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Generalities

The choice of the knee is critical to the success of a femoral prosthesis, because it is the determining element for the safety, comfort and dynamism of the patient.

There are a wide variety of knees on the market. Before making a presentation of the material that has caught our attention for its quality, reliability and functionality we should explain the great principles of the sometimes complex mechanical elements.

The two large families of joints:

Monoaxial articulation: It pivots around a single axis.

Polyaxial articulation: It consists of rods connected to a higher head and lower body. Its pivoting forms an ellipse that approximates the physiological motion of the knee. There is a loss of length of the leg during flexion, thereby increasing the passage of the step. It is also called rod articulation .

Important Concepts

ICR: One of the first things to understand is the position of the  Centre of Instantaneous Rotation.

It is the Prothetic technician's point of reference for mounting the prosthesis.

The ICR is the virtual axis around which the knee joint will rotate at all first degrees of flexion. It is therefore crucial for the positioning of the socket.

For a monoaxial knee , the ICR coincides with the axis of the knee.

For a polyaxial knee , the ICR  is moved backwards and upwards to give greater safety and is a function of its geometry as it is situated at the imaginary intersection of the rods.

The rodded knee offers greater geometrical safety through the movement of the ICR to the rear, and there is a delay in flexion by the movement of the ICR upwards.

 

The two phases of walking:

The movement of a limb during walking consists of two main phases:

The swing phase:  This is the period during which the member is in the air, pendular motion from back to front, since the take off point  of the toe to the heel strike the ground.

Its control is used to set the walking pace, so that the patient is not obliged to await the return of knee extension in order to put the heel on the ground.
It gives the possibility, for knees which have this regulation, to apply resistance to flexion or extension, to adapt to the rhythm of the patient's knee. The weight of the foot and shoe have an influence on its inertia.

This regulation may be pneumatic or hydraulic.
The choice will depend on the activity and vitality of the patient.
The pneumatic system is more flexible, but it is quickly affected by intense activity due to the heating of the circulating air .
The hydraulic system is better suited to very dynamic patients.

The Stance phase: This is the period when the foot touches the ground, from the heel strike, through the course of the step, until the propulsion onto the forefoot prior to it leaving the ground. Its the period during which the limb is in the air in an oscillating movement forward from the back, from the take off of the tip of the foot until the heel strike on the ground.

Some knees have control of the stance phase by progressive braking, allowing the patient descending or going down the stairs, to get support on the prosthesis that flexes in a controlled manner under the influence of body weight  while the sound limb effects its swing phase to arrive in the stance phase a step lower.
This control is adjustable according to the patient's weight and speed of descent.

The criteria for the selection of a knee

They will feature:

  • the abilities of the patient
  • their rehabilitation
  • their weight
  • their age
  • the type of socket
  • the length of the stump
  • activity
  • dynamism
  • etc ...

The choice of the knee must be the result of a multidisciplinary dialogue between the specialist Doctor, the Physiotherapist, Prosthetist and the patient to know their needs and projects.

We propose here a presentation of the principal kinds of knees which have gained our interest and we have classed them into three kinds of categories of patients, although of course every case must be studied individually.