Question
What you would see if a posterior bumper is too stiff? What if the anterior bumper is too soft?
Answer
If the posterior bumper is too stiff, it is going to cause a flexion moment at the knee, and it is going to create a torque moment inside the socket at the distal tibia on a transtibial patient. In a transfemoral patient, you will see a knee flexion moment. Essentially, imagine that you have broken your ankle. You are in a 90 degree cast with nothing on the heel of your cast. You step down, and the front of your foot wants to very quickly rotate to the ground. In doing so, that puts a force at the knee and on the residual limb.
If the anterior bumper was too soft, you would see a drop-off at the end of stance phase. This may include someone who has had an Achilles tendon injury where it was either stretched or was ruptured. They would have no push-off. They would get to mid stance, and then as they came over the toe, the ankle would collapse. If you think of it that way, the posterior bumper is controlling the descent of the foot to the floor during heel strike, and the anterior bumper is controlling the descent of the tibia as it rotates over the foot. That is a good way to understand it.
Anytime there are questions about prosthetic problems, we have to break it down first into what type of problem it is relative to the planes of motion. These bumper questions are in reference to the sagittal plane. When there is too soft of a heel bumper, we are not really concerned with any medial / lateral motion or any transverse motion. We are really just concerned with an anterior / posterior type of motion.
