Choosing the Right Knee Prosthesis

During a knee replacement surgery, the surgeon may choose to place a complete (three-compartment) or partial (unicompartmental) prosthesis depending on the condition of the knee and ligaments.

• Three-compartment prostheses: these replace all of the damaged cartilage in the knee (this is the case for most arthritis) and sometimes require the removal of certain cruciate ligaments. This can include all of the tibiofemoral cartilage and also sometimes that of the patellofemoral joint. There are three-compartment prostheses with sliding or hinged joints, the latter being used less frequently today.
• Unicompartmental prostheses: also called hemi-prostheses or partial prostheses, these replace only the cartilage of the injured compartment without touching the other compartments of the knee or the ligaments. This can be the medial or lateral compartment. They are used when arthritis is limited to a single compartment. The other compartments and ligaments must be healthy.

For a complete knee replacement, you can choose between standard or customized instrumentation. These are the differences:

• Standard instrumentation: this technique consists of planning the operation using standard radiographs and then making the femoral and tibial bone cuts using standard resection templates, which will simply vary according to the size of the patient. The surgeon must therefore rely on the measurements and calculations made from the X-rays in order to correctly position the template and make the desired cuts.
• Customized instrumentation: In recent years, implant manufacturers have developed a three-dimensional preoperative planning system. The patient must first undergo a CT scan or MRI, then customized resection templates are designed using 3D printing. They are often made of nylon. The advantage of this technique is that it is much more practical for the surgeon who uses a resection template perfectly adapted to the patient’s morphology to make the bone cuts. However, the delay for these 3D printed resection templates is longer, which can be an obstacle if the operation has to be performed quickly.

The tibial bearing is one of the components of the three-compartment or unicompartmental sliding knee prosthesis. There are fixed-bearing knee prostheses and mobile-bearing knee prostheses.

Mobile inserts have gained in popularity in recent years as, in theory, they have certain advantages over fixed inserts, particularly in terms of improved movement and service life.

However, to date, there is no concrete evidence of significant clinical improvements related to the use of mobile bearings.

Aston Medical three-compartment mobile-bearing knee prosthesis

Three types of alignment can be chosen when implanting the prosthesis components: neutral mechanical alignment, kinematic alignment or hybrid alignment. These are the differences:

• Neutral mechanical alignment: the bone sections and components are aligned on the mechanical axis of the lower limb (femur + tibia). Historically, this technique was advocated as the only option to avoid premature failure of the prosthesis and the need for early revision surgery. But this was true in the early days of knee replacement, when the quality of the implants and the methods of fixation were more rudimentary than today. Today, kinematic or hybrid alignment techniques are accepted and widely practiced as well.
• Kinematic alignment: in this case the cuts follow the kinematics of the knee in motion, maintaining the alignment of the limb in its original deformation. The alignment angle is therefore defined in a patient-specific way.
• Hybrid alignment: a part of the cuts is aligned with the mechanical axis of the lower limb and another part follows the kinematics of the knee.

Three types of complications can occur during knee replacement surgery: intraoperative complications, early complications and secondary complications. These complications are related to the nature of the procedure itself and don’t include possible risks related to anesthesia or surgical risks related to the patient’s age and medical history.

These are the three types of complications:

• Intraoperative complications: these are exceptional and can be due, for example, to injury to a major artery of the lower limb (popliteal artery) or a nerve (external popliteal sciatic nerve) during the operation.
• Early complications: the main complication of this type is infection. This is a serious complication, but it remains rare. Monitoring in the weeks following the operation makes it possible to detect this complication and to treat the microbe responsible with suitable antibiotics. Another operation is often necessary in order to clean the joint and the prosthesis. Early complications also include phlebitis, hematoma, knee stiffness, etc.
• Secondary complications: these include mechanical complications related to the functioning of the prosthesis, late infection which may be due to contamination of the prosthesis by an infection far from the knee (dental abscess, urinary infection, etc.), or knee stiffness which occurs a while after the operation.