Abstract
Introduction
CT based systems that are used to create custom components and custom cutting guides in total knee arthroplasty (TKA) have variable methods for accounting for the thickness of remaining cartilage that may influence component sizing and bone resection. Little information has been published about the thickness of this cartilage, especially on the posterior femoral condyles. Failure to account for this cartilage may lead to under-sizing of the femoral component, or a reduction in the posterior condylar offset that may adversely affect flexion after TKA.
Methods
This IRB approved, retrospective study included 140 consecutive patients who underwent primary TKA. The medial and lateral posterior condylar bone cuts were performed in the usual manner with mechanical instruments. The resected specimen was sectioned in the sagital plane and the cartilage thickness was measured at the mid portion to the nearest millimeter.
Results
The mean cartilage thickness was 1.7 mm (range, 0 to 4 mm) on the medial posterior condyle and 2.0 mm (range, 0 to 5 mm) on the lateral posterior condyle. There was no correlation between the remaining medial and lateral posterior condylar cartilage thickness (p=0.35).
Conclusions
The thickness of remaining cartilage on the posterior condyles of the femur at the time of TKA is between 0 and 5 mm. This variable cartilage thickness may be poorly visualized on the CT studies used for creating custom femoral components or custom femoral cutting guides. This variability is greater than the 3 to 4 mm differences in AP measurement between femoral sizes in most contemporary TKA systems. Therefore, CT based custom systems may reproduce femoral sizing and posterior condylar offset less well than off the shelf femoral components implanted with conventional instruments. Future studies will be needed to evaluate the accuracy of component sizing between CT versus MRI based systems.
