Instrumented Mechanical Total Knee Arthroplasty Routinely Decreases the Medial Posterior Condylar Offset

Area of Science:

• Orthopedic Surgery
• Biomechanical Engineering

Background:

• Balancing knee flexion gaps in total knee arthroplasty (TKA) often involves external rotation of the femoral component.
• This rotation can lead to reduced posterior condylar offset (PCO), potentially affecting flexion and patient outcomes.

Purpose of the Study:

• To quantify the depth of posterior femoral condylar resection in mechanically aligned TKA.
• To analyze how varying degrees of external rotation (ER) to the posterior condylar axis (PCA) influence PCO changes.

Main Methods:

• A sawbone investigation using seven synthetic knee models.
• Posterior condylar resections were performed with varying degrees of ER (0° to 6°) and internal rotation (IR) (3°, 6°) relative to the PCA.
• Resection depth was measured, and changes to medial and lateral PCO were calculated relative to implant thickness.

Main Results:

• All neutral and externally rotated femoral cuts resulted in a reduction of medial PCO.
• A standard 3° ER cut resected 10 mm medially and 2.5 mm laterally, decreasing medial PCO.
• A neutral cut closely approximated implant thickness, restoring native PCO.

Conclusions:

• Standard surgical workflows involving femoral ER in TKA routinely decrease medial PCO.
• Understanding these PCO changes is crucial for surgeons, especially with the advent of robotic TKA systems.
• Further research is needed to correlate deliberate PCO reduction with patient functional outcomes in TKA.