Frontal plane stability following UKA in a biomechanical study

Area of Science:

• Orthopedic Surgery
• Biomechanics
• Biomedical Engineering

Background:

• Unicondylar knee arthroplasty (UKA) aims to restore native knee function and kinematics.
• Daily activities impose varus and valgus moments on the knee, challenging joint stability.
• Medial compartment replacement via UKA may alter the knee's response to these moments.

Purpose of the Study:

• To investigate the effect of medial UKA on knee joint stability and frontal plane motion.
• To test the hypothesis that UKA stiffens the knee and decreases range of motion in the frontal plane.

Main Methods:

• Six cadaver knees were tested using a robotic system under axial load and varus/valgus moments.
• Measurements were taken before and after medial UKA implantation.
• Tests were repeated with an intentionally "overstuffed" medial compartment to simulate polyethylene inlay thickness variations.

Main Results:

• Frontal plane range of motion decreased after UKA, shifting towards valgus.
• Knees with UKA exhibited increased stiffness compared to native knees when subjected to valgus moments.
• Medial overstuffing exacerbated the stiffening effect and reduced range of motion.

Conclusions:

• UKA replaces the compliant natural anatomy with stiffer prosthetic components.
• Medial overstuffing and lack of natural medial compression tighten the medial collateral ligament, leading to increased joint stiffness.
• Avoiding medial overstuffing during UKA is crucial for preserving knee kinematics and stability.