Case study on a Through Knee Application

Whereas the instructions for use suggest a preferable alignment, in practice not all alignments do or can conform.

Picture A shows that the shin is leaning forward over the ankle by a few degrees, bringing the centre of knee axis forward. This may reflect either an effective dorsiflexion of the foot, or a compensation for an initial socket flexion.

Picture B shows how the knee axis is forward to the hip-ankle line, contrary to the recommendations. However if initial socket flexion HAS to be there, the compromise may be necessary. If socket flexion does not have to be there, a more axial alignment of socket to shin may stretch to a benefit. A more axial alignment will support getting the knee axis posterior to the hip-ankle weight line.


Picture C shows that the low knee axis (inavoidable in a knee disarticulation) causes longer total length from hip to knee axis to tip of toe than would have been in a transfemoral application. The added length of 1 cm may affect the toe clearance in slow walking. This is consequential to geometry. Arguably, with the shin leaning forward, the geometry improves for toe clearance, and may have been a valid reason for this alignment choice to improve toe clearance.


Picture D shows how the initial socket flexion projects to just posterior to the ball of the foot.

Picture E illustrates how the lower knee axis increases the distance from the load line at toe off (represented as a yellow line) to the actual knee centre, and the would have been knee centre. The red line shows a ground reaction force line where the line passes posterior to the would-have-been knee centre and anterior to the actual knee centre.

Picture F shows two blue lines; the one follows roughly the axis of the socket, the other the line between hip and ankle. The image is manipulated at the sole of the shoe, where part of the shoe has been shifted to be brought in line with the socket axis. (In this way there are two pictures in one). In other words, relative to the socket axis, the heel is ‘long’ and the forefoot ‘short’. This may be an appropriate balance to get the mixture of toe clearance, ease of swing initiation, and minimisation of unwanted knee flexion at heel strike right as best as possible.

In conclusion, there were some issues asked about in relation to this case. This study served not as a definitive answer, but as a method to evaluate alignment and segment lengths to shed light on any issues there might be, and may support that despite any perceived issues, deviation from recommended alignment and segment length may represent the best compromise.