Just as important as the socket shape is the alignment of the entire prosthesis. The functionality of the prosthesis will drop very rapidly if it is not aligned properly. A bad alignment will also tend to confuse the patient about how well his/her socket fits. When it comes to above knee prosthetics, everything affects everything else. For that reason everything has to be correct, or the amputee will suffer from lack of mobility and discomfort.
Figure 1, below shows approximately how the alignment should look from the side view of the prosthesis. Referring to the figure 1, the alignment is something other than expected. Since an above knee amputee lacks a God-given knee, the prosthesis needs to be aligned differently than a God-given leg. At first glance, one will notice that the socket is mounted forward of the center of the knee, and that it slopes backward. Though this drawing exaggerates this some, it is the correct alignment. The reason for mounting the socket sloping backward is as follows. As the amputee "follows through" on his/her step, the amputee will find the prosthesis behind the center of the amputee's body. Just like able-bodied people, the step does not end when the foot is under the body. The foot will push off behind the person. Since the Amputee has his/her tailbone mounted in the socket, the socket needs to be slanted backwards. The reason is that if this were not the case, on the follow through portion of the step, the prosthesis would tend to "push" the amputee forward and down. In effect the socket would be slanted forward at this point, causing the "push" into the amputees body. Therefore, the socket needs to be aligned with the rest of the prosthesis, so that it is vertical (straight up and down), at the end of the follow through of the step.
The socket is mounted in front of the knee slightly to make the center of gravity slightly straight through the connection of the prosthesis to the foot. Otherwise the center of gravity would be behind the foot and the prosthesis would be off balance and hard to control. Also, note that the foot's toes are pointed slightly upward (the foot assumes the shoe is on). This may or may not be preferred depending on how much toe resistance the amputee desires. But, the basic idea is to make sure that the toes do not come into effect too early in the step. If the toes come significantly into effect too early in the step, the amputee will have to "fight" the prosthesis through the step.
Also, note the existence of a rotator above the knee. This is an optional component, but some amputees find it desirable. What a rotator allows the amputee to do is to push a button and "rotate" the lower portion of the leg with respect to the socket. This is handy for driving, sitting in movie theaters, etc. Otherwise when the amputee is sitting down, the leg will always extend out from him/her. The rotator provides a way to get the leg "out of the way". However, if the amputee uses a rotator, the amputee needs to double check that it has been locked back in place before the amputee takes any steps.
Alignment, like the socket shape takes some trial and error to get right. Generally there is a four-screw attachment in the rotator or at the connection with the socket that allows the change in alignment. The amputee may want to get permission from the prosthetist to adjust this as needed in the early stages of wearing the prosthesis. After some walking the amputee will find that certain things could be "tweaked". It is very difficult to get it completely right in the prosthetist office during a short visit. Also, settling into the prosthesis socket and changing the number of socks can effect the alignment. This four-screw alignment not only allows the socket to be tilted backward or forward but also side-to-side. Sometimes the amputee may notice that during walking that most of the pressure is on one side of the foot or the other. By adjusting the 2 screws on the sides, this can be fixed.
One last adjustment is the rotation of the knee (See Figure 2). This is usually made with one screw above the knee unit. Generally it is not always obvious in the beginning if this adjustment is correct. Basically if the adjustment is not correct the amputee will notice one of two things. The first one is that the foot is turned in and the leg tends to "whip" before the amputee lands with the foot. If this is the case the knee unit needs to rotated out some. The other problem could be having the knee unit aligned such as the foot is "toed out" too far. The amputee will notice that the foot is toed out and that the amputee cannot get much leverage out of the foot during walking. In this case the knee unit needs to be rotated in slightly. Just a small movement in this adjustment will usually make a big difference. Generally the foot position with the post that goes into the knee unit is fixed and does not need to be changed. That connection is usually made so the foot is ever so slightly toed out with respect to the knee unit.
As a note, verify that the prosthesis you are receiving has the before-mentioned adjustments. You should not accept any limitations in adjusting your prosthesis . Even the best prosthetists will not be able to get the adjustments right without some trial and error.
