Knee Prosthesis. (Part Three)

Posterior Cruciate Ligament

The primary function of the posterior cruciate ligament (PCL) is to allow femoral “rollback” in flexion and to resist posterior displacement of the tibia on the femur. The PCL also controls external rotation of the tibia and increases in knee flexion.

Preservation of the PCL in Knee Prostheses is important as it biometrically ensures normal kinematics of femoral “rollback” on the tibia. This is also important for improving the lever arm of the quadriceps mechanism with the knee in flexion.

Patellofemoral joint

Movements of the patellofemoral joint can be characterized as sliding movements. During knee flexion, the patella moves distally over the femur. This movement is achieved by the insertions in the patella of the quadriceps tendon and the patellar ligament, as well as by the anterior part of the femoral condyles. The muscles and ligaments of the patellofemoral joint are responsible for knee extension. The patella plays the role of a pulley in transmitting forces from the quadriceps muscle to the femur and patellar ligament.

Mechanical Axes

The mechanical axis of the lower limb is an imaginary line through which the body's weight passes. It goes from the center of the coxo-femoral to the center of the talocrural joint passing through the middle of the knee. In the presence of various deformities, this line is altered and must be surgically restored, which allows the normalization of gait and preserves the prosthesis from eccentric loads and early complications.

Contraindications

The absolute contraindications for Knee Prosthesis include: sepsis, active present infection in the organism, malfunction of the extensor mechanism, severe vascular disease, recurvatum deformity as a result of muscle mass loss, and the presence of a functioning arthrodesis.

Relative contraindications include health conditions that do not allow the performance of anesthesia as well as the needs of surgery and rehabilitation. Other relative contraindications include the condition of the skin over the knee to be intervened (e.g., psoriasis), history of osteomyelitis around the knee, neuropathic joint, and obesity.

Results and Prognosis

Most patients are satisfied with the Knee Prosthesis, especially when the main indication for intervention has been pain relief. Knee function usually returns quickly after knee prosthesis. Long-term studies confirm satisfactory functional results and show a survival rate of 91-96% in 14-15 years after prosthesis.

No difference has been found between PCL-retaining prostheses and PCL-sacrificing prostheses. For cementless prostheses, there is not yet any long-term study, but studies 10-12 years after prosthesis show a survival rate of 95%.

The Future and Discussions

Cemented Total Knee Prostheses will remain the standard criterion for knee prosthesis, but also the use of cementless prostheses with bioactive surfaces (e.g., hydroxyapatite) have shown promising midterm results.

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