Purpose: Rotational alignment is important for patellar tracking, ligament balance, and tibiofemoral congruity after total knee arthroplasty (TKA). The posterior condylar axis is often referred to as a rotational alignment landmark. However, articular cartilage wear localized only in the medial condyle might affect the accuracy of rotation, because surgical planning based on CT does not consider the cartilage thickness. The purpose of this study was to clarify whether the cartilage thickness of the posterior condyle affects rotational alignment after TKA.
Methods: A total of 40 osteoarthritis patients waiting for TKA were recruited. MRI of axial sections was performed preoperatively. Scans were controlled to make the cross section perpendicular to the mechanical axis of the femur on the coronal plane and to the tangent line of the distal femur on the sagittal plane, so that the surgical section of the actual femur could be simulated. The condylar twist angle (CTA) was measured with and without articular cartilage. The cartilage thickness on the medial and lateral posterior condyles was surveyed in both MRI images and surgical specimens.
Results: The CTA without cartilage (6.8 ± 2.0°) was significantly larger than the CTA with cartilage (5.2 ± 2.0°) (P < 0.01), and 12 knees (30%) demonstrated differences of more than 2 degrees. The cartilage depicted in MRI showed almost the same thickness as the actual specimens and was significantly thicker on the lateral condyles.
Conclusions: Surgical planning for TKA not considering articular cartilage might lead to the externally rotated malposition of the femoral implant.
Level of evidence: II.