In describing these mechanisms, which may be called intra-condylar, it is necessary to speak at the same time of the stop to limit extension because, as will be seen, it is combined with the extending spring.
We have already said that rigidity in extension when the limb is vertical is essential, but whilst it is necessary for extension to be complete at this moment it is also necessary to prevent the knee being forced into the hyperextended position, as this would quickly strain the joint and render the limb useless.
This limitation of extension can be effected quite easily by the tension of a popliteal cord (see [page 41]. The knee in Marks leg), or by carrying the anterior border of the leg piece upwards in front of the thigh piece so that it impinges against the latter.
This method is not very good because it is noisy.
Moreover, the repeated impact against the leg piece may split the wood, so that if this method is adopted the stop must be reinforced by a binding of several layers of parchment.
We will first describe a mechanism the association of which with the extending sling will be seen on [page 48].
α. To limit extension of the knee all that is necessary is to prolong the antero-posterior diameter of the knee bolt (which turns with the leg) by a horizontal wing, which engages with a corresponding notch in the femoral condyle. We show here [Figs. 50 and 51]) a rather more complicated but still simple mechanism which is interesting because it can be combined with the action of the extending sling (see [page 48]).
It consists of a piece of metal curved on the flat, ending above in a cylinder through which the knee bolt passes, continued below into a cylindrical tail piece, which fits into a ring which is fixed inside the top of the calf. During flexion this plate moves in a median posterior window in the femoral condyle, becoming oblique at the same time as the tail piece sinks into the ring; during extension the tail piece rises in the ring and the interior flat surface engages against a corresponding groove in the femoral condyle (covered with leather to secure silence).
Figs. 50 and 51.—Internal mechanism to limit extension of the knee.