon would prefer to perform an amputation at the most distal level possible.
On the other hand, a far distal tibial amputation is less likely to heal well and less likely to accommodate a prosthesis. Thus, if a patient sustains a mangling 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 the benefits of healing and potential for rehabilitation (see Figure 1).
Figure 1: the optimal length of a below the knee amputation leaves approximately 12-15 centimeters of residual tibia bone (courtesy Indian J Plast Surg. 2019 Jan; 52(1): 134–143.)
If the stump is too short, there is a loss of leverage and the knee will lack power; also, a flexion contracture might develop.
Alternatively, if much more than 15 centimeters is retained, the calf muscles will have become tendons–and tendons don’t heal as well as muscles. Also, if the stump is too long, the prosthesis might not clear the ground.
Ensuring there is an adequate amount of soft tissue to create the padding at the end of the residual limb at the time of the amputation is also important to reduce the risk of skin breakdown and development of wounds.
Once the amputation wound has healed, rehabilitation involves residual limb shaping, desensitization, range of motion exercises, strengthening, cardiopulmonary conditioning, and education on residual limb care.
In the case of transmetatarsal amputation, much more of the limb is preserved, but the energy costs are comparable or higher than a transtibial amputation. This is because the remaining foot is too short to provide any power in push-off and the resultant gait is inefficient. At the same time, too much of the limb is preserved to allow for a functional, energy-transmitting prosthesis (Figure 2). The net effect is that more energy is needed to walk.
Figure 2: A prostheses for a below the knee amputation can be designed to store energy on impact and help propel the body forward on the next step, thereby decreasing metabolic demand. (from Laboratory- and community-based health outcomes in people with transtibial amputation using crossover and energy-storing prosthetic feet https://doi.org/10.1371/journal.pone.0189652)
Additional Points to Consider
In the US, approximately 80% of amputations are for vascular disease. This diagnosis affects wound healing and rehabilitation potential and therefore the level of amputation for vascular disease may not follow the outline here.
Energy Expenditure of Walking with Prostheses: Comparison of Three Amputation Levels https://doi.org/10.3109/03093640903433928