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

To balance a tight medial flexion gap in functionally aligned total knee arthroplasty (FA-TKA), the femoral component is often externally rotated to the posterior condylar axis (PCA) such that more bone is removed from the medial posterior femoral condyle than is replaced with prosthesis. A reduction in the posterior condylar offset (PCO) has been thought to cause decreased flexion and poor outcomes. We performed a sawbone investigation to quantify posterior femoral condylar resection depth in mechanically aligned TKA using conventional resection guides, including the variations that occur with different degrees of external rotation (ER) to the PCA. Seven left synthetic knee sawbone models of the same size were placed in a stable external vice grip and had posterior condylar resections performed for varying degrees of ER. The first trial had the guide set at a neutral (0°) angle to the PCA. The guide was then set to 1.5° increments of increasing ER from neutral (1.5°, 3°, 4.5°, and 6° of ER) and then to 3° and 6° of internal rotation (IR) from neutral. The thickness of the saw blade was added to a caliper measurement of resected bone to give the total resection thickness. The difference between this measurement and the implant thickness was then calculated to assess the resultant change to PCO both medially and laterally. Neutral and externally rotated femoral cuts all yielded a reduction in the medial PCO. In a "standard" 3° externally rotated cut, there were 10 mm of posterior condyle resected medially and 2.5 mm of posterior condyle resected laterally (including bone and cartilage). A neutral cut (8.7 mm) very closely approximated posterior implant thickness (8.5 mm), resulting in the restoration of native PCO. This study provides valuable insights into the changes to PCO during instrumented mechanical TKA that can assist surgeons in considering resection depths when moving to robotic knee arthroplasty systems, where resection depths are preoperatively templated and displayed intraoperatively for the surgeon. A standard workflow with 3° of femoral ER routinely decreases the medial PCO by up to 1.5 mm or, in the case of full-thickness cartilage wear, up to 5 mm. Further research should examine the relationship between the deliberate reduction of PCO and patient functional outcomes in the context of alternative alignment philosophies in TKA.