How To Choose Knee Prosthetic System?

A prosthesis is a device designed to replace a missing part of the body or to make a part of the body work better. Metal prosthetic devices in knee replacement surgery replace cartilage and bone that has been damaged by disease or ageing.

Of all prosthetic components, the knee system is arguably the most complex. It must provide reliable support during standing, allow smooth, controlled movement during walking and permit unrestricted movement during sitting, bending and kneeling.

Prosthetic knees have changed considerably over time, from the simple pendulums of the 1600s to those regulated by rubber bands and springs or pneumatic or hydraulic components. Some knee units now have advanced motion control modulated by microprocessors.

Given the wide range of options and consumer needs, prosthetists and rehabilitation specialists can help amputees choose the prosthetic knee that best suits their individual needs. They can also teach amputees how to use their new knee properly, which is essential to avoid discomfort, falls and stumbles.

Pneumatic Knee Joint

A key way to assess a person's prosthetic needs is to observe his or her walking cycle, which can be divided into two parts: the 'standing phase' (when the leg is on the ground supporting the body) and the 'swing phase' (when the leg is off the ground, also known as "stretching"). The happy medium between these two extremes (standing or stability, versus swinging or bending with ease) varies from person to person.

Although there are over 100 individual knee mechanisms on the market, they can be divided into two main groups: mechanised and computerised. Mechanical knee joints can be further divided into two groups: uniaxial knee joints and polycentric or multi-axial knee joints. All knee mechanisms, regardless of their complexity, require additional stability mechanisms (manual or weight activated locking systems) and additional motion control mechanisms (constant or variable friction and "fluid" pneumatic or hydraulic control).

How do I choose between standard or custom-made instruments?

For a complete knee replacement, you can choose between standard or custom-made instruments. These are the differences.

Standard instruments: The technique involves planning the procedure using standard X-rays followed by femoral and tibial cutting using a standard resection template, which will vary depending on the size of the patient. The surgeon must therefore rely on X-ray measurements and calculations to correctly position the templates and make the required cuts.

Four-Bar Pneumatic Knee Joint

Customised instrumentation: In recent years, implant manufacturers have developed three-dimensional pre-operative planning systems. The patient must first undergo a CT scan or MRI and then use 3D printing to design a customised excision template. They are usually made of nylon. The advantage of this technology is that it is more practical for surgeons who use excision templates that are perfectly adapted to the patient's form for bone cutting. However, the long latency of these 3D printed resection templates can be an obstacle if the operation has to be performed quickly.

Which alignment should I choose?

There are three alignment options available when implanting prosthetic components: neutral mechanical alignment, kinematic alignment or hybrid alignment. These are the differences.

Neutral mechanical alignment: the skeletal component and the component are aligned on the mechanical axis of the lower limb (femur + tibia). Historically, this technique has been advocated as the only option to avoid premature prosthesis failure and the need for early revision surgery. However, this was true in the early days of knee arthroplasty, when the quality of implants and fixation methods were even more rudimentary than today. Today, kinematic or hybrid alignment techniques are also accepted and widely practised.

Kinematic alignment: In this case, the cut follows the kinematics of the knee joint in motion, maintaining the alignment of the limb in its original deformity. The alignment angle is therefore defined in a patient-specific manner.

Hybrid alignment: one part of the cut is aligned with the mechanical axis of the lower limb and the other part follows the kinematics of the knee joint.